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extreme specialization, competition and the
need to quickly and quietly subdue prey have
guided the evolution of marine toxins. Toxins
are products of a complex arms race among
organisms that have granted some the ability to
outcompete, elude or prey upon others.
A toxin is a biologically produced poison.
Venom is a toxin that is administered
through a dedicated delivery system — a
bite or a sting, for example. When toxins
come in contact with living tissue, they may
damage or disrupt enzymes, cell receptors
or nerve channels. Toxins are highly diverse;
University of Hawaii researchers estimate
that about 20 million exist in nature. Some
may cause only minor pain, while others are
quite deadly. Some are relatively simple, and
others are startlingly complex; a textile cone
snail’s venom, for example, includes dozens
of proteins, each targeting a specific nerve
channel or receptor.
Toxins are considered secondary
metabolites because they are not directly
involved in organisms’ growth, development
or reproduction but do play a significant role
in survivability. David Ginsburg, assistant
professor of environmental studies at the
University of Southern California, stated,
“Secondary metabolites are most common
among benthic marine organisms such
as invertebrates and seaweeds, which are
typically the most abundant reef organisms
(as well as the most ecologically significant
players), because benthic organisms are
vulnerable to the highest rates of attack....” In
other words, the slow or immobile creatures
that make up so much of the reefs divers
enjoy are frequently toxin producers.
Sponges
Sponges are among the most common
sedentary animals on reefs. Structural
calcium carbonate and siliceous spicules
provide nominal protection, but similar to
many algae, sponges have evolved a range
of secondary metabolites that successfully
deter most fish and invertebrates. However,
some predators such as angelfish and
nudibranchs have adapted to metabolize
sponges’ toxins. In fact, nudibranchs have
taken the additional step of using their prey’s
toxins for their own purposes. Many of these
colorful and slow-moving slugs, prized by
underwater photographers, dine on sponges,
soft corals or tunicates and incorporate
those organisms’ poisons into their own
tissues. This grants effective defense while
allowing the nudibranchs to avoid the need to
manufacture resource-intensive toxins.
Cnidarians
All corals have stinging cells, but some (soft
corals in particular) have less potent stings
than other cnidarians and thus rely heavily
on chemical defenses to inhibit predation.
Such means also help organisms outcompete
rivals for space and other resources. One
common manifestation of this is the release
of proteins into surrounding water to
hamper the growth of nearby species. This
is especially important for larval or juvenile
coral colonies that would be overrun if not
for their secondary metabolites.
Related to corals are the most dangerous
animals divers can encounter: box jellyfish.
These deadly translucent predators
disable small crustaceans and larval fishes
immediately upon touching them by
unleashing a potent cocktail of neurotoxins,
cardiotoxins and myotoxins, causing rapid
respiratory and muscular paralysis. The
formidable toxins enable the fragile box jelly
to avoid injury while capturing prey.
Mollusks
Most divers know that harmless-looking
cone shells are best left alone, but their stings
are even more wicked than one might guess.
There are more than 600 species of cone shell,
and each one manufactures its own paralytic
neurotoxin that is targeted at specific prey.
Researchers from the University of Hawaii
estimate that 100,000 toxins are present in
cone-snail venoms, which are collectively
known as conotoxins.
Some cone shells use their conotoxin-
filled, harpoonlike projectiles to kill fish,
while other species prey on mollusks or
worms. Fish are much faster prey, so it is
vital that the toxins of the species that prey
on them — e.g., textile and geographic
cone shells (Conus spp.) — operate quickly.
Anything short of an immediate kill might
mean the prey gives up the ghost a few
feet away from the snail, and in a coral reef
environment that would simply make for an
Above, from top: the variably
colored Hypselodoris bullocki
nudibranch; fire urchins
(Asthenosoma spp.);
shortfin lionfish
(Dendrochirus brachypterus)
Opposite: Pelagia jellyfish
(Pelagia noctiluca)