Katherine J. Wu for theatlantic.com, | September 28, 2021

These Sea Slugs Break a Cardinal Rule of Animal Life

Photosynthesis is about the plantiest thing on Earth. The little green thieves manage to pull it off anyway..

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Studying sea slugs in the group Sacoglossa can mean being on the receiving end of some very imaginative emails. Sidney K. Pierce, of the University of South Florida, retired a few years ago. “But to this day,” he told me, “I get questions from little kids in their science classes” who have stumbled upon the marvelous mollusks—and want to know if they could help “end world hunger.”

The answer, Pierce assured me, is no. But the proposal isn’t totally outlandish. Several sacoglossan sea slugs can harvest energy from the sun’s rays and, using only the contents of their cells, turn it into chemical packages of food. In other words, they photosynthesize—arguably the plantiest thing that earthly plants and algae do.

Except sea slugs are, of course, not plants or algae. They’re standard-issue animals that have blurred the boundaries between biological kingdoms, thanks to a spectacular act of thievery: They steal photosynthesizing machinery—in-cell structures called chloroplasts—from the algae they eat, and store the green, light-converting blobs in their body for extended periods. Some species can reap the nutritional benefits of these self-replenishing snack packs for months, perhaps for longer than a year. One sea slug that Pierce has studied extensively, Elysia chlorotica, can go the rest of its life without eating—moseying, mating, vibing—after just one algae-rich binge in its youth. “We collect them in the field,” he told me, “and we never feed them again.”

Humans do not do this; as far as we know, our bodies aren’t set up to carry out these grand heists. But oh, that we could. “What would it be like, if we just ate salad for a week, and then you don’t need to worry about where you’re getting your nutrients ever again?” Karen Pelletreau, of the University of Maine, told me. The slugs’ felonious feat, known as kleptoplasty, is so remarkable that it’s been held up by creationists as proof of intelligent design. (It is, to be clear, evolution.) And researchers still aren’t sure how the plant-pantomiming animals pull it all off.

an Elysia timida sea slug
A sea slug of the species Elysia timida. (Paulo Cartaxana)

Some of the nomming nitty-gritty differs a bit among species, but in general, the slugs, which run up to a couple of inches in length, will latch on to a straw-shaped stretch of algae, puncture it with a tooth, and slurp out its contents like a college student shotgunning a beer. The resultant sludge then floods the slug’s über-branched gut, where it’s captured by cells that hoard the chloroplasts intact, while breaking down or discarding everything else that’s algal. Solar-powered sea slugs tend to hatch translucent or whitish. But the chloroplasts swathe large portions of their flat, billowy bodies in a startling verdigris. In the 1970s, one pioneering biologist who espied E. chlorotica’s emerald hues dubbed them the “leaves that crawl.”

Most sacoglossan sea slugs aren’t of the solar-powered ilk; they digest the chloroplasts along with everything else. That certain species among them manage to keep the little structures operational for days, weeks, or months is, frankly, bonkers—and seems impossible at first, because of how dependent chloroplasts are on their native host cells. Chloroplasts were, millions of years ago, free-living bacteria that were ultimately engulfed by bigger cells; in exchange for room and board, the microbes pumped out energy for their hosts, forging what became a permanent codependency. Nowadays, plant and algae chloroplasts can’t get by without protein cargo that’s manufactured exclusively out of genes in the nucleus, which doesn’t survive the sea slug’s discerning digestion. Sticking a chloroplast into a sea-slug cell and expecting it to run is like asking a car to gun indefinitely down a highway with no gas or oil-change stations. (It’s also why we humans can’t just stick chloroplasts into test tubes and profit.) And yet, even stripped of their nuclear entourage, the chloroplasts persist—and work. “It seems like a biotechnological marvel,” Debashish Bhattacharya, of Rutgers University, told me. “How the hell do they keep the chloroplasts alive?”

Several potential explanations have been put forth over the years. In one, the sea slugs use their own in-house accoutrement to jerry-rig the chloroplasts, making them more durable. In another, the animals manage to ransack algal nuclei, co-opting chloroplast-fortifying genes, though most of the researchers I talked with characterized the evidence for this idea as scant or mixed. A few years ago, a group of scientists proposed another work-around: Perhaps the kidnapped chloroplasts are important to the slug less as photosynthetic factories, and more as self-contained food storesmini, cell-intrinsic calorie caches that could be digested by the animal in times of nutritional need, like a camel’s fat-rich hump. In that scenario, chloroplast maintenance could fall to the wayside.

Pierce told me that idea doesn’t have much support. (It also doesn’t negate the possibility that the sea slugs are solar-powered: Some species, for instance, could be tapping into those reserves after milking the chloroplasts’ photosynthesizing chops for weeks.) And many experts are wholly convinced that, for Elysia chlorotica and several of its closest kin, the biggest benefit of harboring algal contraband centers totally on photosynthesis, especially because “it’s dangerous business to steal a chloroplast,” Paulo Cartaxana, who studies the slugs at the University of Aveiro, in Portugal, told me. Chloroplasts are fragile and fussy; they emit toxic compounds while they work. The structures must be providing big perks, or they’d have been booted long ago. And proof of this has indeed racked up.

Several chloroplast-hoarding sea-slug species will live longer and grow larger when allowed to soak up sunlight. Pelletreau, of the University of Maine, collaborated on prior work showing that E. chlorotica slugs in particular seem to be totally dependent on chloroplasts; without them, the plant wannabes simply perish in their youth. One recent study proposed that the chloroplasts’ energetic oomph may even be powerful enough to sustain certain sea-slug species after they purposefully decapitate themselves and begin the arduous work of sprouting a new body from their severed head.

Further evidence that the animals pack a substantial photosynthetic punch comes from studies that have tracked chloroplast-produced chemical food packets on an atomic scale, as they migrate into a menagerie of sea-slug tissues, where they can presumably facilitate all sorts of sea-sluggy things. Cartaxana’s recent work showcases something new: In Elysia chlorotica’s close cousin Elysia timida, what comes out of gut-cell chloroplasts can end up in reproductive tissues and boost the number of eggs new sea-slug parents lay. (E. timida slugs, while greedy burglars at the dinner table, are very reciprocal lovers. All of them are hermaphrodites, and they mate by colliding head-to-head and mutually inseminating each other with penises that unspool from beneath their right eye.)

Sea slug of the species Elysia viridis, a close cousin of Elysia timida, locked in a mating embrace. (Paulo Cartaxana)

“Spawning is a huge reproductive investment,” Sónia Cruz, also of the University of Aveiro, and another author on the new E. timida study, told me. “It takes a lot of energy out of them.” Each slug has to manufacture hundreds of eggs, each packed with enough nutrients to sustain its offspring during early development. The chloroplasts appear to offer an energetic boon, in some cases doubling the slugs’ output.

Anna Karnkowska, an evolutionary biologist at the University of Warsaw, in Poland, told me that lessons could be learned from the other members of the chloroplast-stealing club, most of which are single-celled creatures such as dinoflagellates (though at least a couple of marine worms seem to briefly hijack the structures too). These unicellular pirates are thought to have an especially intense relationship with their chloroplasts; for them, kleptoplasty might be an intermediate step toward fixing the structures permanently into their cells and making them heritable from generation to generation.

Sea slugs, with their multicelled anatomy and complex lifestyle, would have a much harder time passing pilfered chloroplasts down. As far as scientists can tell, what the slugs accomplish is akin to black-market organ theft, but little more: When the animal dies, its chloroplast cargo dies with it. But even if the chloroplasts’ tenancy is a dead end, it’s a fascinating push to rethink the strange and category-defying ways in which organisms interact, Karnkowska said. The slugs offer the chloroplasts a home, and get to, for a while, masquerade as pseudo-plants; the chloroplasts, in turn, become the sole survivors of carnage, enduring where the rest of their algal comrades could not.