Ceratal autotomy as a defensive mechanism of the sacoglossan sea slug Placida kingstoni against a generalist crustacean predator

Abstract

Sacoglossan sea slugs have developed a variety of defence mechanisms against predation. Research on these mechanisms has focused primarily on the chemical defences of these slugs, and little information is available on nonchemical modes of defence, such as autotomy, a behaviour in which an organism voluntarily detaches body structures at a predetermined breakage point in response to danger or stress. Autotomy is diverse in sacoglossan sea slugs and has been well documented. Within Oxynidae, members can autotomize their tail and parapodial lobes, and slugs in Limapontiidae and Hermaeidae can detach their cerata. More recently, reports have been made of Elysia with the capacity to autotomize most of their body. However, despite the widespread assumption that autotomy in this group serves a defensive purpose, the effectiveness of the behaviour in ensuring survival against predation has seldom been examined. The objective of this study was to evaluate the role of autotomy in sacoglossans by assessing the effectiveness of ceratal autotomy in ensuring survival against the attacks of a generalist predator. Placida kingstoni is a small sacoglossan native to Florida and the Caribbean with the ability to autotomize its cerata. Individual P. kingstoni were exposed to shrimps of the Lysmata wurdemanni species complex for 10-min interactions. Most sea slugs were attacked by the predator, often more than once, but the majority of the slugs readily autotomized cerata and survived. Structure detachment was accompanied by the secretion of a mucus that facilitated the formation of ceratal clumps. Most of these clumps were consumed by the predator and effectively diverted their attention, allowing P. kingstoni to crawl away. In this species, the success of autotomy as a defensive strategy appears to be directly related to the palatability of autotomized cerata. The results of this study show that ceratal autotomy in P. kingstoni is an effective defence against predation. Autotomy is a behaviour with a high-energetic cost; however, it has convergently evolved within Heterobranchia on multiple occasions, and it is highly prevalent in cerata-bearing slugs. Although in sacoglossans much of this behaviour remains a mystery, this study provides a clear example of autotomy as a defensive mechanism.