Once again, Elysia makes a splash with its unusual biology.  This week, it’s an observation that at least two species of Elysia can shed their entire bodies, and regrow them.  This was described in the journal Current Biology (Curr. Biol. 31: R215-R240, 2021).  Because it was so strange, the paper also received attention in the New York Times and Science Magazine.

The authors Sayaka Mitoh and Yoichi Yusa, discovered that slugs in their captive-reared colony of Elysia cf. marginatus (the “cf” apparently being due to uncertainty about identities of members of a species complex) would spontaneously shed their whole bodies from time to time (Figure 1A, below).  They also observed this phenomenon in a field-collected Elysia cf. marginata and in part of a cohort of wild-collected Elysia atroviridis.  The process of shedding a body part, termed autotomy, is well-known among animals, and has been observed before in sacoglossan species related to Elysia that release body parts to avoid predation.  Autotomy on this scale is rare, though, and had never been observed in molluscs.

Mitoh and Yusa Figure 1A. The head is at the top, and the body is below. Arrow indicates the heart.

The pattern was similar for most: the head separated at a line that the authors referred to as a “breakage plane” (Figure 1I, below). 

Mitoh and Yusa Figure 1I. The presumed autotomy plane is indicated by a dashed white line.

The wound healed up, and a new body was regenerated over the course of a few weeks (Figure 1 E through H, below). 

Mito and Yusa Figure 1E through H. Elysia atroviridis autotomizes its body (E and F), then regenerates a new one (G and H).

The authors emphasized that the heart, easily visible on the dorsal surface, remained with the body, and they believed that most of the organs were also lost.  Unfortunately, it was a brief communication, so no anatomical data were provided regarding what exactly was left behind.  Because of the weird development of gastropod molluscs like Elysia, in which there is a twisting of the embryonic body, much of the digestive and reproductive systems end up in the head, so it will be interesting to see in future studies what exactly remains with the head.  They did note that many of the severed heads began to feed within hours of losing their bodies, so some capacity for digestion must have remained.

In any case, it is remarkable that the head could survive at all, much less generate a whole new body.  The authors speculate that kleptoplasty, the famous ability of Elysia species to retain chloroplasts from their food plants and use them to generate energy, could be an important factor in the survival of the head.  The diverticula that contain chloroplasts are certainly found in the head (as shown in the image below of a baby Elysia crispata), so this seems plausible.

Baby Elysia, about three weeks old. Rhinophores well developed, diverticula, including those in the head, are full of chloroplasts from Bryopsis plumosa. 8/17/18

Why do the slugs do it?  It certainly stresses them, as indicated by the fact that all slugs older than 480 days died after autotomy.  It is not likely due to predation, because it is too slow to prevent being eaten, and because simulating predation by pinching the slugs did not cause autotomy.  It is more likely a way to eliminate diseased or parastized tissues.  Among the field-collected Elysia atroviridis, the only ones that performed autotomy were those containing a parasitic copepod.  Curiously, the minority of parasitized animals autotomized their bodies, while many more gradually dissolved the affected parts through a process called autolysis.  When one combines this observation with the fact that lab-reared (and presumably unparasitized) Elysia marginata performed autotomy, it appears that much about the phenomenon remains to be understood. 

Naturally, you are all wondering whether I have observed this phenomenon in Elysia crispata in the lab here.  So far, no.  If they could shed their bodies, I expect they might have done so when some of the tanks were infested with protozoans and the slugs were being eaten away.  Nonetheless, it may still occur under some conditions.  

It makes you wonder how the French Revolution may have been different if the royalty had been Elysia.