Clarification of the identity of the snapping shrimp Alpheus lobidens De Haan, 1849, revalidation of A. crassimanus Heller, 1862, and descriptions of four new species from Japan previously confused with A. lobidens (Decapoda: Caridea: Alpheidae)

Alpheus lobidens De Haan, 1849 (type locality: Japan) and A. crassimanus Heller, 1862 (type locality: Nicobar Islands, Andaman Sea), two taxonomically challenging snapping shrimps previously regarded as conspecific, are rediagnosed based on the holotype and on the newly collected material from the Andaman Sea, respectively. Most importantly, the female holotype of A. lobidens has a balaeniceps ridge on the pollex of the minor chela, whereas in A. crassimanus, the minor cheliped fingers of females are simple, devoid of balaeniceps setae. Other characters diagnostic to A. lobidens include the laterally compressed rostrum reaching the distal margin of the first article of the antennular peduncle, the highly elevated rostral carina widening posteriorly, and the broad, deeply depressed adrostral furrows. These features set A. lobidens apart from other taxa previously referred to A. lobidens or A. lobidens complex in literature. Alpheus polynesicus Banner & Banner, 1975, originally described as a subspecies of A. lobidens and later synonymised with A. lobidens, is reinstated as a full species based on evidence from literature. Four new species, which have been confused with A. lobidens, are described from Japan, based on molecular and morphological data, as well as their colour patterns in life: A. yoshigoui sp. nov., A. nigrofasciatus sp. nov., A. latifasciatus sp. nov., and A. aestuaricola sp. nov. These new species are now referred to the A. crassimanus species complex and compared with allied species, including those from outside the Indo-West Pacific. The Japanese records of A. heeia Banner & Banner, 1975 are all referred to A. nigrofasciatus sp. nov. Molecular phylogenetic analyses using two mitochondrial markers (16S rRNA and COI genes) have shown that two morphologically distinct morphs distinguished by the presence or absence of sclerotised setae on the mesial face of the third maxilliped carpus (= penultimate article) belong to the same species, A. nigrofasciatus sp. nov. The taxonomic identities of several lineages of Alpheus recovered in the phylogenetic analyses remain unresolved due to difficulties in accessing voucher specimens. References to A. lobidens and A. crassimanus have been reviewed as thoroughly as possible.