Taxonomy and morphology of Macrobrachium placidulum species-group (Crustacea: Decapoda: Caridea: Palaemonidae) from the Ryukyu Archipelago

琉球列島産ネッタイテナガエビ種群3種 (ネッタイテナガエビMacrobrachium placidulum (De Man, 1892), マガタマテナガエビ (新称) M. lepidactyloides (De Man, 1892), カスリテナガエビM. sp.) の分類と形態的特徴について, 台湾やフィリピン, インドネシア産の標本も交え, また成長による形態変異も視野に入れ, 詳細に調べた. ネッタイテナガエビ種群は, 雄の第2胸脚の形態・大きさが左右で大きく異なり, 小鉗脚の両指間に形成される大きな隙間を埋めるように剛毛が密生することより, 他のテナガエビ類から区別される (ただし鉗脚が再生した場合は変異あり). カスリテナガエビは, 石垣島よりかつて「M. lepidactyloides」として報告されていたが, 文献と新たに沖縄島から得た同種の標本の詳細な検討により, M. lepidactyloidesではなく, 未記載種である可能性があることが分かった. 一方, 真のM. lepidactyloidesも沖縄島から発見され, これに新標準和名「マガタマテナガエビ」を与えた. マガタマテナガエビは, 雄第2胸脚のうち大鉗脚の指節長と掌部長を比べると, 大きな雄ほど変異幅が大きく, 指節が相対的に長い場合がある. 第2胸脚のうち大鉗脚を自切した雄個体の脱皮に伴う両鉗脚の形態変化を観察した結果, 元小鉗脚が小鉗脚の形態的特徴 (指節が掌部より明らかに長い) を維持したまま大鉗脚に再生するため, 大鉗脚の指節が相対的に長くなる個体が存在する可能性が示された. 琉球列島産ネッタイテナガエビ種群3種は生時の色彩が異なるほか, 成熟した雄個体では第2胸脚の形態が異なることで識別できる. 本報告では, ネッタイテナガエビ種群3種の形態と識別方法を詳細に示すとともに, これら3種を報告した過去の文献にも可能な限りあたり, それらの種同定の信頼性についても検討した.Taxonomy and morphology of Macrobrachium placidulum species-group, i.e. M. placidulum (De Man, 1892), M. lepidactyloides (De Man, 1892) and an unidentified species, were studied on the basis of a good series of specimens collected from the Ryukyu Archipelago and supplemental material from Southeast Asia. Species of this group are characterized by distinctly asymmetric male second pereopods (=chelipeds) and their minor chela being widely gaped and filled by dense setae, although the shape of the chelae can be variable due to regeneration. Our examination has shown that specimens from the Ryukyu Archipelago that have been referred to as M. lepidactyloides actually represent a possibly undescribed species, although formal description of a new species is deferred because of necessity of examination of more extensive material of this species as well as allied species. The occurrence of M. lepidactyloides and M. placidulum in the Ryukyu Archipelago is also verified. The length ratios of the dactylus and palm of male major cheliped in M. lepidactyloides appear to vary more widely in larger males; some males have a proportionally longer dactylus. It was observed, under rearing conditions, that a male that had lost its major cheliped molted and reformed its original-minor cheliped into a major one with remaining minor chelal condition (dactylus being longer than palm). This may explain why larger males have a larger variation in the ratios of palm and dactylus lengths. Species of the M. placidulum species-group from the Ryukyu Archipelago are identified based on body colour in life and cheliped morphology of mature males. The present study also documented diagnostic characters of the three species of the M. placidulum species-group in detail and evaluated the identifications of previous studies as possible