The Modern Bestiary by Joanna Bagniewska

Author:Joanna Bagniewska [Bagniewska, Joanna]
Language: eng
Format: epub
Publisher: Smithsonian
Published: 2022-09-20T00:00:00+00:00

Sacoglossan sea slugs

Elysia marginata, Elysia atroviridis

Many animals love basking in the sun—but could any animals actually be solar-powered? Unlikely as it sounds, yes.

Sacoglossan sea slugs are a group of sea-dwelling molluscs; two species, Elysia marginata and Elysia atroviridis, look like green, winged worms with a pair of tentacles on their heads. These sea slugs feed on algae—not an unusual diet for marine creatures. However, sacoglossans exploit algae in more ways than one: not only do they eat them, they are also able to steal their chloroplasts, the part of a plant cell that enables photosynthesis. The slugs use the biggest tooth of their radula (a sluggy food-scraper mouthpart) to pierce algal cells; they then digest most of the cell contents, but leave the chloroplasts, which are incorporated into the molluscs’ own digestive gland cells. While this incorporation is a feat in itself, what’s even more amazing is that the Elysias manage to keep the chloroplasts photosynthetically active and working for them for months on end. The chloroplasts produce carbon compounds, which the sea slugs then feed on. Chloroplast robbery is called kleptoplasty, and is usually seen in primitive unicellular organisms, the protists. Kleptoplasty is extremely rare in the animal kingdom—the only other creatures capable of it are some marine flatworms.

So, would having active chloroplasts inside their bodies make the sea slugs fat just from sitting in the sun? Not quite. Still, for E. atroviridis, the combination of food and strong light slows down weight loss throughout their lifetime—more so than the combo of food and weak light. These conditions also account for higher numbers of eggs, larger-sized larvae and a better survival rate of the offspring. Plastid snatching clearly pays off.

While E. atroviridis and E. marginata cannot sustain themselves entirely on a chloroplast-provisioned diet (unlike their cousin E. chlorotica, who can live off the chloroplasts for a year or so), they make up for it with an even more extraordinary talent. They can lose their heads—or, rather, their heads can lose their bodies.

This is not a metaphor—the two Elysia species are capable of autotomy, or self-amputation. While the phenomenon has been observed in a number of animals—lizards or salamanders losing their tails in the presence of a predator, for example—it has never been seen elsewhere in such a radical form. In the self-beheading process, the slug severs off around 80–85 percent of its body weight, including the heart and other organs, along a neat “neckline”—and the head wanders off on its own. The body is still alive for a few weeks, or even months, and the heartbeats, more and more faintly, up to the point of decomposition. The head, however, starts a new, solo life, and proceeds to grow a fresh body, in an act of extreme regeneration. The new bod is ready in under three weeks, complete with heart and all.

The process of self-amputation lasts several hours, which means that it is unlikely to be a predator defense—it simply takes too long. Instead, autotomy could be a way of getting rid of parasites; E.

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