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Ontogenetic changes in the visual system of the brown banded bamboo shark, Chiloscyllium punctatum (Elasmobranchii), with special reference to husbandry and breeding

Developmental studies on elasmobranchs are challenging due to the difficulties in obtaining sufficient numbers of animals of different age cohorts. The brown banded bamboo shark, Chiloscyllium punctatum is a good model in this regard as it is abundant and readily available in the wild, is quick to mature, is frequently housed and bred in captivity and is a relatively small species of shark. Whilst there are important factors that must be considered when comparing the retinal development of animals raised in captivity to those caught from the wild, the use of C. punctatum represents an outstanding opportunity to study the development of the elasmobranch visual system from pre-hatching embryonic to adult life stages. In this study, the developing eye and retina of C. punctatum were studied using light and electron microscopy, electroretinography (ERG) and microspectrophotometry (MSP). To provide a source of early-stage animals, and to investigate the effects of environmental factors (such as temperature) on physical development, a captive breeding program was established at the University of Queensland. Sharks sourced from this facility were supplemented with animals bred at UnderWater World, Sea World and caught from the wild. Monitoring the fecundity, embryonic development, growth and viability of captive C. punctatum showed that females lay an average of 115.3 eggs, 38 of which were viable and 21.4% of which hatched. Embryos have an average gestation of 153 days post deposition (dpd; temp: 21 - 25º C) and embryonic growth is most rapid from 99 dpd until hatching. The eye of C. punctatum develops early in embryogenesis, with visible optic vesicles bulging at 27 dpd. Recent advances in fixation and processing techniques for transmission electron microscopy (TEM) have yielded improved levels of ultrastructural detail in a variety of tissue types. Consequently, in addition to conventional chemical fixation (CF) methods, the retina of C. punctatum was also processed using microwave chemical fixation (MCF) and high pressure freezing (HPF), and the resulting ultrastructure compared. Both MCF and HPF produced superior retinal ultrastructure compared to conventional CF, evidenced by higher resolution of ultrastructural detail and fewer artefacts. MCF provided the best, consistent ultrastrucutral results. By examining the time-course of retinal cell differentiation, it was found that ganglion and Müller cells are the first to differentiate, at approximately 81 dpd. The interneurons differentiate next, beginning with the amacrine cells (81 dpd), followed by the bipolar cells (101 dpd) and horizontal cells (124 dpd). The adult retina is duplex and rod and cone photoreceptors are differentiated and synaptic connections are formed by 124 dpd. Topographic analysis of retinal neuron sub-types reveals that C. punctatum undergoes rapid changes in ganglion cell distribution during embryogenesis. High levels of apoptosis, especially around the retinal periphery, result in relatively higher cell densities in the central retina, which progressively extend nasally and temporally to form a meridional band. C. punctatum develops a horizontal streak and shows only minor changes in topography during growth. Only basal levels of apoptosis are seen post-hatching. In the adult shark, the total ganglion cell number reaches 547,881. The mean and highest retinal ganglion cell densities reach a peak around hatching (3,228 cells mm-2 and 4,983 cells mm-2, respectively). Using measurements of lens focal length and ganglion cell density, the calculated maximum spatial resolving power (assuming a hexagonal mosaic) increases from 1.47 cycles degree-1 during embryogenesis to 4.29 cycles degree-1 in adults. The addition of a high ganglion cell density area within the visual streak and an increasing spatial resolving power over post-hatching development suggest an increased prey targeting and capture ability for this species. Using ERG, it is shown that C. punctatum becomes responsive to light at 127 dpd and light sensitivity peaks around the time of hatching, with a slight decrease post-hatching. C. punctatum maintains a flicker fusion frequency (FFF; an indicator for temporal resolution) at 7 - 22 Hz through juvenile stages), which is relatively low compared to other marine predators. ERG results suggest that this species is adapted to low light vision with low temporal resolution. The early differentiation, development and functionality of the visual system in C. punctatum allows for a period of synaptic maturation and potentially the ability of embryonic predator avoidance. The retina of C. punctatum contains a rod visual pigment with a wavelength of maximum absorbance (λmax) at 500 nm and cone visual pigment with a λmax at 532 nm; the max values of these pigments do not change during development. Rod and cone outer segments differentiate at 113 days post deposition (dpd), lengthen during embryogenesis and accumulate pigment throughout life. Although the photoreceptors develop and differentiate well in advance of hatching, there is considerable variation in outer segment length and pigment density during embryogenesis, which suggests that these cells are developing up until hatching. C. punctatum does not appear to have the potential for colour vision based on the lack of two cone photoreceptor types each containing a visual pigment maximally sensitive to different parts of the visual spectrum, but appears specialised for dim-light contrast vision.