Bivalves are unique among the many species widely farmed for human food. Species from the mollusc group, which include mussels, oysters and clams, are dependent on phytoplankton (microalgae) throughout their entire life cycle. Wild stocks of bivalves are under pressure from overexploitation and habitat losses; environmental stresses such as introductions of new diseases and harmful algal blooms; climate change bringing altered temperature regimes and increased incidences of damaging storms; and increasingly severe ocean acidification. The aquaculture industry plays a crucial role in supplying the increasing human demand for bivalves as food, and to maintain natural populations of the species.

1. I N C O R P O R AT I N G
f i s h far m ing t e c h no l og y
January | February 2014
Microalgae: an indispensible feed for bivalves
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2. FEATURE
Microalgae
an indispensible
feed for bivalves
by Eric C. Henry PhD, research scientist, Reed Mariculture Inc, USA
B
ivalves are unique among the many
species widely farmed for human
food. Species from the mollusc
group, which include mussels,
oysters and clams, are dependent on phytoplankton (microalgae) throughout their
entire life cycle. Wild stocks of bivalves are
under pressure from overexploitation and
habitat losses; environmental stresses such
as introductions of new diseases and harmful
algal blooms; climate change bringing altered
temperature regimes and increased incidences of damaging storms; and increasingly
severe ocean acidification. The aquaculture
industry plays a crucial role in supplying the
increasing human demand for bivalves as
food, and to maintain natural populations of
the species.
Cultured microalgae
– hatchery fuel
Hatcheries have long been used to
enhance bivalve reproduction by providing
ample feed for larvae, post-larvae (spat),
and often broodstock animals to increase
their fecundity. Hatcheries are also essential
for selective breeding of desirable qualities
into bivalve strains, and for propagating their
progeny.
Increasing interest in strains of bivalves
selected for superior productivity and disease
resistance will increase the need for husbandry of adult animals in hatcheries. Equally,
further environmental deterioration in natural
beds will increase the importance of broodstock conditioning in hatcheries. Hatchery
production of ‘seed’ can be used to bolster
or repopulate natural production grounds, or
to establish new production sites, sometimes
using entirely artificial installations such as
floating oyster and mussel rafts.
Ample feeding with microalgae is the
key to hatchery productivity. Hatchery
production can be boosted by improved
feeding protocols, which increase the
Image 1: Veliger (larva) of Atlantic
oyster (C. Virginica)
fecundity of the broodstock and improve
the rate of survival and successful metamorphosis of larvae. Better protocols also
make it possible to extend the breeding
season through temperature control and
supplementing diets with cultured microalgae when local seawater conditions do
not permit sufficient production of natural
phytoplankton.
Supplemental feeding with cultured microalgae can also be used to grow settled spat
to larger sizes before outplanting, which
increases the rates of survival and initial
growth. Cultured microalgae can also be used
to speed up depuration of harmful bacteria
(e.g. Vibrio) (Lewis, 2010) and shellfish toxins
(Svensson & Förlin, 2004) that can contaminate harvested bivalves.
Which microalgae are
best for bivalves?
Although hundreds of microalgae strains
have been tested as feeds for aquaculture,
fewer than 20 are in widespread use (Guedes
& Malcata, 2012). Because these algae vary so
greatly in their nutritional profiles, careful consideration is necessary in order to select the
most nutritionally appropriate strains. Such
algae as Spirulina, Chlorella, Haematococcus,
and Dunaliella are easily mass-produced as
36 | InternatIonal AquAFeed | January-February 2014
they can be cultivated in open ponds at low
cost, but they all lack the omega-3, polyunsaturated fatty acids, EPA and DHA content
that is essential for most bivalves.
Although various nutritional components
have been well documented in some algae
strains, complete nutritional profiles are
known for very few of them, so it is very
difficult to predict which strains are the best
choice for a particular application. It is unfortunate that so many studies of the nutritional
performance of microalgae have tested single
strains as the only feed, when it should be
obvious that no single strain is likely to provide
an optimal nutritional profile comparable to
what a natural mixed phytoplankton assemblage can provide.
It is equally unfortunate that so many studies of bivalve feeding have failed to identify
the particular strains of the algae that were
used. Additional uncertainties arise because
the nutritional profiles of microalgae can
be strongly influenced by culture conditions,
including light regime, temperature, nutrient
(e.g. nitrogen, phosphate) availability, and the
growth phase of the culture (exponential,
stationary, declining) when harvested.
Although the PUFA content of many
strains has by now been well documented,
sterol profiles have been more challenging to characterise, since there is far more
strain-to-strain variation. This is even the
case among strains supposedly of the same
species, as revealed in a recent investigation
of over 100 diatom strains (Rampen et al.,
2010). Protein content is less variable, with
a study of 40 strains of microalgae in seven
algal classes finding consistently high contents
of essential amino acids (Brown et al., 1997).
Vitamin contents of microalgae also appear to
be consistently high (Brown & Miller, 1992;
Brown et al., 1999).
The high-PUFA algae most widely used
for bivalves include strains of Tetraselmis
(Prasinophyceae); Isochrysis and Pavlova
(Prymnesiophyceae); Thalassiosira, Chaetoceros,

3. FEATURE
and Skeletonema (diatoms); Rhodomonas
(Cryptophyceae); and Nannochloropsis
(Eustigmatophyceae), the last one especially
used for mussel farming. But which strains to
choose for a particular application?
It can be very difficult, even impossible
to identify a species of microalgae based on
light microscopy alone, even in the hands of
taxonomic specialists. Indeed, it may not even
be sufficient to identify particular strains of
algae from examination of ultrastructural (visible only by electron microscopy) and some
biochemical characteristics. Recent studies
employing molecular genetic analysis show
that strains that are indistinguishable by these
features may nevertheless be genetically distinct.
The extent of this problem can be illustrated by a brief survey of what has been
learned about the differences among some
of the various strains of four microalgae
most often recommended for bivalve aquaculture: Tetraselmis, Isochrysis, Pavlova, and
Thalassiosira.
Tetraselmis
Tetraselmis is widely used as a successful
shellfish feed, probably in a large part due to
high levels of cholesterol and significant EPA
in some strains. Tetraselmis has also been
reported to suppress pathogenic Vibrio spp.
(Austin & Day, 1990; Regunathan & Wesley,
Image 2: Tetraselmis microalgae
Image 3: T-Iso microalga
2004), and some strains are among the
few microalgae containing significant levels of
taurine (Tzovenis et al., 2009; Al-Amoudia &
Flynn, 1989; Flynn & Flynn, 1992).
It is striking that the US National Center
for Marine Algae and Microbiota (NCMA
– formerly CCMP) holds some 118 strains
catalogued as Tetraselmis, but only seven are
identified as to species, and one of the most
frequently recommended Tetraselmis species,
T. chuii, is not among them!
Studies of the fatty acids in nine strains
(Wikfors et al., 1996) and sterols in 11 strains
(Patterson et al., 1993) of Tetraselmis found
wide ranges of total contents and different
forms of these critical nutrients, indicating that
more species diversity exists than has been
recognized by traditional taxonomic stud-
ies. However, molecular genetic analysis of
aquaculture strains has not yet been reported.
Isochrysis
Isochrysis strains are favoured for particularly high levels of the fatty acid DHA,
but the relationships among aquacultured
strains have been unclear. Fortunately, a
recent molecular genetic study (Bendif et al.,
2013) has now shown that the very widelyused ‘Tahitian’ strain of ‘Isochrysis’ (which
has been referred to in different studies as
‘Isochrysis sp.’; ‘Isochrysis galbana,’ ‘Isochrysis
aff. galbana’, or most often simply ‘T-Iso’) is
so different from other species of Isochrysis
that it belongs in its own genus, now named
Tisochrysis. Owing to this research we can
now finally understand why strains that are
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4. FEATURE
(Bendif et al., 2011) has considerably clarified
the relationships among many Pavlova strains
as well as related genera, although additional
strains remain to be examined. Different
strains contain a remarkably diverse range of
unusual sterols (Gladu et al., 1991; Patterson
et al., 1993; Ghosh et al., 1998) and this
sterol content may account for the widespread impression that Pavlova contributes
real, though poorly characterised, value as a
bivalve feed.
One study has surprisingly reported
that the combination of Pavlova lutheri
Image 5: Thalassiosira weissflogii
(unfortunately, strain not specified) and
Image 4: Pavlova microalga
microalgae
Nannochloropsis (not much used for bivalves
other than mussels) provided a feed for the
European oyster Ostrea edulis that proved
indistinguishable by microscopy differ so Pavlova
superior to combinations of Chaetoceros
widely in their PUFAs (I. galbana contains
Pavlova strains, mostly designated P. luthEPA, Tisochrysis has none) and sterols (epi- eri, are also favoured for their high PUFA muelleri and ‘T-Iso’, or Tetraselmis striata
brassicasterol in I. galbana, brassicasterol in content. A recent comprehensive taxonom- and Thalassiosira weissflogii (Ronquillo et al.,
Tisochrysis).
ic Page 1
VICTAMisland:Layout 1 30/8/13 14:22 study incorporating molecular genetics 2012).
Research with scallop larvae
indicates that at least one Pavlova
strain produces a sterol that
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38 | InternatIonal AquAFeed | January-February 2014
Thalassiosira
Strains of Thalassiosira weissflogii and T. pseudonana (in particular the strain known as 3H)
are widely used in aquaculture. T.
weissflogii is easy to culture but
lacks DHA, whereas the 3H strain
has some DHA but requires selenium (Price et al., 1987), and is so
prone to form resting cysts that it
can be difficult to culture reliably
(Dixon & Wikfors, 1997).
T. pseudonana was the first
marine microalga to undergo
whole genome sequencing, and
the functions of many previously
unknown genes are currently
being identified (Armbrust et
al., 2004). For this reason, the
physiology of this species has
the potential to be better understood than that of any other
alga. Diatoms possess cell walls
of silica that are very resistant to
degradation and are ornamented
with finely detailed markings that
permit very precise morphological definition. Diatom species can
therefore be identified with great
confidence by light and electron
microscopy. However, T. weissflogii strains have been isolated
from cool-temperate and tropical

5. FEATURE
danger of introducing diseases along with density culture methods (King, 2004) that
can dramatically reduce water use, by facthe feed.
The best refrigerated products typically tors of hundreds. Minimising water demand
have a shelf-life of six months, and the best is especially important where local water
frozen products may be used several years conditions are unfavourable (e.g. affected by
down the line. This means that a reliable extreme temperatures, acidification, toxic
The microalgae bottleneck
‘The success of a bivalve hatchery depends supply of algae can be kept on hand, available algae blooms) and treatment of sufficiently
on the production of algae. Large quantities for use in any season or if an unexpected large volumes of seawater is prohibitively
of high quality algae must be available when need arises. Algae costs become predictable, costly, or even impossible altogether. This
and often prove to amount to less than on- shows how one innovation in hatchery
needed.’ (FAO Bivalve Hatchery Manual)
Production of microalgae consumes a site production once total production costs technology – in this case, a new form of
major fraction of the infrastructure, labour, and inefficiencies have been accounted for. microalgae feed – can spur other innovations
and other operating costs of a bivalve Success of larvae is so critical to the overall that were never anticipated when the feed
hatchery. It requires specialised equipment success of a hatchery that even a relatively was developed. Bivalve aquaculture clearly
and skilled labour, which entail costs with small improvement in survival or growth rate depends on continued research that will
no return during the seasons when they are due to better feeding can yield great benefits. provide the improvements and innovations
Because these products can be as much in microalgae feeding technologies necessary
not needed. Any shortfall in algae production
can result in reduction or even loss of bivalve as several-thousandfold more concentrated to ensure the future growth of the industry.
production. Algae production can be affected than cultured microalgae, they are ideal
by weather (where natural sunlight is used), for implementing new and innovative high- References available online
FIAAPisland:Layout 1 30/8/13 14:26 Page 1
equipment failures, or human
error, and it must be timed
to match the demands of the
hatchery. Algae produced when
it is not needed (because timing
of production was misjudged, or
an anticipated hatch was not successful) is simply wasted and can
contribute substantially to the
total cost of algae production.
seas, and even fully freshwater environments,
so it is not surprising that different strains,
although nearly identical in appearance, show
different physiological traits.
Microalgae concentrates
One solution to the problem of ensuring reliable supplies of microalgae for hatcheries
can be the use of commercially
available refrigerated or frozen
algae concentrates or ‘pastes’
(Guedes & Malcata, 2012; Shields
& Lupatsch, 2012). These products, which are actually viscous
liquids, have proven to be effective feeds for shellfish and other
filter feeders. In products formulated to provide a long shelf-life,
the concentrated microalgae are
suspended in buffer media that
preserve cellular integrity and
nutritional value, although the
cells themselves are nonviable.
When concentrates with
well-defined biomass densities
are employed, the algae can
be continuously and accurately
dosed into bivalve cultures with
a metering pump, matching feed
delivery to the demands of the
cultures, maximising feeding
efficiency. Nonviability confers
the advantage that the products pose no risk of introducing
exotic algal strains. Concentrates
produced at remote facilities free
eed | January-February 2014
of pathogen vectors reduce the
8 – 10 April 2014 . Bangkok International Trade & Exhibition Centre (BITEC), Bangkok, Thailand
Asia’s foremost exhibition and
conferences for the ingredients
and additives used in the
production of animal feeds,
aquafeeds and petfoods
FIAAP Asia 2014 is the only dedicated trade show and conference organised specifically for feed ingredients,
additives and formulation within the dynamic and growing region of South and South East Asia.
New for 2014
Now including the first
ASEAN Feed Summit
Supported by
The Thailand Convention
and Exhibition Bureau
Specialist conferences
The exhibition will be supported
by its own specialist conferences.
They will include:
The FIAAP Conference 2014
Petfood Forum Asia 2014
Aquafeed Horizons Asia 2014
The Thai Feed Conference 2014
Co-located with
VICTAM Asia 2014
www.victam.com
Contact details
For visitor, exhibition stand
space and conference
information please visit:
www.fiaap.com
January-February 2014 | InternatIonal AquAFeed | 39

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