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.

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Mysids are small shrimp-like crustacean zooplankton that are widely distributed in marine, brackish and fresh waters. Mysids belonging to the tribe Mysini are abundant in near-bottom layers of coastal waters and are important food for juvenile and young fishes. There have been many taxonomic studies of this group, and thus its faunal composition is better known than that of other mysid tribes. Recently, a taxonomic revision was made of Acanthomysis and Parastilomysis. As a result, the taxonomic problems at the genus level were largely resolved in the Mysini. When the geographic distributions of mysinid mysids are arranged by genus, several distinct patterns are evident.

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Dicyemid mesozoans (phylum Dicyemida) are endoparasites that are typically found in the renal sac of benthic cephalopod molluscs. Dicyemid bodies are very simply organized, consisting of only 8 to 40 cells, which is the fewest number of cells in metazoans. The current paper gives an overview of the morphology of dicyemids and reviews current status of the taxonomy of dicyemids. The renal sac of cephalopods is a unique environment providing habitat for a diversity of dicyemids. The current paper also reviews recent advances in several biological aspects, such as prevalence, zoogeography, and host specificity. The degree of host specificity differs among different species of dicyemids, although the recent study reveals narrow host specificity. Most previous descriptions were based on the character of vermiform stages, and several species seem to be incorrectly identified. The current study suggests that the cellular composition and cell types of infusoriform embryos are significant characters used to help identify dicyemids species. In addition, the type host specimen for a new species of parasite is briefly discussed The accurate identification of a host organism is an important component in the taxonomic recognition of a new species of parasite.

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In 1997, Masatsugu Takano and the applied population genetics research group at Tohoku University published a ground-breaking paper on biochemical and morphological evidence indicated that there were two sympatric forms, interpreted as sibling species, of the common crab Hemigrapsus penicillatus (De Haan). This species is very common in intertidal rocky shores, pebble beaches and mud flats in Japan, Korea and China, but prior to Takano et al., no one had noticed that the species comprised two different forms. A recent intensive study by Prof. Seiichi Watanabe, Tokyo University of Marine Science and Technology, and his research group revealed further differences between the two forms, including coloration, allometry, egg size and number, and geographical distribution. Watanabe and myself recognized these as two distinct species and described one of them as a new species Hemigrapsus takanoi in 2005.

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