and lower tail angles, e.g. carpet sharks (Orectolobiformes). The batoids (skates [Rajiformes] and rays [Myliobatiformes and Torpediniformes]) and sawfish and guitarfish (Rhinopristiformes) show dorsoventral flattening of the body and enlarged pectoral fins. Chimaeras (Holocephali) show lateral compression and undulate their pectoral fins rather than their axial body.
Integument
Elasmobranchs produce placoid scales, also known as dermal denticles. These give the sandpaper feel to shark skin as well as focal areas in the skin of batoids. These denticles are formed like teeth with a calcified layer, dentin, and enamel (Moss 1977). This skin represents an abrasion risk to human handlers and exposed skin should be protected. In silky sharks (Carcharhinus falciformis), the denticles are minute which makes the skin softer compared to other sharks (Camhi et al. 2008). In blue sharks (Prionace glauca), females have a significantly thicker skin to withstand mating trauma (Camhi et al. 2008). Many rays have few to no scales e.g. whiptail rays (Dasyatidae), eagle rays (Myliobatidae), mantas and mobulas (Mobulidae). These species tend to have a significant mucus layer which can affect water quality during prolonged restraint. Porcupine rays (Urogymnus asperrimus) and some other rays have “armor” on their dorsum complicating ultrasounds from the dorsal aspect. In many batoids and some sharks, sharp spines can develop. A venomous spine or barb is present in most rays, with the exception of mantas, mobulas, and porcupine rays (Meyer and Seegers 2012). The barbs are covered by integument which includes cells where venom is created. Several barbs can be present. Chimaeras (Holocephali) are scaleless (except in juvenile stages) and are very sensitive to skin trauma (Didier 2004).
Elasmobranch skin has visible, symmetrical epithelial pores known as pit organs and ampullae of Lorenzini (Figure A1.16) (Hueter et al. 2004). Pit organs are free neuromasts which use sensory hair cells to detect water motion. Ampullae are gel‐filled tubular structures that allow elasmobranchs to detect electric fields for navigation, prey and predator detection, and mating (Meyer and Seegers 2012). Clinicians should note that electric fields can exist in aquarium systems, especially where equipment is worn or corroded, and these impact elasmobranchs.
Musculoskeletal System
The entire elasmobranch endoskeleton is cartilaginous. It is made up of a hyaline cartilage‐like core supported by mineralized tesserae (Omelon et al. 2014). Bone does exist in the form of teeth and denticles. While calcification can occur in the vertebrae and jaws, true bone is not found in those areas (Moss 1977). The centrum of the vertebral cartilage is used for aging elasmobranchs (Dean and Summers 2006). If elasmobranch cartilage is fractured, it does not heal fully but rather forms a fibrous “bandage” (Ashhurst 2004).
Many studies have examined elasmobranch skull anatomy, with particular reference to capturing prey. Three modes of prey capture occur (sometimes in combination): biting, ram feeding, and suction feeding (Wilga and Lauder 2004). Clinical relevance comes from the importance of the jaw protrusion capacity. Permanent jaw protrusion is reported to be associated with spinal deformity in sand tiger sharks (Carcharias taurus) (Anderson et al. 2012).
Figure A1.16 Ampullae of Lorenzini in a bamboo shark (Chiloscyllium sp.) (arrows) and across the ventrum of a blue‐spotted stingray (Neotrygon kuhlii).
Musculature is similar to teleosts, with red and white skeletal muscle (Figure A1.17). Most elasmobranchs are poikilothermic, but regional endothermy has been described in some lamniform sharks such as makos (Isurus spp.), white sharks (Carcharodon carcharias), salmon and porbeagle sharks (Lamna spp.), and thresher sharks (Alopias spp.) (Goldman 1997; Bernal et al. 2012; Shadwick and Goldbogen 2012).
Figure A1.17 Cross‐section through the peduncle of a blacktip reef shark (Carcharhinus melanopterus) showing red and white skeletal muscle.
Buoyancy
In cartilaginous fish, buoyancy is attributed to the cartilaginous skeleton, the large, lipid‐dense liver, and urea and methylamine oxides in the blood (Withers et al. 1994; Shuttleworth 2012). No cartilaginous fish have swim (gas) bladders. The sand tiger shark (Carcharias taurus) typically shows a gas shadow in the stomach on imaging as it swallows air for additional buoyancy.
Ocular Anatomy
Eye anatomy in elasmobranchs is diverse. Eyelids are usually fixed, but are mobile in some species, e.g. nurse sharks (Ginglymostoma spp.) and catsharks (Cephaloscyllium), and there is a blink reflex. There is a third eyelid or nictitating membrane in some, e.g. requiem sharks (Carcharhinidae). Pupil type and shape are characterizing features of some species. In rays, the upper iris is modified into an operculum pupillare which covers the iris during light adaptation (Figure A1.18). The pupillary light response is highly variable: diurnal sharks exhibit rapid constriction, nocturnal sharks have an intermediate response, and batoids show the slowest response. Dilation can be produced using topical acetylcholine (Kuchnow 1971). The sclera is thick with a cartilaginous layer. The cornea has the same layers as other vertebrates. Many sharks have a partially or totally occlusible tapetum, meaning that melanophores can migrate over the retina and block photophores to adapt to light. Some species have a fixed tapetum, e.g. catsharks and deep‐sea sharks (Gruber 1977). The retina is avascular and there is no choroid gland (Gelatt 2014). Many elasmobranchs are able to pull their globes back into their eye sockets using extraocular muscles (Jurk 2002; Hart and Collin 2015). They also possess an optic pedicle which is a cartilaginous structure connecting the globe to the cranium (Gelatt 2014). The scleral cartilage, optic pedicle, and the size of the optic nerve, vessels, and muscles make enucleation much more challenging than in teleosts.
Figure A1.18 Modified iris of a clearnose skate (Raja eglanteria); the spiracle is visible caudal to the eye.
Source: Image courtesy of Catherine Hadfield, National Aquarium.
In addition to the eyes, the pineal organ/eye (epiphysis) is well‐developed in most elasmobranchs, although absent in some of the electric rays (Torpediniformes). The photoreceptors are located superficially on the dorsal aspect of the chondrocranium (Gruber 1977).
Auditory Anatomy
The ears of elasmobranchs are similar to other vertebrates and respond to acoustical, vibrational, and gravitational forces. They are located in cartilaginous otic capsules just caudal to the large optic capsules; the only external indication of their position is tiny paired endolymphatic pores (<1 mm diameter) on the dorsal chondrocranium near the medial line (Tester et al. 1972). Each ear has an inner ear labyrinth (utriculus, sacculus, and lagena) but none of the accessory organs seen in teleosts. Within the endolymphatic duct, instead of an otolith, there is an otoconial paste of calcium carbonate granules in gel that functions like the otoliths in teleosts