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222 PROCEEDINGS OF THE CALIFORNIA ACADEMY OF SCIENCES Series 4, Volume 60, No. 10 rather than the coiled shell of gastropods. But complete loss of the shell in the adults is the general rule. The Notaspidea have a “shield” on the dorsal surface that is not equivalent to the head shield of other opisthobranchs. Pleurobranchaea meckelii, shown with its eggs in Photo 45, is a good example. The shell is considerably reduced in all notaspideans, and the mantle cavity has virtually disappeared, so that it no longer covers the gill, which lies along the right side of the animal’s body. This configuration led to the coinage of a rather ludicrous vernacular name “side-gilled slug,” but “pleurobranch” seems a more sensible term. Another name for the group is Pleurobranchomorpha, after the genus Pleurobranchus. Recent phylogenetic studies have suggested that some of the animals traditionally treated as the most basal notaspideans are related to other lineages of opisthobranchs. A group Nudipleura would therefore consist of the remaining notaspideans and their closest relatives, the nudibranchs. The Nudibranchia, or nudibranchs, derive their name from the naked gills. Gills, when present, may occur as a cluster near the posterior end of the body as they do in such dorid nudibranchs as Hypselodoris nigrostriata (Photo 93). Or they may take the form of elongate projections of the dorsal surface of the body (cerata), as in Godiva quadricolor, shown with its eggs in Photo 126. The head almost always bears rhinophores. The shell is absent, except in the larvae. Acochlidiacea is the name for an obscure group of sand-dwelling opisthobranchs that includes the genus Acochlidium. Because these animals have not been studied from the point of view of chemical defense we need only mention them. The last two orders are called “pteropods” on the basis of the wing-like projections of the foot, with which they swim about in the plankton. They were long considered a polyphyletic group, but an increasing amount of evidence seems to indicate that they are a monophyletic assemblage closely related to Anaspidea (see p. 240). Thecosomata, the thecosomes, or thecosomatous pteropods, usually have their bodies protected by a shell or some other firm covering. They are herbivores that feed on phytoplankton by means of mucous nets. Chemical defense has not yet been recorded from this group. Gymnosomata, the gymnosomes, or gymnosomatous pteropods, have naked bodies; in other words they have no shell as adults. They are carnivores, often feeding on thecosomes. Chemical defense is known in only one species. Students of opisthobranch phylogenetics have had to deal with a situation in which snails have repeatedly evolved into slugs. In some cases, it has been a fairly straightforward task to show, for example, that the resemblances between nudibranchs and shell-less sacoglossans are superficial (Russell, 1929). Efforts to arrange opisthobranchs into series of grades, from primitive to advanced (Boettger, 1954), have always failed. However, subsequent efforts to infer branching sequences have suffered somewhat from the use of characters that are apt to evolve repeatedly in separate lineages, and from using only a very limited sample of the available evidence. The morphological trees recently published by Mikkelsen (1996, 2002) and others do not give a consistent picture of the relationships, and the molecular trees (Wägele, Vonnemann & Wägele 2003; Vonnemann, Schrödl, Klussmann-Kolb & Wägele, 2005) leave some of the more interesting relationships undetermined. An effort to use sequences of histone H3 genes to determine deeper relationships within the Euthyneura gave disappointing results (Dinapoli, Tamer, Franssen, Waduvilozhath & Klussmann-Kolb, 2007). Exact branching sequences are notoriously difficult to obtain. Some groups, such as the pleurobranch genus Tylodina, move all over the place, depending on which lines of evidence are utilized. Nonetheless, many relationships are well enough supported by these studies that we can utilize them for our purposes. Research based on a larger and more diverse data base will probably give much better trees than those that have been produced by formal cladistic
CIMINO & GHISELIN: CHEMICAL DEFENSE AND EVOLUTION OF GASTROPODS 223 analyses thus far. For part of the tree we already have that. Meanwhile it will be necessary to make do with the best available evidence. Whatever the evidence that is used, it is obvious that Cephalaspidea in the traditional sense (Cephalaspidea sensu lato) is a paraphyletic grade, from which several of the other orders have evolved. The problem then becomes one of associating the various cephalaspidean lineages with the derived orders, and with one another. There is good evidence for treating some of the orders as sister groups. One of these is Notaspidea + Nudibranchia. This group was called “Acoela” by Thiele (1931); more recent authors have introduced various synonyms. The morphological evidence is reasonably consistent with the molecular trees. To that may be added the chromosome numbers. The pleurobranch Tylodina, as mentioned above, is problematic in the extreme. Its chromosome number is unknown. Beyond the naturalness of Acoela, however, we cannot say very much. It does not show a clear relationship to any other lineage of opisthobranchs. It may be the first lineage of opisthobranchs to have branched off after the separation of the pulmonates. That does not matter very much for the present analysis however. Another clade that may be recognized consists of Anaspidea + Sacoglossa. It is reasonably consistent with the published molecular and morphological trees, and additional evidence, often overlooked, supports it as well. We will discuss that in the appropriate place (pp. 228-229). Diversification within the Opisthobranchia has been marked by the evolution of more specialized herbivores on the one hand and shifts to various kinds of animals as food on the other. That same pattern may be observed within the Cephalaspidea. In many cephalaspideans there are welldeveloped gizzard plates used in processing the food, but others have little more than a cuticle in the same location. Therefore it does not seem likely that there were well-developed gizzard plates in the ancestral opisthobranch or the ancestral cephalaspidean. With shifts to a soft diet, the plates often become lost, so that their absence is not necessarily the original condition. Turning now to cephalaspidean relationships, it is much easier to establish a few groups of close relatives on the basis of derived characters than it is to relate those groups to one another. Traditional classifications generally list a series of families, with familial rank being largely a euphemism for uncertainty as to relationship. We will put these together in three groups, the relationships among which are problematic. To begin, we have four families that are related to Acteon: Acteonidae, Bullinidae, Hydatinidae and Ringiculidae. Leaving the last of these out of consideration, three main points need to be stressed. First, although Acteon is not the living fossil it is sometimes considered, in most phylogenetic analyses it comes out in a basal position and often is associated with other clades. Second, Acteon shares derived characters of the reproductive system with Hydatina (Hydatinidae) and its relatives. And third, the Acteonidae and Hydatinidae are no longer herbivorous, but have switched to feeding upon annelids, which are soft-bodied worms. Therefore gizzard plates would be of no advantage to these animals, and given the switch in diet the absence of such plates could be secondary. On the basis of reproductive anatomy <strong>Ghiselin</strong> (1966b) suggested a possible relationship between the Acteonidae and Hydatinidae on the one hand and the Notaspidea and Nudibranchia on the other. This relationship has been supported by molecular (rRNA) evidence (Vonnemann, Schrödl, Klussmann-Kolb & Wägele, 2005). This puts two groups lacking a gizzard together and it may be a primitive trait. The Ringiculidae, with the genus Ringicula, are of uncertain relationships and have not yet been studied from the point of view of chemical defense. They do, however, have repugnatorial glands on the cephalic shield. The presence in this genus of a (primitive) monaulic (undivided) reproductive system implies that they are the sister group of the rest (Gosliner, 1981). On the basis of new results, Malaquias, Mackenzie-Dobbs, Gosliner and Reid (2009) have advocated recognizing a separate taxon for this lineage, and placing most of the other
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