Redescription of Aphalloides coelomicola Dollfus, Chabaud & Golvan, 1957 (Digenea, Opisthorchioidea) based on specimens from Knipowitschia caucasica (Berg) (Actinopterygii, Gobionellidae) from a Black Sea lagoon, with comments on the systematic position of the genus.

The genus Aphalloides Dollfus, Chabaud & Golvan, 1957 consists of two species parasitic in the body cavity of sand gobies. Its systematic position in the superfamily Opisthorchioidea Looss, 1899 is unresolved and it has been placed by various authors in three families, i.e. Cryptogonimidae Ward, 1917, Heterophyidae Leiper, 1909 and Opisthorchiidae Looss, 1899. Its type-species, Aphalloides coelomicola Dollfus, Chabaud & Golvan, 1957, is here reported from the Caucasian dwarf goby Knipowitschia caucasica (Berg) in the lagoon Atanasovsko Lake, Black Sea coast of Bulgaria (new geographical record). The species is redescribed based on light and scanning electron microscopy demonstrating some characters typical for the Cryptogonimidae but also characters distinguishing it from the other genera of the family such as the lack of tegumental spines and the presence of a short excretory vesicle, which does not extend into the forebody. Phylogenetic analysis of the D2-D3 expansion segments of the 28S rRNA gene suggests phylogenetic relationships of Aphalloides coelomicola with the cryptogonimid Centrovarium lobotes (MacCallum, 1895). These data support the affiliation of the genus Aphalloides to the family Cryptogonimidae. The peculiar morphology of the species in the genus is explained by their unusual life-cycles characterised by progenetic development; sand gobies being simultaneously second intermediate and definitive hosts.