if exposed) as a defense. They also have toxins in their tissues (tetrodotoxin).
Figure A1.22 Sexual dimorphism in brook trout (Salvelinus fontinalis); a female is at the top of the image.
Source: Image courtesy of Catherine Hadfield, National Aquarium.
The ocean sunfish (Mola mola), seen in some commercial aquariums, have a unique shape thanks to the clavus that replaces the caudal fin. They also lack pelvic fins and a swim bladder, which is unique for this order. They have more cartilage than other teleosts, particularly in the skull and fin girdles. The spinal column has fewer vertebrae than any other fish. Females produce the most eggs of any vertebrate (>300 million) (Helfman et al. 2009; Nelson et al. 2016).
Sarcopterygii (Coelacanths, Lungfish)
These are the lobe‐finned fish. Coelacanths (Latimeria spp.) are rarely seen and were considered extinct until a specimen was discovered in the 1930s. Lungfish (Dipnoi) species come from Australia, Africa, and South America. In all species, the pectoral and pelvic fins are on fleshy stalks extending from the body from a type of humerus or femur similar to the teleost model of multiple bones. Lungfish have electrosensory ampullae over the head and trunk. They have tooth‐plates and teeth that are on the interior jaw bones rather than the margins. There is a spiral valve distal to the duodenum, except in Australian species where the valve starts at the esophagus. The atrium and ventricle are partly divided by a partition which more closely resembles the tetrapod four‐chambered heart than typical fish hearts (Jorgensen and Joss 2016).
Australian lungfish (Neoceratodus forsteri) are the largest, with well‐developed tooth‐plates. Scales are cycloid, large, and thick. Four gill arches are present. They are facultative air‐breathers. There is a single lung. Sexual maturation occurs at 15–20 years and longevity is 65–70 years, with some living beyond this time frame.
African lungfish (Protopterus spp.) have pectoral and pelvic fins that are thinner and more filamentous. Their scales are small. The gilled lungfish (Protopterus amphibius) are most easily identified as they retain three large external gills on the sides of the head; otherwise only larvae have external gills. Six gill arches are present. They are obligate air‐breathers with paired lungs (Figure A1.14). They are known for their ability to estivate in dry habitats for remarkable periods of time (seven to eight months, up to four years experimentally). Aestivation involves constructing a burrow which is filled with mucus that dries to form a cocoon. When estivating, the animal breathes air, its heart rate drops, and body proteins are metabolized.
South American lungfish (Lepidosiren paradoxa) have small scales and thin, filamentous fins. They have five gill arches, with external gills as larvae. They are obligate air‐breathers with paired lungs as adults. Estivation occurs in burrows but is poorly documented (Helfman et al. 2009; Jorgensen and Joss 2016; Nelson et al. 2016)
Taxonomy of Cartilaginous Fish (Chondrichthyes)
The cartilaginous fish are divided into the chimaeras (Holocephali) and elasmobranchs (sharks (Selachimorpha) and skates, rays, guitarfish, and sawfish (Batoidea)). We will highlight the more popular aquarium species in taxonomic groups.
Chimaeriformes (Chimaeras, Ratfish, Ghost Sharks)
Chimaeriformes have a cartilaginous skeleton like elasmobranchs, but the upper jaw with grinding tooth‐plates is fused to the cranium. There is one gill opening (a small operculum) over the gills; they lack a spiracle except as embryos. The eyes cannot regulate the light going onto their retinas, so exposure to bright lights is a concern. Additionally, a condition termed “bloody eye” has been reported that is a result of handling‐induced damage to the pseudobranchial artery (Tozer and Didier 2004). The skin is scaleless except for denticles over the pelvic claspers and tenaculum (a club‐like clasping organ on the head). The dorsal spine is venomous and can cause moderate discomfort to humans. They lack a stomach, ribs, epigonal and Leydig organs; there is a spiral intestine and a rectal gland. There are separate anal and urogenital openings, unlike the cloaca of elasmobranchs. Males have claspers and fertilization is internal (Honma et al. 1984; Tozer and Didier 2004; Nelson et al. 2016).
Carcharhiniformes (Ground Sharks)
This is the largest order of sharks. There are two dorsal fins in most and an anal fin, five gill slits, a nictitating membrane, and the mouth extends behind the eyes. They can be oviparous, ovoviviparous, or viviparous. Relevant families include the requiem sharks (Carcharhinidae), catsharks (Scyliorhinidae), hound sharks (Triakidae), and hammerhead and bonnethead sharks (Sphyrnidae).
Requiem sharks look like the “typical shark”. There is an internal nictitans and spiracles are absent. The intestine has a scroll valve. Catsharks have rudimentary nictitans, small spiracles, and a spiral valve. Hound sharks have spiracles and a spiral valve. The hammerheads and bonnetheads have lateral extensions to their heads (cephalofoil) with eyes perched on the lateral aspects. Spiracles are absent and they have a spiral valve (Nelson et al. 2016).
Lamniformes (Mackerel Sharks, White Sharks, Sand Tigers)
There have two dorsal fins and an anal fin. Five gill slits are present that are broad and extend beyond the base of the pectoral fin. Sand tiger sharks (Carcharias taurus) differ in that the gills are in front of the pectoral fin. Spiracles are present behind the eyes but are small. The eyes lack a nictitating membrane. The mouth extends behind the eyes. They have a ring‐type spiral intestine. This group is unique in its practice of oophagy or adelphotrophy (uterine cannibalism) (Nelson et al. 2016). Some members of this group practice endothermy, e.g. white (Carcharodon carcharias) and mako sharks (Isurus spp.).
Squaliformes (Dogfish, Gulper Sharks)
Two dorsal fins are present which may have spines; anal fins are absent. There are five gill slits and spiracles. A small nictitating membrane is present in the lower lid (Nelson et al. 2016). The spiny dogfish (Squalus acanthias) has been a common species in research studies.
Orectolobiformes (Carpet Sharks, Wobbegongs, Bamboo Sharks, Nurse Sharks)
These have two dorsal fins and an anal fin. Five gill slits with spiracles are close to the eyes. There is no nictitating membrane and eyes are dorsolateral on the head. Most have prominent nasoral grooves with barbels. Wobbegong sharks (Orectolobidae) have a dorsoventrally flattened head and body, rostral mouths, and fang‐like teeth. Bamboo (Hemiscyllidae) and nurse sharks (Ginglymostomatidae) are some of the most frequently kept species in aquaria. This group also includes the whale sharks (Rhincodon typus); these are plankton feeders and the largest fish in the world (Nelson et al. 2016).
Rajiformes (Skates)
Skates are dorsoventrally compressed with enlarged pectoral fins that are continuous with their heads. They have a caudal fin and no anal fins; some have dorsal fins. The tail has no barbs. They are oviparous. The skin frequently has thorns. Some animals in the group have weak electric organs (Ebert and Sulikowski 2008; Nelson et al. 2016).
Myliobatiformes (Cownose Rays, Stingrays, Manta Rays)
These animals include the typical stingrays. These show dorsoventral compression with enlarged pectoral fins that are continuous with their heads. The five gill openings are ventral. Eyes and spiracles are on