Evaluation of live food versus artificial food on the growth of juvenile Pocillopora damicornis cultured from planulae

Last year, I wrote a two-part essay for this
magazine (and spoke at several conferences) introducing the idea of growing
corals from larvae. At the conclusion of my discussions I identified
several experiments that needed to be performed in order to increase the
success of culturing corals using this technique. During this past year, I
stepped down from my soap box and retreated into my lab in an effort to put
my words into action. After many unshaven,
pretzels-for-lunch-and-dinner-days, I successfully completed two important
experiments; one that will improve the techniques for growing corals from
larvae, the other, a technique for increasing the growth rates of two
different species cultured together in the same tank. The following is a
report on the first experiment where I examined the effects of different
foods on the growth of juvenile corals.

Introduction

Corals in the Order Scleractinia are popular marine ornamental
invertebrates within the aquarium trade (Delbeek 2001). In 2006, the
Convention on International Trade of Endangered Species (CITES) reported
that over 1 million pieces of live coral were traded globally. Although
there are over 100 commercial facilities worldwide that grow and sell coral
fragments, 99% of the coral fragments introduced into the aquarium trade
still originate directly from tropical reefs (Wabnitz et al. 2003).
Currently, all of the corals offered within the aquarium trade originate
directly from the harvest of coral fragments from parent colonies on the
reef. These fragments are obtained by cutting pieces of coral away from the
main colony and attaching them to substrate. This practice may ultimately
affect the parent colony as the exposed skeleton may promote the settlement
of algae capable of overgrowing and smothering the colony (Nugues and Bak
2006). Further, the reduction in size could reduce the fecundity of the
coral (Tanner 1997), and decrease its ability to contend in a highly
competitive environment (Connolly and Muko 2003).

To reduce the number of corals harvested from reefs, I have developed
techniques for growing corals from larvae. Other marine invertebrates whose
populations have been threatened by the harvesting of wild stock, such as
Giant clams (Tridacna sp.), have benefited substantially from the
research and practice of culturing through sexual reproduction (Ellis
2000). In addition to the potential profits associated with the culture of
Tridanca clams, there has been an increase in wild populations that has
resulted from the reduction in wild stock harvesting (Minogoa-Lucuanan and
Gomez 2002).

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One of the challenges involved with the culture of coral larvae into
adult colonies is the high rates of mortality during the first month post
metamorphosis (Gateno et al. 2000, Szmant & Miller in press, L.
Goldman, pers. obs.). Survivorship increases, however, as the age and size
of the juvenile increases (Gateno et al. 2000, Raymundo & Maypa 2004).
Therefore, to ensure high rates of survival, growth during the first month
must be maximized. The addition of food can make significant contributions
to the growth of marine invertebrates including corals (Ellis 2000,
Borneman 2001, Rhyne & Lin 2004). Different food sources, however, can
have varied performances and production costs which may affect colony
growth and the overall financial investment. With the increase in demand
for corals in the aquarium trade and the continual degradation of natural
reefs around the world, new techniques in coral culture must be
investigated.

Methods

Three treatments, live food (Artemia franciscana,
GSL© Premium, 400μm), artificial food (Golden
Pearls©, 400 μm), and no food (control) were replicated 3 times
for each group of planulae donated from a single colony (Figure 1 and 2).
The experiment was repeated 6 times using planulae donated from 6 different
individual colonies. This experimental design is known as a ‘clonal
design’. Using planulae in all of the treatments donated from a single
colony gives me better control over any genetically-based variation that
may exist between different genotypes. Ultimately, I can conclude with more
accuracy that any differences in growth between colonies may be due to the
different treatments (food types). Replication comes from the 6 times I
repeat this experiment, using a different parent colony each time.

figure1.jpg

Figure 1: Three treatments, live food (Artemia
franciscana
, GSL© Premium, 400µm), artificial food (Golden Pearls©,
400 µm), and no food (control) were replicated 3 times for each group of
planulae donated from a single colony. The experiment was repeated 6
times using planulae donated from 6 different individual colonies.

figure2.jpg

Figure 2: Left: Artificial food treatment, Golden
Pearls© (400 µm); Right: Live food treatment, Artemia
franciscana
, GSL Premium (400 µm).

Each 500ml cup contained one juvenile Pocillopora damicornis
colony and was supplied with aeration via an outdoor, 50W air pump.
Temperature was maintained by placing the cups in a water bath (25º C) and
a 70% shade cloth was used to minimize over exposure to direct sunlight.
Nutritional values for A. franciscana and Golden
pearls© were reported to be similar (Artemia Int’l 2007). I
found that using 0.55 g of GSL© 90% Premium
hatch
(400 μm) and 0.50 g of 400 μm Golden Pearls© in one
liter of water produced equal amounts of food. Each night, I used a 5 ml
pipette to extract 3.5 ml of enriched seawater which was distributed into
each treatment cup. Food remained in the cups overnight. The following
morning 100% of the water was changed and no additional food was added.
Data was analyzed using a two-factor ANOVA (factors: treatments and
clones). Count data was square-root transformed prior to analysis.

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Results

After one month, juvenile P. damicornis who were fed live food
showed a significant increase in the number of polyps (Figure 3, ANOVA;
df = 2, F = 67.83, p = 0.008, Tukey-Kramer test,
p = 0.05) and colony size (Figure 4, ANOVA; df = 2,
F = 145.34, p = 0.0002; Tukey-Kramer test, p =
0.05) compared to juveniles who were fed artificial food or who did not
receive either food item. Corals fed live food had an overall higher
survival rate compared to corals fed artificial food and corals with no
food (Figure 5). There was no significant difference among the responses
between clones (ANOVA: number of polyps: df = 5, F =
0.66, p = 0.6647; ANOVA: diameter; df = 5, F =
0.55, p = 0.7236).

figure3.jpg

Figure 3: Colonies fed live food (Artemia
franciscana
) showed a significantly higher number of polyps than
either the colony fed the artificial food (Golden Pearls©) or fed a no
food control. Treatments with an asterisk were significantly different.
Values were untransformed for clarity.

figure4.jpg

Figure 4: Colonies fed live food (Artemia
franciscana
) showed a significant increase in colony diameter
compared to colonies who were fed either the artificial food (Golden
Pearls©) or fed a no food control. Treatments with and asterisk were
significantly different.


figure5.jpg

Figure 5: Although the differences were not significant,
corals fed live food had a higher average survival rate compared to
corals fed artificial food and corals that received no food.

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Discussion

Although A. franciscana and Golden Pearls© have
similar nutritional values, colonies fed live food showed a significantly
higher growth rate. Because corals are sessile, the exposure to food is
dependant upon water flow (Sebens et al. 1998) or the locomotion of the
prey, as in the case of Artemia spp. Corals consume a variety of
reef organisms (Goreau et al. 1971) and will continue to consume prey as
long as it is available, never becoming satiated (Ferrier-Pages et al.
2004). Observations on the availability of each of the food item revealed
that Golden Pearls© remained buoyant for only one hour after
being distributed into the treatment cup compared to A.
franciscana
which remained continuously active in the water column.
Therefore, because the artificial food did not circulate and sank to the
bottom, it is probable that corals in the artificial food treatment were
not exposed to as much of the Golden Pearls© as corals who were
fed A. franciscana. This conclusion may seem fairly obvious and
pretty straight forward. The manufacturers of Golden Pearls©
promote their product as being ‘neutrally buoyant’; however, this was not
the case (at least for an extended period of time). In their defense, they
developed the food as a replacement for Artemia spp. used to feed
young shrimp; highly mobile creatures that have no qualms about seeking out
their food. Most corals do not have that ability and, thus, any type of
food that cannot consistently remain in the water column may not be the
best choice for corals. Although I believe this to be the major (and
obvious) factor that produced these results, there may have been other
factors at work here.

figure6.jpg

Figure 6: Coral colonies in each of the three
treatments: a: Control; b: Artemia franciscana; c: Golden
Pearls©. Note the substantial algal growth in artificial food treatment
compared to either of the other two treatments.

An equally important observation was how water quality and the colonies
were affected by the different food items. Most aquarists who care for a
variety of corals understand that food additions can be a necessary evil.
One the one hand, feeding corals promotes their health and growth; on the
other, food contributes substantially to the quality of water in which the
corals live. Nutrients associated with food are usually released into the
water and may negatively affect water quality. Since many corals are
vulnerable to even minor changes in water quality, the quality of food must
be considered. In this experiment, I did not test water quality. The growth
responses that I observed in each treatment and previous studies on the
results of elevated nutrients on corals and seawater systems, however,
permitted me to speculate on how the food item affected water quality.

photo1.jpg

Photo 1: Left: Colonies of Pocillopora
damicornis
at 2 weeks post metamorphosis (Diameter = 5 mm); Right:
Colony at 8 months (Diameter = 35 mm)

In 1991, Stambler et al. performed a nutrient enrichment experiment on
Pocillopora damicornis. Motivation for this experiment came from
the need to understand what kind of effects elevated nutrients have on
coral reefs. In the experiment, they examined two the effects of dissolved
inorganic nitrogen and phosphorous on the growth of P. damicornis.
They found that the addition of ammonium (nitrogen) did not lead to an
increase in growth of the coral; rather, it led to an increase in algal
growth. In this case, the algae were symbiotic zooxanthellae contained
within the polyps of the coral. Phosphorous, in combination with nitrogen
produced similar results, but phosphorous alone did not result in any
changes to either the algae or coral. Stambler and his colleagues suggested
that the lack of growth observed in the coral may be due to the higher
energy demands from the increasing algal populations, thus no carbon was
translocated to the coral from the algae. Their findings (and discussions)
were consistent with previous work by Muscatine et al. (1989) who used
Stylophora pistillata as their test subject.

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photo2.jpg

Photo 2: Colonies at 11 months. Most colonies have
reached the minimum size for commercial distribution (Diameter = 50
mm)

In my experiment, I observed similar results. I did not count the number
of zooxanthellae inhabiting each colony so I cannot say definitively
whether one colony had more zooxanthellae than another, however,
descriptively three things were apparent. First, colonies in the artificial
food treatment did not grow significantly more than colonies that were not
fed any food. Second, substrate tiles in the artificial food treatment had
more algae growth than the other treatments. Dozens of studies have shown
that the availability of nutrients affects the growth of algae (Larned
1998; Schaffelke and Klumpp 1998). Third, colonies in the artificial food
treatment were much darker than either of the colonies in the other
treatments. Lisa Chou, a graduate student who studies zooxanthellae at the
University of Guam Marine Lab, suggested, rather cautiously as any good
scientist would, that the darkening of the zooxanthellae may be due to an
increase in the density of zooxanthellae. These observations indicate that
there may have been higher amounts of nutrients in the artificial food
treatment. Analyzed as such, the picture may be more complete: since the
artificial food remained un-consumed on the bottom of the cup, it decayed
and released these nutrients into the water. Therefore, in conjunction with
the lack of available food, there was an increase in nutrients both of
which may have contributed to the lack of significant growth observed for
colonies in the artificial food treatment. Colonies in the control were not
fed, and showed similar a similar lack of significant growth response.
However, their water quality was not affected by the presence of
supplemented nutrients, and thus algal growth was limited. Complete as this
picture now seems, it will take further experiments to confirm these
discussion points.

Conclusions

Live food may incur higher production costs in the form of labor and
equipment than artificial food; however, this study showed that the
increase in colony growth and survival may be an acceptable trade-off.
Although no conclusions can be made about the artificial food and its
effects on coral health and water quality, it is apparent that artificial
food did not promote coral growth, rather only algal growth. In my
experience, substantial algal growth may be detrimental to the health and
survival of juvenile corals. My observations are consistent with other
studies on the growth and survivorship of juveniles on the reef (Sato 1984;
Babcock and Mundy 1996).

This work, in combination with previous studies, has made substantial
contributions to the refinement of this method and, although this
experiment focused on coral recruits, the results can be applied to current
techniques used for maintaining adult colonies as well. Future studies will
investigate other live foods such as rotifers and algae and investigate
other artificial foods and their effects on water quality. By growing
corals from sexually produced larvae, farms can supply the aquarium trade
demand as well as provide corals for conservation programs without having
to harvest or sacrifice coral colonies from existing reefs.

Lee Goldman earned his Masters degree in Marine Biology at the
University of Guam, where he also works as a research associate at the
College of Natural and Applied Sciences, Guam Aquaculture and Development
Training Facility.

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