Aplacophorans

Aplacophora is a class of small wormlike marine animals.  For a long time they were a mystery to marine biologists: up until 1987 they were classified as sea cucumbers (which are echinoderms).  However the shell-free Aplacophorans are not echinoderms at all, no more than penguins are insects.  Aplacophorans are tubular and lack shells, but they are actually mollusks—like clams, belemnites, octoposes, and gastropods.

Aplacophorans are divided into two subclasses: (1) the Solenogasters, which are typically carnivores which feed on feeding on corals and worms; and (2) the caudofoveates which tend to be detritovores feeding on leftover bits rolling around the ocean floor (although some caudofoveates eat foraminiferans (which are large unicellular organisms).

All of this talk of small mollusks which look like little worms…or maybe like little echinoderms…or even possibly like little early chordates (which were basically little tubes) sounds unpromising, but some of the Aplacophorans have a subtle beauty.  Additionally they throw a light upon a bygone time when the mollusks, cnidarians, sponges, worms, and proto-vertebrates really weren’t all that different.  They help illustrate the common bonds of kinship which tie all of the animals together, no matter what airs some of us put on.

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Aplacophora

Cast your imagination down to the bottom of the ocean—not at a beach or a bright coral reef just offshore, but the true ocean floor—the abyssal plains which cover much of Earth’s surface.  Here vast flat swaths of mud lie in black silence.  Only the occasional seamount or shipwreck breaks the monotony of plains as big as continents.  Tides do not particularly affect the bottom of the ocean.  The most violent storms do not perturb the waters.  Even humankind’s restless activities, which have so much affected the rest of the planet, mean little here.  At first it seems bleak, but soon enough you realize that life is everywhere here.  There are spiderfish, lizardfish, deep sea octopuses, bizarre roving sea cucumbers, and all sorts of strange creatures, but we are not here for them.  Instead we are concentrating on an inconspicuous worm-like animal.  The tiny cylindrical creatures are only 5 cm (2 inches long) and they shimmer strangely when exposed to light.  It would be reasonable to assume that they were worms or tiny sea cucumbers, but they are not.  The benthic beasts are members of the aplacophora class of mollusks—the naked mollusks.  They are presumed to be similar in appearance and nature to the basal mollusks from which the other classes of mollusks have evolved (although both fossil and molecular evidence is frustratingly exiguous).  To look at aplacophorans is to see back to the Cambrian (540 million years ago) and to glimpse an even earlier era when the ancestors of the mollusks diverged from the annelids.

Two specimens of the aplacophoran Simrothiellidae (photographed by G. Rouse)

The aplacophoran shine because of tiny calcareous spicules embedded in their skin.  There are about 320 known species split between two clades: the caudofoveates and solenogasters.  To quote the University of California Museum of Paleontology website, “Caudofoveates are burrowers that feed on detritus and bottom-dwelling microorganisms, while solenogasters feed on cnidarians. Both groups have a radula and lack true nephridia.”  There is an even more important distinction between the two different clades: whereas solenogasters are hermaphrodites, caudofoveates have two genders, and reproduce by external fertilization.

Epimenia australis (Photo by R. Willen)

The depths of the ocean are known to harbor animals which have vanished from the rest of the Earth long ago, and such is believed to be the case with aplacophorans.  For a half billion years they have gone about their business in a part of the world which is resistant to outside change.  The next time you fly across an ocean, imagine all of the naked mollusks in the muck at the very bottom and think about the vast amount of time they have been there.