Susan M. Theiss

Vision in elasmobranchs and their relatives : 21st century advances

This review identifies a number of exciting new developments in the understanding of vision in cartilaginous fishes that have been made since the turn of the century. These include the results of studies on various aspects of the visual system including eye size, visual fields, eye design and the optical system, retinal topography and spatial resolving power, visual pigments, spectral sensitivity and the potential for colour vision. A number of these studies have covered a broad range of species, thereby providing valuable information on how the visual systems of these fishes are adapted to different environmental conditions. For example, oceanic and deep-sea sharks have the largest eyes amongst elasmobranchs and presumably rely more heavily on vision than coastal and benthic species, while interspecific variation in the ratio of rod and cone photoreceptors, the topographic distribution of the photoreceptors and retinal ganglion cells in the retina and the spatial resolving power of the eye all appear to be closely related to differences in habitat and lifestyle. Multiple, spectrally distinct cone photoreceptor visual pigments have been found in some batoid species, raising the possibility that at least some elasmobranchs are capable of seeing colour, and there is some evidence that multiple cone visual pigments may also be present in holocephalans. In contrast, sharks appear to have only one cone visual pigment. There is evidence that ontogenetic changes in the visual system, such as changes in the spectral transmission properties of the lens, lens shape, focal ratio, visual pigments and spatial resolving power, allow elasmobranchs to adapt to environmental changes imposed by habitat shifts and niche expansion. There are, however, many aspects of vision in these fishes that are not well understood, particularly in the holocephalans. Therefore, this review also serves to highlight and stimulate new research in areas that still require significant attention.

SEMAT — The Next Generation of Inexpensive Marine Environmental Monitoring and Measurement Systems

There is an increasing need for environmental measurement systems to further science and thereby lead to improved policies for sustainable management. Marine environments are particularly hostile and extremely difficult for deploying sensitive measurement systems. As a consequence the need for data is greatest in marine environments, particularly in the developing economies/regions. Expense is typically the most significant limiting factor in the number of measurement systems that can be deployed, although technical complexity and the consequent high level of technical skill required for deployment and servicing runs a close second. This paper describes the Smart Environmental Monitoring and Analysis Technologies (SEMAT) project and the present development of the SEMAT technology. SEMAT is a “smart” wireless sensor network that uses a commodity-based approach for selecting technologies most appropriate to the scientifically driven marine research and monitoring domain/field. This approach allows for significantly cheaper environmental observation systems that cover a larger geographical area and can therefore collect more representative data. We describe SEMATs goals, which include: (1) The ability to adapt and evolve; (2) Underwater wireless communications; (3) Short-range wireless power transmission; (4) Plug and play components; (5) Minimal deployment expertise; (6) Near real-time analysis tools; and (7) Intelligent sensors. This paper illustrates how the capacity of the system has been improved over three iterations towards realising these goals. The result is an inexpensive and flexible system that is ideal for short-term deployments in shallow coastal and other aquatic environments.

Morphological indicators of olfactory capability in Wobbegong sharks (Orectolobidae, Elasmobranchii).

Elasmobranchs are thought to possess an acute sense of smell, but the relationship between the anatomy of their olfactory organs and their sensory ecology is poorly understood. Moreover, the ecological diversity of elasmobranchs as a group indicates that there might be considerable interspecific variation in the importance of the olfactory sense. Wobbegong sharks, with their sedentary lifestyle and ambush predatory technique, probably utilize their senses differently than other shark species, making it difficult to generalize about their olfactory capabilities and olfaction-dependent behaviors. In this study, the number of olfactory lamellae and the surface area of the olfactory epithelium were measured as a means of assessing relative olfactory sensitivity in four species of wobbegong shark (the Western wobbegong, Orectolobus hutchinsi; the spotted wobbegong, O. maculatus; the ornate wobbegong, O. ornatus; and the dwarf spotted wobbegong, O. parvimaculatus). We also present a phylogenetic comparative analysis between wobbegongs and other elasmobranchs for which published data on olfactory morphology are available. There is a significant difference in the total number of olfactory lamellae between most species, but not between O. hutchinsi and O. maculatus, although the olfactory sensory surface area is comparable between these two species and O. ornatus. Orectolobus parvimaculatus has a significantly larger olfactory sensory surface area than the other three species, and there is a positive relationship between total body length and olfactory sensory surface area for all four species. Assuming that these morphological measures are true indications of olfactory capability, the olfactory abilities of wobbegongs are as good as, or better than, other benthic elasmobranchs. Interspecific differences in olfactory ability within this group of benthic ambush predators could indicate relative differences in prey detection, intraspecific recognition and mate detection.

Cone monochromacy and visual pigment spectral tuning in wobbegong sharks

Much is known regarding the evolution of colour vision in nearly every vertebrate class, with the notable exception of the elasmobranchs. While multiple spectrally distinct cone types are found in some rays, sharks appear to possess only a single class of cone and, therefore, may be colour blind. In this study, the visual opsin genes of two wobbegong species, Orectolobus maculatus and Orectolobus ornatus, were isolated to verify the molecular basis of their monochromacy. In both species, only two opsin genes are present, RH1 (rod) and LWS (cone), which provide further evidence to support the concept that sharks possess only a single cone type. Examination of the coding sequences revealed substitutions that account for interspecific variation in the photopigment absorbance spectra, which may reflect the difference in visual ecology between these species.

Interspecific visual adaptations among wobbegong sharks (Orectolobidae).

Several visual traits have previously been assessed in elasmobranchs; however, few studies have examined and compared multiple visual attributes within a particular genus. The primary advantage of studying closely related species is that any differences between them are more likely to reflect functional ecological adaptations rather than the effects of phylogenetic separation. In this study, the visual capabilities of 4 wobbegong shark species, which 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). The retinae of all 4 wobbegong species are duplex; rod and cone photoreceptors can be distinguished easily on the basis of morphology. Some variation in relative eye size exists, with O. parvimaculatus possessing the largest eyes. The topographic distribution of cells within the ganglion cell layer of O. hutchinsi reveals 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 is similar, with both possessing a dorsal horizontal streak facilitating an increased spatial resolving power of 3.51 cycles deg–1 and 3.91 cycles deg–1, respectively, in the lower visual field. O. 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. While all 4 species have visual systems optimised for increased visual sensitivity, O. maculatus and O. parvimaculatus appear to be particularly well suited to activity under low light conditions.

The distribution and abundance of electrosensory pores in two benthic sharks: A comparison of the wobbegong shark, Orectolobus maculatus, and the angel shark, Squatina australis

Electroreception is an ancient sense found in many aquatic animals, including sharks, which may be used in the detection of prey, predators and mates. Wobbegong sharks (Orectolobidae) and angel sharks (Squatinidae) represent two distantly related families that have independently evolved a similar dorso-ventrally compressed body form to complement their benthic ambush feeding strategy. Consequently, these groups represent useful models in which to investigate the specific morphological and physiological adaptations that are driven by the adoption of a benthic lifestyle. In this study, we compared the distribution and abundance of electrosensory pores in the spotted wobbegong shark (Orectolobus maculatus) with the Australian angel shark (Squatina australis) to determine whether both species display a similar pattern of clustering of sub-dermal electroreceptors and to further understand the functional importance of electroreception in the feeding behaviour of these benthic sharks. Orectolobus maculatus has a more complex electrosensory system than S. australis, with a higher abundance of pores and an additional cluster of electroreceptors positioned in the snout (the superficial ophthalmic cluster). Interestingly, both species possess a cluster of pores (the hyoid cluster, positioned slightly posterior to the first gill slit) more commonly found in rays, but which may be present in all benthic elasmobranchs to assist in the detection of approaching predators.

The mechanosensory lateral line system in two species of wobbegong shark (Orectolobidae)

Comparative studies of the mechanosensory lateral line (MLL) have provided valuable insights into the predatory, escape and navigation behaviours of teleost and elasmobranch fishes. Recent work on the MLL in elasmobranchs has focused on canal morphology and pore topography in batoid species, with considerably less detailed examination of the canal system of sharks. In this study, the spatial arrangement of MLL canals and pit organs, and the morphology of the sensory neuromasts, are described for two species of wobbegong shark: the spotted wobbegong Orectolobus maculatus and the ornate wobbegong O. ornatus. Wobbegongs are benthic species that employ an ambush feeding strategy, and it is hypothesised that the MLL will be adapted for both these ecological traits. The spatial distribution of the MLL system in both species is broadly similar to other elasmobranchs, with a dorsal concentration of canals and pit organs that is ideally suited for detecting water flow over the top of the head. This arrangement may facilitate the detection of prey and predators swimming above and in front of the shark. The non-pored canals positioned directly above the mouth may be used as mechanotactile receptors to optimise ‘touch’ sensation when feeding. Wobbegong canal and pit organ neuromast hair cell morphology is typical of other sharks; however, canal neuromast sensory tissue differs from other elasmobranch species in that it is not continuous throughout the canals. The study provides evidence that the MLL system of wobbegong sharks is well adapted for their rather unique feeding mode and benthic lifestyle.