Visual specializations and light detection in Chondrichthyes

The visual ecology of representatives of the three groups of Chondrichthyes was analysed and compared to identify retinal and pineal specializations for photopic or scotopic vision in species from different habitats. The development of a new spatial analysis methodology to construct and analyse topographic retinal maps is also described. The typical arrangement of retinal photoreceptors and ganglion cells observed is a dorsal streak that affords the animal a high resolution panoramic view of the lower part of the visual field. The visual system in two species of deep-sea chimaeras: Rhinochimaera pacifica and Chimaera lignaria (rod-only retina specialized for scotopic vision with high sensitivity and high convergence of rods to ganglion cells) was compared to a demersal chimaera species Callorhinchus milii (duplex retina with both rods and cones). The visual system of the gummy shark, Mustelus antarcticus, another demersal species but from the Selachii, is similar to C. milii. Both C. milli and M. antarcticus show specializations to demersal habitats, where vertical migration markedly alters the ambient light conditions. Some photopic specializations (retinal duplicity) persist but the convergence between rods and ganglion cells is high, revealing adaptations for enhanced sensitivity. Five sympatric species of coral-reef dwelling stingrays from the Dasyatidae family (Taeniura lymma, Neotrygon kuhlii, Himantura uarnak, Pastinachus atrus and Urogymnus asperrimus) were compared and revealed specialisations for photopic vision with high numbers of cones and high spatial resolving power, in contrast to the other species of chondrichthyan examined (deep-sea and demersal species). The visual specializations within the stingrays reflect different ecological niches that may have promoted speciation or niche separation between the five sympatric species. An immunohistochemical analysis of cone photopigments using a long wavelength-sensitive (LWS) cone antibody in two species of ray (the bluespotted maskray, Neotrygon kuhlii, and the bluespotted fantail ray, Taeniura lymma) reveals that the proportion of labelled LWS cones to unlabelled cones is higher in N. kuhlii than in T. lymma, which directly correlates to the amount of time spent in open sandy areas of the reef (N. kuhlii) versus resting under rocks and caves (T. lymma). The light conditions in shaded areas of the reef (with lower levels of long wavelength light) versus open, bright areas may place intense selection pressure on the type and density of retinal photopigment expressed within the retina. Immunohistochemical labelling of LWS cones in C. milii (in addition to populations of unlabelled cones) corroborates existing theories of the potential for colour vision. The detection of (non-image forming) light via the pineal organ in N. kuhlii and C. milii reveals a direct correlation between the morphology of the pineal and life history traits. Reproduction in C. milii might be triggered by increases in light intensity, as this species moves into shallow water, that is detected by the pineal, which is well developed compared to N. kuhlii and comprises a vesicle with multiple, long photoreceptors projecting into the lumen underlying a pineal window. The research fills a large gap in the visual ecology of the chimaeras and is the first comparative study of the morphology of the pineal organ between two species from different habitats.