AETIDEIDAE) FROM THE JAPAN SEA BY

Based on specimens collected from the Japan Sea, all copepodid stages of the mesopelagic copepod, Gaetanus variabilis are described and illustrated. Among the family Aetideidae, the development of the body segmentation and of the segmentation of the swimming legs in G. variabilis is more similar to that in Gaetanus kruppii than to that in Aetideopsis armata, Chiridius gracilis, or Euchirella messinensis. A sharp, spine-like crest on the frontal dorsal surface of the cephalothorax is a characteristic feature of copepodid stages II-IV of G. variabilis. This frontal crest of G. variabilis, lost in copepodid stages V-VI, is considered as “convergent morphology”, relative to G. kruppii. Nucleotide sequences of the mitochondrial 16S rRNA gene confirmed that copepodid stages II and III with frontal crest and copepodid stage VI without frontal crest are the same species: Gaetanus variabilis. In the adult male, the mandible, maxillule, and maxilla degenerate, whence no feeding takes place: a phenomenon commonly seen in aetideid copepods. RÉSUMÉ A partir de specimens recoltés dans la mer du Japon, tous les stades copépodites du copépode méso-pélagique Gaetanus variabilis sont décrits et illustrés. Parmi la famille des Aetideidae, l