Striped marlin, Tetrapturus audax
Billfish is the common name for any of the large, predatory marine fish comprising the families Istiophoridae and Xiphiidae of the Perciformes order, characterized by large size, elongate premxillary bill, two anal fins, and pectoral fins low on the body. The Xiphiidae family has one extant member, Xiphias gladius, known as the swordfish. The Istiophoriidae has about 11 species commonly (but not exclusively) placed in three genera: the sailfishes comprising genus Istiophorus, the spearfishes of genus Tetrapturus, and the marlins of genus Makaira.
While the various billfishes are most common in tropical and subtropical waters, the swordfish in particular is sometimes found in temperate waters as well.
Billfishes are important apex predators feeding on a wide variety of smaller fish and cephalopods, including squids, octopuses, dolphins, mackerels, and tunas. And young billfish play a role in the marine food chains as food for sharks, among other predators. For humans, billfishes are prized both as food and as game fish, being popular in the later cause both for their large size and strong fight, including acrobat leaps out of the water.
The term billfish generally refers to any of the members of the families Istiophoridae and Xiphiidae (Collette et al. 2006). However, at times the term billfish is reserved for members of the family Istiophoridae (Nelson 2006; Hebrank et al. 1990). In this article, the more inclusive terminology will be used.
Nelson (2006) places the two families of billfishes, Istiophoridae and Xiphiidae, as part of the suborder Scombroidei within the family Perciformes. Perciformes ("perch-like") is the most diverse order of ray-finned fish and includes such familiar members as perches, basses, sunfishes, bluefishes, cichlids, mackerels, and tunas. The Scombroidei suborder includes fish that have an upper jaw that is not protrusible, with the premaxilla fixed (an adaptation to feeding on larger prey). The suborder includes species that likely are the fastest swimming fish in the world, including bluefin tuna, swordfish, and sailfish (Nelson 2006). Other members of the suborder include barracudas, mackerels, and snake mackerels.
However, Collette et al. (2006) maintain that billfish are morphologically and genetically distinct enough from scombroids to be placed together in a separate suborder, Xiphioidei. Nelson (2006) also notes that the two families, Xiphiidae and Istiophoridae, share enough characters that they should be considered sister groups. He notes the following more visible shared characteristics: elongate premaxillary bill (rostrum) in adults; dorsal fin origin over back of head; first dorsal fin lacking true spines and with 37-55 rays; two anal fins; mouth inferior; pectorals low on body; reduced pelvic fins with one spine and two rays or absent; gill membranes free from the istmus; and 24 or 26 vertebrate. Nelson (1994) previously placed the swordfish placed together with the sailfishes, marlins, and spearfishes in the family Xiphiidae.
The swordfish (Xiphias gladius) is the only extant member of Xiphiidae. It is characterized by a bill that is depressed; a lack of scales in the adult; the absence of pelvic fins and girdle; jaws that are toothless in the adult; 26 vertebrae; and a caudal peduncle in the adult with a single medium keel on each side (Nelson 2006).
The other billfishes typically are placed within three genera within Istiophoridae: Istiophorus (sailfishes), Tetrapturus (spearfishes), and Makaira (marlins) (Nelson 2006; Agbayani 2008). Members of this family are characterized by a bill that is rounded; scales present in the adult; a lateral line retained throughout life; elongate pelvic fins; jaws with teeth in the adult; a dorsal fin with a very long base that is sometimes sail-like and is depressible into a groove; 24 vertebrae; and a caudal peduncle in the adult with two keels on each side (Nelson 2006). Note that Collette et al. (2006) and ITIS (2008) recommend Istiophoridae be divided into five genera (see taxonomy).
In the three genera recognized by Nelson (2006), members of Istiophorus are characterized by the first dorsal fin being sail shaped and obviously taller than the body depth and with the rays of the pelvic fin very long. Tetrapturus is characterized by the forward portion of the first dorsal fin being about as high as the body is deep. Makaira is distinguished by the forward portion of the first dorsal fin being not as high as the body is deep (Nelson 2006).
A notable characteristic of the istiophorid billfishes is the remarkable ability to practice a form of endothermy, a type of warm-bloodedness where an elevated body temperature is maintained through internal means. They exhibit a type of endothermy known as cranial endothermy whereby only the brain and eyes are warmed (Nelson 1994; Block et al. 1993). They remain "cold-blooded" (specifically poikilothermic) in that they do not maintain constant internal temperatures and the temperature often mirrors the ambient temperature. But by being able to raise the temperature of their brains and eyes, they can have faster eye movements when hunting, which is valued when diving deep into the ocean where the water is very cold.
The common taxonomy, presented below, recognizes one extant species in family Xiphiidae and eleven extant species, in three genera, in family Istiophoridae, with two species in Istiophorus, six species in Tetrapturus, and three species in Makaira.
However, Nelson et al. (2004) recognize only one worldwide species in Istiophorus (I. platypterus), and Collette et al. (2006) also maintain that there is no genetic evidence to support recognizing two species of sailfish. In addition, some taxonomies only recognize two species within Makaira (syn. Istiomplax), the black marlin and the blue marlin.
Furthermore, Collette et al. (2006) maintain that phylogenetic analysis of molecular data from nuclear and mitochondrial gene sequences show that Makaira is not monophyletic and that it might be better to either group members of Istiophoridae into two genera, with the blue marlin grouped with the sailfish, or to recognize five genera. ITIS (2008) likewise recognizes the five genera of Istiompax (black marlin), Istiophorus (sailfish), Kajikia, Makaira (blue marlin, marlins), and Tetrapturus (spearfishes). ITIS (2008) also follows Collette et al. (2006) in placing Istiophoridae together with Xiphiidae in the suborder Xiphioidei.
Family Xiphiidae (swordfishes)
Billfish are exploited both as food and as game fish. Marlin and sailfish are eaten in many parts of the world, and important sport fisheries target these species, for example off the Atlantic coast of Florida. Because of worries about declining populations, sport fishermen and conservationists now work together to gather information on billfish stocks and implement programs such as catch and release, whereby fish are returned to the sea after they have been caught.
Swordfish are large and have meat that is firm and tasty, and are subject to intense fisheries pressure, and in many places where they were formerly abundant they are now comparatively rare.
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