VISUAL BEHAVIOR, EYE AND RETINA OF THE PARASITIC FISHCARAPUS MOURLANI

Abstract

1. This is the first transmission electron microscope description of the retina of a parasitic fish. 2. The retina is well supplied with capillaries, some of which occur on the inner surface of the retina. An unusual and possibly primitive type of capillary junction is described. 3. The outer segments of the retina of Carapus mourlani, which measure approximately 2 micron in diameter, are of one type only. A certain degree of banking was observed. Outer segments are not isolated from each other by screening pigments; they are, however, surrounded by about 15 calycal processes. 4. Typical spheroids or conoids were not seen in the outer plexiform layer. The inner nuclear layer consists of only one or two layers of nuclei. The inner plexiform layer is unusually wide and some amacrine synapses can be identified. There are few ganglion cells and the ratio of nuclei in the outer nuclear layer to those of the inner nuclear layer to ganglion cells is approximately 100:10:1. 5. The eye of C. mourlani combines features of degenerated photoreceptors, characteristic of cave-organisms, with adaptations which are commonly found in nocturnal and deep-sea forms. 6. The visual behavior of the animal indicates that the eye is fully functional in spite of its corneal keratosis and small number of optic nerve fibers.

First Record of a Pearlfish, Carapus mourlani, Inhabiting the Aplysiid Opisthobranch Mollusc Dolabella auricularia1

“…The ability of Carapus mourlani to feed under subdued light conditions demonstrates a capacity to detect and capture rapidly moving prey at night. Meyer- Rochow and Tiang (1978) showed that the eyes of C. mourlani are fully functional and concluded (p. 583) ''that the eye of C. mourlani is basically a photoreceptor designed for vision in a dark environment characterized by high sensitivity and poor resolution, but that its visual performance drops with age.'' It is also possible that the swimming activities and pressure changes generated by potential prey serve to alert C. mourlani of a feeding opportunity.…”

Further insight on carapid?holothuroid relationships

“…Sight could also play a role, but vision does not appear as important as chemical stimuli. Aronson and Mosher (1951) demonstrated that experimentally blinded C. bermudensis are able to locate their host, while Meyer-Rochow and Tiang (1978) noted that the eyes of C. mourlani have characteristics similar to those of cavernicolous or deep-sea organisms. The present study indicates the capacity of C. boraborensis to locate its host thanks to chemical stimuli.…”

Review of larval and postlarval eye ultrastructure in the lamprey (cyclostomata) with special emphasis onGeotria australis (gray)

“…The thickness of the cornea alone does not necessarily preclude visual perception (Meyer-Rochow and Tiang, 1978). The larval dioptric structures probably transmit some light, at least of the longer wavelengths, given that light of these wavelengths has been found to penetrate into the brains of various vertebrates (Hartwig and Van Veen, 1979).…”