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220 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES Series 4, Volume 60, No. 10 failure to look for them. The four species that have been studied contain unique diterpenoids (Atlas 460) (Manker & Faulkner, 1987, 1996; Díaz-Marrero, Dorta, Cueto, Rovirosa, San-Martín, Loyola & Darias, 2003; Darias, Cueto & Díaz-Marrero, 2006; Van Wyk, Gray, Whibley, Osoniyi & Hendricks, 2008). They are very similar to some metabolites (Atlas 351, 352) found in the notaspidean Pleurobranchaea meckelii (see below). Although these limpets are effective in defending themselves from attacks by sea-stars, those predators that ingest the limpets seem not to be harmed (Rice, 1985). There is no clear answer to the question of whether Trimusculus never evolved propionates or whether it changed from propionates to terpenoids as defensive metabolites. Given, however, the fact that the defensive use of polypropionates has evolved repeatedly in euthyneurous gastropods, the former hypothesis is at least plausible. The other group of marine pulmonates of interest to us is the family Onchidiidae, in which a shell is entirely lacking in the adults. Their stalked eyes suggest that they should be placed in the Stylommatophora together with terrestrial snails and slugs, but it is common practice to put them in a separate group. There is no gill, and they breathe air through an opening at the posterior end of the body. Some respiratory exchange also occurs via the general surface of the body. The dorsum is somewhat leathery, and there are repugnatorial glands that contain defensive metabolites. These metabolites include sesquiterpenoids, depsipeptide acetates, and propionates. In Onchidella binneyi, the secretion consists largely of the enol acetate sesquiterpenoid onchidal (Atlas 216) (Ireland & Faulkner, 1978). This is the protected form of the molecule. When the animal is attacked, onchidal is changed into the active form, ancistrodial (Atlas 217), which has the common dialdehyde structure with an adjacent double bond. Ancistrodial is also present in the defensive secretion of a termite (Baker, Briner & Evans, 1978). Abramson, Radic, Manker, Faulkner and Taylor (1989) found geographical differences in the concentration of onchidal from different species in different places. Onchidella binneyi from Baja California had 230 micrograms per animal, whereas O. borealis from California had only 33 micrograms per animal, showing the usual pattern of more chemical defense in lower latitudes. Nonetheless, the repugnatorial gland secretion of O. borealis is strikingly effective in deterring attacks by seastars, and the slugs are largely left alone by predators (Young, Greenwood & Powell, 1986). Peronia peroni has polypropionates called peronatriols (Atlas 158). These animals are eaten by human beings, but evidently only after the dorsal surface has been cleaned off (Biskupiak & Ireland, 1985). A cytotoxic depsipeptide called onchidin (Atlas 675) has been recorded from an unidentified tropical species of the genus Onchidium (Rodíguez, Fernández, Quiñoá, Riguera, Debitus & Pouchet, 1994). A similar depsipeptide, onchidin B, occurs in a likewise unidentified Onchidium (Fernández, Rodríguez, Quiñoá, Riguera, Muñoz, Fernández-Suárez & Debitus, 1996). The same group of investigators found cytotoxic acetates and propionates in another unidentified tropical Onchidium species (Rodíguez, Riguera & Debitus, 1992). Ilikonapyrone (Atlas 151) esters have been reported from Onchidium verrucatum (Ireland, Biskupiak, Hite, Rapposch, Scheuer & Ruble, 1984). In pulmonates, then, polypropionates are widespread, but other metabolites occur in addition to, or instead of, them in what would seem to be the more modified representatives of the group. This pattern is rather difficult to interpret historically. One possibility is that initially the marine pulmonates were defended chemically by some other metabolite and began to secrete polypropionates when they shifted to the intertidal habitat. The substitution of diterpenoids for polypropionates in Trimusculus would then be a secondary shift, resulting from the availability of new metabolites when a new feeding mechanism evolved. In Onchidium and its relatives an analogous change might have led to additional compounds being added to the secretion of the repugnatorial
CIMINO & GHISELIN: CHEMICAL DEFENSE AND EVOLUTION OF GASTROPODS 221 glands. It may be, however, that the defensive use of polypropionates goes back to the common ancestor of pulmonates and opisthobranchs. CHAPTER V OPISTHOBRANCH SYSTEMATICS AND PHYLOGENY Traditionally, the opisthobranchs have been treated as a sub-class divided into approximately eight orders, namely: Cephalaspidea, Anaspidea, Sacoglossa, Notaspidea, Nudibranchia, Acochlidiacea, Thecosomata, and Gymnosomata. Let us briefly characterize these groups, preparatory to a discussion of their genealogy. The following diagram indicates likely relationships of five of them, leaving out the paraphyletic Cephalaspidea and the groups for which there are no data on chemical defense: ┌ Notaspidea ┌┤ (Acoela) ─┤└ Nudibranchia │┌ Sacoglossa └┤┌ Anaspidea └┤ └ Gymnosomata The Cephalaspidea derive their name from the head-shield at the front of the body. This structure is conspicuous in the specimen of Bulla striata shown with its eggs in Photo 9. They are also called Bullomorpha by allusion to the genus Bulla. The vernacular name, “bubble-shell,” alludes to the fragile shells of many cephalaspideans. The shell may be well developed, but all stages of reduction occur. Cephalaspidea is considered the ancestral stock from which the other opisthobranch groups, or at least some of them, have separately evolved. The group as traditionally conceived, however, is a paraphyletic grade. To make it monophyletic it would be necessary to remove some lineages from this group, and taking this step recently has been suggested by Malaquias, Mackenzie-Dodds, Gosliner, Bouchet and Reid (2009). The Anaspidea derive their name from the secondary modification of the head shield. They are also called Aplysiomorpha by reference to the familiar genus Aplysia, which has been a favorite experimental subject for neurophysiology (Photos 16 & 18). The head shield has been modified so that there are two pairs of rolled projections called “rhinophores” that resemble ears. These have a sensory function. One genus, Akera, however, still retains the unmodified head shield and fragile shell, making it a good transitional form connecting the group with Cephalaspidea (Photo 17). Otherwise the body has expanded considerably and the shell is proportionally reduced. The Sacoglossa, or Ascoglossa, are named on the basis of the sac, or ascus, that holds used teeth. Lobiger serradifalci (Photo 28) and Calyphilla mediterranea (Photo 40) are good examples. There is no vernacular name, but they are sometimes called “sap-sucking slugs” from the way they feed upon algae by piercing the cell with their highly modified radular teeth, and then sucking in the contents. Typically they have a single pair of rolled rhinophores, resembling those of anaspideans, but the retention of a head shield in more primitive representatives of both groups shows that this is a result of evolutionary parallelism or convergence. Some sacoglossans again have shells like those of cephalaspideans, and old-fashioned systematists classified them as such. In one lineage, the shell has been modified so that it looks like the bilaterally divided shell of bivalves
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