Fishes: A Guide to Their Diversity. Philip A. Hastings
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many skin denticles large and thorn-like, in various places on body
2) two relatively small dorsal fins, positioned posteriorly, spines absent
3) all five gill slits anterior to pectoral fin
4) anal fin absent
5) head broad and depressed
6) spiracles small, well posterior to eyes
ILLUSTRATED SPECIMEN:
Echinorhinus cookei, SIO 60–378, 1,700 mm TL.
INSET: Close-up of skin showing thorn-like denticles
SQUALIFORMES—Dogfish Sharks
The dogfish sharks include six families, at least 24 genera, and 130 species. Some authors (e.g., Compagno, 2005; Ebert, 2003; Eschmeyer and Fong, 2013) also include the two species of bramble sharks (Echinorhinidae) among the squaliforms, but others (e.g., de Carvalho, 1996; Vélez-Zuazo and Agnarsson, 2011) place the bramble sharks in their own order. Both groups are characterized by the lack of an anal fin, presence of spiracles, and the location of all five gill slits anterior to the origin of the pectoral fin. Among the squaliforms, the gulper sharks (Centrophoridae) and the lantern sharks (Etmopteridae) are characterized by two dorsal fins, each with a single grooved spine, and by very large eyes. The lantern sharks usually have light organs along the ventral aspect of the body. Some species of sleeper sharks (Somniosidae) have dorsal-fin spines while others do not, and all species are characterized by abdominal ridges between the pectoral and pelvic fins. The roughsharks (Oxynotidae) are easily distinguished by their triangular body (in cross section) and their large, sail-like dorsal fins. The dogfish sharks sensu stricto (Squalidae) and the kitefin sharks (Dalatiidae) are described below.
REFERENCES: de Carvalho, 1996; Vélez-Zuazo and Agnarsson, 2011.
SQUALIFORMES : SQUALIDAE—Dogfish Sharks
DIVERSITY: 2 genera, 30 species
REPRESENTATIVE GENERA: Cirrhigaleus, Squalus
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine; tropical to temperate; continental shelf to continental slope and seamounts, coastal, demersal over soft bottoms
REMARKS: Dogfish sharks have strong dorsal-fin spines, some of which are venomous. Some species commonly form large, social schools that are known to attack and dismember large prey. All dogfishes are yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005), and one species, Squalus acanthias, has extremely large eggs and a gestation period of up to two years. This low reproductive potential, together with their propensity for forming large schools, makes dogfishes quite vulnerable to overfishing, particularly given their importance as a commercially exploited group of sharks (IUCN, 2013).
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; Shirai, 1992a.
SQUALID CHARACTERISTICS:
1) two dorsal fins, each with a strong, smooth spine
2) caudal peduncle with lateral keels and a precaudal pit
3) all five gill slits usually anterior to pectoral-fin base
4) spiracles large
5) anal fin absent
6) nictitating membrane absent
ILLUSTRATED SPECIMEN:
Squalus suckleyi, SIO 08–138, 740 mm TL
SQUALIFORMES : DALATIIDAE—Kitefin Sharks
DIVERSITY: 6 genera, 10 species
REPRESENTATIVE GENERA: Dalatius, Isistius, Squaliolus
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine; tropical to temperate; continental shelf to continental slope, coastal to oceanic, epipelagic to bathypelagic
REMARKS: Kitefin sharks are some of the smallest of all sharks; species in the genus Squaliolus reach maximum sizes of only 22–28 cm TL. These cigar-shaped sharks generally eat small fishes, squids, and crustaceans, but the highly specialized cookiecutter sharks (Isistius spp.) attack much larger prey, removing bite-sized pieces from live marine mammals and large-bodied fishes. Kitefin sharks are yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005) and probably use their ventral light organs to achieve countershading, decreasing their visual profile from below.
REFERENCES: Compagno, 2005; Compagno, in Carpenter, 2003; Compagno and Niem, in Carpenter and Niem, 1998; Compagno et al., 2005; Musick, 2011; Musick and Ellis, 2005; Shirai, 1992a.
DALATIID CHARACTERISTICS:
1) two dorsal fins, without spines, or with a spine in the first dorsal fin only
2) luminous organs usually present along ventral surface
3) caudal fin with a pronounced notch
4) spiracles large
5) anal fin absent
6) nictitating membrane absent
ILLUSTRATED SPECIMENS:
A) Squaliolus aliae, DE 0508, 117 mm TL
B) Isistius brasiliensis, SIO 69–345, 470 mm TL
INSET: Flank of Oarfish (Regalecus russelii, SIO 13–259) with multiple wounds caused by Isistius brasiliensis.
SQUATINIFORMES : SQUATINIDAE—Angel Sharks
DIVERSITY: 1 family, 1 genus, 22 species
REPRESENTATIVE GENUS: Squatina
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine; tropical to temperate; continental shelf to continental slope, benthic on soft substrates
REMARKS: These distinctive sharks resemble rays in many respects, but unlike rays, their pectoral fins are not attached to their heads. In keeping with their benthic lifestyle, their well-developed spiracles are used for respiration, and there are barbels associated with their nostrils. Angel sharks are ambush predators that rely on crypsis to attack their unsuspecting prey (Fouts and Nelson, 1999). They are one of only a few groups of sharks that have protrusible jaws and are capable of producing strong negative pressure for suction feeding. Angel sharks are yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005). Shirai (1992b) studied their phylogenetic relationships.
REFERENCES: Compagno, 2001, 2005; Compagno, in Carpenter, 2003; Compagno and Niem, in Carpenter and Niem, 1998; Compagno et al., 2005; Fouts and Nelson, 1999; Musick, 2011; Musick and Ellis, 2005; Shirai, 1992b.
SQUATINIFORM CHARACTERISTICS:
1) body strongly depressed, ray-like
2) pectoral fins separate from head
3) eyes dorsal
4) two dorsal fins, roughly equal in size, positioned posteriorly
5) spiracles large
6) caudal peduncle with strong, lateral keels
7) anal fin absent
ILLUSTRATED SPECIMEN:
Squatina californica, SIO 65–305, 435 mm TL (dorsal view)
PRISTIOPHORIFORMES : PRISTIOPHORIDAE—Saw