Batoidea includes over 650 species of skates and rays that are classified in four orders, 17 families, and over 70 genera. They are characterized by a variety of features including a dorso-ventrally flattened head and body, enlarged pectoral fins contiguous with the head, mouth and gill slits opening on the ventral side of the head, and eyes and spiracles placed on the dorsal side of the head. The monophyly of this group is well established (Aschliman et al., 2012; McEachran and Aschliman, 2004; McEachran et al., 1998; Naylor et al., 2005) and the group has long been recognized under a variety of names including the Hypotremata, Batidoidimorpha, and Rajiformes sensu lato (Nelson, 2006).
Hypothesized phylogenetic relationships of the Batoidea (rays) after (left) McEachran and Aschliman (2004) and (right) Aschliman et al. (2012).
TORPEDINIFORMES—Electric Rays
There are four families, 11 genera, and 67 species of electric rays, characterized by a nearly circular disc, electric organs located on the pectoral fins, and a completely naked body (without denticles or spines). The Narcinidae is described in more detail below.
REFERENCES: McEachran and Aschliman, 2004
TORPEDINIFORMES : NARCINIDAE—Numbfishes
DIVERSITY: 4 genera, 31 species
REPRESENTATIVE GENERA: Benthobatis, Diplobatis, Discopyge, Narcine
DISTRIBUTION: Atlantic, Indian and Pacific oceans
HABITAT: Marine; tropical to warm temperate; continental shelf to continental slope, benthic on soft substrates
REMARKS: Numbfishes use their electric organs for both defense and feeding. They can be distinguished from the similar torpedo electric rays (Torpedinidae) by their similarsized dorsal fins (first fin larger in torpedo rays), relatively smaller caudal fin, and thin (rather than thick) outer margin of the pectoral-fin disc. Numbfishes prey on benthic invertebrates and small fishes, using their protrusible mouths to provide suction for removing organisms from soft sediments. Numbfishes are yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005).
REFERENCES: Compagno, 2005; de Carvalho et al., in Carpenter and Niem, 1999; McEachran, in Fischer et al., 1995; McEachran and de Carvalho, in Carpenter, 2003; Musick, 2011; Musick and Ellis, 2005.
NARCINID CHARACTERISTICS:
1) two dorsal fins of equal size, positioned posteriorly
2) mouth slot-shaped, jaws protrusible
3) pectoral fins thin around outer edges
4) caudal fin and dorsal fins similar in size
5) lobes of caudal fin continuous
ILLUSTRATED SPECIMEN:
Narcine brasiliensis, SIO 67–89, 300 mm TL
PRISTIFORMES : PRISTIDAE—Sawfishes
DIVERSITY: 1 family, 2 genera, 7 species
REPRESENTATIVE GENERA: Anoxypristis, Pristis
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine and occasionally in freshwater; tropical; continental shelf and coastal, demersal and benthic on soft substrates
REMARKS: The remarkably large rostral blade with teeth on either side distinguishes sawfishes from all other fishes except sawsharks, from which they can be differentiated by their absence of barbels and by their pectoral fins being connected to the head. These impressive predators use their “saw” for disabling swimming prey or digging for buried prey in soft sediments. Sawfishes are known to enter freshwater, with some individuals captured more than 1,000 km upriver, and are known to reproduce in at least one freshwater lake (Thorson, 1976). Sawfishes are yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005). All seven species are listed as critically endangered by the IUCN (2013) as a result of overfishing (sawfishes are especially susceptible to gill-nets) and habitat degradation.
REFERENCES: de Carvalho, 2003; Compagno, 2005; Compagno and Last, in Carpenter and Niem, 1999; McEachran and de Carvalho, in Carpenter, 2003; Musick, 2011; Musick and Ellis, 2005; Thorson, 1976.
PRISTIFORM CHARACTERISTICS:
1) snout elongate, blade-like, with single row of large teeth on each side
2) body large and shark-like, with anterior margin of pectoral fins attached to head
3) nostrils well anterior and not connected to mouth
4) spiracles large
5) two large dorsal fins of equal size, widely separated
6) barbels absent
ILLUSTRATED SPECIMEN:
A) Pristis pectinata, UAZ uncatalogued, 940 mm to end of broken snout (dorsal view)
B) head of Pristis pectinata, UAZ uncatalogued (ventral view)
RAJIFORMES—Skates
The skates are the most diverse order of chondrichthyan fishes, with 361 species and over 30 genera in four families. Their disc is diamond- or heart-shaped, and the dorsal surface of the tail has one or more longitudinal rows of thorns but no stinging spine. Skates have large, well-developed spiracles that usually contain visible pseudobranchs.
REFERENCES: Aschliman et al., 2012; McEachran and Aschliman, 2004.
RAJIFORMES : RHINOBATIDAE—Guitarfishes
DIVERSITY: 11 genera, 62 species
REPRESENTATIVE GENERA: Aptychotrema, Platyrhinoides, Rhina, Rhinobatos, Zapteryx
DISTRIBUTION: Atlantic, Indian, and Pacific oceans
HABITAT: Marine, rarely entering freshwater; tropical to temperate; continental shelf to continental slope, benthic on soft substrates and rocky reefs
REMARKS: Guitarfishes are elongate batoids with a wedge-shaped head and a wide tail. Their lower caudal-fin lobe is not well defined. In addition to dermal denticles covering the body and fins, they can have enlarged thorn-like spines on the dorsal surface of the snout and the midline of the body and tail. Some authors (e.g., McEachran and Aschliman, 2004; Nelson, 2006) separate them into two or more families, considered here as subfamilies. These include the monotypic Bowmouth Guitarfish (Rhininae), the six species of wedgefishes (Rhynchobatinae), and the six species of thornbacks (Platyrhininae), once considered members of the Myliobatiformes (Nelson, 2006). Guitarfishes feed on a variety of benthic invertebrates, as well as small bony fishes. They are yolk-sac viviparous (Musick, 2011; Musick and Ellis, 2005).
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