Consider the Sacoglossa sea slug, also known as the “solar-powered sea slug” – or, when scientists are feeling unkind, the “sap-sucking sea slug.” Or we could call it the “self-decapitating sea slug that bears an uncanny resemblance to Shaun the Sheep” and still be entirely fair to this tiny creature, Costasiella kuroshimae.
Actually there are (at present) 284 known species of Sacoglossa sea slugs in all temperate and tropical waters worldwide, and only C. kuroshimae looks like an adorable hybrid between Shaun the Sheep and a Medusa. It lives in the waters of Southeast Asia.
Shaun the Sheep, by the way, is a cartoon character who featured in, among other things, “Farmageddon.” He is adorable. But let us get back to the sea slugs.
Some of the other Sacoglossans bear a passing resemblance to lovable ovines, but some look mainly like seaweed – and this is not coincidence.
Sacoglossans comprise what zoologists call a “superorder” within the world of mollusks and can be split into two main types: the ones that retain a shell like their cousin the snail, and are basically bivalve gastropods; and some that lost their shells in the course of their evolution. “Shaun the Sheep” Sacoglossans are shell-less. These are the ones affectionately known as “sea sheep,” which obviously they are not.
What these different Sacoglossan branches have in common is an unusual relationship with algae.
Unlike snails, which sport a flexible band called a radula bearing thousands of microscopic teeth that rasp particles of food from – you don’t want to know – Sacoglossans sport merely one row of tiny teeth on their radula.
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The “sap-sucking sea slugs” use their relatively un-horrifying radula to extract plasma from algae cells. Shelled Sacoglossans have a particular predilection for Caulerpa seaweed, which looks not unlike a soybean plant of the sea. The sap-sucking sea slugs simply digest this plasma. This is not particularly unusual. Non-shelled Sacoglossa have diversified and will also dine on siphonalean or septate green algae.
It’s the other Sacoglossans that “weed out” chloroplasts from algae they eat, and sequester the chloroplasts inside their own tissues, and use these embezzled chloroplasts to do the same thing the algae did with them – photosynthesize – which stand out from the pack of every other multicellular animal in the world.
Chloroplasts are the organelles in plant and algae cells that perform photosynthesis (they capture and convert sunlight into chemical energy). Plants and algae are green because of the green-hued chlorophyll in their chloroplasts. And this explains why Sacoglossans, including Shaun the Sheep, are green or have green areas, because of the chloroplants they stole and are using to make “food.” Thus, their method of eating is called “kleptoplasty.”
The heisted chloroplasts can remain photosynthetically active for months, according to a 2020 report in Nature. How the sea slug achieves this is not clear. But it has been proven that that’s what they’re doing – stealing and utilizing chloroplasts.
Animals get their nutrition from eating, not synthesizing food from sunlight as plants do. In not only helping themselves to chloroplasts but keeping them active for months, the kleptoplastic sea slugs are unique. The only other known successful kleptoplasts are unicellular animals.
“Such long-term maintenance of kleptoplasts photosynthetic activity in Sacoglossan sea slugs is puzzling, as the algal nucleus has been digested,” the 2020 paper observed.
They also point out that it has not been proven that the sea slugs who do this actually need to do this; they could potentially survive as every other animal in the world does – by eating.
But perhaps the best part is something they can survive that other animals cannot: self-decapitation.
From here to eternity
Some lizards can famously detach their tails when attacked, to distract the predator who will hopefully remain engaged with the squirming appendage and let the rest of the reptile escape. This is called “autotomy.”
But the Sacoglossan sea slug makes lizards look like amateurs. This hardy denizen of the deep can, in a pinch, detach its head from its body.
To be clear, the “self-decapitating” slugs aren’t cutting off their heads; they’re cutting off their bodies and leaving their heads. Why are they engaging in this practice – not just autotomy but extreme automoty? For the same reason as octopuses may lose an arm, a crab its claw, an insect its leg – to avoid being eaten. In fact, the property of automoty exists throughout the (relatively lower) animal world and is thought to have independently evolved no less than nine times!
In these slugs, this remarkable ability was described in March in a Current Biology article evocatively headlined “Extreme autotomy and whole-body regeneration in photosynthetic sea slugs.”
Worry not, the floating heads regenerated new bodies. Feel better now?
But the shed body did not regenerate a head. Even so, it could live on for months, its heart still beating presumably because it was being “fed” by its captive chloroplasts – until eventually it would, well, wind down and decay. Still feel OK?
This self-decapitation capacity was discovered by accident, according to New Scientist, when researchers studying the sea slugs, as one does, noticed “a living, severed head in their laboratory,” and then noticed that the whole lot had a sort of groove. Let us now look at more pretty pictures of sea sheep to recover our equanimity.
But again, a tail is one thing, one of six or eight legs is another, but the whole body? What in the name of Darwin and all his finches could explain that?
Well, the Sacoglossans seem to go back to the Cretaceous, based on the not-many fossils of them found so far. They have had time to evolve, and as the Current Biology article on the phenomenon posits, once autotomy has evolved, evolutionary selection “appears to act on the removable appendage to increase the efficacy and/or efficiency of autotomy.” One must salute the self-decapitating Sacoglossan sea slug, because there’s nothing more efficient than losing your whole body. The end.