Morphology and distribution of taste papillae and oral denticles in the developing oropharyngeal cavity of the bamboo shark, Chiloscyllium punctatum.

Gustation in sharks is not well understood, especially within species that ingest food items using suction. This study examines the morphological and immunohistochemical characterisation of taste papillae and oral denticles in the oropharynx of the brown-banded bamboo shark Chiloscyllium punctatum and compares their distribution during development. Taste papillae of the brown-banded bamboo shark Chiloscyllium punctatum are located throughout the oropharyngeal region and are most concentrated on the oral valves (2,125-3,483 per cm(2) in embryos; 89-111 per cm(2) in mature adults) close to the tooth territories. Papillae appearance is comparable at all stages of development, with the exception of the embryos (unhatched specimens), where no microvilli are present. Oral valve papillae are comparable in structure to Type I taste buds of teleost fishes, whereas those of the rest of the oropharyngeal region are comparable to Type II. Both types of papillae show immunofluorescence for a number of markers of taste buds, including β-Catenin and Sox2. Taste papillae densities are highest in embryos with 420-941 per cm(2) compared to 8-29 per cm(2) in mature adults. The total number of papillae remains around 1,900 for all stages of development. However, the papillae increase in diameter from 72±1 µm in embryos to 310±7 µm in mature individuals. Microvilli protrude in multiple patches at the apical tip of the papilla covering ∼0.5% of the papillar surface area. We further document the relationship between taste papillae and the closely associated oral denticles within the shark orophayngeal cavity. Oral denticles first break through the epithelium in the antero-central region of the dorsal oral cavity, shortly after the emergence of teeth, around time of hatching. Denticles are located throughout the oropharyngeal epithelium of both immature and mature stages, with the highest concentrations in the antero-dorsal oral cavity and the central regions of the pharynx. These denticle-rich areas of the mouth and pharynx are therefore thought to protect the epithelium, and importantly the taste papillae, from abrasion since they correlate with regions where potential food items are processed or masticated for consumption. Taste papillae and denticles are more dense in anterior oropharyngeal regions in close association with the oral jaws and teeth, and in the juvenile/hatchling shark taste units are functional, and innervated, allowing the shark to seek out food in utero, at birth or on emergence from the egg case

Underwater hearing in sea snakes (Hydrophiinae): first evidence of auditory evoked potential thresholds

The viviparous sea snakes (Hydrophiinae) are a secondarily aquatic radiation of more than 60 species that possess many phenotypic adaptations to marine life. However, virtually nothing is known of the role and sensitivity of hearing in sea snakes. This study investigated the hearing sensitivity of the fully marine sea snake Hydrophis stokesii by measuring auditory evoked potential (AEP) audiograms for two individuals. AEPs were recorded from 40 Hz (the lowest frequency tested) up to 600 Hz, with a peak in sensitivity identified at 60 Hz (163.5 dB re. 1 µPa or 123 dB re. 1 µm s⁻²). Our data suggest that sea snakes are sensitive to low-frequency sounds but have relatively low sensitivity compared with bony fishes and marine turtles. Additional studies are required to understand the role of sound in sea snake life history and further assess these species' vulnerability to anthropogenic noise.Lucille Chapuis, Caroline C. Kerr, Shaun P. Collin, Nathan S. Hart and Kate L. Sander

The effect of underwater sounds on shark behaviour

The effect of sound on the behaviour of sharks has not been investigated since the 1970s. Sound is, however, an important sensory stimulus underwater, as it can spread in all directions quickly and propagate further than any other sensory cue. We used a baited underwater camera rig to record the behavioural responses of eight species of sharks (seven reef and coastal shark species and the white shark, Carcharodon carcharias) to the playback of two distinct sound stimuli in the wild: an orca call sequence and an artificially generated sound. When sounds were playing, reef and coastal sharks were less numerous in the area, were responsible for fewer interactions with the baited test rigs, and displayed less ‘inquisitive’ behaviour, compared to during silent control trials. White sharks spent less time around the baited camera rig when the artificial sound was presented, but showed no significant difference in behaviour in response to orca calls. The use of the presented acoustic stimuli alone is not an effective deterrent for C. carcharias. The behavioural response of reef sharks to sound raises concern about the effects of anthropogenic noise on these taxa

Ontogenetic shifts in brain scaling reflect behavioral changes in the life cycle of the pouched lamprey Geotria australis

Very few studies have described brain scaling in vertebrates throughout ontogeny and none in lampreys, one of the two surviving groups of the early agnathan (jawless) stage in vertebrate evolution. The life cycle of anadromous parasitic lampreys comprises two divergent trophic phases, firstly filter-feeding as larvae in freshwater and secondly parasitism as adults in the sea, with the transition marked by a radical metamorphosis. We characterized the growth of the brain during the life cycle of the pouched lamprey Geotria australis, an anadromous parasitic lamprey, focusing on the scaling between brain and body during ontogeny and testing the hypothesis that the vast transitions in behavior and environment are reflected in differences in the scaling and relative size of the major brain subdivisions throughout life. The body and brain mass and the volume of six brain structures of G. australis, representing six points of the life cycle, were recorded, ranging from the early larval stage to the final stage of spawning and death. Brain mass does not increase linearly with body mass during the ontogeny of G. australis. During metamorphosis, brain mass increases markedly, even though the body mass does not increase, reflecting an overall growth of the brain, with particularly large increases in the volume of the optic tectum and other visual areas of the brain and, to a lesser extent, the olfactory bulbs. These results are consistent with the conclusions that ammocoetes rely predominantly on non-visual and chemosensory signals, while adults rely on both visual and olfactory cues

The deep-sea teleost cornea, a comparative study of gadiform fishes

The corneal structure of three deep-sea species of teleosts (Gadiformes, Teleostei) from different depths (250-4000 m) and photic zones are examined at the leve1 of the light and electron microscopes. Each species shows a similar but complex arrangement of layers with a cornea split into dermal and scleral components. The dermal cornea comprises an epithelium overlying a basement membrane and a dermal stroma with sutures and occasional keratocytes. Nezumia aequalis is the only species to possess a Bowman's layer, although it is not well-developed. The scleral cornea is separated from the dermal cornea by a mucoid layer and, in contrast to shallow-water species, is divided into three main layers; an anterior scleral stroma, a middle or iridescent layer and a posterior scleral stroma. The iridescent layer of collagen and intercalated cells or cellular processes is bounded by a layer of cells and the posterior scleral stroma overlies a Descemet's membrane and an endothelium. In the relatively shallow-water Microgadus proximus, the keratocytes of the dermal stroma, the cells of the iridescent layer and the endothelial cells al1 contain aligned endoplasmic reticulum, which may elicit an iridescent reflex. No alignment of the endoplasmic reticulum was found in N. aequalis or Coryphanoides (Nematonurus) armatus. The relative differences between shallow-water and deep-sea corneas are discussed in relation to the constraints of light, depth and temperature

The morphology of the retina and lens of the sandlance, Limnichthyes fasciatus (Creeiidae)

The sandlance, Limnichthyes fasciatus (Creeiidae), is a small teleost (30 mm in length), which lives beneath the sand. It has minute dorsally placed eyes (1.04 mm in diameter), which move independently of one another. The structure of the retina and lens was examined by both light and electron microscopy. A deep convexiclivate fovea lies on the visual axis of the eye, and regional increases in photoreceptor and ganglion cell densities occur within the area surrounding the foveal depression. The sandlance possesses a pure cone fovea, with a regular square mosaic of a single cone bordered by four equal double cones distributed over most of the retina. Rods are rare and are distinguishable from cones on ultrastructural morphology. A pigmented choriocapillaris extends behind the retina, closely apposing the retinal pigment epithelial cell layer and Bruch's membrane. Surrounding the optic nerve, and adjacent to the choriocapillaris, is a vascularised, horseshoe-shaped choroidal gland, or rete mirabile. A small system of vitreal blood vessels from the hyaloid artery near the optic nerve, supplies the large number of ganglion cells, arranged in up to five sub-laminae, within the ganglion cell layer. The retina is jacketed by an uveal argentea within the sclera. This argentea contains plates of guanine crystals, oriented with their flat surfaces approximately perpendicular to the incident light path, and discrete bundles of melanosomes apposing the sclera. A non-spherical lens, previously described only in deep-sea teleosts, was found, and its refractive properties are discussed in relation to the deep pit fovea

The foveal photoreceptor mosaic in the pipefish, Corythoichthyes paxtoni Syngnathidae, Teleostei

The foveal and non-foveal retinal regions of the pipefish, Corythoichthyes paxtoni (Syngnathidae, Teleostei) are examined at the level of the light and electron microscopes. The pipefish possesses a deep, pit (convexiclivate) fovea which, although lacking the displacement of the inner retinal layers as described in other vertebrate foveae, is characterised by the exclusion of rods, a marked increase in the density of photoreceptors and a regular square mosaic of four double cones surrounding a central single cone. In the perifoveal and peripheral retinal regions, the photoreceptor mosaic is disrupted by the insertion of large numbers of rods, which reduce spatial resolving power but may uniformly increase sensitivity for off-axis rays. In addition to a temporal fovea subtending the frontal binocular field, there is also a central area centralis subtending the monocular visual field. Based on morphological comparisons with other foveate teleosts, four foveal types are characterised and foveal function discussed with respect to the theoretical advantage of a regular square mosaic