Autotomy. There’s a word you don’t see every day – but those familiar with lizards may well have seen the result. For autotomy is the scientific name for what I suppose we could also call “self-amputation”: the process whereby an animal deliberately sheds a part of its body (a tail, limb, or other appendage). Lizards do it, but apparently so do arthropods, starfish, amphibians – and slugs & snails.
And in the case of the sea slug in this story, the self-amputation is rather extreme, as it involves shedding the whole body (heart, guts, & all) and regenerating an entire new one at the amputation site. (I first came across this in a New York Times article, but you can read the original research paper here.)
The sea slugs belong to a group of molluscs known as sacoglossans, or “sap-sucking sea slugs” (and try saying that one quickly, several times!). They eat algae, slurping up their cell contents, which is where the sap-sucking bit comes from. Some sacoglossans (including the ones we’re discussing here) keep the chloroplasts from their algal meals alive in their own cells, and can live off the glucose the chloroplasts make for quite some time.
Sayaka Mitoh studies sea slugs, and was in the right place at the right time to notice that a sea slug raised in one of her lab’s tanks had literally lost its head. More – the head was still alive, cruising around & eating algae, and appeared to have separated itself from the body (which was also still twitching). Much to her surprise, despite the fact that it had lost its heart & guts, the head not only continued to live, but regenerated a whole new body in just 3 weeks¹. (The disemheaded bodies continued moving for a while, but eventually died.) This initial observation led to an investigation into not just the “how”, but also the “why” of this feat of self-amputation and regeneration.
First up, Mitoh & her colleague looked at Elysia specimens caught in the wild – did they also show this ability to behead themselves. The answer was yes: a small proportion of wild slugs did indeed separate head & body – along a pre-existing groove on the ‘neck’ region – and this behaviour seemed to be related to the presence of parasites (in this case, small crustaceans called copepods). Other parasitised slugs weren’t quite so extreme, removing smaller amounts of tissue (aka autolysis). But none of the slugs found to be parasite-free showed any autotomy.
The researchers wondered if autotomy by Elysia could possibly be a defence against predators. However, they noted that these particular sea slugs don’t have many predators, partly because they have an aversive taste. More importantly, they found that autotomy took several hours, so not exactly an effective way to avoid being eaten!
So, on the basis of their observations that only slugs parasitised by copepods showed this tendency to separate head and body, they concluded that the behaviour could function as a means of removing internal parasites, albeit a rather extreme one, commenting that
The parasites occupy most of the main body of E. atroviridis and strongly inhibit its reproduction during [its] lifetime (data not shown). Thus, removing such parasites by autotomy likely enhances the host’s reproductive success.
It’s an interesting hypothesis, although I’d like to see further studies done as the number of slugs in the study that showed full autotomy is pretty small. I’d also love to know whether the slugs’ behaviour is in turn placing some sort of selection pressure on the ousted parasites.
Mitoh, S. & Yusa, Y. (2021) Extreme autotomy and whole-body regeneration in photosynthetic sea slugs. Current Biology 31(5), DOI: https://doi.org/10/1016/j.cub.2021.01.014
¹ How does the head survive and – perhaps more importantly – obtain energy & nutrients to regenerate the body? That’s where the chloroplasts from its algae-eating habits come into play. The head, like the lost body, contains living functioning chloroplasts from its meals, and can use the materials they produce for its own benefit.