By farming photosynthetic cells stolen from their food and using the nutrients they produce, these spectacularly beautiful creatures have effectively become solar-powered, part-time plants.
Snails without shells
Unlike the unattractive, slimy brown slugs we see in our gardens, sea slugs are among the most spectacular and morphologically diverse animals on earth. From metre long giants to tiny creatures that easily move between grains of sand, sea slugs can be found crawling over rocks, seaweeds, sponges and corals from the polar regions to the tropics. Some 3000 species of sea slugs have already been described worldwide.
Although they lack shells, sea slugs are in fact molluscs — related to clams, oysters, squids and snails. And while the loss of the shell has given sea slugs the freedom to evolve spectacular body shapes and brilliant colours, it has come at a price — they are tempting prey for other animals.
Very much at the mercy of the elements, sea slugs live for a maximum of one and a half years, depending on the availability of food and suitable conditions. And food availability for these fussy eaters varies enormously. For many, the plants they consume are distributed patchily through the shallow waters. For others, food sources appear in huge quantities for a few weeks then suddenly disappear again. This can mean a long time between meals for the slow-moving slugs. But some shallow-dwelling tropical species have found a unique way to turn the odds in their favour.
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Generally speaking, it is a plant’s ability to directly convert the sun’s energy to food through photosynthesis that distinguishes it from animals. In the tropics, corals and other colonial animals have a symbiotic relationship with microscopic algae known as zooxanthellae, and have evolved a means of farming the algae in their bodies to take advantage of sugars they produce.
Two groups of sea slugs — sacoglossans and nudibranchs — have taken this idea a step further.
According to Dr Bill Rudman, an invertebrate zoologist from the Australian Museum, because sacoglossans are herbivorous and nudibranchs are carnivores, they have evolved different methods of capturing the photosynthetic plastids (like chloroplasts) from their food and putting them to work converting sunlight into nutrients.
Vegetarian solar power trippers
“Herbivorous sea slugs are suctorial feeders. Most simply suck out and digest the cellular contents of the algae they feed on,” he says. “But the sacoglossan sea slugs have evolved branches of their gut that protrude through the body wall. This allows them to store the still-functioning chloroplasts (from the algae) in this digestive gland where they are exposed to sunlight through the transparent layer covering the sea slug’s body.”
So unlike corals, which keep entire algal organisims alive within them, sacogossan sea slugs keep only the photosynthetic chloroplasts from their food — the rest of the algae is digested.
One of the most widespread sacoglossan genera is Elysia, or lettuce sea slug — so named because of the green coloration from its stolen chlorophyll food source coupled with its frilly exterior.
“Different species exhibit varying stages of evolution and sophistication. The most primitive take up the chloroplasts and digest them after a short period of time before consuming more,” says Rudman. “Other, more evolved species are able to store their chloroplasts for months.”
Captive chloroplasts remain functional for up to nine months. Once they lose their photosynthetic ability, they are rapidly digested and the slug replenishes its store during another seaweed feast.
The other solar-powered sea slugs are the carnivorous aeolid nudibranchs. The name nudibranch comes from the prominent tufts of gills displayed on these slugs’ dorsal surface (nudi=naked, branch=gill). Like corals, nudibranchs have developed the ability to ‘farm’ entire symbiotic algal cells, called zooxanthellae. These single celled plants are adapted to living within the tissues of animals.
Rudman says nudibranchs feed on soft corals, like hydroids, and are able to remove the algae intact from the coral’s tissues and store them in their own cerata — finger-like projections that contain ducts of the slug’s digestive gland. Within the cerata the algae continue to photosynthesise and provide their host with an ongoing supply of manufactured sugars.
“One of the most spectacular nudibranchs is Phyllodesmium longicirrum. It captures and farms the symbiotic, microscopic plants from its soft-coral prey Sarcophyton in its large flattened cerata. The cerata effectively act as ‘solar paddles’, with the digestive gland forming a ring-like garden around the paddles.”
Another species, the blue dragon nudibranch, Pteraeolidia ianthina, is found throughout tropical and subtropical Indo-west Pacific, and along the east coast of Australia. Studies on nudibranchs have shown that juvenile animals must develop their own crop of photosynthetic algae, probably by feeding regularly on small hydroids with symbiotic zooxanthellae. Adults can last some time without feeding, obtaining enough nutrition from their algal gardens, and snacking on large solitary hydroids when their energy levels run low.
Rudman says that although unusual, this process of recycling useful components of their food for their own purposes is not without precedent in nudibranch families. One group of nudibranchs normally feed on cnidarians, like sea anemones, absorbing their stinging cells and storing them in the tips of their cerata for their own defence. Other sea slugs acquire chemicals from the sponges and ascidians they eat — and likewise, these chemicals deter the slugs’ predators from feasting on them. For a shell-less slug, this kind of strategy — along with their plant-derived colouration — is an important form of protection.
As for the algae farming, nudibranchs are doing it because they can, says Rudman: “This is a very efficient way of feeding — corals certainly do well out of getting plants to do the energy production work.”
In return the algae, which are unable to survive on their own, are well cared for: Studies on Pteraeolidia ianthinashowed that the zooxanthellae within the nudibranch’s body breed rapidly, and at the same time produce nutrients far in excess of their own requirements, suggesting that they are living in a healthy protected environment, akin to growing plants in a greenhouse.
Evolution of a part-time plant
Although Rudman says that retaining the green pigmented chloroplasts probably started as a convenient form of camouflage, research has shown that some species now depend on their chloroplast captives for as much as a quarter of their energy needs.
And the dabbling with biological conventions doesn’t end there. Recently, US researchers discovered that for chloroplasts to function, they require instructions from the plant cell nucleus — the chloroplast’s own DNA is not enough. This raised the question of how they continue to thrive onboard sea slugs in the absence of their parent plant. The researchers found that in a complex process, some sacoglossans actually absorb the plant DNA into their own cell nucleii then pass the transgenic information on to future generations, which presumably continue adding to the mix.