Despite being the world’s largest batoid fish, manta rays have been the focus of very little research in the last century. Almost nothing is currently known about their ecology or distribution throughout the world’s oceans. Manta rays are considered to be near-threatened to vulnerable by the World Conservation Union’s Redlist of endangered species, yet many populations still face threats from target fishing for their fin cartilage, branchial filaments and meat. The taxonomic history of the genus Manta has been questionable and convoluted,with Manta having one of the most extensive generic and species synonymies of any living genus of cartilaginous fish. Currently this genus is considered to be monotypicwith a single recognized species, Manta birostris. Behavioural and morphological data collected during this study showed that there are two distinct, sympatric species of manta ray in Mozambique. This discovery has led to an examination of manta ray distribution worldwide, with the two species hypothesis receiving further support from study sites in the Indian and Pacific Oceans. This thesis examined large populations of both putative species in Mozambique. Development and application of a photographic-identification methodology allowed identification of individuals based on their unique ventral surface markings (spots). The ability to recognise individuals enabled many aspects of both populations to be examined. A total of 449 individual rays were identified of Manta birostris variant A and 101 individual rays were identified of Manta birostris variant B. Both populations exhibited a highly significant female bias in observed sex ratios of 1:3.5 and 1:7.4 respectively. The photographic identification approach was used in studies of population size and structure and to examine the reproductive ecology of these two distinct populations. Population estimates incorporating four years of re-sighting data on Manta birostris variant A and five years of data on Manta birostris variant B were conducted using the program MARK resulting in super population estimates of 890 and 600 individuals respectively. The region encompassing the study site was identified as a mating ground for Manta birostris variant A based on observations of mating events and fresh pectoral fin tip scars on female rays produced when the male rays bite onto the fins during copulation. The distribution of scars was highly biased, with 99% on the left pectoral fin, indicative of a strong lateralised behavioural trait in this species. No other elasmobranch has been reported to display behavioural lateralisation. The study region also acts as a birthing ground, with individuals giving birth in the summer after a gestation period of approximately one year. Reproductive periodicity in M. birostris variant A was most commonly biennial, but a few individuals were seen to be pregnant in consecutive years, confirming an annual ovulatory cycle. The production of a single pup appears to be the normal situation, although observations in the wild as well as during opportunistic dissections of individuals killed in fisheries revealed that two pups are conceived on occasion. Morphometric analysis of late-term foetus (M. birostris variant A) was contrasted with measurements taken from adult rays. These are the only detailed measurements on M. birostris variant A from the western Indian Ocean. Predatory scarring and bite injuries on individuals of both variant A and B were consistent with attacks from sharks of various sizes. The frequency and effect of these predatory injuries on the two populations were examined over a three-year period. Acoustic tags were attached to fourteen rays and their presence/absence at sites around the major inshore reefs was explored to evaluate how the rays utilised their environment on a temporal scale. Cleaning activity of both putative species of manta rays by small fishes occurred on these reefs and was considered to be the main reason why the rays were present at these sites. Parasite removal and wound healing were implicated as the benefits received. Host cleaner fish species partition the manta ray body to avoid interspecific competition while the two putative species appear to partition cleaning habitats, with very little overlap apparent. This research on manta rays is the first of its kind in African waters. Many aspects of the study have contributed to the limited baseline data currently available for this genus. The study additionally provided sufficient empirical evidence to warrant the separation of these two putative species of Manta and a full revision of the genus with the systematic examination of specimens throughout Manta’s range. The results of this study may be useful in this endeavour as diagnostic characteristics have been isolated that may help to differentiate members of this genus. The results of this research study are directly applicable to management strategies for both putative species of manta rays off the East Coast of Africa as well as populations worldwide and have highlighted the potential need for different conservation strategies

Despite being the world’s largest batoid fish, manta rays have been the focus of very little research in the last century. Almost nothing is currently known about their ecology or distribution throughout the world’s oceans. Manta rays are considered to be near-threatened to vulnerable by the World Conservation Union’s Redlist of endangered species, yet many populations still face threats from target fishing for their fin cartilage, branchial filaments and meat. The taxonomic history of the genus Manta has been questionable and convoluted,with Manta having one of the most extensive generic and species synonymies of any living genus of cartilaginous fish. Currently this genus is considered to be monotypicwith a single recognized species, Manta birostris. Behavioural and morphological data collected during this study showed that there are two distinct, sympatric species of manta ray in Mozambique. This discovery has led to an examination of manta ray distribution worldwide, with the two species hypothesis receiving further support from study sites in the Indian and Pacific Oceans. This thesis examined large populations of both putative species in Mozambique. Development and application of a photographic-identification methodology allowed identification of individuals based on their unique ventral surface markings (spots). The ability to recognise individuals enabled many aspects of both populations to be examined. A total of 449 individual rays were identified of Manta birostris variant A and 101 individual rays were identified of Manta birostris variant B. Both populations exhibited a highly significant female bias in observed sex ratios of 1:3.5 and 1:7.4 respectively. The photographic identification approach was used in studies of population size and structure and to examine the reproductive ecology of these two distinct populations. Population estimates incorporating four years of re-sighting data on Manta birostris variant A and five years of data on Manta birostris variant B were conducted using the program MARK resulting in super population estimates of 890 and 600 individuals respectively. The region encompassing the study site was identified as a mating ground for Manta birostris variant A based on observations of mating events and fresh pectoral fin tip scars on female rays produced when the male rays bite onto the fins during copulation. The distribution of scars was highly biased, with 99% on the left pectoral fin, indicative of a strong lateralised behavioural trait in this species. No other elasmobranch has been reported to display behavioural lateralisation. The study region also acts as a birthing ground, with individuals giving birth in the summer after a gestation period of approximately one year. Reproductive periodicity in M. birostris variant A was most commonly biennial, but a few individuals were seen to be pregnant in consecutive years, confirming an annual ovulatory cycle. The production of a single pup appears to be the normal situation, although observations in the wild as well as during opportunistic dissections of individuals killed in fisheries revealed that two pups are conceived on occasion. Morphometric analysis of late-term foetus (M. birostris variant A) was contrasted with measurements taken from adult rays. These are the only detailed measurements on M. birostris variant A from the western Indian Ocean. Predatory scarring and bite injuries on individuals of both variant A and B were consistent with attacks from sharks of various sizes. The frequency and effect of these predatory injuries on the two populations were examined over a three-year period. Acoustic tags were attached to fourteen rays and their presence/absence at sites around the major inshore reefs was explored to evaluate how the rays utilised their environment on a temporal scale. Cleaning activity of both putative species of manta rays by small fishes occurred on these reefs and was considered to be the main reason why the rays were present at these sites. Parasite removal and wound healing were implicated as the benefits received. Host cleaner fish species partition the manta ray body to avoid interspecific competition while the two putative species appear to partition cleaning habitats, with very little overlap apparent. This research on manta rays is the first of its kind in African waters. Many aspects of the study have contributed to the limited baseline data currently available for this genus. The study additionally provided sufficient empirical evidence to warrant the separation of these two putative species of Manta and a full revision of the genus with the systematic examination of specimens throughout Manta’s range. The results of this study may be useful in this endeavour as diagnostic characteristics have been isolated that may help to differentiate members of this genus. The results of this research study are directly applicable to management strategies for both putative species of manta rays off the East Coast of Africa as well as populations worldwide and have highlighted the potential need for different conservation strategies

Worldwide declines in populations of large elasmobranchs and the potential cascading effects on marine ecosystems have garnered considerable attention. Far less appreciated are the potential ecological impacts of changes in abundances of small to medium bodied elasmobranchs mesopredators. Crucial to elucidating the role of these elasmobranchs is an understanding of their habitat use and foraging ecology in pristine conditions. I investigated the trophic interactions and factors driving spatiotemporal variation in abundances of elasmobranch mesopredators in the relatively pristine ecosystem of Shark Bay, Australia. First, I describe the species composition and seasonal habitat use patterns of elasmobranch mesopredator on the sandflats of Shark Bay. Juvenile batoids dominated this diverse community and were extremely abundant in nearshore microhabitats during the warm season. Stomach content analysis and stable isotopic analysis revealed that there is a large degree of dietary overlap between common batoid species. Crustaceans, which tend to be found in seagrass habitats, dominated diets. Despite isotopic differences between many species, overlap in isotopic niche space was high and there was some degree of individual specialization. I then, investigated the importance of abiotic (temperature and water depth) and biotic (prey and predator abundance) factors in shaping batoid habitat use. Batoids were most abundant and tended to rest in shallow nearshore waters when temperatures were high. This pattern coincides with periods of large shark abundance suggesting batoids were seeking refuge from predators rather than selecting optimal temperatures. Finally, I used acoustic telemetry to examine batoid residency and diel use of the sandflats. Individual batoids were present on the sandflats during both the warm and cold seasons and throughout the diel cycle, suggesting lower sandflat densities during the cold season were a result of habitat shifts rather than migration out of Shark Bay. Combined, habitat use and dietary results suggest that batoids have the potential to seasonally impact sandflat dynamics through their presence, although foraging may be limited on the sandflats. Interestingly, my results suggest that elasmobranch mesopredators in pristine ecosystems probably are not regulated by food supply and their habitat use patterns and perhaps ecosystem impacts may be influenced by their predators.

Longline fishing is the most common elasmobranch capture method in the world, both for commercial fishing, and to a lesser extent for scientific surveys. The capture of an animal on a longline initiates a series of physiological responses designed to promote survivorship in the short term, but if unchecked, can cause reduced individual fitness and/or mortality in the long term. Given widespread declines in shark populations, an improved understanding of the physiological costs of longline capture is needed. The aim of this thesis was to investigate the physiological response of sharks to capture and restraint, to assess novel, non-invasive alternatives to scientific longline surveys, and to generate scientific insight into poorly understood elasmobranch populations in The Bahamas. The results presented herein suggest that some species of shark are able to recover from the physiological stress of capture despite the presence of persistent negative stimuli. Tonic immobility was assessed as a means of generating baseline blood chemistry data, but was found to be inappropriate given that it increases the magnitude of physiological perturbation in the short term. To avoid the stress of capture altogether, Baited Remote Underwater Video Surveys (BRUVS) were considered as a non-invasive alternative to capture based surveys, however, it was concluded that they lack the resolution necessary to answer fine scale demographic questions. For the Caribbean reef shark, longline surveys yielded high resolution data allowing the identification of fine scale spatiotemporal shifts in demographic population structure with minimal cost (mortality). Nevertheless, the ethics of using capture based surveys on sensitive species are questionable when alternative techniques are available. Deep water sharks caught on longline surveys exhibited high mortality rates, however, for these very poorly understood species moribund specimens have great scientific value which in some cases can offset the high ecological costs of the surveys. The results presented in this thesis highlight the on-going need for improved biological and ecological research into the majority of elasmobranch populations, particularly with regards to anthropogenic interactions such as capture. Given the tenuous conservation status of many species, the acquisition of applied, management focused data should remain the priority of elasmobranch scientists.

The ability of an organism to modulate its feeding behavior is an important focus of feeding ecology studies. Modulation is the ability to distinctly and consistently alter a behavior to accommodate different stimuli. The goal of this study was to examine the ability of the nurse shark Ginglymostoma cirratum to modulate its food capture behavior with different sizes and types of food items. This was carried out through kinematic and electromyographic analysis. Eight sub-adult specimens of G. cirratum were filmed feeding on two different food types (squid and fish) and sizes (gape size and larger than gape size). Filming consisted of high-speed videography utilizing a low-light digital video system. Kinematic variables related to lower jaw movement, mouth width, and head angle were measured from video footage. Up to twelve muscles in each of six specimens were implanted with bipolar electrodes to measure the onset and duration of motor activity. There were no significant differences between food sizes and any of the kinematic variables. Only two muscles showed significant differences in onset time based on food size. In regards to food types, squid bites were significantly faster than fish bites, but when examined proportionately to bite duration only the time to jaw closure remained significantly different. The motor pattern of G. cirratum demonstrates an anterior to posterior sequence, which corresponds to the anterior to posterior kinematic sequence. Little cranial elevation is present during feeding sequences and is not thought to contribute significantly to feeding. Ginglymostoma cirratum is a stereotyped, inertial suction feeder. There is little evidence that there is modulation in feeding behavior based on food size or food type. If modulation does exist in the feeding behavior, it is more likely to occur after prey capture while the prey is being processed and manipulated prior to transport. Initial observations suggested that a novel behavior termed 'spit-suck manipulation' is utilized for larger prey items.

Cephalic lobes are unique structures derived from the anterior pectoral fins, found in select myliobatid stingrays. Many benthic batoids utilize undulatory locomotion and use their pectoral fins for both locomotion and prey capture. Pelagic myliobatids that possess cephalic lobes utilize oscillatory locomotion, using their pectoral fins to locomote and their cephalic lobes for prey capture. Despite differences in habitat usage and locomotor modes, these batoids feed on very similar benthic organisms. The purpose of this study was to 1.) compare the morphology of the cephalic lobes and anterior pectoral fins in lobed and lobeless species, looking at skeletal elements, musculature and electrosensory pore distributions; 2.) compare prey capture kinematics in lobed and lobeless species and examine the role of the cephalic lobes in prey capture modulation due to elusive/non-elusive prey; 3.) analyze multiple morphological and behavioral variables to establish any correlations to the presence of cephalic lobes. Radiography, dissections and staining techniques were employed to examine the morphology of the cephalic lobes and anterior pectoral fins in six species of batoids. High speed videography was used to film prey capture behavior in five batoid species, using elusive and non-elusive prey. Continuous morphological and behavioral variables were used to determine any correlations with the presence of the cephalic lobes, taking phylogeny into account. Results indicate that the skeletal components of the pectoral fins of oscillatory species are very different from pectoral fins of undulatory species as well as the cephalic lobes. Second moment of area (I), showed that the cephalic lobes and pectoral fins in undulatory species had greater resistance to bending in multiple directions and were also more flexible. The cephalic lobes had a novel muscle layer compared to the pectoral fin musculature. Electorsensory pores were absent from the anterior pectoral fins in oscillatory batoids, but numerous on the cephalic lobes and anterior pectoral fins in undulatory batoids. The distribution of the electrosensory pores was uniform with the exception of Rhinoptera bonasus, which possessed higher pore numbers along the edges of the cephalic lobes. Overall, the morphology of the cephalic lobes is distinct, but more similar to the pectoral fins of undulators compared to oscillators. Kinematic data showed that species with cephalic lobes localize prey capture to the cephalic region of the body. Lobed species were faster at pouncing and tenting prey, but slower during biting. The cephalic lobes were able to move more in the vertical and horizontal plane compared to the anterior pectoral fins. All species were able to modulate prey capture behavior to some degree. Species lacking lobes spent more time handling elusive prey compared to non-elusive prey. For all species, elusive prey were farther from the mouth during biting but prey escapes were rare. Lobed species were overall faster in prey capture, but did not display more modulation or feeding success than lobeless species. Phylogenetically corrected correlations showed that most morphological variables correlated to the appearance of the cephalic lobes, while kinematics variables did not. There was also a correlation among habitat, locomotion and the cephalic lobes. The cephalic lobes may have played a key role in partitioning prey capture to the head region, maintaining dexterity in the lobes while allowing the pectoral fins to shift to oscillatory locomotion and consequently a pelagic lifestyle.

Elasmobranchs (sharks, skates and rays) possess a sophisticated array of sensory systems that are, undoubtedly, of great importance to their survival. Representing the earliest group of extant jawed vertebrates, detailed study of elasmobranch sensory biology can provide much-needed information on the evolution of vertebrate sensory systems. Some sensory modalities have been studied in detail in several species, but few studies have examined and compared multiple sensory systems within a particular genus. By examining the morphology and physiology of the different sensory systems, correlations can be made within both an ecological and a phylogenetic context. The primary advantage of studying the sensory systems of closely related species is that any differences between them are more likely to reflect functional ecological adaptations rather than the effects of phylogenetic separation. Wobbegongs sharks (Orectolobidae) are a distinctive group of benthic sharks that are characterised by a highly patterned, dorso-ventrally compressed body. Wobbegongs are ambush predators that employ a unique ‘sit and wait’ strategy. Their morphologically distinct body shape, sedentary lifestyle and mode of predation suggest that wobbegong sharks may differ from other elasmobranchs in how they employ their different sensory systems. In this study, four wobbegong species that vary in life-history and/or habitat were examined: the Western wobbegong, Orectolobus hutchinsi, the spotted wobbegong, O. maculatus, the ornate wobbegong, O. ornatus and the dwarf spotted wobbegong, O. parvimaculatus. Vision and olfaction were assessed in all four species. Detailed assessment of electroreception and mechanoreception (lateral line) was conducted only for O. maculatus and O. ornatus. Morphology, physiology and molecular genetics were examined in the visual system, and morphological assessment was conducted for the olfactory, electroreceptive and mechanosensory lateral line systems. The retinae of all four wobbegong species are duplex; rod and cone photoreceptors can be distinguished easily on the basis of morphology. The wavelength of maximum absorbance (λmax) of the rod visual pigment is 496 nm in O. hutchinsi, 484 nm in O. maculatus, 498 nm in O. ornatus and 494 nm in O. parvimaculatus. Absorbance spectra of cone visual pigments were only obtained from O. maculatus and O. ornatus. Only one spectral type of cone was measured in each species, with max values at 553 nm and 560 nm, respectively. Partial sequences were obtained for the rh1 opsin gene in all four species, and for the lws opsin gene in every species except O. parvimaculatus. The apparent presence of only one cone pigment raises the possibility that wobbegongs do not have colour vision. The topographic distribution of cells within the ganglion cell layer of Orectolobus hutchinsi show a weakly elongated central visual streak of increased cell density, mediating a higher spatial resolving power of 2.06 cycles deg-1 in the frontal visual field. Retinal topography of O. maculatus and O. parvimaculatus are similar, with both possessing a dorsal horizontal streak facilitating increased spatial resolving power in the lower visual field. Orectolobus parvimaculatus also possesses an area of increased cell density in the naso-ventral region of the retina mediating acute vision in the upper caudal region of the visual field. Spatial resolving power reaches 3.51 cycles deg-1 and 3.91 cycles deg-1 in O. maculatus and O. parvimaculatus, respectively. The topographical variation in retinal sampling indicates that different regions of the visual field are relatively more important and may reflect interspecific differences in behaviour and habitat. The mean number of lamellae in the olfactory rosette is 47.0 for Orectolobus hutchinsi, 48.7 for O. maculatus, 40.7 for O. ornatus and 55.7 for O. parvimaculatus. Olfactory sensory epithelial surface area is comparable in O. hutchinsi, O. maculatus and O. ornatus, while O. parvimaculatus has a significantly larger surface area, relative to body size, compared to the other three species. Olfaction appears to be relatively more important in O. parvimaculatus, especially during low light conditions, when vision is limited. The distribution of ampullary electroreceptive pores and mechanosensory lateral line pores (pored and non-pored canals) is almost entirely concentrated on the dorsal region of the head in both O. maculatus and O. ornatus. This suggests that both sensory systems are well-adapted and specialised to detect prey swimming overhead when the wobbegong is sitting motionless, thereby facilitating its unique, predatory, “lie-in-wait’ ambush strategy. Orectolobus hutchinsi and O. ornatus appear to be well-suited to both diurnal and nocturnal activities, whereas O. maculatus and O. parvimaculatus are probably most active under low light conditions. Sensory system information inferred from this study correlates well with what is known of the diet and habitat of the four wobbegong species examined. Therefore, in the absence of other biological data, sensory neurobiological approaches can be used to predict such bio-ecological factors as predatory strategy, habitat preference, and behaviour. Electrophysiology and behavioural approaches will provide major advances in future studies in order to understand how each of the different senses is integrated at both peripheral and central levels and how such studies are vital to our understanding of evolutionary and ecological processes.

The elasmobranch fishes possess a remarkable diversity of feeding mechanisms for a group containing relatively few species (~1200). The three most prevalent of these mechanisms involve prey capture during which the predator overtakes its prey (ram), prey is drawn into the mouth of the predator (suction), and relatively stationary consumption of sessile or substrate affixed prey (biting). Biomechanical modeling of cranial force distributions, in situ bite performance trials, and kinematic analysis of prey capture behaviors were employed to identify morphological and behavioral specializations and constraints associated with these feeding mechanisms in lemon Negaprion brevirostris (ram), whitespotted bamboo Chiloscyllium plagiosum (suction), and horn Heterodontus francisci (biting) sharks. Biomechanical modeling of the forces generated by the cranial musculature was used to theoretically estimate the maximum bite force and mechanical loadings occurring throughout the hyostyl ic jaw suspension mechanisms of each species, characterized by suspensory hyomandibular cartilages between the back of the jaws and cranium and anterior ligamentous attachments. To assess the mechanical factors involved in the evolution of elasmobranch jaw suspension mechanisms, the feeding mechanism of the sharpnose sevengill shark Heptranchias perlo was modeled as well. Heptranchias perlo possesses an ancestral amphistylic jaw suspension mechanism including non-suspensory hyomandibular cartilages, a large post-orbital articulation between the jaws and cranium, and anterior ligamentous attachments. Theoretical estimates of maximum bite force were compared to voluntary bite forces measured during in situ bite performance trials. Voluntary bite force measurements allowed the quantification of discrete behavioral attributes of bite force application in each species. To further assess the behavioral specializations associated with these feeding mechanisms, high-speed digital videography w as used to analyze the prey capture cranial kinematics of species. Collectively, these analyses have developed a morphological and behavioral basis from which to understand the functional diversity of the ram, suction, and biting feeding mechanisms in elasmobranchs.

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.

1) Both the partially euryhaline Scyliorhinus canicula and the fully euryhaline Carcharhinus leucas significantly modify plasma concentrations of urea and chloride (Cl-) (and sodium (Na+)) in response to changes in environmental salinity, in order to maintain overall plasma osmolality slightly hyper- or isosmotic to the environment. C. leucas has a greater capacity for urea retention in dilute environments. In S. canicula all of these changes occur within 12 hours of transfer, with the notable exception of increasing plasma urea in response to acute transfer to elevated salinity. 2) A new technique, 51Cr-labelled erythrocytes, was developed to assess blood volume in elasmobranch fish. S. canicula displays significant haemodilution and concentration during chronic acclimation to decreased and increased environmental salinity respectively. Significant changes in blood volume were seen within 6 hours of acute salinity transfer. 3) In vivo secretion rates were measured in the rectal gland of S. canicula during both chronic and acute salinity transfer. Significant changes in Cl- clearance occur during acute transfer, as plasma Na+ and Cl- levels are modified, but do not persist in chronically acclimated animals. This is achieved through modifications in the volume and Cl- concentration of the secretory fluid. 4) C. leucas is able to significantly alter the abundance and/or recruitment of Na+, K+-ATPase in both the rectal gland and the kidney during chronic acclimation to salinity transfer. This is presumably in response to increased requirements for NaCl secretion in SW and osmolyte retention in FW respectively. S. canicula do not significantly alter abundance and/or recruitment of Na+, K+-ATPase in the principle osmoregulatory organs following chronic acclimation to salinity transfer. 5) Chronically SW acclimated C. leucas modify the proportion of ouabain-sensitive oxygen consumption in the tissues of the rectal gland in response to the secretory endocrine stimulus C-type natriuretic peptide (CNP). No such modification occurred in the rectal glands of FW acclimated C. leucas. This represents a change in the sensitivity and response to endocrine control factors during chronic acclimation to salinity transfer in this species. No such modification was seen the in the proportion of ouabain-sensitive oxygen consumption in the rectal glands of chronically acclimated S. canicula in response to CNP. These results were discussed in relation to the capacity for modification of osmoregulatory organs in partially and fully euryhaline elasmobranchs.