Chemical Evolution
Opisthobranch gastropods provide impressive example of parallel evolution resulted from natural selection. Reduction of shell has occurred independently within distinct lineages, transforming snail into slugs repeatedly (Cimoni 1998). Presence of secondary counpounds animals’ bodies and the reduction or outright absences of shell is highly correlated. Evidences from Faulkner & Ghiselin (1983) shows that chemical defense was like a preadaptation that allowed shell reduction, causing further changes in the biology of the organism.
Less modified representatives of the order Cephalaspidea is very to primitive Sacoglossan (Cimoni 1998). The Cephalaspidea was an herbivour with a cuticular plates gizzard for trituration of food. Most Sacoglossans are mainly herbivore, primitive Sacoglossans feed on a small range of siphonaceous green algae and then shifted to wider range of algae and often cause the lost of gizzard. Family Elysiidae display two main evolutionary themes: 1) extraction of chloroplasts from algae and use them as a source of energy via photosynthesis (Kay 1968; Clark & De Freese 1987) and 2) Shifts of algae host usually withing the same order.
Chloroplasts from algae, in photosynthesis, are used more effectively if the disgestive gland becomes highly ramified rather than compact. This can be apply to family Elysiidae (Placobranchidae), where organisms evolve a leaf like appearance with a pair of lateral extensions of foot or parapodia (Cimoni 1998). Thuridilla is unique among Sacoglossans because of distinct colouration and being conspicuous rather than cryptic upon its food (Gosliner 1995). Therefore, Gosliner (1995), suggested that it is possible that the organism may have effect chemical defense. However, with relevant data avaible, only T. hopei show chemical defensive mechanism with three diterponoids derived from its food (Gavgnin et al. 1993).
On the other hand, Sacoglossan radula is divergently adapted to algae feeding (Gavgnin et al. 1993). Feeding upon a wide range of algae allow variety of defensive chemical, however, it involves complicated step including detoxifying, transporting, storing, and releasing chemical. Therefore, further investigated is needed for better understanding of the evolution of this family.
Figure 1: Thuridilla bayeri from Shark Bay, Heron Island, on green algae (21 Sep 2011). |