Systematics and Evolution of Small Crustaceans : Their Diversity and Dynamism

Abstract

Free-living copepods are regarded as one of the most ecologically important animal groups in aquatic ecosystems, but systematic and phylogenetic surveys of these crustaceans are still incomplete. In recent years my colleagues and I have newly established five families, 11 genera, three subgenera, and 59 species of copepods. Our success in finding previously unknown forms is partly a result of our focus on collections from hitherto poorly sampled hyperbenthic layers. The peculiar swarming behavior, distinct vertical migrations, and unknown life cycles of many species also make their collection difficult. We were the first to discover copepods of the order Platycopioida in the Indo-Pacific region; based on a circumtropical distribution and has thus come to be understood as a Tethyan relict. Another example of biogeographical significance is a new cavernicolous species of the calanoid copepod genus Ridgewayia that we described from Palau, which shows a closer relationship to the Atlantic-Mediterranean species group of this genus than to the Indo-West Pacific species group. This suggests dispersal by the westward circumtropical current that existed from the late Jurassic to the Miocene. We have also described three families, three genera and four species of parasitic copepods that infect fishes, sea urchins, bivalves and mysids. In addition to copepods, four tantulocaridans, one ascothoracidan, and three peracaridans were newly described as a result of collaboraive work with specialists on these groups. Copepods with a basic developmental pattern comprising six naupliar, and six copepodid stages, are a relatively easy group to tracehomologous features when compared to other crustaceans. This fact makes phylogenetic analyses of copepods feasible. Better understanding of the phylogeny of copepods not only leads to revisions of their classification system, but also allows us to make certain deductions concerning evolutionary patterns and processes that are related to, for example, their habitat exploitation, direction of dispersal, and switching of feeding mode or host. Our analyses have thus led us to infer that members of the calanoid superfamily Arietelloidea have exploited a wide range of habitats, both horizontally from coastal to oceanic regions and vertically from the surface to the deep hyperbenthic layers, with recolonization therefrom into the original, coastal benthic habitat. We have also been able to reconstruct a switch in feeding from suspension feeding to carnivory in the calanoid family Heterorhabdidae, a trend leading to carnivorous taxa that employ a venom-injecting system for capture of prey. This sophisticated feeding structure seems to have been constructed by modification of prototypes that were present in the suspension-feeding ancestor, resulting in a drastic functional change in feeding without much alternation of its form.