groove anterior to their elongate eyes, while the monotypic Barbled Houndshark (Leptochariidae) has notably long labial furrows and nostrils with barbels. The weasel sharks (Hemigaleidae) are characterized by their wavy, upper caudal-fin lobe. The Carcharhiniformes as a group are difficult to characterize, but all included species have two dorsal fins without spines, an anal fin, five gill slits (last one to three positioned over the pectoral fin), and a large mouth extending behind the eyes (Compagno, 1988). In contrast with the similar Lamniformes, the eyes of most carcharhiniforms are protected by a nictitating membrane and most species lack spiracles. Three families (Carcharhinidae, Sphyrnidae, and Triakidae) are described in more detail below.
REFERENCES: Compagno, 1988, 2001.
CARCHARHINIFORMES : TRIAKIDAE—Hound Sharks
DIVERSITY: 9 genera, 47 species
REPRESENTATIVE GENERA: Galeorhinus, Mustelus, Triakis
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine, occasionally in river mouths; tropical to temperate; coastal to continental slope, demersal over rocky reefs and soft substrates
REMARKS: Hound sharks are moderately sized (to 2.4 m) and feed on benthic and midwater invertebrates and fishes. Some species specialize on crustaceans and others on cephalopods. Hound sharks are either yolk-sac viviparous or placental viviparous (Musick, 2011; Musick and Ellis, 2005) and produce litters with as many as 52 pups. Their phylogenetic relationships were studied by Lopez et al. (2006).
REFERENCES: Compagno, 1988, 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; Lopez et al., 2006; Musick, 2011; Musick and Ellis, 2005.
TRIAKID CHARACTERISTICS:
1) mouth small, subterminal, snout long
2) dorsal-fin base well anterior to pelvic fins
3) eyes oval with nictitating membranes
4) fourth and fifth gill slits over pectoral-fin base
5) nostrils with flaps, often broad, not barbel-like
ILLUSTRATED SPECIMEN:
Triakis semifasciata, SIO 62–213, 1,035 mm TL (dorsal and lateral views)
CARCHARHINIFORMES : CARCHARHINIDAE—Requiem Sharks
DIVERSITY: 12 genera, 60 species
REPRESENTATIVE GENERA: Carcharhinus, Galeocerdo, Prionace, Rhizoprionodon
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine and occasionally freshwater; tropical to temperate; neritic to epipelagic to demersal over reefs and adjacent soft substrates
REMARKS: Requiem sharks are medium to large (up to 7.4 m) predators and are well known for their occasional migrations into freshwater (sometimes exceeding 1,000 km). Species in this family are among those most commonly encountered by divers and sport fishers, and several have been known to attack swimmers. The Bull Shark, Tiger Shark, and Oceanic Whitetip Shark are responsible for most human fatalities. Requiem sharks are generalist predators, taking a wide variety of prey. While some species are yolk-sac viviparous, most species are placental (Musick, 2011; Musick and Ellis, 2005). Some species are highly fecund: the Blue Shark (Prionace glauca) is known to have litters of up to 100 pups.
REFERENCES: Compagno, 1988, 2001, 2005; Compagno et al., 2005; Compagno et al., in Fischer et al., 1995; Compagno and Niem, in Carpenter and Niem, 1998; Garrick, 1982; Musick, 2011; Musick and Ellis, 2005; Naylor, 1992.
CARCHARHINID CHARACTERISTICS:
1) mouth large, subterminal
2) caudal peduncle with a precaudal pit
3) eyes with nictitating membrane
4) dorsal fin with lateral undulations along posterior margin
5) spiracles usually absent
6) nasal grooves and barbels absent
ILLUSTRATED SPECIMEN:
Galeocerdo cuvier, SIO 66–44, 1,310 mm TL
CARCHARHINIFORMES : SPHYRNIDAE—Hammerhead Sharks
DIVERSITY: 2 genera, 9 species
REPRESENTATIVE GENERA: Eusphyra, Sphyrna
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine; tropical to warm temperate; continental shelf and seamounts, neritic or demersal over reefs and soft bottoms
REMARKS: Their characteristic hammer-shaped heads distinguish these sharks from all other fishes. This unusual feature increases capabilities of both vision and the electromagnetic sense and also improves maneuverability. The phylogenetic relationships of hammerheads were studied by Naylor (1992) and Lim et al. (2010). Hammerheads are top predators that feed on bony fishes, sharks and rays, and squids and other invertebrates. Unlike most sharks, hammerheads often form large schools near seamounts during the day, likely dispersing to hunt individually at night. They are viviparous, with yolk-sac placentas (Musick, 2011; Musick and Ellis, 2005). Hammerheads are particularly vulnerable to overfishing and are often captured as bycatch in longline and net fisheries.
REFERENCES: Compagno, 1988, 2005; Compagno, in Carpenter, 2003; Compagno, in Carpenter and Niem, 1998; Compagno et al., in Fischer et al., 1995; Compagno et al., 2005; Gilbert, 1967; Lim et al., 2010; Musick, 2011; Musick and Ellis, 2005; Naylor, 1992.
SPHYRNID CHARACTERISTICS:
1) head flattened and broad, hammer-shaped
2) eyes and nostrils near ends of hammer-like extensions
3) mouth relatively small, subterminal
4) usually one or two gill slits above pectoral-fin base
5) spiracles absent
ILLUSTRATED SPECIMEN:
A) Sphyrna zygaena, SIO 64–528, 1,035 mm TL (dorsal view)
B) head of Sphyrna zygaena, SIO 64–528 (lateral view).
HEXANCHIFORMES—Six-gill Sharks
The hexanchiforms were once thought to be the most primitive extant shark group, but recent research (e.g., Naylor et al., 2005; Vélez-Zuazo and Agnarsson, 2011) has shown them to be allied with the squaliform and related sharks. This group is characterized
