Akira Tsukagoshi

Taxonomy, morphology and speciation of the Semicytherura henryhowei group (Crustacea, Ostracoda)

The genus Semicytherura belongs to the family Cytheruridae, and was distinguished from Cytherura on the basis of carapace features. Species of Semicytherura from Japan and adjacent seas can be divided into two groups. One is represented by Semicytherura miurensis Hanai, 1957, characterized by a thin, oval carapace covered with fine reticulation. The other is represented by Semicytherura henryhowei Hanai & Ikeya, 1977, characterized by a thick sub-rectangular carapace in lateral view. Semicytherura henryhowei, which is distributed from Hokkaido to Okinawa in Japan, has been regarded as having several morphotypes distinguishable on outline and reticulation of carapace. However, as a result of detailed observations on the copulatory organ, carapace outline and distributional pattern of pore systems, remarkable differences are shown to exist between the two most frequently occurring morphotypes. In order to recognize S. henryhowei sensu stricto, the carapace of the holotype was re-examined. Consequently, neither of the two morphotypes are considered to belong to S. henryhowei due to differences of carapace outline and distribution of pore systems. The two morphotypes are here regarded as independent taxa, described as new: S. kazahanan. sp. and S. sasameyukin. sp. The geographical distributions of the two new species overlap, but their micro-habitats differ from each other; the former lives on calcareous algae on rocky shores, the latter lives on silty sand bottom within the inner bay. A third new species, S. slipperi sp. nov., is also described. In view of their present geographical distributions and fossil records, the origin of this group of species would appear to be the Japanese islands or adjacent areas in and after the Miocene. This group then migrated to the Arctic Ocean and East Pacific Ocean during or before the middle Pliocene.

Carapace formation of the podocopid ostracode Semicytherura species (Crustacea: Ostracoda)

Details of ostracode carapace structures were examined by SEM and TEM. The podocopine ostracode Semicytherura kazahana has major ridges on the carapace surface and develops its prismatic layer inside the adult carapace. Electron microscopy at the final molt reveals that the major ridges arise from the highly dense formation of pits within the underlying swollen epidermis, and that disappearance of the epidermis in the presumptive area of the prismatic layer occurs after the calcification of the outer lamella cuticle, and just before synthesis of the membranous layer. These facts suggest that the formation of the carapace in Semicytherura takes place via a more complex process than that of the other podocopid ostracodes.

Trunk segmentation of some podocopine lineages in Ostracoda

The trunk segments of podocopine ostracods, which have previously been regarded as having a non-segmented body, are comprehensively illustrated in some lineages for the first time. Descriptions are given of the trunk segmentation of representatives from eight podocopine families: Bairdiidae, Eucytheridae, Leptocytheridae, Cytheridae, Hemicytheridae, Cytheruridae, Loxoconchidae and Xestoleberididae. As observed in the family Leptocytheridae, the maximum number of segments in the Podocopina examined is 11. The boundary between the thorax and abdomen of Podocopina is not distinct. It was found that in Podocopina, the paired copulatory organs are derived from between the fifth and seventh most terminal segments in females and the second most terminal segment in males. It is suggested that the female body plan of Podocopina is plesiomorphic because it is common to platycopid structure and approximately satisfies the supposed ground plan of the Maxillopoda.

A bridge between original and novel states: ontogeny and function of "suction discs" in the Branchiura (Crustacea).

SUMMARY The emergence of novel structures in the course of evolution faces an explanatory problem, leaving the gap from the ancestral structures difficult to bridge. This difficulty is caused by the lack of intermediate stages. Branchiurans are ectoparasitic crustaceans which use a pair of “suction discs” to attach to their host. These structures are modified first maxillae. During ontogeny, the first maxillae transform from a normal cephalic appendage to the specialized suction disc. However, supposedly ancestral branchiurans lack the suction discs in the adults and the first maxilla remains a normal appendage throughout. We describe the muscular arrangements in the developing first maxillae in Argulus coregoni. The suction discs originate as a fusion of the first and second podomeres. The sucker muscles of the suction discs are homologous to the muscles that insert in the second podomere at the early larval stages. The developmental process of the suction disc can be seen as a “recapitulation” of the evolutionary process. We thus show how the first maxilla can maintain not just the biological role but also a functional continuity during the evolution of the novel structure. From this example it is obvious that the intermediate stages of the emerging novelty, if present in the ontogeny, can help solve at least some of the enigmatic appearances of novel structures.

The Interspecific Relations between Three Close Species of the Genus Cythere O. F. Miüler, 1785

The genus Cythere was first proposed by O. F. Muller in 1785, with the European C. lutea as the type species. Almost a century after this proposal, the genus had expanded to contain some 1,000 species. Hanai (1959) researched Cythere , but restricted it, however, to species that are closely related to C. lutea , the type species of this genus. This communication deals with the interspecific relationships between C. uranipponica Hanai, 1959 and C. nishinipponica Okubo, 1976. These, both fossil and Recent, are distributed around the Japanese Archipelago and are very similar to each other. The two species were compared in detail, particularly with respect to morphological elements such as the size, outline, ridges, pits, distribution patterns of the normal pore and radial pore canals, muscle scars, hingement, chitinous appendages, and ecological elements as well as their geographical and stratigraphical distribution. The study has revealed that the morphology of the carapace is very similar in the two species, and that it is difficult to distinguish between them on this basis. In particular, the distribution pattern of the sieve-type normal pore canals, including both their number and position, which has been considered to be a criterion unique to each species, is not necessarily a very stable character in either of the two species studied, and the difference between them was found to be very small. The soft parts were also studied, but no significant difference was observed between the chitinous appendages, antennules, antennae, and legs. However, a striking difference was found in the copulatory organs of the males of the two forms. The species reported by Schornikov (1974) as C. uranipponica was obviously different from C. uranipponica in the distribution pattern of the sieve-type normal pore canals and the morphology of the copulatory organs of the male, and therefore we propose it here as a separate species C. schornikovi n. sp.

The taxonomic utility of the male upper lip morphology in the ostracod genus Parapolycope (Crustacea), with descriptions of two new species

The male copulatory organ is recognized as a significant character for species taxonomy in the sexually reproducing Ostracoda. However, the genus Parapolycope has a simply shaped male copulatory organ consisting almost entirely of just the copulatory duct. In this study, two new species, Parapolycope psittacina sp. nov. and Parapolycope uncata sp. nov., are described, and the characters that have taxonomic value were examined based on a morphological comparison of these new species. Carapace size, shape and surface ornamentation, and the male upper lip are valuable characteristics for species identification. The morphology of the male upper lip, especially, exhibited a remarkable sexual dimorphism, which might have a function associated with reproductive isolation. http://www.zoobank.org/urn:lsid:zoobank.org:pub:D2F2F902-C496-4861-8872-813BC310AD9F

Preface: The phylogeny, fossil record and ecological diversity of ostracod crustaceans

After more than two centuries of research, more than 65,000 living and fossil ostracod species have been described and studied, yet much remains to be learned about this ancient, widespread and diverse group of bivalved arthropods. Their higher classification and phylogeny are subjects of vigorous debate, as is their position in the broader picture of crustacean phylogeny. At the same time, major advances in our understanding of ostracod lineages and their relationships are resulting from the application of innovative approaches and techniques. This preface provides a contextual overview of the 15 contributions to this volume, which resulted from the 14th International Symposium on Ostracoda (ISO2001) held in 2001at Shizuoka, Japan. As such it provides a cross-section of topics at the forefront of research on the evolution and diversity of Ostracoda, and indicates directions for future work.

Origin of the novel chemoreceptor Aesthetasc “Y” in Ostracoda: morphogenetical thresholds and evolutionary innovation

SUMMARY The morphology and developmental processes of the two types of ostracod chemoreceptors, the Aesthetasc “Y” and the “Grouped setae,” were compared. Cypridoidea and Pontocypridoidea, belonging to Cypridocopina, have a large baseball bat‐like seta as an autapomorphic character on the second antenna, whereas most ostracod taxa with plesiomorphic characters bear “Grouped setae” consisting of multiple setae on the second antenna. Their budding positions, morphology, and ontogenetic changes were compared, and our deduction is that the Aesthetasc “Y” originated from “Grouped setae‐like” organ in the Paleozoic. The morphogenetic processes in the molting period of these chemoreceptors were compared at the cellular level. The observations suggest that the “Grouped setae” are formed by hypodermal cells and share sheath cells corresponding to those of the Aesthetasc “Y” as a common constraint in the molting process of setae. We conclude that modification of the morphogenetic processes in the molting period of the “Grouped setae” gave rise to the Aesthetasc “Y” as a novel organ in the evolutionary pathway of the Ostracoda.

Description and scanning electron microscopic observation of a new species of the genus Polycopetta (Crustacea, Ostracoda, Cladocopina) from an interstitial habitat in Japan

Abstract A new species of the genus Polycopetta Chavtur, 1981, Polycopetta quadrispinata sp. n. is described from the interstitial environment of Mihomasaki Beach in Japan. These observations showed some morphological peculiarities of Polycopetta quadrispinata sp. n. compared with its congeners; Polycopetta monneroni Chavtur, 1979, Polycopetta curva Chavtur, 1979, Polycopetta bransfieldensis (Hartmann, 1987), and Polycopetta pax Kornicker and Harrison-Nelson, 2005. Three characteristics are described for the first time: (1) a seta with serrated tip on the male antennula, (2) the endopodite of the fifth limb consisting of two podomeres, (3) the long spermatozoa in the male posterior body. More detailed observations of the type species are needed in order to update the generic diagnosis.

Male Dimorphism in a New Interstitial Species of the Genus Microloxoconcha (Podocopida: Ostracoda)

Abstract The marine interstitial Microloxoconcha dimorpha n. sp. has two morphotypes in the male, “L type” and “S type”; they occur sympatrically and can be distinguished by the size of their carapaces and the morphology of the male copulatory organs. Conversely, the genetic relationships based on the partial mitochondrial COI gene did not demonstrate an independent clade as belonging to only one type. The L and S types are therefore not reproductively isolated from each other, and they express an intra-sexual dimorphism. We also argue that the morphological features of the male copulatory organ could have changed prior to the establishment of reproductive isolation.