Halfbeak

The halfbeaks (family Hemiramphidae) are a geographically widespread and numerically abundant family of epipelagic fish inhabiting warm waters around the world. The family Hemiramphidae is divided into two subfamilies, the primarily marine Hemiramphinae and the freshwater or estuarine Zenarchopterinae. The halfbeaks are named for their distinctive jaws, in which the lower jaws are significantly longer than the upper jaws. The halfbeaks show an exceptionally wide range of reproductive modes. These include egg-laying, ovoviviparity, and true vivipary where the mother is connected to the developing embryos via a placenta-like structure. In some of the livebearing species, developing embryos are also known to exhibit oophagy or intrauterine cannibalism, where developing embryos feed on eggs or other embryos within the uterus.

Though not commercially important themselves, these forage fish support artisanal fisheries and local markets worldwide. They are also fed upon by other commercially important predatory fishes, such as billfishes, mackerels, and sharks. Some halfbeaks are maintained as aquarium fish.

Taxonomy

In 1775, Carolus Linnaeus was the first to scientifically describe a halfbeak, Esox brasiliensis. In 1775 Peter Forsskål described two more species as Esox, Esox far and Esox marginatus. It was not until 1816 that Georges Cuvier created the genus Hemiramphus; from then on, all three were classified as Hemiramphus. In 1859, Gill erected Hemiramphidae, deriving its name from Hemiramphus, the family's type genus.

The family Hemiramphidae is currently divided into two subfamilies, the Hemiramphinae and the Zenarchopterinae, each containing about half the known species. A 2004 review of the family recognized two subfamilies, 13 genera, and 117 species and subspecies.

The Hemiramphinae are primarily marine and found in the Atlantic, Pacific, and Indian Oceans, though some inhabit estuaries and rivers. The Zenarchopterinae are confined to the Indo-West Pacific zoogeographic region, an area running from East Africa to the Caroline Islands. The Zenarchopterinaeexhibit strong sexual dimorphism, practicing internal fertilisation, and in some cases ovoviviparous or viviparous. Three genera in this subfamily are exclusively freshwater fish and several, such as the wrestling halfbeak, have become commonly traded aquarium fish. Apart from differences in the length of the upper and lower jaws, recent and fossil halfbeaks are distinguished by the fusion of the third pair of upper pharyngeal bones into a plate. The phylogeny of the halfbeaks is in a state of flux.

On the one hand, there is little question that they are most closely related to three other families of streamlined, surface water fishes: the flyingfishes, needlefishes, and sauries. Traditionally, these four families have been taken to together comprise the order Beloniformes.

Morphology

The halfbeaks are elongate, streamlined fish adapted to living in open water. Halfbeaks range in size from standard length (SL) in Hemirhamphodon tengah to over SL in the case of Euleptorhampus viridis. The scales are relatively large, cycloid (smooth), and easily detached. There are no spines in the fins.

Halfbeaks carry several adaptations to feeding at the water surface. The eyes and nostrils are at the top of the head and the upper jaw is mobile, but not the lower jaw. Combined with their streamlined shape and the concentration of fins towards the back (similar to that of a pike), these adaptations allow halfbeaks to locate, catch, and swallow food items very effectively.

Besides modifications to the anal fin, other differences include size, coloration, and the beak's length or shape. Female Normorhamphus are much larger than males but aren't as brightly colored and have shorter beaks.

Range and habitat

Halfbeaks inhabit warm seas, predominantly at the surface, in the Atlantic, Indian, and Pacific oceans. Most species of marine halfbeaks are known from continental coastlines, but some extend into the western and central Pacific, and one species is endemic to New Zealand. For some subtropical species at least, juveniles are more predatory than adults. Some tropical species feed on animals during the day and plants at night, while other species alternate between carnivory in the summer and herbivory in the winter. They are in turn eaten by many ecologically and commercially important fish, such as billfish, mackerel, and sharks, and so are a key link between trophic levels.

The freshwater species are more predatory than the marine species, and typically orient themselves into the current and take aquatic insect larvae, such as midge larvae, and small insects, such as flies that have fallen on the surface of the water, particularly mosquitoes and spiders.

Behavior

Marine halfbeaks are typically pelagic schooling forage fish.

Reproduction

Halfbeaks exhibit a remarkably wide variety of reproductive modes ranging from straightforward egg-laying (oviparity) through to various form of livebearing (ovoviviparity and viviparity). There is a taxonomic split in this however: all Hemiramphinae are egg-layers, while many of the Zenarchopterinae are either ovoviviparous or viviparous.

Oviparity in the Hemiramphinae

Hemiramphinae species are all external fertilizers. The eggs of Hemiramphus brasiliensis and H. balao are typically in diameter and have attaching filaments. They hatch when they grow to about in diameter. {| class="wikitable sortable" border="1" !Type!! Egg location!! Superfetation (sperm storage)!! Yolk sac!! Maternal connection!! Broods per mating!!width=15% class=unsortable|Example!!width=15% class=unsortable|Notes |- |1|| ovarian follicle || No ||Large||None||- || Southeast Asian populations of Dermogenys pusilla|| |- |2|| ovarian follicle || Yes ||Small||via the coelomic cavity and pericardial sac||3||Dermogenys pusilla from Sabah and Dermogenys orientalis|| |- |3|| ovarian follicle, briefly, then along full ovary length|| Yes ||Small||Via expanded belly sac||2||Dermogenys viviparus|| |- |4|| ovarian follicle, briefly, then along full ovary length|| No||Large||No||||Nomorhamphus megarrhamphus, Nomorhamphus weberi, and Nomorhamphus towoetii |- |5|| ovarian follicle, briefly, then along full ovary length || Yes ||Small||Partial||||Nomorhamphus ebrardtii|| Late-stage embryos are oophagous (eating eggs and small embryos)

Relationship to humans

Halfbeak fisheries

Halfbeaks are not a major target for commercial fisheries, though small fisheries for them exist in some places, for example in South Australia where fisheries target the southern sea garfish (Hyporhamphus melanochir). and the eastern sea garfish (Hyporhamphus australis). Halfbeaks are caught by a variety of methods including seines and pelagic trawls, dip-netting under lights at night, and with haul nets.

In some localities significant bait fisheries exist to supply sport fishermen.

Gambling

They are small and generally peaceful towards other species, although males can be aggressive to one another.

In the aquarium

Some of the smaller freshwater species are kept as ornamental aquarium fish., particularly genera Dermogenys and Nomorhamphus, but also Hemirhamphodon and Zenarchopterus, less commonly.

To be kept successfully, halfbeaks require an aquarium with plenty of space at the surface. Depth is not critical, so a wide tank is better than a deep one. They are sensitive to low oxygen levels but are otherwise relatively hardy, except that they are intolerant of sudden changes in salinity, pH, hardness, or temperature. Consequently, they must be introduced to a new aquarium gently, and small but frequent water changes are best, so the water chemistry does not change suddenly. A few species, most notably Dermogenys pusillius, have traditionally been kept in slightly brackish water, though some authors aver that those found in brackish water are estuarine juveniles.

Halfbeaks are sensitive and shocks like sudden changes in illumination can evoke frantic swimming activity. They may crash into the glass, injuring their beaks, or jump out of the tank. Such beak injuries usually heal within a few weeks. They will eat insect larvae such as bloodworms readily, as well as crustacean eggs, shrimp, fruit flies, and small pieces of chopped white fish. Halfbeaks sometimes eat flake foods as well. Some aquarists also offer them tiny pieces of algae wafer on the basis that most species are omnivorous in the wild, and so plant food probably suits them.

Halfbeaks breed in captivity, but despite being livebearers not easily.

Conservation status

A small number of freshwater halfbeaks are listed in various categories on the IUCN Red List assessing their risk of extinction. None of these species are traded as aquarium fish. Most are simply rare in the wild, and consequently at particular risk from habitat destruction. Dermogenys megarramphus – Lower Risk, Near Threatened Dermogenys weberi – Vulnerable Nomorhamphus celebensis – Data Deficient Nomorhamphus towoeti – Vulnerable Tondanichthys kottelati – Vulnerable Zenarchopterus alleni – Data Deficient Zenarchopterus robertsi – Lower Risk, Least Concern

See also

Live-bearing aquarium fish

USS Halfbeak (SS-352) American submarine named after these fish

References

Category:Ovoviviparous fish Category:Viviparous fish Category:Fishkeeping Category:Fish of Hawaii