Nudibranchs & relatives Research Study 1
At least one west-coast species of opisthobranch, the notaspidean Berthella californica (Fig. 1), secretes acid onto its skin for defense, but there are many more species worldwide that employ this strategy. The acid, a mixture of sulphuric and hydrochloric with a pH of 1 - 2, is contained within vesicles in the acid-secreting cells in the skin. When the nudibranch is irritated, special muscles around the vesicles contract and force the acid onto the skin surface (Fig. 2). Preliminary laboratory experiments on the defensive role of the secretions in related species in Port Erin, England show that the flesh of the nudibranchs is rejected as food by several fishes, including blennies, flatfishes, and pollacks. As for Berthella, pufferfishes in the Gulf of California will bite at them, but then spit them out.
Gillette et al. 1991 J Exp Biol 156: 335
Research Study 2
Another acid-secreting west-coast notaspidean is Pleurobranchaea californica (Fig. 1), which releases its acid in response to skin abrasion or compression. Interestingly, exposure of Pleurobranchaea and other acid-secreting species to small aliquots of acidified seawater (HCl) elicits aversive behaviour, including withdrawal of affected parts, retraction of oral veil and head, turning, and rapid locomotion. The responses, shown in the photo series below for an individual P. californica (Fig. 1) attached to the side of an aquarium, last for several minutes. In such cases the defensive acid seems to be acting as an intraspecific warning signal. Additionally, in the case of P. californica, an acid-treated individual may commence swimming using dorsal-ventral flexions to lift itself off the substratum. Application of dilute solutions of the amino-acid taurine, a feeding stimulant in many marine invertebrates, also induces acid secretion in Pleurobranchaea.
