2000-12 Side Gilled Sea Slug

At first I thought it was an unusual cephalopod, and although it is related to them, it was definitely not a squid, cuttlefish or octopus. This odd-looking creature was a pleurobranch or side-gilled sea slug that, like the cephalopods, belongs to the mollusk family.

Its scientific name is Euselenops luniceps, which probably refers to the crescent moon shape of its oral veil or mouth. I know of no common name.

Pleurobranchs evolved from snails and are the precursors of nudibranchs. They are generally larger (over four inches long) than nudibranchs and have gills on the right side of the body, between the fleshy mantle and foot. They have either lost their shells completely or retain only a remnant of them.

Even without a shell, E. luniceps has no problem defending itself. It has the nasty habit of secreting sulfuric acid when it senses danger—an effective deterrent against most predators. On the other hand, it is itself a formidable hunter.

If you look closely, you will see fringed tissues under the oral veil. They help the slug detect food. The rhinophores (sometimes mistaken for an-tennea) also sense potential food sources. E. luniceps has a varied diet and reportedly feeds on invertebrates, including other slugs, and fish.

This slug is well-suited for life on the ocean floor. The flattened body shape makes it easy to move over and burrow under sand. When E. luniceps buries itself, the tips of its rhinophores remain exposed so as not to miss any tasty morsels that happen to venture near. The mantle rolls up into a siphon that extends just above the sand and draws water over the slug’s buried gills.

Although E. luniceps is not common, it is found throughout the Indo-Pacific region. Look for it in sandy areas after dusk when it is likely to be out feeding. During the day look for the telltale tips of the rhinophores and mantle while the body lies safely buried under the sand.