"Facelinella quadrilineata (Baba, 1930)", a Pair of Cryptic Species of Facelina from Japan

Abstract

One of the two colour variations in Facelinella quadrilineata (Baba, 1930) is found to be an undescribed species, which has been commonly collected from the Pacific coast of Boso Peninsula, central Japan. Although both species are similar enough morphologically to be considered cryptic species, there is a marked difference in the penial structure. The new species possesses an ejaculatory penis, while Facelinella quadrilineata does not. Considering close affinity between the two species and the reasons for separating the two genera of Facelinella and Facelina, invalidity of Facelinella becomes obvious and it is proposed to merge Facelinella into Facelina. The new species is herein described and illustrated as Facelina bilineata and compared with Facelina quadrilineata comb. nov.