Setal Developmental Patterns of Thoracopods of the Cyclopidae (copepoda: Cyclopoida) and Their Use in Phylogenetic Inference

Thoracopod developmenl was analyzed for 25 species from 18 genera among the Cyclopidae. One species from each of 7 genera of presumed older cyclopoids. 5 poeciloslomaloid genera. 3 harpacticoid genera, 5 calanoid genera, and a siphonoslomatoid were studied to help establish ancestral patterns of thoracopod development. Ancestral character states were inferred from the presence of identical states in species from presumed older fainilies, from the presence of stales which show diverse and frequent occurrence among the Copepoda, or from the presence of states shared by serially homologous structures presumed to be determined by the same pleiotropic regulatory process. Developmental patterns of the 4 swimming legs were assumed to result from the action of 2 different regulatory processes. A pleiotropic process acting early in development determines the morphology of all 8 rami together and results in 3 states among cyclopids, ancestral, and the independently derived delayed and truncated patterns. A second set of up to 8 regulatory processes acts later and one of the 8 determines the morphology of each individual ramus. The ancestral stales of the resulting individual rami are. by default, the morphology that results from the pleiotropic regulatory process. Variations in developmental patterns were used to generate a phylogenetic hypothesis. Cyclopids have separated into a lineage of 10 species which has delayed the development of the swimming legs or has modified some individual rami from the delayed condition, and a lineage of 8 species which has truncated the development of the swimming legs or has modified some individual rami from that truncated condition. Four species have retained the ancestral process regulating swimming-leg development and 3 species have modified development of some individual rami from the ancestral condition. Copepod development is anamorphic. A new somite is added anteriorly from the posterior somite during each stage of development. Thoracomeres 1-4 form during naupliar stages 3-6, respectively, although naupliar somites are not separated from one another. Thoracomeres 5-7, which usually articulate, form during copepodid stages I-III. In general, copepod thoracopods initially appear as setose buds one stage after the formation of their somite. However, the maxilliped of cyclopids is not present as a setose bud during naupliar development and swimming legs 1 and 2 initially are present as setose buds at nauplius 6. During the molt to copepodid I, the maxilliped and buds of the swimming legs 1 and 2 are transformed into complex appendages, which are more similar to their adult morphology. Appendage development during the copepodid phase usually involves the addition of at most one segment and one or more setae at each new copepodid stage. While there is significant variation among copepods in this general pattern of development, there are only a few studies in which this variation has been used to draw inferences about copepod phylogeny. Distinctions in appendage development have been used to group species (Claus, 1893; Ito, 1984) or higher taxa (Lang, 1948), but only occasionally have characters expressed during development been used to draw phylogenetic inferences (Dahms, 1990; Dahms et al, 1991; Ferrari, 1991). This use of characters expressed during development can be problematical for three reasons. If juvenile and adult characters are analyzed from a simple matrix, an incorrectly robust hypothesis may result if juvenile and adult states are invariably dependent, because they are determined by the same genetic regulatory process. Phylogenetic inferences from analyses which are limited to a specific developmental stage, however, cannot make use of the full expression of developmental variability. Finally, when results from analyses of several stages are compared, inferences may be complicated if stage-specific hypotheses are not concordant. A method which overcomes many of these