Who Needs a Shell?

Nudibranch

Bar cephalopods, most marine molluscs move slowly, if at all. While this leaves them open to predation, tough calcium based shells provide excellent protection. The nudibranchs buck this trend. Over time, they have lost their protective armour, yet in spite of their apparent vulnerability they are rarely the victims of predators (1). The loss of shell in nudibranchs appears to be associated with the development of chemical defense mechanisms (2). In most cases these are derived from the diet of the nudibranch, however there are some species of synthesising their own chemical defenses (1).  Aeolid nudibranchs procure stinging cells from their cnidarian prey (3) and ascoglossan and aplysiid nudibranchs obtain toxic secondary metabolites as they graze on algae (4). Others obtain their chemical defenses from their diets of sponges, bryozoans and tunicates. Hypserlodoris webbi selects sponges with high levels of secondary metabolites for predation, which it then sequesters in dorsal glands for pretection (2). Similarly Hexabranchus sanguineus obtains its defense chemicals from Halichondria spp. sponges but also passes these compounds to its egg ribbons, which are otherwise defenseless (5).

Nudibranch

References:

1. Fontana et al. 1994, Cellular and Molecular Life Sciences 50 pp. 510-516
2. Pawlik et al. 1988, Journal of Experimental Marine Biology and Ecology 119 pp. 99-109
3. Thompson 1976, Biology of Opisthobranch Molluscs p. 207
4. Lewin 1970, Pacific Scientific 24 pp. 356-358
5. Avila and Paul 1997, Marine Ecology Progress Series 150 pp. 171-180