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Bythotrephes longimanus
''Bythotrephes longimanus'' (also ''Bythotrephes cederstroemi''), or the spiny water flea, is a planktonic crustacean less than long. It is native to fresh waters of Northern Europe and Asia, but has been accidentally introduced and widely distributed in the Great Lakes area of North America since the 1980s.〔〔(''Bythotrephes longimanus'' ) USGS NAS – Nonindigenous Aquatic Species〕 ''Bythotrephes'' is typified by a long abdominal spine with several barbs which protect it from predators.
==Description and taxonomy==
''Bythotrephes longimanus'' is a cladoceran crustacean (water flea) recognizable with its straight tail spine averaging about 70% of its length. Adult individuals have three to four barbs on the spine while juveniles have only one pair. The females grow to become larger than the males. The animal has one large eye that is usually black or red. It also possesses a pair of swimming antennae and four pairs of legs, of which the first pair is used to catch prey. Mandibles are used for consumption of prey.
The spiny water flea is similar to another introduced cladoceran of the same family, the fishhook waterflea ''Cercopagis pengoi'', which however has a more slender spine featuring a prominent loop-like hook at its end (see figure).〔 There is also variation in the shape of the more robust spine of ''Bythotrephes'' itself. Specimens arising from sexually produced eggs have a completely straight and relatively shorter spine. In parthenogenetically produced animals the spine features a kink in the middle (figure). Previously, the kinked-spined animals were thought to be a separate species ''Bythotrephes cederstroemi''.〔 After genetic analysis it is now considered to be a form of ''B. longimanus'', making ''Bythotrephes'' a monotypic genus, one with only a single species.〔Therriault, T. W., et al. (2002). (Taxonomic resolution of the genus ''Bythotrephes'' Leydig using molecular markers and re‐evaluation of its global distribution. ) ''Diversity and Distributions'' 8(2), 67-84.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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