The ectopic compound ommatidium-like pallial eyes of three species of Mediterranean (Adriatic Sea) Glycymeris (Bivalvia: Arcoida). Decreasing visual acuity with increasing depth?

The undescribed ommatidium‐like ectopic pallial eyes of three Mediterranean species of Glycymeris have been studied and compared with those of the eastern Atlantic G. glycymeris. Wide interspecific variation in eye size between the taxa is recorded with those of G. bimaculata being the largest, G. nummaria the smallest and G. pilosa and G. glycymeris of intermediate and similar dimensions. Such differences result primarily from interspecific variations in the numbers and sizes of the ommatidium‐like rhabdomes. The consequences of this will be the differences in abilities to create a summed image and thereby reduce visual acuity. Other bivalves, such as the Pectinoidea, have shown eye size differences in relation to depth. Glycymeris bimaculata, with the largest eyes, occupies the shallowest depths; G. nummaria bears the smallest eyes and occupies the deepest depths. The genetically sister clusters of G. pilosa and G. glycymeris occupy intermediate depths. Accurate depth records for the four taxa blur this hypothesised trend, but comparative eye dimensions suggest it to be valid.