Nudibranchs & relatives Research Study 1
The cerata of many species of aeolids are readily autotomised, such as after contact with rainwater in tidepools or with cnidarians such as cup corals. For a while after their autotomy, the cerata may wriggle about, perhaps acting as an “attention-getters”, or decoys, for potential predators. If cerata are experimentally removed from the aeolid nudibranch Hermissenda crassicornis, they regenerate within a few weeks. The cerata of Hermissenda are conspicuously coloured with white- or light-coloured tips surrounded by orange bands, and white lines trailing down the sides (Fig. 1). Inside each can be seen the brown diverticula of the digestive gland.
Miller & Byrne 2000 Invert Biol 119: 167
Research Study 2
A strong aversive stimulus, such as a firm pinch, to a ceras of Melibe leonina will cause autotomy. Initiation of autotomy of a ceras is mediated by two rings of nerves originating from the ceratal nerve (Fig. 1), the latter of which derives from the pleural nerve running the length of the body (Fig. 2). Severance occurs at the ceras base in an area surrounded by two bands of sphincter muscles known as the autotomy plane. The actual autotomy appears to be a disruption of connective-tissue components at the autotomy plane that causes the tissues to separate. Contraction of the sphincter muscles seals off both sides of the wound (Fig. 3). The entire process, from stimulus to autotomy, occurs within 10sec at temperatures of 9 - 12oC. The author notes that autotomised cerata in Melibe show little spontaneous movement. Pinching of a ceras or other aversive treatment may also initiate swimming. The author observes “galloping” from contact with certain sea stars (but no cerata loss), and swimming, and ceratal loss from being pinched by crabs. The question still remains as to the function of ceras autotomy.
Research Study 3
Detailed studies on ceratal autotomy and regeneration in Hermissenda crassicornis at the Bamfield Marine Sciences Centre, British Columbia show that a ceras detaches through severence along an autotomy plane located at its basal constriction (see also Research Study 2 above). The cross-sectional drawing in Fig. 1 shows these constrictions, as well as the presence of mature cerata along the mid-sagittal axis and smaller, immature cerata along the outside margins of the body. About 4d after autotomy, regenerating cerata appear as small protuberances. By 24d their structural organisation is complete and by about 40d they are indistinguishable from surrounding cerata (Figs. 2 - 7). In laboratory experiments, kelp crabs Pugettia producta induce autotomy in H. crassicornis by grasping cerata in their chelipeds. Later, the crabs are observed to eat the "decoy" cerata while the previous owner crawls rapidly away. Although this is highly suggestive of a defensive function for autotomy, other tests with hermit crabs and sculpins fail to induce similar behaviours in Hermissenda. However, the authors suggest that ceratal autotomy may nonetheless be an important means of escape from Pugettia, which is common in their habitat. The study is the first to detail ceratal regeneration in an aeolid nudibranch.
