Lettuce Sea Slug Care: Bryopsis, Diet and Light

Your bryopsis came back for the third time. It grips the rock like little ferns, the snails walk past it, and pulling it only seems to spread it. Somebody at the club tells you to get a lettuce slug. They are right about the bryopsis. What they may not tell you is what happens to the slug once it wins.

The lettuce sea slug, Elysia crispata, is a frilly green slug from the Florida Keys and the Caribbean. It is sold as a “lettuce nudibranch,” but it is not a nudibranch. It belongs to a small group of sea slugs called sacoglossans, the sap-suckers. It pierces algae one cell at a time and drinks what is inside. Then it does something almost no other animal can: it keeps the algae’s chloroplasts alive in its own body and uses them to make food from light.

Quick facts

Scientific name	Elysia crispata. Older names include Tridachia crispata and Elysia clarki.
Where it lives	Tropical western Atlantic and the Caribbean, including the Florida Keys
Size	Up to about 2 inches long
What it is	A sacoglossan sea slug, not a nudibranch and not a sea hare
Diet	Green algae in the bryopsis group (Bryopsidales). Bryopsis is the one that matters in a reef tank.
Reef safe	Yes. It eats algae only.
Light	Normal reef lighting. It uses light to stretch its food.
Flow	Low to moderate, with every intake covered
Main risk	Starvation once its algae is gone

Not a nudibranch, not a sea hare

Three kinds of soft sea slugs get mixed up in the hobby, and they do different jobs.

Animal	How it eats	Job in a reef tank
Lettuce sea slug (a sacoglossan)	Pierces single algae cells and sucks them out	Bryopsis and related green algae
Sea hare	Bites and chews whole strands with a rasping tongue	Heavy hair algae outbreaks, for a few weeks
Nudibranchs	Most eat animals: sponges, hydroids, soft corals	Usually none. Some are coral pests.

If you want a big grazer for long green hair algae, a lettuce slug is the wrong animal. Sea hares and urchins do that job. See our guide to what eats hair algae.

Solar powered: how it keeps stolen chloroplasts

When a lettuce slug drinks an algae cell, it does not digest everything. It keeps the chloroplasts, the green parts that run photosynthesis, and moves them into cells along its body. Those frilly folds on its back are full of them. That is why the slug is green, and why it opens its folds under light.

Scientists call this kleptoplasty, “stolen plastids.” In this species the chloroplasts keep working for around 40 days. Chloroplasts from different algae can work side by side in the same slug. Researchers have studied it for decades because almost no other animal can do it.

Here is the part that matters for your tank. Light helps it last longer between meals. It does not replace food. A 2023 study fed lettuce slugs bryopsis on different schedules. Slugs fed every two days for 56 days grew to about 46 times their starting weight. Slugs fed every two weeks did not even double. Slugs with no food for 45 days shrank to between 17 and 41 percent of their starting weight. Solar power slows starvation. It does not stop it.

The Florida Keys connection

Some of the key research on this slug was done on animals from the Keys. In 2006 scientists described a “new” species, Elysia clarki, from Florida Keys mangrove lagoons. It was later folded into Elysia crispata, and researchers now talk about a Keys lagoon form and a Caribbean reef form of the same slug.

Work on Keys animals also gave us the clearest picture of what they eat. Adults collected at Vaca Key carried chloroplasts from mermaid’s shaving brush (Penicillus), Halimeda and bryopsis. Newly hatched slugs from Grassy Key would only eat bryopsis or a fine hair alga called Derbesia, and died when offered anything else. A 2019 study looked at slugs from five Caribbean locations and found that every location had slugs feeding on bryopsis.

What it eats, and what it will not

Every source agrees on bryopsis. That is the feathery, fern-like green algae that grips rock and that most snails, hermits and fish leave alone. It is also the reason most people buy this slug.

The research on common hair algae is thin. Young slugs ate Derbesia in the lab, and Derbesia is the soft green hair algae in most reef tanks. Adults in the wild live mostly on bryopsis, shaving brush and Halimeda. Reefers report mixed results with ordinary hair algae. We would not buy a lettuce slug for long green hair algae. Buy it for bryopsis.

It will not eat cyanobacteria, dinoflagellates, diatoms, bubble algae, coarse wiry algae or prepared foods.

The end date

A lettuce slug is a fix with an end date. When the bryopsis is gone, it has nothing left to eat, and the study above shows how fast it shrinks. Watch for these signs: the body gets thinner, the folds stay closed, the green fades to pale yellow.

Plan its exit before it goes in. A friend’s tank with bryopsis, a refugium with bryopsis growing in it, or back to us. Start with one. A large outbreak in a big tank may justify two or three, but every slug you add needs a home afterward.

Pumps, overflows and filter socks

Slugs crawl everywhere, and they do not let go when water pulls on them. A powerhead or return pump intake is the quickest way to lose one. Before you add one:

• Cover every powerhead and pump intake with foam or fine mesh.
• Check that your overflow has a cover with small slots.
• Turn off or cover the skimmer intake in the sump if the slug can reach it.
• Look in the filter sock for the first few days. Slugs that go over the overflow often end up there alive.

Tankmates that eat slugs

Many fish ignore it. Some do not. Puffers, butterflyfish, some wrasses and larger angelfish may pick at it. Large crabs can catch it at night. Peaceful reef fish, snails, small hermits and urchins are fine. If you keep a fish that picks at everything new, the slug is a gamble.

Acclimation

Slugs are sensitive to fast salinity changes. Turn the lights off and float the sealed bag for 15 minutes. Open it into a clean container and drip tank water in slowly for 45 to 60 minutes. Move the slug in a cup of water, not through the air, and set it right on the bryopsis. Throw the bag water away. Never use copper in the tank. Copper kills invertebrates.

FAQ

The Underwater Gardener is a reef aquarium store in Key West, Florida. Our lettuce sea slugs are collected by our team in the Florida Keys. Livestock ships UPS Next Day Air on Tuesdays and Wednesdays.