Kleptoplasty: These colorful animals can steal from plants to become solar-powered!

Algae is often overlooked by divers on a coral reef, but it's here that we find some of the most interesting and colorful little animals in the Caribbean.
Sap-sucking slugs (or sacoglossans) feed on algae but do not fully digest it. Instead, they keep the chloroplasts alive as a continuing source of food. These tiny plants cells are trapped inside the slug's body, still producing sugars by normal photosynthesis. Only now they are feeding an animal instead of a plant!
This is called kleptoplasty, and these are the only animals known to feed in this way.

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Inside the mouth is a tiny serrated organ, called a radula, used to saw open the tough walls of plant cells. The contents of the cell are then sucked up and the chloroplasts are saved, either in the lining of the gut or up on the fleshy folds on the animal's back, so they can be closer to the sunlight. These chloroplasts act like solar-powered batteries and can be kept kept alive and functioning (for up to a month in some species) before they are depleted and need to be replaced.

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The largest and most common of these plant-stealing animals is the Lettuce Sea Slug (Elysia crispata), with it's ruffled back looking like intricate leaves of lettuce. It's no accident, these folds allow more sunlight to reach the plant material stored in its body. The colors of the Lettuce Sea Slug can change depending on which kinds of algae it has been feeding on. As an added bonus, this helps to keep the animal camouflaged on it's food source. When they are very young or haven't fed in a while, they have little color and appear almost white.

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The bodies of many of these sap-sucking slugs are almost completely transparent. This allows more sunlight to pass through and reach the plant material living inside their bodies, but makes them harder to spot.

Harlequin Glass Slug

Mourgona murca

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Where to Find them:

Lined Elysia with eggs.

Obviously the best way to start is by looking closely into blades of algae, but there is a lot of it scattered around the reef, especially in the shallows and the rocky inshores and lagoons. Sea slugs may leave you clues that they are in the area by laying eggs, often in a spiral pattern. Find these eggs first and chances are that the little animals will not be far away. After hatching, some species live in a planktonic stage, but others will hatch and begin feeding immediately, so the eggs are often on their favorite food source. Each species of sap-sucking slug seems to prefer different kinds of algae. Look closely at the common Pinecone Alga to find the Lined Elysia, or onto the  Paddle Blade Alga, Saucer Blade Alga, Mermaid's Fans or even Turtle Grass to find other species.

Mermaid's Fan

Turtle Grass

Paddle Blade Alga

Saucer Blade Alga

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plenty of New Discoveries to make:

It's always exciting to find new things on the reef, and hunting for these little creatures has become almost an obession for me over the last year. With the help of a group of friends (who have much better eyes than I do!) and doing long slow shore dives, the updated third edition of Caribbean Reef Life now has sixteen distinct species of sap-sucking slugs, and the list continues to grow!

Elysia zemi

Introducing Zemi's Elysia: This little sea slug was a mystery until recently. It has a distinctive olive to orange-brown body and keeps it's parapodial flaps open to the sunlight as it moves about. I found this one at 50 feet, deeper than most Elysias. It was only given it's name last year from a collected specimen from Martinique. (Geeks can read: Systematics of Caribbean Elysia, to see this and  additional species and photos.) “Named after zemí, an embodiment of natural forces and ancestral spirits of the Taíno, an indigenous people of the Lesser Antilles."

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So that's one more species I can now move out of the "Unknowns" folder and add to my list. Who knows what's still waiting to be found? Give the coral a break every now and then; take the time to go slow and poke about in the algae. The new finds are worth it. Happy hunting!
Mickey Charteris