420 Dive Log Australasia OCTOBER 2026

STAYIN ALIVE THE DEFENCES OF SEA SLUGS - Part 10 Reactive Defences continued

Autotomy - Sacrifice/Distraction response

muscular coordination to readily facilitate the self amputation and subsequent immediate sealing of the fractured joint. In some instances, the autotomy is not just limited to the sacrificial offering. Those cast-off sections of mantle may contain glands of distasteful chemicals to further parry the predator. In the case of the cast off cerata of some aeolids, like the Phyllodesmium for example, they will wriggle temptingly for quite some time creating further distraction, that is, a greater attraction to the predator – although the cerata of Melibe do not. The autotomised cerata may also exude sticky and distasteful mucus that will anchor one end to the substrate thus preventing dispersal or even become attached to the would-be predator. In that situation the tempting distraction then becomes a nasty deterrent. There are some dorids that autotomize sections of their mantle as a stress distraction sacrifice. Speci mens of Sebadoris fragilis , for example, can cast off significant pieces from both sides of the mantle. This behaviour was known back in 1864 when the species was first described - hence the specific epi thet of fragilis . The fracture lines are very clean and clear-cut and a large quantity of mucus is usually produced.

Another effective method of avoiding an attack by a predator, once detected, is to confuse or distract it. A number of different sea slugs utilise a reactive process called autotomy . With autotomy a portion of the sea slug’s body can be cast off, i.e. sacrificed to the predator, hopefully distracting and keeping it engaged to enable an escape. It is therefore a sacrificial response. The bodily portion thus sacrificed is not vital to the immediate survival of the sea slug and can in time be regenerated in most species. Examples of body parts that can be cast off in this manner include, depending on species, outer portions of the mantle, the tail, parapodial lobes and the cerata - all portions that are exposed and seemingly attractive to a predator. Not all sea slugs possess this ability and in those that do some will do so much more readily than others. Detachment most usually requires direct predatory contact upon the detachable portion. This is a complex process made possible by special cells located along the fracture line, known also as the autotomy plane, together with nervous and

Left: An entire specimen of Tomoberthella martensi having all three mantle segments intact. The fracture lines are apparent though. Right: This specimen of the side-gilled sea slug Tomoberthella martensi has autotomised all three of its mantle segments. With its right segment cast off, the “side gill” has become exposed. DIVE LOG Australasia #420 Oct ‘ 26 66 www.divelog.net.au