The ontogeny and phylogeny of copepod antennules

Abstract

Comparative analysis of the development of antennulary segmentation and setation patterns across six orders of copepods revealed numerous common features. These features are combined to produce a hypothetical general model for antennulary development in the Copepoda as a whole. In this model most compound segments result from the failure of expression of articulations separating ancestral segments. In adult males, however, compound segments either side of the neocopepodan geniculation are typically formed by secondary fusion at the last moult from CoV (stage 5). The array of segments distal to the articulation separating segments XX and XXI is highly conserved both in ontogeny and phylogeny: typically the distal segmentation of the adult female is already present in the CoI. A maximum of three setae is added to the distal array during the entire copepodid phase. This morphological conservatism is interpreted as evidence of the functional continuity of the distal setal array as a mechanosensory system providing early warning of approaching predators. Sexual dimorphism typically appears late in development; the male undergoing modifications especially at the final moult to sexual maturity. These modifications include the formation of the neocopepodan geniculation at the XX to XXI articulation and, in some orders, the formation of a proximal geniculation at the XV to XVI articulation. A proximal geniculation is reported here from the Calanoida for the first time. The geniculations allow the male to grasp the female during any mate guarding and during spermatophore transfer. Particular setae on segments either side of the neocopepodan geniculation are modified as basally fused spines in at least some representatives of the Calanoida, Misophrioida, Cyclopoida, Harpacticoida and Siphonostomatoida. The antennulary chemosensory system, comprising primarily the aesthetascs, is enhanced at the final moult in many male copepods. In planktonic copepods this enhancement may take the form of a doubling of the aesthetascs on almost every antennulary segment, as in the eucalanid calanoids, or of an increase in size of existing aesthetascs, as in the siphonostomatoidPontoeciella, or of the transformation of possibly originally bimodal, seta–like elements into distally thin–walled, more aesthetasc–like elements, as in some calanoids, harpacticoids and poecilostomatoids. Enhancement of the chemosensory capacity of adult males appears to be linked with their mate–locating role. Copepods inhabiting the open–pelagic water column are more likely to exhibit enhancement of the chemosensory system than neritic or benthic forms. Enhancement may confer a greater sensitivity to chemosensory signals, such as pheromones produced by receptive females, which may retain their directional information at lower concentrations and, therefore, for longer periods, in oceanic waters than in more turbulent neritic waters. Aesthetascs appear to be more evolutionarily labile than other setation elements, apparently being lost and regained within well–defined lineages. Caution is urged in the use of aesthetasc patterns in phylogenetic analysis. The ontogenetic analyses suggest that the timing of expression of intersegmental articulations during development may in future provide the most informative characters for phylogenetic study, rather than either segment numbers or the patterns of fused or undivided segments.