Naked Gills

When we see one in our gardens we cringe with disgust at the slimy, grayish brown, shapeless body leaving a trail of goo as it creeps along. What is it? It is a
terrestrial slug. But, when we take the time to look for the land slug’s
ancient ancestors that still inhabit the world’s seas, we find a sight to
behold; with all their glorious colors, shapes, and
sizes ranging from a fraction of an inch to over 12 inches and weighing up to 2

While one sea
slug species, the Lettuce Leaf nudibranch, found in
the Caribbean, looks like a bit of ruffled leaf matter undulating in the
currents, others look like delicate blue lace, tiny orange anemones, pale arms
of multi-branched coral, or yellow green sargassum
weed. Waters of the Caribbean, Florida
and the Bahamas
hold over 40 identified species of nudibranchs with
more likely to be discovered.

Nudibranchs, meaning naked gills, inhabit all the oceans
including the freezing waters of Antarctica. There
are over 3,000 species of nudibranchs worldwide and
they can be found from shallow tidal pools to some 700 meters in
depth. They are gastropods, members of the shellfish family Mollusca, which appeared some 500 million years ago.

All nudibranchs have two antennae-like tentacles on their heads
known as rhinophores which scientists believe serve
as locators for food and mates.

Although nudibranchs are a division of the Mollusca
family known as the opisthobranch mollusks,
only larval nudibranchs have shells, which are shed
and replaced, in some nudibranch
species, by cerata. The cerata
are tiny sacs that store the nematocysts, stinging cells, of the prey on which
these nudibranchs feed and to which they are
immune. When threatened, these nudibranchs fire
the captured nematocysts in defense, warding off
their predators. Those nudibranchs without cerata exude strong and sometimes toxic chemicals that
deter predators.

“naked gills” are external and appear in an amazing array of
shapes, colors, and configurations. The multi-lobed
ruffles of the Lettuce Leaf nudibranch serve not only
as gills but also as camouflage, giving it the appearance of leaf
matter. The tiny grape-like clusters of cerata
circling the Grape Cluster nudibranch’s body
not only exchange gasses but also ward off predators when they fire their
stored nematocysts. The yellow-green appendages along the Sargassum nudibranch’s back
make it appear to be just another shoot of sargassum

Nudibranchs are hemaphrodites,
thus possessing both male and female reproductive abilities
which greatly increases their chances of reproduction, since they can
become the opposite of any potential mate they may encounter. After
mating, they can lay a million eggs or more in a variety of colors,
some in single egg chains and others in intricate coiling or flower-like
configurations. After a five to 50-day
incubation, the eggs hatch, producing hard-shelled larvae that are carried by
currents until they settle on the bottom, where they grow to adulthood. In
the absence of predators, nudibranch lifespans range from 2-3 months to 4-5 years with an
overall average of 1 year.

Nudibranchs are grazing carnivores, with most species
slowly inching their way along the bottom and on reefs, living their lives in a
rather small area, while a few of the larger species are known to swim to new
locations using their cerata as swim fins. They
eat a wide variety of marine life specific to each species. While some eat
sponges, others feed upon anemone, coral, hydroids, tunicates, barnacles and
even other nudibranch species. Their mouths are
located on the underside of their bodies and contain teeth called radula that vary in size and shape depending upon the prey
each species feeds upon.

Some of the
coral feeding species of nudibranchs, such as the
Lettuce Leaf, ingest algae along with the coral. These species have
evolved the ability to photosynthesize the algae, thus becoming known as
‘solar powered’ nudibranchs, using the
resultant sugars to fuel their bodies.

Most divers, swimmers, and snorkelers never notice the beautiful nudibranchs
that inhabit our seas; too busy looking for the big things and missing the
glorious small things often in plain sight but never seen. Whether walking
through tidal pools or swimming along a reef, look first for what the local nudibranchs feed upon and then look very closely for a
small, delicate, fleshy looking animal with two tiny antennae. If you
look, you will find one of these amazing creatures but, as always, only look
and do not touch, for nudibranchs are not only very
delicate creatures, easily injured; but, they may also exude a noxious chemical
or inflict a painful sting.

Becky Bauer is a scuba instructor and award-winning journalist covering the marine environment in the Caribbean. She is a contributing photographer to NOAA.