Biology Photo of the Month - March 2002

Lettuce Sea Slug - Elysia crispata (Tridachia crispata*)

*Tridachia is an older outdated genus name for this animal

Photos and text by Bill Capman

Update - June 2002: Click here for expanded version of article in Reefkeeping magazine.

Move cursor over photo to see wider view

The beautiful iridescent ruffles in the photo above might look like the leaves or petals of some exotic plant, but in fact they are the ruffled flaps of tissue (parapodia) that outline each side of the back of a two inch sea slug that lives in the waters of the Caribbean and Florida Keys. This animal is commonly known as the lettuce sea slug, so named because when the animal is at rest the parapodia make this snail without a shell look a lot like a small head of ruffled lettuce.

The resemblance of the lettuce sea slug's parapodia to leaves is more than just a coincidence, since these structures, and indeed the entire surface of the animal, function somewhat like leaves. In fact, this sea slug carries out photosynthesis within its body! Like many of its relatives, this animal feeds by sucking the cytoplasm from the cells of marine algae, but this species and some other sea slugs don't completely digest what they ingest. The lettuce sea slug keeps the chloroplasts (the structures within the algal cells that carry out photosynthesis) intact and relocates them to its skin. There the stolen chloroplasts continue to function, using energy from sunlight to build energy-rich food molecules (sugars) from carbon dioxide and water, but rather than feeding the algal cells that made these chloroplasts, the sugar that is produced feeds the sea slug. Just as the leaves of a plant provide the plant with more surface area for the capture of sunlight, the parapodia of the lettuce sea slug provide more surface area for light capture. These parapodia also increase surface area for gas transfer (oxygen uptake and carbon dioxide release), so they serve double duty, functioning somewhat like gills.

In the photo below the green color that comes from the stolen choroplasts is clearly visible:

Lettuce sea slug (the same individual as in the other photos) crawling on glass in Augsburg College's coral reef aquarium (pink birdsnest coral in background)

Closeup of ruffled frill of tissue along back of lettuce sea slug

The above photo shows the lettuce sea slug sitting on a patch of the marine alga Bryopsis, which is one of the sources of the stolen chloroplasts. This sea slug spends much of its time simply sitting like this in a well-lit location, providing the chloroplasts with the light energy necessary for the manufacture of food.

The lettuce sea slug is not alone in its habit of gaining extra functionality using cellular structures stolen from the organisms it eats. Some of its relatives (certain of the sea slugs known as nudibranchs) arm themselves with stolen nematocysts (tiny stinging structures produced by the cells of anemones, hydroids, and their relatives) from the cnidarians they eat! Somehow they manage to eat their prey without discharging all of the nematocysts, and they relocate some of these nematocysts to certain regions of their skin, where they continue to function. Many would-be predators of the nudibranchs are stung upon contact with the nudibranchs.

More information, and many more photos can be found by clicking on the following links:

For further information on the lettuce sea slug, go to: http://www.seaslugforum.net/elyscris.htm. This page also has some wonderful photos showing the great variation in color within this species, and some really nice photos of tiny juvenile lettuce sea slugs in an aquarium where reproduction occurred. I highly recommend you take a look at this site, if only to see the beautiful lettuce sea slug photos.

For information on solar powered sea slugs in general, go to: http://www.seaslugforum.net/solarpow.htmw.htm

For more information on nudibranchs that arm themselves with stolen nematocysts, go to: http://www.seaslugforum.net/defcnid.htm

For more information on nematocysts (and the cnidarians that produce them), go to: http://www.animalnetwork.com/fish2/aqfm/1999/dec/wb/default.asp

For a great deal more information on sea slugs in general, and many beautiful photos of colorful sea slugs, explore the pages of the Sea Slug Forum: http://www.seaslugforum.net/