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 pounds.
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.
Their “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 seaweed.
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 theLettuce 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.