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a powerful capacity to form lasting relationships
with other organisms, including bacteria and other
microorganisms as well as crustaceans, worms,
clams and fish. Reef fish that shelter in corals, for
example, supply a nutritious gift in return: feces and
dissolved nitrogenous waste. Fish that bite and break
apart corals in search of food provide the service of
spreading coral fragments.
Asexual “Vegetative” Reproduction
The advice to not touch the corals may prompt you to
think about how delicate corals are, but it is far more
likely that you would be injured by the contact. I’m not
advocating that divers stomp on reefs, but breakage
is an important mode of reproduction for corals. The
fragments (“frags”) are clones of the parent that can
grow into whole colonies. This vegetative reproduction
is easily exploited for coral farming.
The Coral Restoration Foundation (CRF) produces
coral cuttings from a number of diverse genotypes that
they maintain to grow on “trees” in the sea. Fragments
quickly grow into large colonies that are further
fragmented to grow more colonies. These spawn, and
so the farm itself is a significant source of coral larvae
with diverse genetic parentages. Anyone who loves
corals should support the work of CRF and take the
time to join them chopping up corals, hanging them in
the farm and transplanting them on reefs.
Aside from “fragging,” asexual reproduction in corals
includes the simple process of growth (polyps bud
or divide so the colony expands), “polyp bail-out” in
which polyps separate from the skeleton and settle in
a new location and even the formation of swimming
larvae by internal vegetative budding. Perhaps the
most amazing feat of coral asexual reproduction is
the capacity to be “resurrected.” A dead coral is often
not a dead coral. Small bits of tissue — a few cells
maybe — can survive deep within the skeleton after an
event that seems to have killed the coral. Like seeds,
the cells survive there for months and then grow into
polyps when conditions are right. The entire coral may
regrow over the original skeleton from these vestiges;
alternatively they may produce buds called anthocauli
that generate baby corals from the original “mother.”
Sex and Reproduction
Corals have sex lives, too. While being sessile limits
the range of positions, sex offers a chance to make
new genetic combinations, which promotes diversity
and vigor. Corals release eggs and sperm into the
water, where they combine to form planula larvae.
Corals may be male, female or hermaphrodites that
release eggs and sperm simultaneously. In some corals
only sperm are released into the water while eggs are
retained in the polyps. For these corals fertilization is
internal, and the larvae are “brooded.”
Since fertilization success between siblings is
commonly low, outcrossing with genetically distinct
varieties is critical to maintenance and expansion of
populations. Even hybridizing is a part of this survival
strategy; the synchronized mass broadcast spawning
of corals puts together the gametes of many different
species and genera in a soup of chance combinations,
producing hybrids — much like cross pollination in
plants,
1
and hybrids are often fertile. New genetic
combinations are key to corals’ ability to adapt to
changes in their environment.
Corals spawn en masse at the time when water
temperature and day length put them at risk for
bleaching and heat shock. Why would they spawn at
the very time when the survival of their delicate larvae is
most at risk? The larvae produced during this time must
be fit to survive the heat stress, so the strategy provides
a rapid way to adapt to increasing temperatures. Thus
there are two benefits to the synchronized spawning
occurring at this time: outcrossing, which promotes
From far left: A member of the
CRF works to cultivate staghorn
coral. A Platygyra brain coral
and an Acropora coral have
bleached and dying tissue where
the free-living Fungia coral has
made contact. A coral chimera
is formed by two fused fragments
of distinctly colored Cycloseris
coral. Today corals can be found
on land in aquariums. Anthias
live within coral for shelter and
supply the coral with nutrients.
TONY VARGAS
STEPHEN FRINK
