A Beautiful Slug?

As I mentioned last time, the Craig Key Channel swim offers an opportunity to float on your stomach and let the currents drift you over top of some pretty impressive, large sea creatures out in the middle of the channel. But even more fun for me is to go inch by inch over the rocks and mangroves along the shore. The mangrove roots seem to be a nursery for many juvenile fish. Angelfish, parrotfish, groupers, puddingwives, scorpionfish, the invasive lionfish, green morays, and especially species of colorfully decorated damselfish all hide among the roots and rocks and young fish seem especially prevalent here. More fascinating still are the array of invertebrates that live in the channel. Shrimp, sea cucumbers, sea stars, brittle stars, sponges, hard & soft corals, anemones, zoanthids, tunicates, feather duster worms, and many molluscs including the beautiful conchs and cowries are just a few of the fascinating invertebrates regularly seen on the Craig Key Channel dive.

Atlantic Deer Cowrie

Cushion Sea Star (genus Oreaster)

Every dive we record all the species of plants and animals we are able to identify and the list from a Craig Key Channel Dive is always long and interesting.
But there is one unique animal we always make a special effort to see every time we swim the channel. It is a species we see almost no place else. It is called the Lettuce Sea Slug, or Elysia crispata (formally Tridachia crispata) It is usually not much more than an inch long and usually shows some combination of green, yellow, and purple hues. It loves to hide on the large, crunchy Halimeda algae beds where it is almost impossible to find due to it's nearly foolproof camouflage. Some of the slugs however crawl onto the rocks and mangrove roots where they are easier to spot and when they are found, they quickly become one of the favorites of most students.
The first thing that catches a diver's eye is the slug's frilly design on it's back. These fancy ruffles which look a bit like a head of lettuce and give the slug it's name, are actually folds of tissue called parapodia. These skin folds provide two key life functions for Elysia, one which is a particularly fascinating handiwork of the Creator.
The thoughtful observer would quickly guess that the design of these parapodia is likely increasing the surface area for some reason and this is the case. The ruffles function much like gills, taking in oxygen and releasing carbon dioxide so the design aids in gas transfer.
More fascinating still, these parapodia function in providing the animal food. The lettuce slug feeds by sucking the cytoplasm out of cells of algae. Not all the cytoplasm, however, is digested. The slugs are able to keep the chloroplasts intact. The chloroplasts are the structures within the algal cells that carry out photosynthesis.

chloroplasts

Elysia then stores the intact, stolen chloroplasts in it's frilly parapodia and there, incredibly, the chloroplasts continue to function, gathering sunlight and converting it to food which, rather than feeding the algal cells, instead feeds the crafty slug. So incredibly, this creature can feed by sticking food in it's mouth like an animal AND it can feed by gathering sunlight like a plant! In fact, some have called Elysia a fusion of plant and animal. That's pretty cool! Actually, the Lettuce Sea Slug is a mollusc, particularly from the Class Gastropoda. It is basically a snail without a shell. Whatever you call it though, the Lettuce Sea Slug is one of those creatures that we find that is incredibly beautiful, but when you learn a bit more about it, you discover it is as interesting as it is sweet to look at.