Growing corals from sexually produced larvae and planulae is
a relatively new approach to farming corals for the aquarium
trade. Although coral settlement and juvenile growth has been
previously studied (Babcock 1985, Morse et al. 1991, Negri and
Heyward 1999, Rinkevich 1979), there are few individuals,
myself included, using this approach in an attempt to meet the
demands of the aquarium trade. There are several reasons why
this technique should be realized, with the conservation of our
natural reefs leading the pack by a thousand fold. Colony
morphology and genetic diversity within the trade are distant
seconds, but important reasons none-the-less. However, as with
any type of aquaculture, this form of coral farming is not
without it challenges.
Corals have two modes of sexual reproduction (Richmond and
Hunter 1990). Corals that release their gametes (egg and sperm
packaged together called the egg/sperm bundle) into the water
column for external fertilization are called broadcast
spawners. Sperm is attached to the positively buoyant eggs and
the bundle floats to the surface where it will collide with
bundles released from other corals of similar species. This
mode requires that all of the corals in the area use specific
seasonal cues to ensure that their gametes will mix with
gametes from other similar corals. The term ‘mass spawning’ was
ascribed to this event as many corals will release their
gametes at the same time (Harrison, et. al.). Further research
has revealed that different species of corals on the same reef
will release their gametes on the same night, but not at the
same time. For instance, in Guam, Acropora surculosa
will release gametes on the same nights as Acropora
humulis. However, each species will release them at
specifically different times (Richmond, Pers. Comm.). This will
help increase the chance of colliding with species specific
gametes and further minimize the chance of cross-breeding.
Fertilization of the eggs by the sperm results in the formation
of larvae. Larvae further develop for a few days in the water
column before they drift down towards the reef and settle. A
brooding coral internally fertilizes the egg and sperm and
releases developed planulae into the water. The planulae, like
the larvae, will then seek a suitable substrate on which to
settle and grow.
In either case, corals are capable of releasing anywhere
from hundreds to millions of larvae or planulae per year. Some
will release a only few planulae per day, as in the case of
Leptoria purporea (Pers. Obs.) while others, like some
Pocilloporids, can release thousands of planulae timed to each
lunar cycle (Harrigan 1972, Richmond 1984, Rinkevich 1970). In
the case of broadcast spawners, one specific night out of the
year can trigger the release of millions of gametes into the
water column (Richmond and Hunter 1990).
When the larvae and planulae are ready to search for a
suitable substrate, they are deemed competent to settle. Corals
can use biofilms, crustose coralline algae, or bacteria found
on the reef substrate as cues to begin the transformation from
the larvae into a benthic coral (Harrigan 1972, Iwao, et. al.
2002, Morse and Morse 1991, Negri, et. al. 2001). This natural
process is the path that larvae and planulae would take to
survive and grow on the reef. It is not much different than the
steps I would have to perform to get the same results.
Collecting the larvae or planulae, consistently and
quantitatively settling the coral larvae onto substrates, and
growing the juveniles to adulthood represent these steps
towards success. My challenge is infinitely harder than the
just fulfilling these steps. As a farmer trying to raise enough
corals to meet the demands the obstacle that I face is not in
ushering some of them along the natural path towards adulthood,
rather it’s that I need to usher all of them along this path.
To do this successfully, I have to put them on a path that is
un-natural and defies the strategy that corals have employed
for millions of years.
r/K selection theory relates, in a general sense,
to the two reproductive strategies that species are determined
to take based on environmentally selective pressures (Pianka
1970, Heylighen 2000). The terms are familiar to theoretical
ecologists who study population growth and derive from the
where ‘r‘ is growth rate of a given population (N)
and K is the carrying capacity. ‘r‘ selective
species or ‘r strategists’ are usually found in highly
unstable and often competitive environments (Heylighen 2000).
Resources that the individual use is often allocated towards
adjusting to and surviving in their environment. Any chance to
allocate resources to reproduction must be capitalized on
before the conditions change again or external pressures
require their immediate attention. r-strategists tend
to have short life spans and are small in size. They reproduce
quickly and with high fecundity (the number of offspring
produced by an organism). As a function of their investment
into the creation of many offspring, organisms invest only a
minimal amount of resources into each offspring. Mortality
rates among the offspring are exceedingly high, with the
expectation that only a few will survive to adulthood. They
also release them into the environment where they will be
dispersed widely throughout their range (such as in the water
column or in the air). Finally, no protection or resources from
the parents are offered once the offspring are released.
Examples of organisms using this strategy are frogs, weeds,
clams, oysters, some insects and many fish. ‘K
strategists’ have almost opposite considerations when producing
offspring. They tend to reside in more stable environments,
live longer and grow bigger. They invest highly in only a few
offspring, and provide plenty of parental care and resources to
the offspring. Since they are well adapted to a more stable
environment, resources can be allocated to producing offspring
that will also be well equipped to survive in the given
environment. Examples are elephants, whales, birds, trees in an
old growth forest, and of course, humans. Ultimately, the two
strategies can be defined by the investment each organism makes
into their perspective offspring. Organisms either invest in
producing millions of ill-equipped offspring or a few
Like many organisms, corals as a whole group do not fit
neatly into either category however; they do tend to be
r-strategists. Corals exist in a highly competitive
often unpredictable environment and demonstrate many of the
characteristics that are important qualifications for that of
r-strategists. Even corals that are thought to be
K-strategists use many of the mechanisms for
reproduction that are generally related to
r-strategists. For example massive, long lived corals
that reproduce once per year, like some Poritiids, release tens
of thousands of gametes into the water column and do not equip
their offspring with abundant internal resources nor give
parental care once the gametes are released.
The selective pressures that drive corals to be
r-strategists is further understood when their habitat
is taken into consideration. Reasons for coral mortality, both
as adults and more so as juveniles, are much more plentiful
than reasons for their survival. Algal overgrowth, temperature
changes, sedimentation (from storms or terrestrial run-off),
diseases, predators and herbivores all contribute to creating a
less than stable environment, especially for the offspring
(Hunte and Wittenberg 1992, Kuffner, 2004, Sammarco 1980,
Tanner 1995, Te 1992). Further, resources available to corals
on the reef are at a premium and competing successfully against
other corals for a favorable position on the reef (availability
to sunlight, water movement, and food) often requires a large
amount of energy (Lang and Chornesky 1990). If resources went
into producing only a few, but well developed offspring, it is
possible that they would not have the resources to compete
successfully with neighboring corals.
It is for these reasons that corals spend their reproduction
resources on producing as many offspring as possible. It
ensures that a small percentage out of the thousands of
released larvae will make it past the open ocean predators; and
that a small percentage of those who escape predation will find
a spot on the reef that is suitable for settlement; and that a
small percentage of those that find a spot on the reef will
metamorphose quickly enough to grow and defend themselves
against the myriad of potential life ending confrontations.
Further, the ability to distribute offspring to a wide range of
areas will ensure that a few may make it to areas that are
temporarily more favorable than the area from which they were
released (Richmond 1988). On the natural reef, the ‘r‘
strategy that corals employ is obvious. If we apply a simple
mathematical model for exponential increase to Pocillopora
damicornis it might look like this: A single colony
releases 1000 planulae and half survive (opposing r-strategy
and its selective pressures). If, in a few years of growth the
survivors (including the original colony) release another 1000
planulae of which half survive, there would be 250,500 new and
existing (the single colony who started it all) colonies. A few
years later, keeping in proportion with the previous years,
there would be 125,500,500 new and existing colonies. This is
the math for only one initial colony and one species on the
reef. It gets very obvious when we throw 300 different species
of corals each with 20 local representatives into the mix.
Regardless of the type of selective pressure, it’s the nature
of the strategy that corals use that only a few offspring
survive to adulthood.
As previously mentioned, my job as a researcher dedicated to
this type of farming is to have as many offspring make it to
adulthood (or, at least, marketable sizes) as possible. Another
way to look at it is that I have to create a supportive habitat
for larvae and planulae that were destined to die the moment
they were released. I have to further rear them through their
steps towards adulthood, which requires an understanding of
their nutritional and environmental needs. If I rely on the
natural percentages for offspring survival in corals, I may
only have a handful of corals reach marketable sizes. This is
further complicated by the limited spawning seasons for many of
the corals popular in the aquarium trade. Would I be able to
compete against other farms that currently fragment corals,
meet financial obligations or convince other facilities to
subscribe to this type of farming if I have only a dozen corals
per annual season? The short answer is: No. At the very least,
it is an extremely intimidating task; at most it borders on
impossible. The task, however, is not without its positive role
models. Another very popular animal that is cultured with great
success are the Pacific White Shrimp (Litopenareus
vannamei). These shrimp have been successfully grown in
hatcheries, mostly for human consumption, for decades. Shrimp
are also r-strategists. They produce millions of
offspring with the hopes that a few survive to adulthood. The
beginning of shrimp culture started out similar to that of the
corals. Through exhaustive research over the years, hatcheries
have gained the knowledge to be able to rear large percentages
of shrimp into adulthood (Victor Camacho, Pers. Comm). I
believe this is the next step towards successfully rearing
corals from larvae. Research, research, and more research into
all aspects of their requirements is the future of this method.
At my lab, this is my goal. It’s a lofty goal but with benefits
that my baby girl can enjoy in the future; beautiful and
natural reefs worthy of her admiration.
When people ask me what the hardest challenge I face in my
pursuit of this goal is, I simply state, “oh, it’s nothing. I
just have to defy the strategy that corals use for reproduction
and rear the animals past their extremely high trend towards
fatality”. It’s easy…yeah ‘r‘ight.
Lee Goldman currently divides his time between his
Master’s work at the University of Guam and his research on
growing corals from larvae for the aquarium trade. He will be
speaking about his work at the upcoming MACNA XVIII conference
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