Diploma Thesis

"Vergleichend-morphologische Untersuchungen an den Augen coleoider Cephalopoda"

Coleoid cephalopod retinae - a comparative morphological approach

We investigated the retinae of four coleoid cephalopod species collected from the Mediterranean off Banyuls-sur-mer (France), two from shallow water: Octopus vulgaris and Sepia officinalis and two from greater depths: Eledone cirrhosa and Sepietta neglecta. Eyes were excised, fixed in buffered glutaraldehyde and subsequently each retina was dissected into 24 pieces. The retina fragments were bisected, embedded in epoxy resin and cut in radial and tangential direction respectively. Semithin sections were used to demonstrate retinal histology, to measure layer extensions and to determine photoreceptor densities. Ultrathin sections were made to investigate fine structures. To plot retinal maps of various morphological characters (e.g. distal segment length and photoreceptor density) the polar coordinates of each measured value were determined and isolines were calculated after linear interpolation with a self-made computer algorithm (IDL).

Longitudinal sections through the retinae of all examined species show the characteristic stratification of the photoreceptors with proximal and distal segments, supporting cell nuclei in between and the plexiform layer close to the eye cartilage (see Yamamoto et al. 1965). The length of distal segments (containing the rhabdomers) varies strikingly with retinal location, culminating in a horizontal streak near the eye-equator. Tangential sections at the level of the distal segments show the photoreceptor profiles for enumeration. The corresponding density maps show horizontal streaks or an area of increased density, more or less concurrent with the topography of distal segment lengths.