Comparative morphology of the eyes of Sagitta (Chaetognatha) in relation to depth of habitat.

A survey of the eye structure in 10 species of Sagitta (phylum Chaetognatha) which differ in habitat was carried out: 5 epipelagic, 4 mesopelagic and 1 bathypelagic species. Paying attention to the dimension of the pigment cell, and assuming that the perforated lamellae are the photoreceptive regions (PRs), we classified the eyes of the 10 species into 5 types. In type I and II eyes there is one large pigment cell. The maximum length of the pigment cell relative to that of the eye (PC/E) is more than 30% in dorsal view and more than 50% in transverse sections. The pigment cell is surrounded by a wide area of the PR that we designated "central PR". The type II eye possesses, in addition, near the periphery of the eye, masses of PRs which measure about 15% of the central one in size. We call these positionally separated PRs "peripheral PR". In type III eyes the pigment cell and the central PR are small (PC/E is about 10% in dorsal view and less than 40% in transverse sections) and the peripheral PRs are scattered. In type IV eye the pigment cell is also small and the central PRs extend to the periphery. In type V eye the pigment cell is absent and the PR occupies a wide area of the dorsal half of the eye. The type I and II eyes were found mostly in epipelagic species, the type III and IV, in mesopelagic species, and the type V, in bathypelagic species. The adaptational significance of the small pigment cells, the peripheral PRs, and the pigmentless eye is discussed. In addition, we report a new type of photoreceptive cell in mesopelagic Sagitta zetesios, in which two receptoral processes emerge from single cells, in contrast to one in all the other species of Sagitta.