| Postzygotic care of offspring exclusively by males is the rarest form of parental investment in animals. Hence, taxa that exhibit paternal care provide excellent systems for examining the proffered links between sexual selection and mating systems. In the Class Pycnogonida, or sea spiders, egg-carrying males have been found in nearly all of the 1200+ species. Despite such taxonomically widespread expression of paternal care, pycnogonids have received little attention in studies of behavioral ecology and evolution.;For this dissertation, I developed microsatellite DNA markers for each of three species (Ammothea hilgendorfi, Ammothella biunguiculata, and Pycnogonum stearnsi), and I employed these markers to elucidate, for the first time in pycnogonids, patterns of genetic paternity and maternity in natural populations. In addition, I quantified the magnitude of sexual size dimorphism in these taxa. Using quantitative measures of mating systems, I interpreted these results in the light of sexual selection theory.;In general, the species examined were similar in three mating system characteristics: (i) none of the males assayed genetically could be excluded as the sire of all the progeny they carried (i.e. no cuckoldry was detected); (ii) progeny found in the same egg cluster were invariably full-siblings; (iii) both sexes routinely engaged in multiple mating within the same window of time. Although male P. stearnsi carry large egg clusters for prolonged periods of time and are ~30% smaller than females, I found no evidence of sex-role reversal in this species, as evidenced by strikingly similar Bateman gradients and opportunities for sexual selection in the two sexes. Ammothea hilgendorfi and Ammothella biunguiculata exhibited comparable magnitudes of sexual size dimorphism, albeit in opposite directions. In contrast to predictions from sexual selection theory, males of the species with female-biased dimorphism (Ammothea hilgendorfi ) showed a potential for sexual selection that was two to four times higher than that of males of the species with male-biased dimorphism ( Ammothella biunguiculata). In neither species, however, was male body size a good predictor of mating or reproductive success. Hence, experimental manipulations are needed to determine the phenotypic targets of sexual selection in pycnogonids. |
