Olfactory and Gustatory Anatomy
Olfactory bulbs are located within the rostrum and are a part of the forebrain (telencephalon); they are well‐developed in elasmobranchs. The bulbs detect amino acids, bile salts, and pheromones (Evans et al. 2004). Meningitis occurs in elasmobranchs and a possible entry route is via the nares or the endolymphatic pores. Taste buds occur in the oropharyngeal cavity as with other vertebrates (Hueter et al. 2004).
Olfaction is used for feeding, particularly within 3–15 m of a prey item, while vision becomes more important at closer range (<3 m) (Hart and Collin 2015). One study showed that sharks become conditioned to the odors from normal, healthy fish within the same system. Fish can produce different odors when frightened, stressed, or excited that can stimulate predation (Tester 1963).
Oral/Pharyngeal Cavity
The types of teeth or dental plates are dictated by the feeding strategy (biting, crushing, shearing, filter feeding, etc.) and can often be used to speciate animals (Kemp 1999). The teeth or plates erupt and roll out continuously, with the caudal‐most gradually replacing the front; this is polyphyodont dentition. Teeth are lyodont (embedded in the oral mucosa, not ankylosed to the jaw) and consist of dentine and enameloid. Tooth replacement rates vary among species, from 8 to 10 days per row up to 5 weeks per row (Motta 2004). If animals that normally eat crustaceans and hard‐bodied prey are not provided with these foodstuffs, overgrowth of the plates can occur. Gingival hyperplasia and neoplasia have been described in sand tigers (Carcharias taurus) (Borucinska et al. 2004).
Denticles are present in the pharynx of most sharks (except carpet sharks, Orectolobiformes) and some other elasmobranchs such as guitarfish (Rhinobatidae). This may decrease drag for ram ventilators, prevent trauma from prey, and improve predation success (Atkinson and Collin 2012).
Gastrointestinal System
The gastrointestinal (GI) tract is short and simple, essentially a slightly S‐ or J‐shaped tube. The ileum includes a spiral or valvular intestine that significantly increases surface area for absorption. This structure is colloquially referred to as a spiral colon but is not technically colonic tissue. The valvular intestine has four distinct variations: a spiral winding around a central column, cones directed caudally or cranially, and scroll‐shaped (Hamlett 1999). The valvular intestine empties into a short tube interchangeably termed the colon and rectum (Wood et al. 2007; Theodosiou and Simeone 2012). A few elasmobranch species have a pyloric cecum or ceca, e.g. deepwater dogfish (Somniosus spp.) (Hamlett 1999).
The rectal gland is a unique intestinal appendage located caudal to the valvular intestine at the colon/rectum (Figure A9.7). It has an osmoregulatory function and the fluid secreted contains sodium and chloride at concentrations almost twice those of the plasma; this is in contrast to elasmobranch urine which is not concentrated (Shuttleworth 2012). The rectal gland is significantly reduced in freshwater elasmobranchs (Evans et al. 2004).
There are bilateral coelomic (abdominal) pores in the cloaca suspected to have an excretory function (Figure A6.6). They provide a possible access point for catheterization or endoscopy of the coelom.
Liver and Gallbladder
The liver is the primary lipid storage organ in elasmobranchs, which helps maintain neutral buoyancy. A large volume of lipid must be stored in the liver, and the size and density are critical in maintaining position in the water column. As the primary energy storage organ, the liver is also an indicator of general health and caloric intake, although size may fluctuate with the reproductive cycle or age (Hamlett 1999; Hussey et al. 2009). A hepatosomatic index (HSI), the ratio of liver weight to body weight, has been identified for many species as a “fitness” indicator (Sherman and Gilliam 1996; Hoffmayer et al. 2006). Grossly the liver should be tan‐colored, fill a large percentage of the ventral coelom (Figure A1.19), and float in formalin; if it does not, there are inadequate lipid reserves. Normal elasmobranch livers may easily be misinterpreted as hepatic lipidosis on histology. The gallbladder is dynamic and appears to increase in size with inappetence.
Figure A1.19 Gross appearance of a normal liver (a) and a small liver (b) at necropsy of a southern stingray (Hypanus americanus). The open coelom shows the liver (Lv), spleen (Sp), valvular intestine (VI), ovary (Ov), and epigonal organ (Ep).
Respiratory System
There are usually five gill arches in elasmobranchs, although there may be up to seven. They consist of a hemibranch cranially (with one row of filaments) and holobranchs for the remaining arches (with two rows of filaments). The interbranchial septum extends to form gill slits on the external surface of the animals. The cranial‐most gill slit is modified into a spiracle in some species (Figure A1.18). These are well‐developed on the dorsal surface of skates and rays and they are present in some slow‐moving sharks (Butler 1999). The spiracle is absent in the more pelagic sharks.
Damage to the gills can affect gas exchange as well as other physiological needs. In saltwater species, gills play an important role in acid–base balance and in freshwater species, gills are important in salt regulation. Gills are also critical for urea retention, particularly in some species like the spiny dogfish (Squalus acanthias) (Ballantyne and Robinson 2010).
The oral cavity is divided into orobranchial and parabranchial cavities. In species that show buccopharyngeal pumping, these areas have a double pumping action which delivers oxygenated water through the mouth or the spiracles to the gills. In ram‐ventilating species, where water continually flows over the gills, the mandibular muscles control the opening of the mouth to manage the water flow (Shuttleworth 2012).
Cardiovascular System
The heart lies in a rigid pericardial chamber surrounded by a large volume of pericardial fluid. The pericardial lumen communicates with the coelomic cavity via a pericardiocoelomic canal, which is usually closed unless the pericardial fluid pressure exceeds that in the coelomic cavity. The fluid in the pericardial cavity is reported to be different from the plasma and coelomic fluid (Tota 1999; Shuttleworth 2012). The elasmobranch heart is sometimes considered four‐chambered although that is a misnomer as it is not equivalent to the mammalian model. The heart consists linearly of the sinus venosus, atrium, ventricle, and conus arteriosus (similar to the bulbus arteriosus of teleosts). The sinus venosus is thin‐walled and very compliant. The atrium is flaccid with a volume larger than the ventricle, the thickest of the myocardial tissues. The conus arteriosus is tubular and thick with prominent valvular structures. Detailed circulatory anatomy of elasmobranchs is available (Hoar et al. 1983; Muñoz‐Chápuli and Satchell 1999). There is often a synchrony noted between respiratory and cardiac beat, but this is inconsistent and has no obvious clinical consequence (Shuttleworth 2012). Electrocardiograms are similar to other vertebrates with the exception of a V‐wave (depolarization of the sinus venosus) prior to the PQRS (Shuttleworth 2012).
There is a secondary vascular system (SVS) in elasmobranchs that has different blood