Makrinos and Bowden 2016

Natural environmental impacts on teleost immune function
Daniel L. Makrinos*
, Timothy J. Bowden
School of Food & Agriculture, University of Maine, Orono, ME, USA
a r t i c l e i n f o
Article history:
Received 11 February 2016
Accepted 7 March 2016
Available online xxx
Keywords:
Teleost
Fish
Environment
Immune
Immunity
Climate change
Temperature
Salinity
Acidity
Particulates
Oxygen capacity
a b s t r a c t
The environment in which teleosts exist can experience considerable change. Short-term changes can
occur in relation to tidal movements or adverse weather events. Long-term changes can be caused by
anthropogenic impacts such as climate change, which can result in changes to temperature, acidity,
salinity and oxygen capacity of aquatic environments. These changes can have important impacts on the
physiology of an animal, including its immune system. This can have consequences on the well-being of
the animal and its ability to protect against pathogens. This review will look at recent investigations of
these types of environmental change on the immune response in teleosts.
© 2016 Published by Elsevier Ltd.
Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
2. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
3. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
4. Acidity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
5. Salinity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
6. Particulates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
7. General conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00
1. Introduction
The environment in which we live in can have profound impacts
on our immune function. The study of these interactions is usually
classified as either environmental physiology or ecophysiology.
Many animals rely on behavioral strategies to overcome detri-
mental changes in their environment. They can move away from an
immediate environmental stressor and find a less stressful envi-
ronment. However, sessile organisms do not have that ability and
instead have to ‘tough it out’. To this end, sessile organisms will
likely have a physiological adaptation that will allow them to limit
the impacts of the environmental perturbation, at least in the short
term. Although, aquacultured fish, such as salmon raised in sea
cages are not sessile, they do not have the ability to relocate to avoid
environmental stressors. Thus it is important to understand how
such stressors can impact relevant organisms.
Long-term solutions to environmental change may occur
through epigenetic changes or evolutionary adaptations. These
adaptations can be defined by the acute or chronic nature of the
* Corresponding author. School of Food and Agriculture, University of Maine,
5735 Hitchner Hall, Orono, ME 04469-5735, USA.
E-mail address: danmakrinos@gmail.com (D.L. Makrinos).
Contents lists available at ScienceDirect
Fish & Shellfish Immunology
journal homepage: www.elsevier.com/locate/fsi
http://dx.doi.org/10.1016/j.fsi.2016.03.008
1050-4648/© 2016 Published by Elsevier Ltd.
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environmental change [1]. Environmental impacts can be either
abiotic (physical and chemical factors) or biotic (direct or indirect
effects of other organisms) in nature. Some environments, such as
the deep ocean, can be viewed as stable on timescales that are
relevant to living organisms. Evolutionary or geological timescales
can encompass changes such as sea level rises or the erosion/
deposition of materials. More rapid changes can be defined on
annual, lunar or daily cycles. Finally, changes can occur in seconds,
minutes or hours such as the weather. Size of an organism can also
impact its ability to respond to environmental change. Recent
research is indicating that small adult animals are more resistant
(tolerant) to a temperature change than larger animals, a finding
that contradicts the variation in the volume/surface area ratio or
isometric scaling [2]. Thus an organism does not have to resist a
change in its environment, it can just be tolerant to those changes.
Highly variable environments put a selective pressure on organ-
isms that are physiologically versatile or tolerant rather than those
which have precise adaptations.
When does an environmental change become a stressor and at
what point does your stressor switch between being chronic or
acute? This may be defined by the intensity, duration, predictability
and controllability of the stressor, leading to an assessment of the
severity. In addition, stressors, of many kinds and especially in
natural environments, can occur in combination, which adds
complexity to the understanding of the intensity [3]. Ultimately, if
the stress becomes too severe or long-lasting the animal may no
longer be capable of maintaining its homeostasis.
The physiological responses to environmental changes/stressors
have been grouped into a simplistic primary, secondary and tertiary
response model [4]. Primary responses involve the initial neuro-
endocrine responses, such as the release of catecholamines. Sec-
ondary responses involve changes to metabolism and respiration
but also involve changes in immune function. Finally, tertiary re-
sponses affect the whole animal and include metabolic activity, and
overall resistance to disease.
In a previous review we looked at the modulation of teleost
immune function by their environment [5]. In the intervening time,
researchers have continued to look at how the environment in-
fluences immune function. One area that has been increasingly
spotlighted has been the impact of climate change and its various
facets. Climate change has the potential to deliver a whole range of
environmental impacts [6]. These could include temperature
changes through perturbation of the climate, salinity changes
through variations in fresh or salt-water input, perturbation of
acidity due mainly to anthropomorphic pollution inputs and many
of these can lead to variations in oxygen capacity or biological
availability in water. Consequently, the factors that will be covered
in this review include:
1. Temperature
2. Oxygen level
3. Acidity
4. Salinity
5. Particulates
Since the environment is rarely static, animals usually have
some level of ‘plasticity’ in their response to environmental con-
ditions. Genetic variability can differentiate this response within
populations, among populations and between species [7]. This
plasticity can be seen as acclimation. However, the conditions
necessary to trigger an acclimation response will also vary between
species. This complicates any attempt to generalize the response
across a group of animals as diverse as fish.
New techniques are changing the way that we analyze the
response to the environment. The development of transcriptome
analysis, using techniques such as next generation sequencing and
global gene expression analyses, has opened up the possibility to
study the complete shift in genetic expression patterns due to
controlled environmental challenges [8]. Another technique that
can provide a similar level of detail is the use of microarrays to
deliver partial gene expression profiles.
The scope of this review is intended to encompass those natural
environmental parameters that can impact immune function,
highlighting reports that have been published since our last review.
2. Temperature
Fish are usually considered poikilothermic, or more accurately
ectothermic, in that they cannot maintain a constant body tem-
perature against changes in the surrounding environmental tem-
perature since they are reliant on external heat sources. Some
species, such as tuna and other members of the suborder Scom-
broidei, use heat-exchange mechanisms to maintain elevated core
temperature to improve swimming efficiency [9].
Short term acute changes in temperature can be compensated
for, certainly at a cellular level, by processes such as the heat shock
response [10]. However, more subtle chronic temperature changes
are less likely to induce such responses and yet may impact the
physiology of an organism [11e13]. Yet, there is little published data
on the impacts of the rate of temperature change (ramp) directly.
One paper investigated growth rates and stress responses under
various regimes and found little variation in stress responses and
reduced growth in some scenarios [13]. But these reports did not
look at the impacts on the immune response.
One of the most common phenomena of the interaction be-
tween immune response and environmental temperature has been
an increase in the antibody levels with increases in the water
temperature. This has been reported in various fish species
including; sea bass (Dicentrarchus labrax), blue tilapia (Oreochromis
aureus), olive flounder (Paralichthys olivaceus), Atlantic halibut
(Hippoglossus hippoglossus), ayu (Pecoglossus altivelis), Nile tilapia
(Oreochromis niloticus), Atlantic cod (Gadus morhua), and turbot
(Scophthalmus maximus) [5,14e21]. In the study on turbot, animals
were initially acclimated to 16 C. Then the temperature was raised,
at a rate of 3 C every 48 h, to a range of different temperatures
between 16 C and 28 C. The authors reported temperature
dependent expression changes in lysozyme, IgM, hepcidin and IL-
1b [14]. Lysozyme is a common innate immune enzyme involved in
the breakdown of the cell walls of gram-positive bacteria [22].
Immunoglobulin-M (IgM) form the predominant class of antibodies
found in teleost species [23]. Hepcidins have been identified in
more than 20 different teleost species and are associated with both
anti-microbial function and iron metabolism [24]. Interleukin-1b is
an important mediator of the inflammatory response [25].
A temperature trial on farm raised Atlantic cod involved raising
the temperature from 10 C to 19 C [26]. They reported elevations
in b2-M, MHC class 1 and IgM-L mRNA when the temperature was
raised up to 16 C. These parameters then fall back to baseline at
higher temperatures. Only the levels of IL-1b rose at 19 C. The
study on ayu raised the water temperature from 18 C to 28 C and
observed increased agglutinating antibody titers against Fla-
vobacterium psychrophilium and indicated that elevated water
treatments could help induce protective immunity against this
pathogen [21].
A recent study on turbot confirmed the importance of temper-
ature as a driver of immunocompetence compared to, in this case,
salinity, with increases in IgM levels in liver and kidney in animals
exposed to elevated temperatures [27]. A study on Catla catla
looked at expression of Toll-like and NOD-like receptors at elevated
and lowered temperatures and found that expression of TLR2, TLR4
D.L. Makrinos, T.J. Bowden / Fish Shellfish Immunology xxx (2016) 1e82
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and NOD2 were increased at elevated temperatures and that TLR5
and NOD1 expression was increased at both elevated and lowered
temperatures [28].
It has been proposed that at low temperatures fish may rely
more on non-specific immune responses, while at higher temper-
atures there may be more reliance on specific immunity [29e33]. A
recent study on perch (Perca fluviatilis) indicated that pattern
recognition by glucan binding proteins was more prevalent in fish
acclimated at lower temperatures and that opsonin, or specific,
recognition was more effective at higher temperatures, thereby
reinforcing this conclusion [34]. Another recent study on gene
expression profiles in the skin of Atlantic salmon (Salmo salar)
maintained at 4 C, 10 C and 16 C reported a rise in skin-mediated
immune activity at high and low temperatures [35]. It was noted
that IL-1B, IL-8, and TNF were expressed at low temperatures. A
study involving Atlantic cod (Gadus morhua), which were injected
with either PBS or poly-IC at 10 C or 16 C, reported a large number
of differences in gene expression in animals receiving poly-IC in-
jection [36]. However, in the control-portion of the study, animals
that weren't injected but were only exposed to the increase in
temperature from 10 to 16 C only displayed differential expression
of six genes, none of which were identified or assigned a function.
Other studies have used similar injection techniques, but have not
reported data from uninjected animals that were solely exposed to
the temperature change [37]. It seems clear from these studies that
a significant portion of the gene expression variations are linked to
the injection procedure itself rather than being a natural response
to the temperature shift. A study on shortnose sturgeon (Acipenser
brevirostrum) showed a transient rise in expression of interferon
regulatory factors (IRF) 1 and 2 at higher temperatures [38]. A study
on three-spined sticklebacks (Gasterosteus aculeatus) varied the
temperature profiles in multiple experiments [39]. Firstly, animals
were maintained at various temperatures within the estimated
permissive range (13 C, 18 C and 24 C). Results indicated respi-
ratory burst activity and lymphocyte proliferation were lower at
24 C and higher at 13 C. In addition, another experiment exposed
them to a ‘heat wave’ of 28 C for 2 weeks, which resulted in long-
lasting immune disorders. The authors suggested when such events
occur naturally, they may result in animals becoming immuno-
compromised, which could facilitate the spread of infectious dis-
ease within these populations [39]. Another study looked at
expression of a range of anti-viral genes such as; IL-1b, iNOS, TNF-a,
TLR3, IFN-I, IFNg, IRF3, MDA-5 and Mx, in larval zebrafish (Danio
rerio) maintained at either 15 C or 28 C and showed generally
higher levels of expression at the higher temperature [40]. A study
on black porgy (Acanthopagrus schlegeli) that were raised and 20 C
and 30 C showed increased expression of anti-oxidant enzymes
SOD and catalase at higher temperatures [41]. A study on orange-
spotted grouper (Epinephelus coioides) that were initially main-
tained at 27 C and were subsequently placed at 19 C and 35 C
showed depression of respiratory burst activity, phagocytic activity,
alternative complement activity and lysozyme activity at these
altered temperatures [42]. The fall in the respiratory burst response
here contradicts the result reported in the previous study. Another
study investigated an abrupt change in temperature from 25 C to
30 C on Japanese medaka (Oryzias latipes) [43]. They measured
lymphocyte proliferation and respiratory burst associated super-
oxide production by isolated kidney phagocytes and showed
elevated levels after the stressor. However, whether such a stressor
constitutes a realistic natural phenomenon is debatable, although,
as the authors noted, such a stressor could occur in an aquaculture
environment.
A number of studies have looked at gene expression in relation
to cold tolerance. One study conducted a transcriptomic analysis of
gilthead sea bream (Sparus aurata) exposed to two temperatures
(16 C as a control and 6.8 C as a cold-exposed group) for 21 days
and showed under-expression of anti-oxidant genes such as cata-
lase and glutathione S-transferase [44]. Another study investigated
the transcriptomic profile of two populations of barramundi (Lates
calcarifer), one from a cool region, one from a warm region, grown
subsequently at 22 C, 28 C and 36 C [45]. This analysis high-
lighted only a small number of immune genes, specifically for
complement cascade components. These were suppressed in the
cool-adapted animals that were subsequently exposed to warmer
environments, indicating a stress-induced process of immuno-
suppression that has been considered previously [46].
One issue is to discern the difference between a chronic and an
acute thermal change and the rate of change or the ramping rate is
an important consideration. One protocol listed temperature ac-
climations using adjustments of 1 C/h with a maximum of 7 C/day
[47]. A second listed acclimation changes of 3 C every 48 h over a
maximum range of 15 C [27]. Others presented a 6 C change in
temperature using an acclimation program of 1 C/day, which is
likely to be a more gentle acclimation procedure [36]. Given that
the latter study reported only six gene expression differences this
may indicate a markedly less impactful acclimation process. A
study in sea bass looked at seasonal variation of humoral immune
parameters and found little variation [48]. It is interesting to
consider the rates of change here are natural, varying on a seasonal
basis and that as a consequence the immune functions were not
impacted. A study on gene expression in the erythrocytes of
rainbow trout (Onchorynchus mykiss) that were exposed to a tran-
sient rise in temperature from 13 C to 25 C, with a ramp of 3 C/h,
showed an increase in the expression of a number of genes
including; apolipoprotein and Ig light chain [49].
Some studies look at multiple stressor interactions. One inves-
tigated the interaction between temperature and salinity in turbot
noted that expression of both Hsp70 and IgM were positively
correlated to both temperature and salinity and that temperature
was the dominant factor [27]. Another study on cod found little
variation in natural antibody levels when animals were reared at
different salinities and temperatures [50].
Conclusion
While there are trends in the information available, there are
still contrasting or contradictory results. Many papers indicate a
bias towards innate immune function at lower temperatures. Many
also indicate an up-regulation of antibody related functions at
increasing temperature. But the development of clear response
patterns to temperature changes across all teleosts is not apparent
and probably shouldn't be expected given the number of species
involved.
Another facet that may bear consideration is that these changes
in immune response are associated with operation within
permissive temperatures and that exceeding these temperatures,
16 C in the case of cod, 23 C for carp, 25 C for turbot and 28 C for
tilapia, can often put these animals into an environment where
they have exceeded their optimum range for effective immune
function [16,51e53].
3. Oxygen
Oxygen is the key to aerobic respiration and the efficient gen-
eration of metabolic energy. Oxygen dissolves in water bodies at
rates that can vary from 0 to 18 mg/l. It diffuses from the atmo-
sphere, from the aeration of water movements such as waves and
waterfalls, and as a waste product of photosynthesis. Dissolved
oxygen levels can fall as a consequence of the temperature of the
water rising. Oxygen levels can also fall when the biological oxygen
demand rises such as when algae or bacteria proliferate. In addi-
tion, oxygen solubility in water varies with respect to salinity,
D.L. Makrinos, T.J. Bowden / Fish Shellfish Immunology xxx (2016) 1e8 3

whereby oxygen solubility falls with increasing ionic concentration.
It has become clear that oxygen levels can influence fish
behavior. A review of the behaviors of Atlantic salmon in com-
mercial scale cages indicated some of the published information,
but concluded that further research was need to understand the
‘dynamics and hierarchical effects’ between hypoxia and other
factors' [54]. So how does the level of available oxygen impact
immune function in a fish?
There are few studies that have looked at hypoxia or hyperoxia
from an immune standpoint. One study investigated the impact of
moderate hypoxia (35% oxygen saturation) on Atlantic cod (Gadus
morhua) acclimated to varying temperatures (5 C, 10 C and 15 C)
on the expression of hsp70 and showed significantly elevated
expression in hypoxic conditions at the lower temperatures [55].
Another study looked at hypoxia in Atlantic salmon (Salmo salar)
exposed to oxygen levels of 74% and 52% saturation [56]. Firstly,
isolated macrophages were stimulated with poly I:C after 29 days.
Later the animals themselves were stimulated with poly I:C at day
58. The macrophages were screened for expression of interferon,
Mx, IL-1b and TNF-a and showed significantly higher levels in
interferon expression in cells from non-hypoxic animals indicating
that chronic hypoxia can modulate the innate immune response,
which could affect the susceptibility of these animals to infection.
Conclusion
With only a few studies to draw from, there is little ability to
synthesize any distinct trends from the impacts of chronic or sub-
lethal hypoxia. However, it does appear that such hypoxia im-
pacts immune function and should be avoided when possible.
4. Acidity
Natural sources of acidity that can be considered environmental
include volcanic emissions, decomposing organic matter, and
drainage from newly exposed igneous rocks that are rich in sulfides
[57]. However, these phenomena can be considered small-scale
contributors in comparison to anthropogenic acidification. Most
of this is a result of the burning of fossil fuels, resource extraction,
which results in acid drainage from mines, and food production,
primarily the manufacture and application of nitrogen-based fer-
tilizers. Acid deposition as a consequence of acidic rainfall has been
witnessed for well over 100 years. It has been estimated that over
260 billion tonnes of carbon have been released into the atmo-
sphere since the mid 1700s [58]. Over half of those emissions have
occurred since the 1970s. The largest sources of CO2 have been from
fossil-fuel combustion (57%), and deforestation and biomass decay
(17%) [58]. Approximately one third of the CO2 released in the last
two centuries has been taken up by the ocean [59]. The current
annual rate of global CO2 uptake by the ocean is estimated to be just
over 2 billion tonnes [60]. As the oceans absorb CO2, the pH is
consequently lowered. Recent estimates suggest that the average
decrease in ocean pH of 0.1 since the start of the industrial revo-
lution [61]. Although the ocean should still be considered an
alkaline solution, this fall in the pH of the ocean is a cause for
concern [6]. Inevitably, marine organisms will become exposed to
this change in environment and little is known about how these
organisms will react to this change.
One investigation looked at tissue specific proteomics of Atlantic
halibut exposed to various environments for approximately three
months [62]. Animals were exposed to different temperatures
(12 C and 18 C) and different pH levels (pCO2 m atm (~pH 8.0,
present day), 1000 m atm (~pH 7.6, potential end-of-century)). They
found increased expression of complement component C3 and
fibrinogen b chain precursor at high pCO2, irrespective of the
temperature, suggesting that these changes are a consequence of
the altered pH. In addition, IgM heavy chain constant region was
downregulated in high-CO2/low temp animals. Complement
component C3 supports the activation of the three complement
system pathways and thus plays a central role in the innate im-
mune system. Fibrinogen expression is involved in regulation of the
inflammatory response and high levels of fibrinogen in the serum
are considered an indicator of a pro-inflammatory state [63].
Conclusion
While there has been considerable interest on the impacts of
climate change and specifically ocean acidification on a range of
physiological process, we could only find one study that looked at
acidification impacts on immune function. This study indicated an
increase of innate function with increasing acidification. However,
it is not possible to draw any more general conclusions based on the
findings from a single. Given the political importance of climate
change and ocean acidification, this is surprising. This clearly in-
dicates an area that requires considerable increase in research. The
importance of the immune function in maintaining a healthy status
for animals impacted by climate change cannot be stressed enough.
5. Salinity
The oceans serve as basins for all water sources to naturally
drain towards. Fresh water environments and the estuarine waters
in between them experience natural fluctuations in osmolality
through dilutions by rainwater and tidal mixing. Although eury-
haline fish are often well adapted to the changes in osmotic con-
centrations, stenohaline species may not be so resistant [64] and in
either case, immune functionality may be altered. The complexity
of seawater and its regional variation in simple expressions of
salinity may not articulate the variability in natural seawater
composition. Other scales, such as TEOS-2010 and PSS-1978, have
been developed that try and compensate for that variability. Prac-
tical salinity units (PSU or PSSepractical salinity scale) originates
from 1978 and the use electrical conductivity to estimate the ionic
activity in seawater. The introduction of a more recent standard
[TEOS-10, TEOSethermodynamic equation of seawater] attempted
to combine electrical conductivity measurements with other in-
formation to accommodate regional changes in the seawater
composition. Attempting to compare studies that have used a wide
variety of measurements of salinity or ionic composition is difficult.
Some studies merely inform the reader that fresh or seawater was
used. Others provide an estimate of ionic strength through one of
the scales indicated above, but this can still render comparison with
another study that uses an alternate scale challenging. Lastly, many
studies on salinity's impact on teleosts tend to focus on growth
[50], metabolism [65e67], hormonal control [68e70] and gene
expression [71e75]. The present review will examine how salinity
specifically affects the immune response in fish.
A study on pipefish (Syngnathus typhle) measured the prolifer-
ation of immune cells at three practical salinity units (PSU) (con-
trol: 18 PSU, lowered: 6 PSU, increased: 30 PSU) when infected with
Vibrio spp. [76]. At day one post infection, the pipefish in high
salinity treatments compared to low and ambient salinity displayed
significantly higher phagocytosis values. Lymphocyte proliferation
was higher in all salinities during vibrio infection. However, the
tradeoff for increased osmoregulation during Vibrio infection led to
a decrease in activation of monocyte and lymphocyte proliferation,
suggesting pipefish are immunocompromised at higher salinities.
At lower salinity, monocyte production was induced over a longer
time scale compared to higher salinity, which yielded more active
cells, but lasted for a shorter period of time.
In Nile tilapia (Oreochromis niloticus), decreasing salinity ex-
periments (20e15, 10 and 5 g/L) increased phagocytosis and the
percentage of lymphocytes and monocytes at 15 g/L [77]. During
increased salinity (0e5, 10 and 20 g/L), no significant effect on

phagocytosis or the amount of lymphocytes, monocytes, or neu-
trophils was observed. It is believed that acute changes in envi-
ronmental salinity could lead to increased susceptibility to
infectious diseases in farmed and wild fish.
A study on juvenile golden pompano (Trachinotus ovatus) reared
for 30 days at salinities of 10, 18, 26, and 34 (control) ppt were
studied for a range of immune and metabolic related activities [78].
In this study, salinity was reduced by 1 ppt/day and fish at salinities
of 18 ppt and 34 ppt displayed significantly higher superoxide
dismutase (SOD) activity than fish at 10 and 26 ppt. In another
study, tilapia (Oreochromis mossambicus) were placed in either
fresh water (control) or briefly exposed to 25 ppt seawater in order
to analyze the effects of serum on head kidney and spleen leuko-
cyte phagocytic ability [79]. A significant decrease in phagocytosis
was observed when both types of leukocytes were exposed to
tilapia serum from SW. Proteomic analysis indicated that acute
exposure to seawater caused an increased expression of C3 com-
plement protein in tilapia. In a previous study, tilapia exposed to
the same conditions showed increased lysozyme activities in head
kidney homogenate and plasma after 1 h and 24 h [80]. Transfer to
a hyperosmolar environment displayed an up-regulation of plasma
lysozyme at 1 and 24 h, phagocytic capacity at 1 h, and respiratory
burst intensity at 8 h.
In Turbot (Scophthalmus maximus) initially reared at 30 ppt,
Hsp70 and IgM gene expression levels in the kidney and liver were
analyzed between 18 and 42 ppt [27]. IgM levels in the kidney
significantly decreased upon initial transfer and then increased
after salinity conditions reached the desired level. They report the
use of a control group, which presumably was maintained at the
original temperature and salinity. However, their experimental
matrix suggests that while they implemented another group that
changed the temperature but not the salinity, there was no com-
parable group where they changed the salinity but not the tem-
perature. Thus, it is difficult to separate the interactions that are
occurring between these two environmental factors.
Gilthead sea bream (Sparus aurata L.) are a commonly aqua-
cultured species that is tolerant of a wide range of salinities from
brackish to hyper-saline [81]. A study on gilthead sea bream that
had been acclimated at 38 ppt, investigated the impact of salinity
changes on immune function in two independent experiments
[82]. In the first experiment, fish were placed in brackish water
(12 ppt), hyper-saline water (55 ppt), or 38 ppt (control) and
sampled after two weeks. Fish acclimated to hyper-saline water
significantly increased total IgM levels compared to control, while
IgM levels in brackish water remained unaltered. In the second
experiment, fish were divided between low saline water (6 ppt),
brackish water (12 ppt), or 38 ppt (control) and sampled after 100
days. In the low saline acclimated fish, peroxidase content and
alternative complement activity were significantly lower than the
control group, however IgM levels in plasma did not appear to be
affected [82].
In yellow drum (Nibea albiflora), juveniles were placed under
salinity stress for 7 days at 9, 16, and 23 ppt (control) to test the
immunity and antioxidant levels of serum [83]. At a salinity of
9 ppt, IgM levels significantly decreased, while total serum anti-
oxidant capacity increased. Although lysozyme and IgM gradually
changed, it is thought that yellow drum can effectively adapt to the
salinity levels tested, but lower salinity levels could potentially
compromise immunity.
Atlantic cod (Gadus morhua) that were raised at four salinities
(6e32 ppt) for a short period (19e57 days), then returned to
normal seawater (32 ppt) for (20e391 days) displayed no signifi-
cant changes in plasma protein concentration, natural antibody
activity or anti-trypsin activity, representing its benefit in com-
mercial aquaculture for being tolerant to high and low
environmental salinities [50]. After 57 days at 6 ppt, plasma cortisol
levels were around 10 ng/ml higher compared to other salinities,
indicating that 6 ppt is near the limit of chronic stress. Coho salmon
(Oncorhynchus kisutch) initially reared in fresh water were trans-
ferred to hypersaline water concentrations of (0.5 (control), 8
(low), 16 (medium), and 32 ppt (high)) with an increase of 4 ppt
every 2 days [66]. Using microarray analysis, the authors found
gene expression induced by salinity was highly tissue-dependent.
Overall, it was suggested that the response to salinity acclimation
caused a general immunosuppressive effect in immune genes of the
gills, liver, and olfactory rosettes.
Hematological parameters of juvenile great sturgeon (Huso
huso) acclimated to salinities of 0, 3, 6, 9, and 12 ppt were surveyed
after 20 days [84]. Fish that were transferred from fresh to brackish
water, when compared to control fish, experienced decreased red
blood cells, hematocrit, and hemoglobin. Although there were no
changes in white blood cells, monocytes, and eosinophils, a sig-
nificant increase in lymphocytes was seen only with an increase
from 0 to 12 ppt.
Conclusion
During acute salinity changes, phagocytic activity can increase
[77,85]. Innate immune cells during salinity changes tend to in-
crease, while the adaptive immune responses are often compro-
mised due to resource allocation. Furthermore, long term salinity
stress can cause the adaptive immune response to be suppressed.
The variations in study parameters such as increasing and
decreasing salinity or transfer from fresh water to salt water can
complicate the overall effects of salinity changes. To analyze the
overall effects of salinity on teleost immune parameters frequently
studied, a meta-analysis of published findings could indicate
common trends between different salinity changes.
6. Particulates
The aquatic environment can be full of particulates or sus-
pended solids, which can fluctuate temporally and spatially
depending on seasonal changes in flow rates [86,87]. Turbidity and
total suspended solids (TSS) represent two parameters commonly
monitored when analyzing particulates [88]. Anthropogenic
pollution and increasing storm events have been shown to leave
ecosystems vulnerable due to the increase in suspended particu-
lates [89e91]. It is important to note that organic or inorganic
particulate levels in fresh water environments can fluctuate
differently compared to saltwater environments.
Environment-specific guidelines are being put in place to
monitor suspended particulate matter (SPM) [87,92e94]. In a UK
study, suspended solids in 10 contrasting river sites varied 9-fold,
with mean concentrations between 3 and 29 mg/L [87], further
supporting the results from a previous study [93]. Standard
guidelines can be useful in future assessments of sediments, as they
represent a means to analyze global ecosystems collectively.
Turbidity in particular, is often relied upon when studying sus-
pended solids, however it measures parameters other than just
suspended solids [92]. The effects of particulates on fish immune
functionality can be difficult to summarize due to the extensive
amount of environmental factors at play including, but not limited
to; seasonal changes in flow rates, characteristics of river topog-
raphy, and catastrophic weather events [86]. Although studies have
clearly defined the effects of varying concentrations and sizes of
suspended solids on fish immunity [95e97], we direct the reader to
the previous review of these impacts [5]. Furthermore, here we
evaluate the most recent published literature on particulates and
their effects on fish immunity since the previous review [5].
Particulate accumulation in freshwater environments can be
quite variable, as the influx and efflux of water rapidly fluctuates

from temporal and spatial seasonal changes [86,87]. The effects of
suspended solids on salmonids are often studied in freshwater,
because anadromous fish spawn in gravel and other fine river
sediment [86,91]. Increased fine sedimentation has also been found
to have negative impacts on developing eggs in Pacific salmon
(Oncorhynchus spp.) [99,100], walleye (Sander vitreus) [101], and
robust redhorse (Moxostoma robustum) [102]. The factors that in-
fluence salmon embryo development are argued to be too complex
to make assessments about the effects of fine sedimentation [103].
A meta-analysis was performed on data from experiments on
sediment-tolerant northern pike (Esox lucius) and intolerant brook
trout (Salvelinus fontinalis) to determine the overall impacts of
increased sedimentation on fish embryo development, feeding
behavior, and species richness [98]. Sedimentation was found to
have an overall negative impact on these factors, however it is
emphasized that the lack of homogeneity between studies and
sediment measurement leaves gaps in the research [98]. Although
particulates can impair fish embryo and larval development,
breathing, and feeding behavior [98], we sought to determine the
direct effects on immune function.
In one study, Gulf killifish (Fundulus grandis) and sea trout
(Cynoscion nebulosus) exposed to anthropogenic oil particulates in
the Gulf of Mexico experienced decreased lymphocyte counts
compared to hatchery-reared control fish [89], however alligator
gar (Atractosteus spatula) were found to be unaffected. Gulf killifish
experienced a significant increase in blood monocyte levels, while
sea trout eosinophil levels were shown to increase upon oil expo-
sure [89]. Although alligator gar lymphocyte levels were previously
unaffected by oil particulates [89], a follow up study displayed sig-
nificant reductions in peripheral blood lymphocytes at 0.5 gm oil/L
tank water for 48 h [104]. Inconsistencies between studies and
sample parameters such as particle exposure further support the
need for consistent measuring techniques. Although most of these
findings represent a direct negative impact on fish immunity,
immunotoxic perturbations of PAH's (polycyclic aromatic hydro-
carbons) are the most likely cause of these alterations [105], rather
than the physical inert particles.
When rainbow trout (Oncorhynchus mykiss) were exposed to
particle pulses for 24 days, structural and metabolic changes were
observed due to the increased turbidity in the water column and
not due to physical damage [106]. Splenic melano-macrophages
generally increased during particle exposure at 8 days and 24
days. After 8 days of medium and high particle treatment levels,
immature erythrocytes increased twofold, while at 24 days of
exposure, erythrocyte release was no longer observed. Further-
more, it was suggested that trout can adapt to increased sedi-
mentation rates, although turbidity may cause increases in stress
related immune parameters [106]. Another study on individual
rainbow trout exposed to varying sediment concentrations dis-
played decreased hematocrit levels and swimming ability, however
it was suggested that swimming in groups could provide benefit to
reduce the severity [107].
Conclusions
Suspended particulates in the aquatic environment, particularly
in rivers, pose a threat to fish immunity. It is generally believed that
increased suspended solids can physically damage fish, decrease
feeding efficiency, and cause physiological stress, therefore
impacting growth rate, reproduction, and survival [108]. Reports
commonly state that turbidity is the underlying factor that impairs
functionality rather than suspended particulates physically
impairing them. Suspended solids can trigger an upregulation of
innate immune parameters, but depending on the length and
severity of altered water quality, fish have been shown to adapt to
their surroundings [106]. Further guidelines of environmental
suspended solids are essential to improve the overall study of these
impacts on fish over various temporal and spatial scales [87].
7. General conclusion
The disparity in the number of published studies in each of the
areas highlighted in the present review is interesting. Clearly, a
significant number of studies have investigated the impacts of
temperature on immune function. Other areas, while receiving less
attention, have enough data to begin to draw some generalized
ideas. It is somewhat surprising that the impact of ocean acidifi-
cation on teleost immune function has not received more attention.
Perhaps this review will help highlight areas that require further
study. The development of new analytical tools such as next gen-
eration sequencing, genomics, and proteomics has created a
considerable expansion of information, but these techniques are
still developing and hopefully future work will bring greater clarity
to the impacts of the environment on teleost immune function.
Additionally, one avenue that may provide greater clarity of the
trends associated with these studies would be some form of meta-
analysis. Clearly, shifts in the environment from the median often
involve some form of immune system change. There is contradic-
tion in some reports verses others, which makes generalizations
difficult. But environmental perturbations from the normal can
only be viewed as detrimental with animals often preparing for
immune insults as a consequence.
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