(2012) concluded that these groups are reciprocally monophyletic, that is, each is monophyletic and they are sister groups, sharing a unique common ancestor. Naylor et al. (2012) recently summarized information on the valid species of elasmobranchs.
SELACHII—Sharks
The Selachii includes all species of sharks and is characterized by lateral gill openings and pectoral fins separate from the head (Nelson, 2006). This group includes 518 species, classified in eight orders, 35 families, and over 100 genera (Compagno, 1984a, 1984b, 2005; Compagno et al., 2005; Eschmeyer and Fong, 2013; Naylor et al., 2012). Phylogenetic relationships of sharks have been hypothesized by several researchers (e.g., de Carvalho, 1996; Naylor et al., 2005; Shirai, 1996; Vélez-Zuazo and Agnarsson, 2011), including Maisey et al. (2004), who recognized two major lineages, the Squalomorphii and the Galeomorphii.
Hypothesized phylogenetic relationships of the Selachii (sharks) after Maisey et al. (2004) and Musick and Ellis (2005).
HETERODONTIFORMES : HETERODONTIDAE—Bullhead Sharks
DIVERSITY: 1 genus, 9 species
REPRESENTATIVE GENUS: Heterodontus
DISTRIBUTION: Indian and Pacific oceans
HABITAT: Marine; tropical to warm temperate; continental shelf (one species on continental slope), benthic to demersal including shallow rocky or coral reefs
REMARKS: Members of the family of bullhead sharks are characterized by their distinctive dorsal fins and blunt snouts. They are usually nocturnal and generally feed on benthic invertebrates and occasionally on small fishes. Bullhead sharks are oviparous (Musick, 2011; Musick and Ellis, 2005) and produce distinctive screw-shaped, keratinoid egg cases.
REFERENCES: Compagno, 2001, 2005; Compagno, in Carpenter, 2003; Compagno and Niem, in Carpenter and Niem, 1998; Compagno et al., in Fischer et al., 1995; Compagno et al., 2005; Musick, 2011; Musick and Ellis, 2005.
HETERODONTIFORM CHARACTERISTICS:
1) five external gill slits, two to three behind pectoral-fin origin
2) two dorsal fins, each with a broad-based spine
3) crests above eyes
4) eyes without a nictitating membrane
5) spiracles small
6) nostrils and mouth connected by a groove
ILLUSTRATED SPECIMEN:
Heterodontus francisci, SIO 64–33, 765 mm TL
INSET: Lower jaw of Heterodontus francisci (SIO 60–23, 865 mm TL) showing anterior rows of pointed teeth and posterior rows of pavement-like teeth.
ORECTOLOBIFORMES—Carpet Sharks
The carpet sharks comprise seven families, 14 genera, and 42 species of mostly benthic sharks, noted for sitting perfectly still on the bottom of the ocean. Their nostrils have barbels and are connected to the relatively small mouth by a groove. The wobbegongs (Orectolobidae) are sit-and-wait predators and have a head covered in skin flaps, cryptic coloration, large spiracles, and large fang-like teeth. Their phylogenetic relationships were studied by Goto (2001) and Corrigan and Beheregaray (2009). Two families (Ginglymostomatidae and Rhincodontidae) are described in more detail below. The remaining families (Parascyliidae, Brachaeluridae, Hemiscyliidae, and Stegostomatidae) include relatively few species and occur in the tropical Indo-West Pacific.
REFERENCES: Corrigan and Beheregaray, 2009; Goto, 2001.
ORECTOLOBIFORMES : GINGLYMOSTOMATIDAE—Nurse Sharks
DIVERSITY: 1 family, 3 genera, 3 species
REPRESENTATIVE GENERA: Ginglymostoma, Nebrius, Pseudoginglymostoma
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine; tropical to subtropical; continental shelf, benthic on shallow reefs and adjacent sandy areas
REMARKS: Nurse sharks are characterized by their brownish coloration and by dorsal fins located far back on the body. They are generally nocturnal and can be observed resting in small groups on the reef or sandy surfaces during the day. The small mouth with a large oral cavity is capable of suction-feeding benthic invertebrates and small fishes. Nurse sharks are yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005).
REFERENCES: Compagno, 2001, 2005; Compagno, in Carpenter, 2003; Compagno, in Carpenter and Niem, 1998; Compagno et al., in Fischer et al., 1995; Compagno et al., 2005; Musick, 2011; Musick and Ellis, 2005.
GINGLYMOSTOMATID CHARACTERISTICS:
1) five small gill openings (slits), fifth slit nearly overlapping fourth
2) dorsal fins without spines, positioned posteriorly on body
3) mouth short, subterminal, not extending to level of eyes
4) eyes without nictitating membrane
5) spiracles small, located just behind eyes
6) nostrils with barbels
ILLUSTRATED SPECIMEN:
Ginglymostoma cirratum, SIO 64–229, 521 mm TL (dorsal view)
ORECTOLOBIFORMES : RHINCODONTIDAE—Whale Sharks
DIVERSITY: 1 genus, 1 species
REPRESENTATIVE GENUS: Rhincodon
DISTRIBUTION: Atlantic (absent from Mediterranean), Indian, and Pacific oceans
HABITAT: Marine; tropical to warm temperate; usually epipelagic but occasionally mesopelagic or neritic
REMARKS: While the Whale Shark is distinguished by its huge size (the largest fish on Earth), it feeds on planktonic organisms and fish eggs, using its long, thin gill rakers for filter feeding. Its brain anatomy was studied by Yopak and Frank (2009). The Whale Shark is yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005), but, curiously, produces keratized egg capsules that hatch within the female. These enormous fishes are highly fecund compared to other members of the Chondrichthyes; one female caught by fishermen held 300 pups. In many areas, the Whale Shark has been overfished; it is considered “vulnerable” by the IUCN (2013) and it has been given protected status by many nations.
REFERENCES: Compagno, 2001, 2005; Compagno, in Carpenter, 2003; Compagno, Carpenter and Niem, 1998; Compagno et al., in Fischer et al., 1995; Compagno et al., 2005; Musick, 2011; Musick and Ellis, 2005; Yopak and Frank, 2009.
RHINCODONTID
