Billfish

The billfish are saltwater predatory ray-finned fish from the superfamily Xiphioidea, characterised by prominent pointed bills, and by their large size; some are longer than. Extant billfish include sailfish and marlin, which make up the family Istiophoridae; and swordfish, sole member of the family Xiphiidae. They are often apex predators which feed on a wide variety of smaller fish, crustaceans and cephalopods.
Billfish are pelagic and highly migratory, and are found in all oceans. Although they usually inhabit tropical and subtropical waters, swordfish are also found in temperate waters. Billfish use their long spear/sword-like upper beaks to slash at and stun prey during feeding. Their bills have been known to impale prey, and have sometimes even accidentally impaled boats and people, but they are not intentionally used for this purpose. They are highly valued as game fish by sports fishermen.

Taxonomy

These two families were previously classified as belonging to the order Istiophoriformes. Other classifications treated them as being closely related to the mackerels and tuna within the suborder Scombroidei of the order Perciformes. However, the 5th edition of the Fishes of the World does recognise the Istiophoriformes as a valid order, albeit including the Sphyraenidae, the barracudas. Most recently, comprehensive phylogenetic analyses have found the billfish to be deeply nested within an expanded treatment of Carangiformes as the sister group to the moonfishes. For this reason, Eschmeyer's Catalog of Fishes now places both families of billfish within the suborder Menoidei of the Carangiformes.

Evolution

Several extinct families of billfish are known from the early Paleogene, including the Blochiidae, Xiphiorhynchidae, Palaeorhynchidae, and Hemingwayidae; all of these already have the elongated rostrum present in modern billfish. The earliest fossil billfishes are a Blochius-like fish from Peru and Hemingwaya from Turkmenistan, both of which are known from the Late Paleocene or earliest Eocene. Some studies suggest that Palaeorhynchidae and Hemingwayidae are stem-group billfish, while Blochiidae and Xiphiorhynchidae are crown group-billfish related to the Xiphiidae and Istiophoridae respectively. Most of these early billfish were small in size, but members of the Xiphiorhynchidae could grow to very large sizes, suggesting that billfish had already evolved large body sizes early in their evolutionary history.
Some studies have found the divergence between the Xiphiidae and Istiophoridae to date to the Late Cretaceous, while more recent studies have dated it to the early Paleocene. This ancient divergence is based on the early remains of apparent crown-group billfish such as the blochiids and xiphiorhynchids.
The enigmatic Cylindracanthus, known from the Late Cretaceous to the Eocene, is sometimes considered a "billfish" related to blochiids on the basis of its presumed rostral spines, but no other fossils are known of it aside from its rostral spines, leading to the suggestion that it had a cartilaginous body and may even be a relative of sturgeons. Similarly, the pachycormid fish Protosphyraena and the plethodid fish Rhamphoichthys from the Late Cretaceous had both convergently evolved a highly billfish-like body plan, but are known to be very distantly related to actual billfish; these genera may have instead served as a Cretaceous ecological analogue to billfish.

Species

The term billfish refers to the fishes of the families Xiphiidae and Istiophoridae. These large fishes are "characterized by the prolongation of the upper jaw, much beyond the lower jaw into a long rostrum which is flat and sword-like or rounded and spear-like."

True billfish

The 12 species of true billfish are divided into two families and five genera. One family, Xiphiidae, contains only one species, the swordfish Xiphias gladius, and the other family, Istiophoridae, contains 11 species in four genera, including marlin, spearfish, and sailfish. Controversy exists about whether the Indo-Pacific blue marlin, Makaira mazara, is the same species as the Atlantic blue marlin, M. nigricans. FishBase follows Nakamura in recognizing M. mazara as a distinct species, "chiefly because of differences in the pattern of the lateral line system".

Family	Genus	Common name	Scientific name	Maximum length	Common length	Maximum weight	Maximum age	Trophic level	FishBase	FAO	IUCN status
Xiphiidae	Xiphias	Swordfish	Xiphias gladius	455 cm	300 cm	650 kg	years	4.49			Near threatened
Istiophoridae	Istiophorus Sailfish	Atlantic sailfish	Istiophorus albicans	315 cm	cm	58.1 kg	17 years	4.50			Not assessed
Istiophoridae	Istiophorus Sailfish	Indo-Pacific sailfish	Istiophorus platypterus	340 cm	cm	100 kg	years	4.50			Vulnerable
Istiophoridae	Istiompax	Black marlin	Istiompax indica	465 cm	380 cm	750 kg	years	4.50			Data deficient
Istiophoridae	Makaira	Indo-Pacific blue marlin	Makaira mazara	500 cm	350 cm	625 kg	4.5 – 6 years	4.46			Not assessed
Istiophoridae	Makaira	Atlantic blue marlin	Makaira nigricans	500 cm	290 cm	820 kg	years	4.50			Vulnerable
Istiophoridae	Tetrapturus	White marlin	Tetrapturus albidus/''Kajikia albida Poey, 1860	300 cm	210 cm	82.5 kg	years	4.48			Least concern
Istiophoridae	Tetrapturus	Shortbill spearfish	Tetrapturus angustirostris Tanaka, 1915	200 cm	cm	52 kg	years	4.50			Data deficient
Istiophoridae	Tetrapturus	Striped marlin	Tetrapturus audax/Kajikia audax	350 cm	cm	200 kg	years	4.58			Least concern
Istiophoridae	Tetrapturus	Roundscale spearfish	Tetrapturus georgii Lowe, 1841	184 cm	cm	24 kg	years	4.37			Data deficient
Istiophoridae	Tetrapturus	Mediterranean spearfish	Tetrapturus belone Rafinesque, 1810	240 cm	200 cm	70 kg	years	4.50			Least concern
Istiophoridae	Tetrapturus	Longbill spearfish	Tetrapturus pfluegeri'' Robins and de Sylva, 1963	254 cm	165 cm	58 kg	years	4.28			Least concern

Billfish-like fish

A number of other fishes have pronounced bills or beaks, and are sometimes referred to as billfish, despite not being true billfish. Halfbeaks look somewhat like miniature billfish, and the sawfish and sawshark, which are cartilaginous fishes with long, serrated rostrums. Needlefish are sometimes confused with billfish, but they are "easily distinguished from the true billfish by having both jaws prolonged, the dorsal and anal fins both single and similar in size and shape, and the pelvic fins inserted far behind the pectorals." Paddlefish have elongated rostrums containing electroreceptors that can detect weak electrical fields. Paddlefish are filter feeders and may use their rostrum to detect zooplankton.

Structure and function of the bill

Billfish have a long, bony, spear-shaped bill, sometimes called a snout, beak or rostrum. The swordfish has the longest bill, about one-third its body length. Like a true sword, it is smooth, flat, pointed and sharp. The bills of other billfish are shorter and rounder, more like spears.
Billfish normally use their bills to slash at schooling fish. They swim through the fish school at high speed, slashing left and right, and then circle back to eat the fish they stunned. Adult swordfish have no teeth, and other billfish have only small file-like teeth. They swallow their catch whole, head-first. Billfish do not normally spear with their bills, though occasionally a marlin will flip a fish into the air and bayonet it. Given the speed and power of these fish, when they do spear things the results can be dramatic. Predators of billfish, such as great white and mako sharks, have been found with billfish spears embedded in them. Pelagic fish generally are fascinated by floating objects, and congregate about them. Billfish can accidentally impale boats and other floating objects when they pursue the small fish that aggregate around them. Care is needed when attempting to land a hooked billfish. Many fisherman have been injured, some seriously, by a billfish thrashing its bill about.

Other characteristics

Billfish are large swift predators which spend most of their time in the epipelagic zone of the open ocean. They feed voraciously on smaller pelagic fish, crustaceans and small squid. Some billfish species also hunt demersal fish on the seafloor, while others descend periodically to mesopelagic depths. They may come closer to the coast when they spawn in the summer. Their eggs and larvae are pelagic, that is they float freely in the water column. Many grow over three metres long, and the blue marlin can grow to five metres. Females are usually larger than males.
Like scombroids, billfish have both the ability to migrate over long distances, efficiently cruising at slow speeds, and the ability to generate rapid bursts of speed. These speed bursts can be quite astonishing, and the Indo-Pacific sailfish has been recorded making a burst of 68 miles per hour, nearly top speed for a cheetah and the highest speed ever recorded for a fish.
Some billfish also descend to considerable mesopelagic depths. They have sophisticated swim bladders which allow them to rapidly compensate for pressure changes as the depth changes. This means that when they are swimming deep, they can return swiftly to the surface without problems. "Like the large tuna, some billfish maintain their body temperature several degrees above ambient water temperatures; this elevated body temperature increases the efficiency of the swimming muscles, especially during excursions into the cold water below the thermocline." See heater cells for more information about these specialized modified muscle cells.
In 1936 the British zoologist James Gray posed a conundrum which has come to be known as Gray's paradox. The problem he posed was how dolphins can swim and accelerate so fast when it seemed their muscles lacked the needed power. If this is a problem with dolphins it is an even greater problem with billfish such as swordfish, which swim and accelerate faster than dolphins. In 2009, Taiwanese researchers from the National Chung Hsing University introduced new concepts of "kidnapped airfoils and circulating horsepower" to explain the swimming capabilities of swordfish. The researchers claim this analysis also "solves the perplexity of dolphin's Gray paradox". They also assert that swordfish "use sensitive rostrum/lateral-line sensors to detect upcoming/ambient water pressure and attain the best attack angle to capture the body lift power aided by the forward-biased dorsal fin to compensate for most of the water resistance power."
Billfish have prominent dorsal fins. Like tuna, mackerel and other scombroids, billfish streamline themselves by retracting their dorsal fins into a groove in their body when they swim. The shape, size, position and colour of the dorsal fin varies with the type of billfish, and can be a simple way to identify a billfish species. For example, the white marlin has a dorsal fin with a curved front edge and is covered with black spots. The huge dorsal fin, or sail of the sailfish is kept retracted most of the time. Sailfish raise them if they want to herd a school of small fish, and also after periods of high activity, presumably to cool down.