1. Myctophid feeding ecology and carbon transport along the northern
Mid-Atlantic Ridge
Jeanna M. Hudson a,n
, Deborah K. Steinberg a,nn
, Tracey T. Sutton b
,
John E. Graves a
, Robert J. Latour a
a
Virginia Institute of Marine Science, College of William & Mary, P.O. Box 1346, Gloucester Point, VA 23062, USA
b
NSU Oceanographic Center, 8000 North Ocean Drive, Dania Beach, FL 33004, USA
a r t i c l e i n f o
Article history:
Received 25 April 2014
Received in revised form
4 July 2014
Accepted 8 July 2014
Available online 24 July 2014
Keywords:
Lanternfish
Diet
Zooplankton
Mesopelagic
Diel vertical migration
Biological pump
a b s t r a c t
Myctophids are among the most abundant fishes in the world's ocean and occupy a key position in
marine pelagic food webs. Through their significant diel vertical migrations and metabolism they also
have the potential to be a significant contributor to carbon export. We investigated the feeding ecology
and contribution to organic carbon export by three myctophid species, Benthosema glaciale, Proto-
myctophum arcticum, and Hygophum hygomii, from a structurally and ecologically unique ecosystem- the
Mid-Atlantic Ridge (MAR). Similar to the results of previous studies, the diet of these fishes was
primarily copepods and euphausiids, however, gelatinous zooplankton was identified in the diet of
B. glaciale for the first time. Ridge section and time of day were significant explanatory variables in the
diet of B. glaciale as determined by canonical correspondence analysis, while depth was the only
significant explanatory variable in the diet of P. arcticum. Daily consumption by MAR myctophids was
less than 1% of dry body weight per day and resulted in the removal of less than 1% of zooplankton
biomass daily. Although lower than previous estimates of carbon transport by myctophids and
zooplankton in other areas, MAR myctophid active transport by diel vertical migration was equivalent
to up to 8% of sinking particulate organic carbon in the North Atlantic. While highly abundant,
myctophids do not impart significant predation pressure on MAR zooplankton, and play a modest role
in the active transport of carbon from surface waters.
& 2014 Elsevier Ltd. All rights reserved.
1. Introduction
Fishes of the family Myctophidae are an important component
of the marine pelagic food web due to their abundance and duality
as both prey and predator in the epi- and mesopelagic zones.
Myctophids are important prey for deep-sea and epipelagic
piscivorous fishes, marine mammals, and sea birds (Hopkins et al.,
1996; Beamish et al., 1999; Pusineri et al., 2008; Pereira et al.,
2011). As predators, myctophids feed primarily on crustacean
zooplankton, but are also known to feed on gelatinous zooplank-
ton, pteropods, and other non-crustacean prey including fishes
(Kinzer, 1982; Sameoto, 1988; Hopkins et al., 1996; Moku et al.,
2000). Within midwater fish assemblages, myctophids can be the
most important consumer (Hopkins et al., 1996), and have a
significant impact on zooplankton populations, consuming 8–16%
of the total copepod daily production and 2% of the overall
zooplankton biomass each night in the Gulf of Mexico (Hopkins
and Gartner, 1992), and 2–31% of the zooplankton standing stock
daily in the equatorial Pacific (Gorelova, 1984).
In this study, we focus on the feeding ecology of myctophids
from a structurally and ecologically unique ecosystem – the Mid-
Atlantic Ridge (MAR). The MAR was the location of a project to
describe and understand the patterns of distribution, abundance,
and trophic relationships of organisms inhabiting the northern
MAR between Iceland and the Azores (MAR-ECO; Bergstad, 2002).
Sutton et al. (2008) characterized the midwater fish composition
at the MAR during June–July, 2004 and reported that the family
Myctophidae was the numerically dominant fish family (59% of all
fishes collected), with one species, Benthosema glaciale, the most
abundant species collected.
Diet studies of myctophids from seamounts (Pusch et al., 2004,
Colaço et al., 2013) show elevated feeding, which is hypothesized to
be due to turbulent mixing resulting from the unique hydrography
associated with these structures (Pusch et al., 2004). Structurally
similar to seamounts, mid-ocean ridges have the potential to increase
food availability to benthic and planktonic consumers through
resuspension of sediment (Genin and Boehlert, 1985; Dower et al.,
1992; Muriño et al., 2001) and trapping of laterally advected and
vertically migrating zooplankton by the raised bottom of the ridge
Contents lists available at ScienceDirect
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Deep-Sea Research I
http://dx.doi.org/10.1016/j.dsr.2014.07.002
0967-0637/& 2014 Elsevier Ltd. All rights reserved.
n
Corresponding author. Tel.: þ1 804 684 7165.
nn
Corresponding author. Tel.: þ1 804 684 7838.
E-mail addresses: jeannak@vims.edu (J.M. Hudson),
debbies@vims.edu (D.K. Steinberg).
Deep-Sea Research I 93 (2014) 104–116

2. (Genin and Dower, 2007; Porteiro and Sutton, 2007), which may
sustain a unique community and trophic structure compared to off-
ridge waters. Although several studies have reported on the feeding
ecology of myctophid species from seamounts or other environments
(Hopkins et al., 1996; Pakhomov et al., 1996; Pusch et al., 2004;
Petursdottir et al., 2008; Dypvik et al., 2012), none exist for
myctophids from mid-ocean ridge systems.
Furthermore, little quantitative data exist on the role of these
abundant consumers in carbon cycling. Many myctophid species
make daily vertical migrations to the epipelagic zone at night to
feed on zooplankton, and migrate to deeper water ($400–
1000 m) during the day where apparently most of the food is
digested (Baird et al., 1975). By metabolizing this surface-derived
food in the mesopelagic zone, myctophids actively transport
organic and inorganic carbon to depth, a process which is an
important component of the biological pump (Ducklow et al.,
2001). Active transport of carbon via mortality, egestion of fecal
pellets, respiration of CO2, and excretion of dissolved organic
carbon at depth have been determined for vertically migrating
zooplankton in a variety of environments (e.g., Longhurst et al.,
1990; Steinberg et al., 2000; Al-Mutairi and Landry, 2001), but
only two previous studies have quantified active transport by
vertically migrating myctophids. Hidaka et al. (2001) reported
myctophids in the equatorial Pacific actively transported the
equivalent of 15–28% of passively sinking particulate organic
carbon (POC) measured by sediment traps, and Davison et al.
(2013) estimated myctophid vertical migration could account for
$8% of passively sinking POC (as estimated from satellite data) in
the northeast Pacific Ocean. There are no previous estimates of
active transport by myctophids for the Atlantic Ocean.
The first goal of our study was to describe and quantify the
feeding ecology of three myctophid species from the northern
MAR (Benthosema glaciale, Protomyctophum arcticum, and Hygo-
phum hygomii) to investigate how the presence of a mid-ocean
ridge may affect myctophid diet composition. Our second goal was
to provide an estimate of active carbon transport by myctophids at
the MAR, a first for the North Atlantic Ocean. Diet composition,
daily consumption rates, and carbon export of these fishes were
used to assess their role in the overall trophic structure and
carbon cycle of this topographically and hydrodynamically unique
ecosystem.
2. Methods
2.1. Sampling procedure
Myctophids were collected during the R/V G.O. Sars research
expedition to the northern MAR (Iceland to the Azores) during
June–July, 2004. Two double-warp, multi-cod end midwater trawls
were used to sample the ridge fauna in discrete depth zones. The
macrozooplankton trawl has a 6 Â 6 m mouth opening, 6 mm
stretched mesh throughout its length, and was equipped with five
opening and closing cod ends. The Åkra trawl has a 20–35 m
vertical mouth opening, 110 m door-spread, graded mesh to
22 mm (stretched), and was equipped with three multiple opening
and closing cod ends. Volume of water filtered was calculated
using the trawl mouth area, towing speed, and distance traveled.
Predefined stations along the ridge were sampled discretely
within five depth categories: 0–200, 200–750, 750–1500, 1500–
2300, and 42300 m in four ridge sections (Fig. 1). Samples were
classified as day (D), dusk (DN), night (N), or dawn (ND) with dusk
and dawn samples defined as the start time of the net being one
hour before to one hour after sunset and sunrise, respectively
(Sutton et al., 2008). Once on board, specimens were sorted and
either frozen whole or preserved in 10% buffered formalin.
Preserved samples were identified and transferred to 70% ethanol
in the laboratory. For additional details concerning net sampling
aboard the R/V G.O. Sars, (see Wenneck et al., 2008).
Trawl gear provides measures of relative fish biomass (Koslow
et al.,1997; Kaartvedt et al., 2012), which implies information on
gear efficiency is needed for the estimation of any biomass-
normalized rate processes. Using hydroacoustics, Kaartvedt et al.
(2012) reported a value of 0.14 for the sampling efficiency of B.
glaciale collected by a large ($400 m2
mouth area) Harstad trawl
net. For the gears used in the present study, Heino et al. (2011)
determined the Åkra trawl was more efficient at sampling fishes
than the macrozooplankton trawl, with a relative catchability of
2.3. Despite the improved efficiency of the Åkra trawl, we elected
to base our myctophid biomass and carbon transport calculations
on data obtained by the macrozooplankton trawl since the fixed
mouth area of this gear allowed for a more accurate calculation of
volume of water filtered. For all calculations based on data
collected by the macrozooplankton trawl, both the actual biomass
and biomass corrected for an assumed sampling efficiency of 0.14
are reported.
2.2. Sample selection, dissection, and prey identification
Three myctophid species were selected for analysis – Bentho-
sema glaciale, Protomyctophum arcticum, and Hygophum hygomii –
based on their diverse geographic distributions and availability of
samples. A subset of specimens from the total cruise catch was
selected for measurement and dissection from the available
geographic-location, depth, and time-of-day combinations. A total
of 380 fishes were processed for diet composition information
broken down by species as follows: 265 B. glaciale, 76 P. arcticum,
and 39 H. hygomii. The standard length of each fish was recorded
to the nearest 0.1 mm and the stomach and intestines were
excised. Prey was identified microscopically to the lowest possible
Fig. 1. Trawl sampling stations at four ridge sections along the northern Mid-
Atlantic Ridge from Iceland to the Azores during the 2004 R/V G.O. Sars MAR-ECO
expedition. Black line represents the approximate location of the Sub-polar Front
(301W, 521N) at the time of the cruise.
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116 105

3. taxonomic level using a Nikon SMZ 1000 dissecting microscope.
Diet descriptions and analyses included prey from stomachs only;
intestinal prey were not included due to the tendency for
advanced digestion resulting in low taxonomic resolution. Prey
types that were observed infrequently were grouped using genus
or family classifications to increase sample size. Diet indices were
calculated for the lowest taxonomic level of grouped prey types
that provided adequate sample size, as well as for broad prey
categories at the subclass level (e.g., copepods) (Table 1).
2.3. Prey measurements
The length and width of each prey item was recorded to the
nearest 0.01 mm using Image Pro Plus 5.0 software. Cephalosome
and urosome length and width measurements were determined
for copepods. Total length and width were determined for all prey
for which body measurements were possible. For well-digested
prey, average body measurements from intact, related taxa were
used, while hook length was used to determine body length and
width of chaetognaths (Pearre 1980; Terazaki 1993). Body mea-
surements of prey items were used to calculate body volume using
formulae for the most similar geometric shape. Prey volume was
then used to estimate wet weight (assuming specific gravity-
¼1.0 g ccÀ1
). Crustacean dry weight was calculated as 20% of wet
weight and carbon as 40% of dry weight (Silver and Gowing, 1991;
Steinberg et al., 1998). Conversion factors for other, less abundant,
prey taxa were utilized as described in Larson (1986) and
Steinberg et al. (1998).
2.4. General diet description
The diet of each myctophid species was summarized using
three diet indices: percent frequency of occurrence, percent
composition by number, and percent dry weight (Hyslop, 1980).
The indices, represented generally as %Ik, were calculated using a
cluster sampling estimator (Buckel et al., 1999; Latour et al., 2008)
of the following form:
%Ik ¼
∑n
i ¼ 1Miqik
∑n
i ¼ 1Mi
 100 ð1Þ
such that
qik ¼
wik
wi
ð2Þ
where k represents prey type, n is the number of cod ends
containing a predator, and Mi is the number of predators collected
in cod end i (with one cod end representing one depth zone at an
individual station). In Eq. (2), qik represents the proportion of
occurrence, abundance, or weight of each prey type in each cod
end. Diet composition indices were calculated as a weighted
average of qik with the abundance of each predator, Mik, as the
weighting factor. The variance for each diet index was calculated
as
varð%IkÞ ¼
1
nM
2
∑n
i ¼ 1M2
i ðqik ÀIkÞ2
nÀ1
 1002
ð3Þ
after Latour et al. (2008), where M is the average number of a
particular predator collected in a cod end, and was used to
calculate standard error for each diet index.
2.5. Ontogenetic and spatial changes in diet
To examine spatial and ontogenetic patterns in the diet,
individuals of B. glaciale and P. arcticum (there were too few H.
hygomii to conduct these analyses) were grouped into narrow,
5 mm size classes with the members of each class having a
relatively similar diet composition. The proportion of dry weight
of each prey type was then calculated for each size class. Dry
weight was used as the reference index to investigate dietary
patterns as it is the least biased compared to abundance and
frequency of occurrence indices which are heavily influenced by
the size and number of prey. The narrow size classes were grouped
into broader categories with similar prey composition based on
prey weight using cluster analysis (Euclidean distance, average
linkage method, Sokal and Michener, 1958). A scree plot was used
to determine the number of clusters based on the average distance
between clusters.
Canonical correspondence analysis (CCA, ter Braak, 1986),
a method which extracts the major gradients in a data set that
can be accounted for by the measured explanatory variables
(McGarigal et al., 2000), was used to investigate the relationship
between the diets of B. glaciale and P. arcticum and ridge section
(RR, CGFZ, FSZ, AZ), depth zone (DZ 1, 0–200 m; DZ 2, 200–750 m;
DZ 3, 750–1500 m), and time of day (day, dusk, night, dawn). Each
element of the response matrix for the CCA was the mean percent
dry weight of a given prey type in a particular depth, ridge section,
and time-of-day combination. Variability is explained by the
canonical axes, which are linear combinations of the independent
explanatory variables. Significance of the explanatory variables
was determined using ANOVA, and a biplot was constructed to
explore the relationships between the explanatory variables and
prey weight. The CCA and cluster analysis were performed using
R version 2.12.0.
2.6. Gastric evacuation and daily consumption
Daily consumption for myctophids in specific ridge sections
was calculated using an evacuation rate model based on Elliott and
Persson (1978). Consumption (Cdkr, μg DW dÀ1
fishÀ1
) was calcu-
lated as
Cdkr ¼ 24ErSkr ð4Þ
where 24 is the number of hours in a day, Er is the evacuation rate
(hÀ1
) at ridge section r, and Skr is the mean stomach content dry
weight of predator k (μg DW prey fishÀ1
) from ridge section r
(Durbin et al., 1983; Link and Garrison, 2002; Tanaka et al., 2013).
A temperature-based evacuation rate model was derived from a
regression analysis of compiled myctophid gut evacuation rate
information (E¼0.0942e0.0708t
, where t represents temperature in
1C, Table 2; Pakhomov et al., 1996). The bulk of digestion by
myctophids was assumed to occur within depth zone 2 (200–
750 m) and day time temperature information collected synopti-
cally with fish sampling in depth zone 2 was used to derive ridge
section-specific evacuation rates. Daily consumption rates per fish
were converted to consumption per unit of average myctophid
biomass and were multiplied by the biomass of all myctophid
species combined. Myctophid biomass was integrated to 2300 m
to provide a range of possible daily consumption estimates
(μg DW mÀ2
dÀ1
) of MAR myctophids.
2.7. Active carbon export by diel vertical migration
Active carbon export to below 200 m via myctophid respiration
of CO2, excretion of dissolved organic carbon (DOC), and egestion
of POC in the form of fecal pellets were calculated for ridge
sections at which both day and night tows were performed in
depth zone 1 (0–200 m) and for which night time myctophid
biomass was greater than day time myctophid biomass in depth
zone 1. The RR and AZ were the only ridge sections at which these
criteria were met, with two day and two night tows performed at
each ridge section. Migrator biomass was defined as the difference
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116106

4. Table 1
Prey composition from stomachs of Benthosema glaciale, Protomyctophum arcticum, and Hygophum hygomii collected during a cruise to the Mid-Atlantic Ridge during June–
July, 2004. n is sample size, %W is percent dry weight, %N is percent composition by number and %F is percent frequency of occurrence of each prey taxa. (À) indicates prey
was absent.
Prey B. glaciale P. arcticum H. hygomii
n %W %N %F n %W %N %F n %W %N %F
Copepoda
Calanoida
Aetideus sp. 1 0.36 0.70 9.46 4 0.66 1.38 2.99 À À À À
Aetideus armata À À À À 9 0.45 1.97 7.75 À À À À
Aetideopsis sp. 1 0.00 0.00 0.02 À À À À À À À À
Chiridiella sp. À À À À 1 0.29 0.43 1.71 À À À À
Euchirella sp. 1 0.00 0.00 0.02 À À À À À À À À
Gaetanus sp. 1 0.01 0.01 0.34 2 1.33 0.61 2.51 À À À À
Pseudochirella sp. 1 0.00 0.01 0.34 À À À À À À À À
Other Aetideidae 71 1.28 2.84 24.2 22 3.13 4.84 16.5 6 0.26 1.80 18.6
Calanus sp. 164 9.00 16.3 44.4 47 5.88 7.81 22.6 À À À À
Calanus finmarchicus 388 27.9 51.4 71.7 94 8.14 12.2 14.8 À À À À
Other Calanidae 116 2.88 4.56 5.80 11 1.18 1.87 7.68 46 1.33 7.16 54.8
Candacia sp. À À À À 39 1.13 6.99 65.4
Candacia armata 1 0.00 0.00 0.02 À À À À À À À À
Other Candaciidae 5 0.04 0.06 0.07 À À À À À À À À
Euchaeta sp. 1 0.00 0.00 0.02 À À À À À À À À
Paraeuchaeta sp. 1 0.01 0.00 0.01 À À À À 1 0.09 0.12 2.06
Paraeuchaeta norvegica 37 5.99 1.87 20.6 17 14.7 3.88 17.7 6 0.87 0.70 8.22
Paraeuchaeta tonsa 1 0.00 0.00 0.02 À À À À 1 0.09 0.12 2.06
Other Euchaetidae 22 0.37 0.87 11.6 12 3.20 3.30 6.84 À À À À
Heterorhabdus sp. 10 0.04 0.13 0.67 3 0.24 0.53 1.62 À À À À
Heterorhabdus compactus 1 0.00 0.00 0.02 À À À À À À À À
Heterostylites longicornis 1 0.00 0.00 0.02 À À À À À À À À
Heterostylites sp. 12 0.00 0.03 0.20 1 0.26 0.43 1.50 À À À À
Other Heterorhabdidae À À À À 1 0.01 0.15 1.54 À À À À
Metridia sp. 87 0.21 1.25 4.21 71 6.27 12.2 22.5 À À À À
Metridia curticauda 1 0.00 0.01 0.13 À À À À À À À À
Metridia lucens 1 0.00 0.00 0.03 46 4.79 7.84 6.45 À À À À
Pleuromamma sp. 218 1.42 4.18 39.4 37 4.67 8.45 27.2 82 2.17 17.2 80.2
Pleuromamma abdominalis 5 0.01 0.02 0.15 1 0.11 0.19 1.02 11 0.19 1.28 8.22
Pleuromamma borealis 5 0.01 0.02 0.08 À À À À À À À À
Pleuromamma gracilis 2 0.00 0.00 0.04 À À À À À À À À
Pleuromamma robusta 1 0.00 0.00 0.02 À À À À À À À À
Pleuromamma xiphias 1 0.00 0.00 0.02 À À À À À À À À
Other Metridinidae 48 0.19 1.03 3.87 20 3.06 4.54 17.4 À À À À
Paracalanus sp. 1 0.00 0.00 0.02 À À À À À À À À
Other Spinocalanidae 1 0.00 0.00 0.02 À À À À À À À À
Other Calanoida 129 1.26 3.04 8.86 50 7.95 9.65 38.2 39 1.55 7.53 56.4
Cyclopoid
Oncaea sp. 21 0.02 0.77 9.84 2 0.02 0.29 2.62 27 0.06 5.36 55.2
Poecilostomatoida 1 0.00 0.00 0.02 À À À À À À À À
Sapphirina sp. 1 0.00 0.01 0.06 À À À À À À À À
Other Copepoda 20 0.52 0.98 12.4 10 1.39 2.11 10.4 4 0.15 1.02 14.8
Amphipoda 19 0.34 0.12 2.21 À À À À 17 4.92 5.12 51.6
Themisto sp. 10 0.09 0.06 0.40 À À À À À À À À
Themisto compressa 2 0.10 0.07 1.64 À À À À À À À À
Phronima sp. 1 0.45 0.11 0.30 À À À À À À À À
Other Hyperiidae 14 0.21 0.28 1.37 À À À À 1 0.25 0.27 1.83
Euphausiacea 34 28.7 1.82 17.3 14 24.9 2.61 13.4 34 68.8 7.49 56.6
Euphausiidae 25 8.35 0.51 4.08 À À À À À À À À
Myodocopoda 247 3.11 3.72 19.2 55 4.87 9.70 35.2 162 14.4 37.1 80.4
Conchoecia sp. 2 0.01 0.01 0.04 À À À À À À À À
Conchoecia magna 2 0.01 0.01 0.04 À À À À À À À À
Conchoecetta acuminate 1 0.00 0.00 0.02 À À À À À À À À
Conchoecilla daphnoides 1 0.00 0.00 0.02 À À À À À À À À
Other Conchoecinae 6 0.01 0.01 0.10 13 1.52 2.30 4.26 À À À À
Pteropoda 1 0.00 0.01 0.02 À À À À À À À À
Limacina sp. 1 0.00 0.01 0.02 À À À À À À À À
Cavoliniidae À À À À À À À À 2 0.00 0.23 2.06
Gelata 6 0.10 0.37 1.02 À À À À À À À À
Thaliacea 1 0.01 0.11 0.30 À À À À À À À À
Chaetognatha 13 6.08 1.05 5.48 À À À À À À À À
Polychaeta 1 0.01 0.01 0.05 À À À À À À À À
Osteichthyes 2 0.08 0.04 0.00 À À À À 1 3.49 0.20 5.40
Crustacea 8 0.07 0.05 0.18 2 0.51 0.55 2.14 À À À À
Unidentified prey 23 0.79 1.56 11.8 1 0.40 0.17 2.05 2 0.14 0.31 7.46
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116 107

5. between night and day integrated myctophid biomass in the
0–200 m depth interval.
A sensitivity analysis was used to estimate CO2, DOC, and POC
flux using a range of values where available (Table 3). Respiratory
flux for all myctophid species at ridge section r was calculated
using the following equation (Dam et al., 1995):
Fr ¼ BrRr12 ð5Þ
where Fr is the CO2 flux (mg C mÀ2
dÀ1
), Br is the migrator
biomass integrated to 200 m (mg DW mÀ2
), Rr is the weight-
specific migrator respiration rate (mg C mg DWÀ1
hÀ1
) at tem-
peratures experienced during day time at depths of 200–750 m,
and 12 is the assumed number of hours spent at depth over a
24-h day.
The term R was calculated using the myctophid oxygen con-
sumption vs. temperature (1C) regression reported in Donnelly and
Torres (1988): y¼(0.016 n temperature)À0.07, where y is oxygen
consumption in units of ml O2 mg WWÀ1
hÀ1
converted to CO2
respiration in units of mg C mg DWÀ1
hÀ1
using carbon and
oxygen atomic ratios. A respiratory quotient (RQ; the ratio of
oxygen consumed to carbon dioxide released) was used in the
calculation of R. A common RQ value for zooplanktivorous fishes is
0.8 (Brett and Groves, 1979); however, a range of 0.7–1 was used
due to the lack of myctophid-specific data. DOC excretion data for
fishes are lacking, so the relationship of CO2 respiration to DOC
excretion in zooplankton (DOC excretion¼31% of CO2 respiration)
reported by Steinberg et al. (2000) was used to estimate DOC
excretion in myctophids, with a range of 20–40% used in the
sensitivity analysis. POC egestion at depth was calculated for B.
glaciale and P. arcticum using the carbon conversions discussed
above. We assumed that 100% of stomach and intestinal contents
of fish caught at the surface would be released below 200 m based
on myctophid gut clearance times ranging from 4 to 40 h
(Pakhomov et al., 1996, Pepin, 2013). Similar to Eq. (5) above,
POC egestion at depth (mg C mÀ2
dÀ1
) was calculated by multi-
plying prey carbon per mg fish DW by the integrated fish biomass
at the RR or AZ and by 12 h. Myctophid dry weight was deter-
mined using a wet weight-standard length regression for each
species (Fock and Ehrich, 2010) and a wet weight–dry weight ratio
(DW/WW¼22.64720%) based on an average of compiled wet
weight–dry weight ratios for many individual myctophid species
(Carmo pers. comm.).
3. Results
3.1. General diet description
Fifty-nine different prey types were identified in 201 nonempty
B. glaciale stomachs to an array of taxonomic levels depending on
the extent of digestion. Copepods constituted the bulk of the diet
by dry weight (52%), were the most frequently occurring prey
category (93%), and were the most abundant prey by number
(90%). The copepod Calanus finmarchicus was the predominant
organism in the diet of B. glaciale and made up over one quarter of
the diet by dry weight alone. Euphausiids were another major
component of the diet, constituting 37% of the diet by dry weight.
The remaining prey categories constituted 6% or less of the diet
by dry weight, and included chaetognaths, ostracods, amphipods,
gelatinous zooplankton, fishes, polychaetes, pteropods, and
digested crustaceans.
The diet of P. arcticum comprised 26 different prey types from
74 nonempty stomachs and contained primarily copepods and
euphausiids by dry weight (68% and 25%, respectively). Nearly all
P. arcticum stomachs contained copepods (93%) with the large
copepod Paraeuchaeta norvegica constituting the highest propor-
tion of copepod dry weight (15%). Ostracods, digested crustaceans,
and unidentified prey were the only other prey found in the diet of
P. arcticum.
Eighteen different prey types from 28 nonempty stomachs
were identified in the diet of H. hygomii. Euphausiids constituted
the highest proportion of the diet by dry weight (53%) with a mid-
level frequency of occurrence (57%) and a relatively low percent by
number (7%). Ostracods were consumed frequently (84%) and in
high numbers (49%), while copepods, mainly Pleuromamma spp.,
Candacia spp., and euchaetid species, were the most frequently
consumed prey category (84%) and made up half of the diet by
number. Fishes, amphipods, pteropods, and unidentified prey each
made up 5% or less by dry weight, although amphipods were
found in over half of stomachs.
3.2. Ontogenetic and spatial changes in diet
The sizes of B. glaciale included in this study ranged from 11 to
67 mm standard length (Fig. 2). Aside from a lack of euphausiids
in the diets of myctophids o20 mm (cluster A), there were no
further detectable ontogenetic changes in prey type consumed
over the size classes examined as indicated by cluster analysis
(Fig. 3). The diets of B. glaciale in cluster B contained the highest
proportion of euphausiid weight, while those in cluster C
Table 2
Evacuation rates at minimum, average, and maximum water temperatures (200–
750 m) and daily consumption at each ridge section (RS) (RR, Reykjanes Ridge;
CGFZ, Charlie–Gibbs Fracture Zone; FSZ, Faraday Seamount Zone; and AZ, Azorean
Zone) by individual myctophids (μg DW dÀ1
) and by the uncorrected and corrected
biomass (corrected biomass adjusted for 14% gear efficiency) of all myctophid
species (μg DW mÀ2
dÀ1
) at each ridge section integrated 0–2300 m using the
average consumption per unit fish biomass based on Benthosema glaciale stomach
content weights. Minimum individual consumption was always less than
1 μg DW dÀ1
.
RS T 1C Evac rate
(hÀ1
)
Individual Total uncorrected
biomass
Total corrected
biomass
Avg Max Min Avg Max Min Avg Max
RR 4.2 0.13 11 119 6 32 162 41 231 1155
6.6 0.15 13 141 7 42 194 49 300 1386
9.6 0.19 16 174 8 323 2264 60 2310 16,173
CGFZ 3.8 0.12 7 21 6 26 52 44 188 369
4.9 0.13 7 22 7 28 103 48 203 737
8.5 0.17 9 29 9 37 103 62 264 737
FSZ 4.5 0.13 1 4 2 5 51 11 39 363
7.6 0.16 2 5 2 7 63 14 49 451
11.7 0.22 2 7 3 9 75 18 64 536
AZ 7.1 0.16 0 0 0 0 5 0 4 33
11.8 0.22 0 0 0 1 7 0 5 47
15.6 0.29 0 1 0 1 9 0 7 66
Table 3
Minimum and maximum values of parameters used in active carbon transport
analysis (RR, Reykjanes Ridge; AZ, Azorean Zone; WW, wet weight; and DW, dry
weight).
Parameter Minimum Maximum
Temperature (1C) RR 200–750 m 5 10
Temperature (1C) AZ 200–750 m 7 16
Respiratory quotient 0.7 1
O2 consumption slope 0.012 0.02
O2 consumption intercept À0.0875 À0.0175
CO2:DOC ratio 0.2 0.4
Myctophid DW:WW (%) 18.1 27.2
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116108

6. contained a smaller proportion. The size distribution of P. arcticum
was 18–45 mm standard length (Fig. 2). The cluster analysis and
scree plot indicated there was no ontogenetic change in the diet
(data not shown). Due to the lack of evidence of ontogenetic shift
in the diet of B. glaciale and P. arcticum, fish size was not included
as an explanatory variable in subsequent analyses. The size
distribution of H. hygomii was 28–56 mm standard length (Fig. 2)
but the small sample size of H. hygomii did not allow for cluster
analysis.
Canonical correspondence analysis (CCA) indicated significant
B. glaciale dietary changes in relation to ridge section (p¼0.005)
and time of day (p¼0.025), but nonsignificant changes in relation
to depth zone (Fig. 4). The three explanatory variables included in
the CCA, depth, ridge section, and time of day, accounted for 30%
of the variability in the diet, collectively. The first and second
canonical axes accounted for 34% and 29% of the explainable
variation, respectively. Ridge section corresponded more closely
with the first canonical axis than the second and accounted for a
NumberofSpecimens
0
10
20
30
40
50
60
10−14.915−19.920−24.925−29.930−34.935−39.940−44.945−49.950−54.955−59.960−64.965−69.9Standard Length (mm)
n = 265
Benthosema glaciale
NumberofSpecimens
0
10
20
30
40
50
60
15−19.9
20−24.9
25−29.9
30−34.9
35−39.9
40−44.9
Standard Length (mm)
n = 76
Protomyctophum arcticum
NumberofSpecimens
0
10
20
30
40
50
60
25−29.9
30−34.9
35−39.9
40−44.9
45−49.9
50−54.9
55−59.9
Standard Length (mm)
n = 39
Hygophum hygomii
Fig. 2. Standard length (mm) frequency histograms for myctophids included in this
study. n is the number of each species dissected.
10−14.9
15−19.9
60−64.9
20−24.9
65−69.9
40−44.9
55−59.9
35−39.9
45−49.9
25−29.9
30−34.9
50−54.9
0
0.2
0.4
0.6
AverageDistanceBetweenClusters
Size Class (mm)
A B C
2 4 6 8 10
0.2
0.3
0.4
0.5
0.6
Number of Clusters
AverageDistanceBetweenClusters
Fig. 3. Cluster diagram (A) and scree plot (B) for Benthosema glaciale. The cluster
diagram represents the relationships among the diet compositions of 5 mm size
classes of B. glaciale. The scree plot was used to determine the number of clusters
into which the size classes of B. glaciale should be grouped.
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116 109

7. greater proportion of the explainable variation associated with
that axis.
The diet of B. glaciale differed among ridge sections in that prey
types constituting the bulk of the diet from the three northern
ridge sections were similar to each other, but different from prey
types from the southern AZ (Fig. 5A). The diets of B. glaciale from
the RR, CGFZ, and FSZ comprised mainly euphausiids – 35%, 44%,
and 65% by dry weight, respectively. The copepod C. finmarchicus
ranked second in the RR and CGFZ, constituting 31% and 25% of the
diet by weight, respectively. The abundance of C. finmarchicus, the
numerically dominant calanoid copepod north of the SPF and in
the diet of B. glaciale, declined by two orders of magnitude to the
south of the front and was not identified in the diets from the AZ.
The diets of B. glaciale from the AZ differed in that ostracods made
up 59% of the diet by dry weight. Pleuromamma copepods (7%)
were the only other prey to make up greater than 5% of the diet by
dry weight in B. glaciale from the AZ.
The diet of B. glaciale summarized by dry weight differed with
time of day. Stomach contents of fish collected during the day
were comprised mainly of C. finmarchicus (31%) and euphausiids
(29%), whereas at night copepods of the family Calanidae were the
primary prey (42%), and at dawn chaetognaths (67%) were the
predominant prey (Fig. 5B). Calanus finmarchicus, pteropods, and
fish prey were only identified in specimens collected during the
day and polychaetes were only found in dawn samples.
CCA of P. arcticum diet indicated depth zone (p¼0.03) was the
only significant factor; ridge section (p¼0.46) and time of day
(p¼0.23) were not significant. The three explanatory variables
included in the CCA accounted for 38% of the variability in the diet,
collectively (Fig. 6) and the first and second canonical axes
accounted for 38% and 32% of the explainable variation, respec-
tively. Depth zone corresponded more closely with the second
canonical axis. The main difference in P. arcticum diet by dry
weight in relation to depth zone was the high proportion of
euphausiids (31%) in fish collected from depth zone 2 (200–
750 m) and the absence of euphausiids in fish from depth zone
1 (0–200 m; Fig. 7). The main components in the diets by dry
weight of fish from depth zone 1 were the copepod Metridia sp.
(17%), ostracods (16%), C. finmarchicus (13%), and copepods of the
family Aetideidae (12%).
−2.0 −1.5 −1.0 −0.5 0.0 0.5 1.0
−1.5
−1.0
−0.5
0.0
0.5
1.0
Aetideidae
Amphipoda
Calanidae
Other Calanoida
Calanus finmarchicus
Calanus sp.
Candaciidae
Chaetognatha
Copepoda
Crustacea
Euchaetidae
Euphausiacea
Euphausiidae
Gelata
Heterorhabdus sp.
Heterostylites sp.
Hyperiidae
Metridia sp.
Metridinidae
Myodocopoda
Oncaea sp.
Paraeuchaeta norvegica
Pleuromamma sp.
Polychaeta
Pteropoda
Osteichthyes
Unidentified prey
DZ 1
DZ 2
DZ 3
AZ
CGFZ
FSZ
RR
Day
Night
Dawn
Canonical Axis 1
CanonicalAxis2
Fig. 4. Canonical correspondence analysis biplot for Benthosema glaciale. Bolded
labels represent the centroids for each level of the ridge section (Reykjanes Ridge,
RR; Charlie–Gibbs Fracture Zone, CGFZ; Faraday Seamount Zone, FSZ; Azorean
Zone, AZ) time of day, and depth zone (0–200 m, DZ 1; 200–750 m, DZ 2; 750–
1500 m, DZ 3) explanatory variables. Points represent prey types in the diet. The
canonical axes represent linear combinations of the explanatory variables. Ridge
section and time of day were significant at α¼0.05.
0
10
20
30
40
50
60
70
Reykjanes Ridge
n=84
0
10
20
30
40
50
60
70
PercentWeight
Charlie Gibbs Fracture Zone
n=40
0
10
20
30
40
50
60
70
Faraday Seamount Zone
n=66
Aetideidae
Calanidae
Calanussp.
C.finmarchicus
Euchaetidae
P.norvegica
Metridinidae
Metridiasp.
Pleuromammasp.
Heterorhabdussp.
Heterostylitessp.
Candaciidae
OtherCalanoida
Oncaeasp.
OtherCopepoda
Amphipoda
Hyperiidae
Euphausiacea
Myodocopoda
OtherCrustacea
Pteropoda
Gelata
Chaetognatha
Polychaeta
Osteichthyes
UnidentifiedPrey
0
10
20
30
40
50
60
70
Azorean Zone
n=64
0
20
40
60
80 Day
n=151
0
20
40
60
80
PercentWeight
Night
n=68
Aetideidae
Calanidae
Calanussp.
C.finmarchicus
Euchaetidae
P.norvegica
Metridinidae
Metridiasp.
Pleuromammasp.
Heterorhabdussp.
Heterostylitessp.
Candaciidae
OtherCalanoida
Oncaeasp.
OtherCopepoda
Amphipoda
Hyperiidae
Euphausiacea
Myodocopoda
OtherCrustacea
Pteropoda
Gelata
Chaetognatha
Polychaeta
Osteichthyes
UnidentifiedPrey
0
20
40
60
80 Dawn
n=12
Fig. 5. Diet composition (percent weight) of Benthosema glaciale presented by
(A) ridge section and (B) time of day. Error bars represent standard error of the
percent weight values of each prey type in the diet of B. glaciale calculated from
variance estimates (Eq. 3). n is the number of stomachs dissected from each ridge
section or time of day.
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116110

8. 3.3. Gastric evacuation and daily consumption
A range of gastric evacuation rates was calculated for each
myctophid species using minimum, average, and maximum water
temperatures experienced in the 200–750 m depth zone in each
ridge section. From the northernmost ridge section to the south-
ernmost, average water temperatures (1C) were 6.6 (RR), 4.9
(CGFZ), 7.6 (FSZ), and 11.8 (AZ) (Table 3), with the decline in
temperature at the CGFZ due to the unique physical characteristics
of the Subpolar Front present in this area (Søiland et al., 2008).
Myctophid evacuation rates ranged from 0.13 to 0.29 hÀ1
(Table 2).
The average total dry weight of prey in the stomachs of B. glaciale
(μg DW prey fishÀ1
) was highest at the RR (3.6, range 0–39) and
for P. arcticum at the CGFZ (0.09, range 0.003–0.37). H. hygomii was
only caught at the AZ and the average total weight of prey in the
stomachs was 0.92 μg DW prey fishÀ1
(range 0.24–1.97).
Estimated daily consumption of prey (μg DW mÀ2
dÀ1
)
inferred for all MAR myctophid species at average water tempera-
tures based on B. glaciale consumption rates was 42.0 at the RR,
28.4 at the CGFZ, 6.83 at the FSZ, and 0.72 at the AZ. Daily
consumption in proportion to myctophid body weight was always
less than 1% for each species regardless of ridge section, with B.
glaciale from the RR from 200 to 750 m having the highest
consumption of 0.7% of body weight per day at maximum water
temperatures. The daily consumption (μg DW mÀ2
dÀ1
) using
sampling efficiency corrected myctophid biomass values averaged
300 at the RR, 203 at the CGFZ, 48.8 at the FSZ, and 5.12 at the AZ.
Using integrated zooplankton biomass values from Gallienne et al.
(2001) for RR estimates and Steinberg et al. (2012; http://bats.bios.
edu/) for AZ estimates and average myctophid consumption rates,
results in an estimated removal of o1% of zooplankton biomass at
each ridge section every night.
3.4. Active carbon export by diel vertical migration
Myctophid biomass in the surface 200 m increased at night by
3.1-fold at the RR and 3.8-fold at the AZ, indicating diel vertical
migration (Fig. 8). This resulted in a migrant myctophid biomass
(uncorrected for gear sampling efficiency) of 5.2 and 40 mg C mÀ2
in the RR and AZ, respectively. Respiration of CO2 at depth by the
migrant myctophid biomass at the RR ranged from 0.01 to 0.03
(average 0.01) mg C mÀ2
dÀ1
(minimum water temperature from
200 to 750 m in the RR was lower than the minimum tempera-
tures used to generate the oxygen consumption regression in
Donnelly and Torres (1988) and thus the minimum estimate above
may be unreliable) while respiration at the AZ ranged from 0.05 to
0.27 (average 0.13) mg C mÀ2
dÀ1
(Table 4). Myctophid excretion
ranged from o0.01 to 0.01 mg C mÀ2
dÀ1
at the RR and from 0.01
to 0.11 mg C mÀ2
dÀ1
at the AZ. Export of POC by myctophid
egestion at depth was low, o0.01 mg C mÀ2
dÀ1
, regardless of
ridge section or myctophid species used to estimate weight-
specific egestion rate, and considering the range of values deter-
mined in the sensitivity analysis. Estimated total carbon transport
by the migrant myctophid biomass in the RR ranged from 0.01 to
0.04 (average 0.02) mg C mÀ2
dÀ1
and in the AZ ranged from 0.06
to 0.38 (average 0.17) mg C mÀ2
dÀ1
. Using sampling efficiency-
corrected migrant myctophid biomass (38 and 285 mg C mÀ2
in
the RR and AZ, respectively), active transport ranged from 0.04 to
0.26 mg C mÀ2
dÀ1
in the RR and from 0.40 to 2.78 mg C mÀ2
dÀ1
in the AZ, equivalent to 0.01–3.2% of sinking POC at 150 m and
0.05–7.5% at 300 m (based on POC flux values from studies in the
North Atlantic during April and May, Table 4).
4. Discussion
4.1. General diet description
Of the three myctophid species included in this study, more is
known about the feeding ecology of B. glaciale than P. arcticum or
H. hygomii. The diet of B. glaciale from the MAR observed in this
study consisted predominantly of copepods by dry weight, fre-
quency of occurrence, and abundance. Calanus finmarchicus and
euphausiids made up the bulk of the diet by dry weight, with
many other copepod taxa, amphipods, ostracods, chaetognaths,
pteropods, polychaetes, fishes, and unidentified gelatinous zoo-
plankton consumed as well. Myctophids consume gelatinous prey,
although how commonly they do so remains unclear. Several
studies from the Gulf of Mexico reported some larval and adult
Fig. 6. Canonical correspondence analysis biplot for Protomyctophum arcticum.
Explanatory variables include ridge section, time of day, and depth zone. Depth
zone was significant at α¼0.05. For description of explanatory variables see Fig. 4.
0
20
40
Aetideidae
Aetideussp.
Calanidae
Calanussp.
C.finmarchicus
Euchaetidae
P.norvegica
Metridinidae
Metridiasp.
Metridialucens
Pleuromammasp.
Heterorhabdidae
OtherCalanoida
OtherCopepoda
Euphausiacea
Myodocopoda
OtherCrustacea
UnidentifiedPrey
Depth zone 1, n = 15
Depth zone 2, n = 54
40
20
0
PercentWeight
Fig. 7. Diet composition (percent weight) of Protomyctophum arcticum, presented
by depth zone (depth zone 1, 0–200 m; depth zone 2, 200–750 m). Error bars
represent standard error of the percent weight values of each prey type in the diet
of P. arcticum calculated from variance estimates (Eq. 3). n is the number of
stomachs dissected from each depth zone.
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116 111

9. myctophid species fed on gelatinous prey, with some larvae
feeding almost exclusively on gelatinous zooplankton (Hopkins
and Gartner, 1992, Conley and Hopkins, 2004). Hopkins and
Gartner (1992) reported gelatinous prey in the diet of Benthosema
suborbitale; however, to our knowledge, gelatinous prey has not
previously been reported in the diet of B. glaciale. Gelatinous prey
was identified in the stomachs of seven B. glaciale 440 mm from
the MAR, equating to less than 1% of the numerical abundance of
prey in the diet. Due to the rapid digestion of gelatinous material,
this represents an underestimate of gelatinous zooplankton in the
diet of this species (Arai et al., 2003). The diet of B. glaciale from
the MAR was similar to the results of other studies in that
copepods (predominantly C. finmarchicus, Pleuromamma spp.
Metridia spp., and Paraeuchaeta norvegica) comprised the bulk of
the diet, with euphausiids, ostracods, amphipods, pteropods,
chaetognaths, and fish also consumed (Gjøsæter, 1973; Kinzer,
1977; Kawaguchi and Mauchline, 1982; Roe and Badcock, 1984;
Petursdottir et al., 2008; Pepin, 2013).
The diet of P. arcticum from the MAR was also comprised
mainly of copepods, with P. norvegica constituting the bulk of the
copepod component. Euphausiids and ostracods were the only
other identifiable prey in the diet. In a study from the Rockall
Trough in the North Atlantic, the diet of P. arcticum was also
comprised mainly of copepods, specifically Pleuromamma spp.,
(Kawaguchi and Mauchline, 1982). In the Davis Strait west of
Greenland, C. finmarchicus was the only identifiable prey
(Sameoto, 1989). H. hygomii fed mainly on euphausiids and
copepods; however, fish prey were also abundant, constituting
5% numerically. In previous studies at Great Meteor Seamount and
in the Gulf of Mexico, H. hygomii fed on a wide variety of prey
including copepods, euphausiids, and gelatinous prey (Hopkins
and Gartner, 1992; Pusch et al., 2004).
4.2. Ontogenetic and spatial changes in diet
Ontogenetic changes in the diet of P. arcticum were not
identified by cluster analysis, although there was a slight tendency
for larger P. arcticum to consume larger prey such as euphausiids
and the copepod P. norvegica, while smaller P. arcticum consumed
primarily small copepods. The grouping of B. glaciale size classes
in the cluster analysis was driven primarily by the weight of
euphausiids in the diet and did not indicate ontogenetic changes,
although the smallest size classes did group together based on a
lack of euphausiid consumption and all size classes above 20 mm
did consume euphausiids (Fig. 3). The presence of euphausiids in
the diets of smaller B. glaciale from the MAR, which has not been
previously reported in other regions (Kinzer, 1977; Kawaguchi and
Mauchline, 1982), in addition to the presence of gelatinous
zooplankton, suggests the diet of MAR myctophids may be distinct
compared to those in off ridge waters.
The diets of B. glaciale, P. arcticum, and H. hygomii are influ-
enced by the prey fields which are markedly different to the north
and south of the Sub-Polar Front (SPF). Gaard et al. (2008) found
that the SPF appeared to act as a boundary to the horizontal
distribution of several copepod genera, with the northward dis-
tribution of southern copepod genera restricted more so than the
reverse. CCA revealed the diet of B. glaciale to be influenced
primarily by ridge section, which serves as a proxy for latitude,
rather than by depth or time of day. This suggests that hydro-
graphic features, such as the Sub-Polar Front and physical proper-
ties of water masses, may play a greater role in myctophid diet
than the MAR itself, if explanatory variables in the diet of
B. glaciale are applicable to other myctophid species. This may be
true more so for mesopelagic myctophids than bathypelagic fishes
because the MAR in the study area resides primarily below
1000 m, deeper than the range of most vertical migrators.
Aside from ridge section, patterns in the diet with respect to
time of day were also present; although, such patterns were
driven primarily by a few prey species and were not observed
for the diet as a whole. The absence of C. finmarchicus from night
and dawn samples, while constituting a third of the diet by weight
during the day, contributed to the significant differences in
B. glaciale diet with respect to time of day. The absence of
C. finmarchicus from the diet at night and dawn could be due to
mismatch in vertical distribution of predator and prey, although
Gaard et al. (2008) reported C. finmarchicus was present from
0–2500 m at the MAR during the same sampling period. Alter-
natively, the absence of C. finmarchicus could be an artifact of poor
taxonomic resolution in prey identification, as copepods of the
family Calanidae made up 42% of night diets by weight. Although
chaetognaths made up a greater proportion of the diet by weight
of B. glaciale during dawn (67%) than other times of day (4% or less
in day and night samples), this trend is driven by two particularly
0 50 100 150
NightDay
RR Biomass (mg C m−2)
150 100 50 0
0
200
750
1500
2300
Depth(m)
0 50 100 150
NightDay
AZ Biomass (mg C m−2)
150 100 50 0
0
200
750
1500
2300
Depth(m)
Fig. 8. Vertical distribution of all myctophid species (uncorrected for gear effi-
ciency) during day and night from (A) the Reykjanes Ridge (RR) and (B) the Azorean
Zone (AZ). Error bars represent standard error.
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116112

10. large chaetognaths found in fish stomachs at dawn and may not be
indicative of diel diet changes.
Depth was the only significant explanatory variable in the diet
of P. arcticum as indicated by CCA; although, P. arcticum was only
collected from r750 m and, as in the case of B. glaciale, dietary
patterns may not be directly attributable to the MAR itself. The
main difference in P. arcticum diet between depth zones 1 and
2 was the presence of euphausiids only in stomachs collected
during the day in the deeper depth zone 2, possibly due to
euphausiid diel vertical migration. However, abundances of
euphausiids were much higher in depth zone 2 than in depth
zone 1 during both day and night (MAR-ECO unpublished data).
Alternatively, fish size may be the underlying factor dictating
euphausiid consumption at depth. Larger predators are capable
of consuming larger prey, and larger fish of a species are com-
monly found deeper than smaller individuals (Collins et al., 2008;
Sweetman et al., 2013). However, there was only a 4 mm increase
in average P. arcticum size from depth zone 1 (28.4 mm SL) to
depth zone 2 (32.1 mm SL) and the size range of P. arcticum that
consumed euphausiids encompassed the average length of fishes
in each depth zone (28–44.5 mm SL). Protomyctophum arcticum
diet characteristics with respect to depth included aetideid
copepods, C. finmarchicus, Metridia sp., and ostracods, all of which
are known vertical migrators (Al-Mutairi and Landry, 2001;
Irigoien et al., 2004), and which constituted a considerably greater
proportion of the diet by weight in depth zone 1, while P. norvegica
made up a greater proportion in depth zone 2. Other prey were
generally consumed in similar proportions at both depths.
An alternative to significant diet differences being due to the
vertical distribution of prey is differences in the vertical dis-
tribution of P. arcticum. Results from Nafpaktitis et al. (1977) and
Hulley (1984) suggested P. arcticum in the North Atlantic may
exhibit a weak diel vertical migration, with depths of P. arcticum
maximum abundance during the day vs. night of 350 m vs.
250 m, respectively, and depth ranges of occurrence of 250–
850 m vs. 90–325 m, respectively. During the day, P. arcticum
from the MAR exhibited typical patterns of abundance corre-
sponding to diel vertical migration, with abundance in depth
zone 2 more than three times that in depth zone 1. However,
night time abundance was nearly equal in depth zones 1 and 2,
suggesting a sizeable proportion of the P. arcticum population
remains at depth and does not migrate (Watanabe et al., 1999,
Kaartvedt et al., 2009, Dypvik et al., 2012). This is supported by
Cook et al. (2013) who report approximately two-thirds of the
P. arcticum population remained at depth at night based on
samples collected from a subsequent MAR-ECO expedition to
the CGFZ in 2009.
4.3. Daily consumption
The maximum daily consumption as a percentage of dry body
weight for myctophids in this study was 0.7%, which agrees well
with the estimates of Pepin (2013) who found average rations of B.
glaciale to be less than 1% of body weight per day and hypothe-
sized that this species does not feed daily, but once every two
days. Although a variety of methods has been used and direct
comparisons are not possible, the consensus is that myctophids
generally consume o1–6% of dry body weight per day, with most
estimates closer to 1% (see Table 6.2 in Brodeur and Yamamura,
2005, and references therein). The similarity of these results to off
ridge estimates indicate that the MAR may not significantly
influence the daily consumption of myctophids, although daily
consumption estimates for MAR myctophids presented here are
likely to be underestimated as a result of the sampling design
employed on the G.O. Sars cruise. Shallow depths were sampled
first and, thus, myctophids collected in depth zones 1 and 2 would
continue digesting for many hours after capture while the deeper
depths were sampled. This prolonged digestion would result in an
underestimation of prey abundance in stomachs and, conse-
quently, underestimation of prey weight as a percentage of fish
body weight.
Estimated removal rates at the MAR were comparable with
estimates by Pepin (2013) who reported B. glaciale fed primarily on
copepods and consumed 0.002–1.8% (midpoint 0.15%) of copepods
in the Labrador Sea per day. Consumption rates reported by Pepin
(2013) and in the current study are similar to or lower than daily
zooplankton removal rates by myctophids reported in previous
studies: 1–4% of zooplankton standing stock in the upper 150 m in
the western North Pacific (Watanabe et al., 2002), 2% in the upper
200 m in the Gulf of Mexico (Hopkins and Gartner 1992), and
5–20% in the upper 300 m in the Southern Ocean (Pakhomov et al.,
1999). Minimal feeding impact of myctophids on zooplankton
observed in our study could be a result of the time of year
sampling occurred, which likely coincided with maximum zoo-
plankton abundance following the spring bloom. The impact of
myctophid predation at the MAR has the potential to be greater in
other seasons when zooplankton abundance is lower.
Table 4
Active transport of carbon (mg C mÀ2
dÀ1
) by diel migrating myctophids from the Reykjanes Ridge (RR) and Azorean Zone (AZ). Integrated (0–200 m) migrant myctophid
biomass (mg C mÀ2
), uncorrected and corrected for 14% gear efficiency, was 5.2 and 38 in the RR and 40 and 285 in the AZ, respectively. Active transport calculated using
both uncorrected and corrected myctophid biomass is presented. Active transport of CO2 and dissolved organic carbon (DOC) is for the 0–200 m integrated biomass of all
myctophid species combined. Active transport of particulate organic carbon (POC) is the average carbon content of prey from night time 0–200 m Benthosema glaciale diets
multiplied by the 0–200 m integrated biomass of all myctophid species combined. Minimum, average, and maximum values were obtained from a sensitivity analysis of
carbon transport parameters. Total myctophid carbon export across 200 m using the corrected myctophid biomass is compared to average POC flux in the North Atlantic
Ocean from different locations and depths n
during spring (April and May), i.e., (active transport/sediment trap POC flux) Â 100%; % POC.
Ridge Section Uncorrected biomass Corrected biomass
CO2 DOC POC Total CO2 DOC POC Total % POC
RR
Average 0.013 0.004 o0.001 0.017 0.090 0.028 o0.001 0.118 0.04–0.3
Min 0.005 0.001 o0.001 0.006 0.030 0.006 o0.001 0.036 0.01–0.1
Max 0.027 0.010 o0.001 0.037 0.190 0.073 o0.001 0.263 0.1–0.7
AZ
Average 0.129 0.040 o0.001 0.169 0.920 0.290 o0.001 1.210 0.5–3.3
Min 0.046 0.009 o0.001 0.055 0.330 0.070 o0.001 0.400 0.2–1.2
Max 0.271 0.110 o0.001 0.381 1.940 0.840 o0.001 2.780 1.1–7.5
n
POC flux ranged from 86 to 259 mg C mÀ2
dÀ1
at 150 m and from 37 to 72 mg C mÀ2
dÀ1
at 300 m (Bender et al., 1992, Buesseler et al., 1992, Ducklow et al., 1993, Harrison
et al., 1993, Martin et al., 1993). Sampling during the G.O. Sars cruise was performed during a post-bloom period and during which passive POC flux could be lower than
during the spring bloom.
J.M. Hudson et al. / Deep-Sea Research I 93 (2014) 104–116 113

11. 4.4. Active carbon transport by myctophid diel vertical migration
We compare our estimates of myctophid active carbon trans-
port to passive POC flux measured by sediment traps, and to
zooplankton active transport, to explore their relative importance
as components of the biological pump (Steinberg et al., 2000;
Hidaka et al., 2001). Diel vertically migrating zooplankton con-
tribute to the vertical export of carbon from the euphotic zone
through mortality, respiration, excretion, and egestion at depth
during the day of food consumed in surface waters at night
(Longhurst et al., 1990; Dam et al., 1995; Zhang and Dam 1997;
Steinberg et al., 2000, 2008; Al-Mutairi and Landry 2001;
Schnetzer and Steinberg 2002; Kobari et al., 2008). This ‘active
transport’ could be an important source of carbon for non-
migrating mesopelagic zooplankton, and for mesopelagic bacteria,
which are ultimately reliant on surface-derived production
(Steinberg et al., 2008). Diel vertically migrating fishes may also
contribute to active transport of carbon, and as myctophids are the
most abundant vertically migrating mesopelagic fishes at the MAR
and many other regions, it is important to consider their role in
the biological pump. Indeed, acoustic estimates of mesopelagic
fish biomass by Irigoien et al. (2014), although not ground-truthed
by net sampling, are an order of magnitude higher than reported
in the classic study of Gjøsaeter and Kawaguchi (1980) which
would significantly increase contributions by vertically migrating
fishes to carbon flux.
Integrated (0–200 m) migrant myctophid biomass (mg C mÀ2
),
uncorrected and corrected for sampling efficiency, was 5.2 and 38
in the RR and 40 and 285 in the AZ, comparable with total fish
biomass estimates in the North Atlantic by Angel and Pugh (2000)
of up to ca. 40 mg C mÀ2
in the top 200 m. Active transport of CO2,
DOC, and POC during summer by the corrected MAR myctophid
biomass was as much as 3.2% of passively sinking POC in the North
Atlantic at 150 m and 7.5% at 300 m. Although not quantified in
our study, mortality of diel migrating myctophids at depth would
further increase export, with previous such estimates for fishes
ranging from o0.1 to 20.3 mg C mÀ2
dÀ1
(Williams and Koslow,
1997; Angel and Pugh, 2000; Davison et al., 2013). Another study
investigating carbon export by myctophids in the western equa-
torial North Pacific reports myctophid biomass (uncorrected
for sampling efficiency) of 249–462 mg C mÀ2
(0–160 m) and
active transport through respiration and egestion of 1.2–
2.2 mg C mÀ2
dÀ1
, equivalent to 2.0–3.7% of passively sinking
POC (Hidaka et al., 2001, stations 15 and 16). Hidaka et al.,
(2001) adopted the 0.14 sampling efficiency estimated by Koslow
et al. (1997), which significantly increased their myctophid bio-
mass estimate (1778–3303 mg C mÀ2
) and, subsequently, carbon
export (8.4–15.4 mg C mÀ2
dÀ1
, 14.3–26.4% of sinking POC).
Davison et al. (2013) estimated the average biomass of mesopela-
gic fishes off the continental U.S. to be 24.7 g WW mÀ2
(corrected
for a sampling efficiency of 0.06) and mediates, on average, 15–17%
of total carbon export. Approximately half of this export passes
through vertically migrating fishes, over 90% of which are mycto-
phids. The high myctophid biomass in both Pacific studies con-
tributes to the considerably higher active carbon transport
estimated there compared to the MAR. Myctophid active transport
along the MAR is also lower than the long-term (1994–2011)
annual average zooplankton active transport in the subtropical
North Atlantic measured at the BATS station. There zooplankton
transported 4.1 mg C mÀ2
dÀ1
via respiration, excretion, and eges-
tion at depth, which was on average 15% of sinking POC flux at
150 m (Steinberg et al., 2012).
At deeper depths, active transport by myctophids becomes
increasingly important as myctophids, of which some species verti-
cally migrate 1000 m or more (Fig. 8), have the potential to transport
a greater proportion of sinking POC, due to the rapid decline of
sinking POC with increasing depth. As a rough comparison, Honjo
and Manganini (1993) report passive POC flux at 1000 m in the
North Atlantic during April of 4.1 mg C mÀ2
dÀ1
. Using corrected 0–
200 m integrated MAR myctophid biomass, the myctophid carbon
export calculated in this study during summer would be equivalent
to 1–73% of sinking POC at 1000 m. A few factors would decrease the
amount of carbon transported to this depth, however. A smaller
proportion of the migrating myctophid biomass resides at 1000 m
during the day, and greater migration distance means increased time
for digestion to occur resulting in a smaller proportion of POC
actively exported out of the euphotic zone being released at
1000 m (although active carbon transport is comprised predomi-
nantly of respiration and excretion, with egestion of POC constituting
a small proportion). Nevertheless, the high proportion of carbon
exported by myctophids in relation to POC flux at greater depths
reaffirms the potential importance of myctophids in the biological
pump, and suggests that active transport by fishes should be
considered in biogeochemical models.
5. Conclusions
The diet of the three species of myctophids from the MAR was
consistent with previous investigations of these species in off-
ridge areas of the North Atlantic Ocean. However, the most
abundant myctophid, B. glaciale, possessed unique dietary char-
acteristics not observed in this species elsewhere, such as con-
sumption of gelatinous prey, and of euphausiids and amphipods
earlier in the fish's life history (i.e., at smaller fish sizes), evidence
that the MAR may support a distinct food web structure. Sampling
during other times of the year is necessary to determine if the diet
patterns observed in this study during summer are characteristic
of annual patterns at the MAR. Despite the temporal scale of this
study being limited to summer, our results will be useful for
comparison of MAR food web structure to that of continental
slope regions of the Atlantic. This study also provides the first
estimate of active carbon transport for Atlantic myctophids and
for a mid-ocean ridge. Carbon transport by myctophids at the
MAR during summer was low compared to sinking POC flux in
the upper mesopelagic zone during the spring bloom at off ridge
areas, but may account for a greater proportion of exported
carbon at lower mesopelagic and bathypelagic depths. Addi-
tional spatial and temporal sampling and information on sam-
pling efficiency of an array of trawl types are needed to develop
more robust estimates of active carbon transport by myctophids
and other migrating fishes, resulting in a more comprehensive
view of the biological pump.
Acknowledgments
The authors acknowledge the crew of the R.V. G/O. Sars for
expert vessel operations during sample collection. Thanks are due
to I. Byrkjedal for shipping myctophid samples and to M. Vecchione,
V. Carmo, and C. Sweetman for statistical and scientific guidance for
this research and manuscript. This research was funded by NSF,
United States Ocean Sciences Division-Biological Oceanography
Program (Grant OCE 0623551 to T.T.S.) and by the VIMS Office of
Academic Studies. This paper is Contribution no. 3388 of the
Virginia Institute of Marine Science, College of William and Mary.
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