Megasternum japonicum Shatrovskiy, 1989

Fikáček, Martin, Jia, Fenglong & Prokin, Alexander, 2012, A review of the Asian species of the genus Pachysternum (Coleoptera: Hydrophilidae: Sphaeridiinae), Zootaxa 3219, pp. 1-53 : 48

Megasternum japonicum Shatrovskiy, 1989 , stat. restit.

Megasternum gibbulum auctt. nec Motschulsky, 1866 : Sharp (1874: 420); Knisch (1924: 158); Nakane (1966: 55); Satô (1985: 211).

Megasternum japonicum Shatrovskiy, 1989: 286 .

Megasternum japonicum: Shatrovskiy (1992: 368, subsequent more detailed description) ; Hansen (1999: 305, catalogue); Hoshina & Satô (2005: 13, incorrectly synonymized with M. gibbulum ).

Material examined. JAPAN: Tochigi: 8 spec. ( PJWP, NMPC): Nikko City env. [coordinates ca. 36°43'N 139°42'E], 23.ix.2006, lgt. P. Jaloszynski. RUSSIA: Kuril Islands: 1 spec. ( MPU): Kunashir Is. , UV okr. oz. Peschanoye ( Sernovodsk-Alekhino ) [ South-East vicinities of Peschanoye lake ( Sernovodsk-Alekhino road)], 8.vii. 2008, lgt. K. Makarov .

Taxonomic notes. There is a single species of Megasternum occurring on the Japanese islands which differs from the continental M. concinnum (Marsham, 1803) by finely but distinctly punctate ventrites 2–5 (impunctate in M. concinnum ). The Japanese species was for the first time recognized and diagnosed as separate from M. concinnum by Sharp (1874), who called it Megasternum gibbulum Motschulsky, 1866 even though he did not examine the Motschulsky’s types and noticed some discordance between his Japanese specimens and Motschulsky’s (1866) short description of M. gibbulum . Sharp’s (1874) interpretation of M. gibbulum was followed by all subsequent Japanese authors. While working on the keys to identification of the Hydrophilidae of Russian Far East, A. G. Shatrovskiy reexamined the types of Megasternum gibbulum in Motschulsky collection, designated the lectotype of this species, correctly recognized that the type specimens do not belong to the genus Megasternum but Pachysternum and realized that the Japanese species of Megasternum is therefore unnamed. For that reason, he described the Japanese species as Megasternum japonicum (see Shatrovskiy 1989, 1992). Unfortunately, the transfer of Megasternum gibbulum to Pachysternum , its synonymization with P. haemorrhoum and the designation of its lectotype was never published, and the reasons for describing Megasternum japonicum remained therefore unexplained. For that reason, Hoshina & Satô (2005) synonymized Megasternum japonicum with M. gibbulum , considering some paratypes of M. japonicum from the Lewis collection in BMNH as being at the same time syntypes of M. gibbulum . All paratypes deposited in BMNH and mentioned by Shatrovskiy (1992) were, however, collected after the description of M. gibbulum by Motschulsky (1866) and therefore cannot be syntypes of the latter species. Hoshina & Satô (2005) did not examine the specimens of M. gibbulum from the Motschulsky collection deposited in ZMUM, which are all conspecific with Pachysternum haemorrhoum (see above under this species). For that reason, the synonymy of Megasternum japonicum with M. gibbulum proposed by Hoshina & Satô (2005) is incorrect, and Megasternum japonicum has to be resurrected as a valid name for the Japanese species of the genus Megasternum .