Evolution of zooplankters with special reference to pelagic copepods
2004
Ohtsuka, S. (Hiroshima Univ. (Japan))
The functional morphology, phylogeny, and zoogeography of the recent pelagic copepods have revealed some distinct patterns and processes of their evolution. Benthic ancestors of copepods may have originated in the Paleocene on the basis of the occurrence of diversified maxillopodans in the Cambrian. Calanoid copepods appear to have independently invaded the pelagic realm several times. The most primitive arietelloideans may have first colonized this realm in the Devonian. Calanoids consist of two phylogenetically different groups, the more advanced group of which might have successfully resided in open waters by enhancement of sensory and genital systems. Copepods could have evolved with radical functional changes, not altering structures as much, which made switching from suspension feeding to carnivory possible in an arietelloidean family. Cladistic analyses postulate that arietelloideans have repeatedly exploited the pelagic and benthic realms in harmony with environmental changes. The present day distribution of a marine cavernicolous species of the most primitive superfamily Pseudocyclopoidea strongly alludes to the existence of the westward Tethys-Equator Currents and passive dispersal of the ancestor from the Caribbean-Mediterranean to the Central Pacific. A phylogenetic and zoogeographical study of a neritic genus Tortanus hypothesizes its origin, colonization and dispersal routes during the Miocene to the Pleistocene.
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