News / Species Spotlight / Elyssid Lettuce Slugs (aka Lettuce Nudibranchs) (12/30/14)

Elyssid Lettuce Slugs (aka Lettuce Nudibranchs)



by Charles J. Hanley III

Scientific Name: Elysia crispata, E. chlorotica, E. diomedea, E. ornata, E. viridis, E. Halimedae, E. rufescens, E. degeneri
Common Name: Lettuce Slug, Lettuce Nudibranch, Ornate Elysia, Crawling Leaf Slug

Taxonomy:

Domain: Eukarya
Kingdom: Animalia
Phylum: Mollusca
Class: Gastropoda
Order: Sacoglossa
Family: Elysiidae
Genus: Elysia

Description:

The diversity of the animals found in the marine environment is profoundly astonishing. Scientist are discovering new animals, or new things about old ones, just about every day. One such recent discovery relates to a very close relative of a not uncommon aquarium habitat, the so-called Lettuce Nudibranch (Elysia crispata; formerly Tridachia crispata), known as the Crawling Leaf Slug (Elysia chlorotica). Though Elysia species slugs (they are not actually nudibranchs) are a challenge for aquarists to keep alive, and their willingness to eat nuisance algae is overrated, they are of significant evolutionary interest. These little sea slugs are very specialized herbivores that actually steal parts of their algal prey (kleptoplasty), and incorporate them into their own tissues. Members of the genus Elysia have varying abilities to absorb photosynthetic organelles, foul tasting chemicals, and even genes from their food! Perhaps the most amazing fact is that these slugs can photosynthesize using chloroplasts they took straight from their algal food. The ability to feed oneself via photosynthesis is highly advantageous, especially for a motile animal that can also seek external food sources. The photosynthetic properties of this genus may lend to their aquarium hardiness; Elysia species may be the easiest sea slugs to keep in captivity. In truth, however, lettuce slugs are still only moderately appropriate aquarium animals at best. They are certainly cool though, and well worth learning about.

Lettuce slugs are Sacoglossan sea slugs, which means they are shell-less gastropod mollusks that can use chloroplasts absorbed from their food. Unlike the nudibranchs they are often misreported to be, they do not have exposed gills (nudi=naked, branch=gill). Though all nudibranchs are sea slugs, not all sea slugs are nudibranchs. The misnomer is easy to understand, however, since lettuce slugs also have soft, fleshy projections emerging from their backs. The projections, called parapodia, are the source of the animals common nametheir ruffled appearance resembles a leafy lettuce plant. These parapodia may actually facilitate some gas exchange, as well as house stolen chloroplasts.

The dietary habits of individual E. crispata vary quite a bit, which might also coincide with the remarkable variation in appearance manifested in this species. Then again, it could also mean that certain varieties of the species E. crispata may require their own species name. More scientific data is needed to ascertain whether the notion is valid, but the variation itself is undeniable. Some individuals are pale green, olive green, or green with white or tan markings. Others have bright blue or yellow bands. There are even red ones, and pale whitish ones. Another species, E. diomedea, also has vegetative looking parapodia, and is easily confused with E. crispata. Both species may be sold as Lettuce slugs, but do not necessarily eat the same fooda potentially fatal misunderstanding.

In spite of the variety in what they, how they all eat is very similar. In general, snails and slugs have a fused bed of course teeth, called a radula. In most, the radula is used to scrape food off of a flat surface. However, unlike the others, Elysia slugs have a radula shaped more like a piercing hypodermic needle. They use this apparatus to slice open the cells of their algal prey, and its hollow core to suck out the inside of the cell. Upon doing so, the slugs digest the cellular cytoplasm and sequester the chloroplasts into their own tissues. The chloroplasts will continue to work for a short time, photosynthesizing to produce sugars that feed the slug.


Ecology:

Elysia slugs that are of interest to the aquarium trade are Caribbean in origin, though there are Pacific Elysiids as well. They are shallow water species that tend to stick to lagoonal areas that are conducive to the growth of green algae. Because they do not stick well to surfaces, the water must also be fairly calm or the slugs would simply be washed away.

As Sacoglossan slugs, Elysiids have the amazing ability to absorb chloroplasts and use them to photosynthesize. Certainly, other animals can photosynthesize as well, corals being the premier example. But this is different. Corals house living algal cells (zooxanthelle, among others) within their own. Although the algal cells typically need the coral to survive, they have been shown to survive on their own under laboratory conditions. Elyssids eat the algae, which die, and then do not digest the chloroplast organelles.

The ability to utilize the cellular organelles of a different organism is astounding. Even more astounding is that the Atlantic species E. chlorotica does not stop there. It actually absorbs genes from the algas genome, and splices it into its own. Doing so allows the slug to maintain the proper conditions to keep chloroplasts alive. To be precise, the functioning chloroplast genes (chloroplasts have their own genes that are remnants of the archaic organisms they once were) used to be expressed in the algal genome. They are then transferred to the slugs genome, thereby preventing chloroplast deterioration.


Aquarium Care:

The main aquarists have purchase lettuce slugs throughout the years is that they have been reputed to eliminate problems with nuisance algae. Unfortunately, this reputation has been downfall of many unfortunate little creatures. While lettuce slugs do in fact eat algae, and voraciously by all accounts, they must be given the appropriate type. When not provided the correct alga, lettuce slugs slowly starve to death, withering away in weeks or months.

Suffice to say, caring for lettuce slugs poses in interesting conundrum. On the one hand, if you have a real problem with green algae, and you get the right variety of slug, it may actually rid you of your verdant pest. On the other, if you get the wrong kind, it will surely die. There is no way to tell for certain, unless the salesperson in your local fish store can assure you that the individual you are buying has been witnessed eating the algae your tank provides. For example, Bryopsis, Chlorodesmis, Codium, Derbesia, Halimeda, and Caulerpa genera algae are all listed as possible algal prey for lettuce slugs. Yet, a given individual may refuse all but one of theses offeringssometimes only a single species of a single genus is accepted.

Lettuce slugs are picky eaters, and that sounds like bad news. Fortunately, if you do manage to match your animal to its algae, it will probably survive fairly well. They are otherwise hardy aquarium inhabitants, and they will not often get picked on by your other livestock. Their ability to absorb usually chemical from their food, coupled with their penchant for eating stuff others find distasteful, means they are quite unappealing to most predators. Never fear however, they will not soon take over your tank. Though not entirely unheard of, lettuce slugs rarely breed in captivity. And most reports indicate that they eat slowly enough that they usually will not exhaust the supply of food your tank can produce.

Lettuce slugs will benefit from intense lighting. They are found in shallow, well-lit waters where green algae thrive. In the past, aquarists have failed to realize the role good lighting plays in lettuce slug care. It will certainly enhance the chances of yours surviving for a decent amount of time.

Sadly, lettuce slugs may have a lifespan as short as six months. The good news is that an individual may be well on its way to old age by the time it has cleaned out that unsightly green algae. If so, you can be happy knowing that it acquitted itself well going into its golden years. The flip side of the coin is the violent, premature ending I like to call Death by Impeller. Lettuce slugs have the tragic tendency to find their way into pump intakes, powerheads, and overflow boxes, mainly because they do not stick well to substrates. Strong currents can easily rip them from the rocks, and send them tumbling around a tank. All too often, the trip ends badly. As if that were not enough, lettuce slugs also like to explore, so if they do not get blown into the intake, into it they bring themselves. It is thus necessary to maintain moderate flow and cover pump intakes whenever a lettuce slug is in the system.


Quick Notes:
  • Lettuce slugs can absorb chloroplasts, toxins, and even genes from their food
  • The ability to absorb and express genes is a fundamentally important scientific finding
  • Do not purchase lettuce slugs unless you are certain you can feed it adequately
  • Provide bright lighting to supplement the slugs nutritional needs
  • Cover all pump and powerhead intakes with mesh, or darken the opening