halfbeak n : tropical and subtropical marine and freshwater fishes having an elongated body and long protruding lower jaw

English

Noun

1. Any slender, marine fish of the genus Hemirhamphus, having the upper jaw much shorter than the lower; -- called also balahoo.

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 are remarkable for showing 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 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 in the fishkeeping hobby.

Taxonomy

The first halfbeak to be scientifically described was Esox brasiliensis, by Carolus Linnaeus in 1758. Two more species were described as Esox, Esox far and Esox marginatus, by Peter Forsskål in 1775. It was not until 1816 that Georges Cuvier created the genus Hemiramphus; from then on, these three species were classified as Hemiramphus species. Hemiramphidae was erected by Gill in 1859, 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. In a 2004 review of the family, two subfamilies, 13 genera, and 117 species and subspecies were recognised.

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 Zenarchopterinae are remarkable for exhibiting strong sexual dimorphism, practicing internal fertilisation, and in some cases being 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 controversial and currently 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 4 centimetres (1.6 in) standard length (SL) in Hemirhamphodon tengah to over 40 cm (16 in) 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 demonstrate a number of adaptations to feeding at the surface of the water. The eyes and nostrils are at the top of the head and it is the upper jaw that is mobile, 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, colouration, and the length or shape of the beak. Female Normorhamphus are much larger than males but aren't as brightly coloured and have shorter beaks.

Distribution

Halfbeaks are found in warm seas, predominantly at the surface, occurring 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 have been observed to 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 predatory 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 water 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.

Behaviour

Marine halfbeaks are typically pelagic schooling fish that swim close to the surface of the sea.

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 1.5–2.5 millimetres (0.059–0.098 in) in diameter and have attaching filaments. They hatch when they are about 4.8–11 mm (0.19–0.43 in) in diameter.

• Type 1: Fertilised eggs retained within the ovarian follicle. Superfetation, that is storage of sperm, does not occur. The eggs are provided with a large yolk sac and have little or no connection to the maternal blood supply. Example: Southeast Asian populations of Dermogenys pusilla.
• Type 2: Fertilised eggs retained within the ovarian follicle. Superfetation does occur, with up to three broods resulting from a single mating. The eggs are provided with a small yolk sac but the embryos instead have a connection to the maternal supply through the coelomic cavity and pericardial sac. Examples: Dermogenys pusilla from Sabah and Dermogenys orientalis.
• Type 3: Fertilised eggs retained within the ovarian follicle only for the early stages of development, with the embryos later developing along the full length of the ovary. Superfetation does occur, and up to two broods can develop simultaneously in the ovary. The eggs are provided with a small yolk sac but the embryos have a connection to the maternal supply through an expanded belly sac. Example: Dermogenys viviparus.
• Type 4: Fertilised eggs retained within the ovarian follicle only for the early stages of development, with the embryos later developing along the full length of the ovary. Superfetation does not occur. The eggs are provided with a large yolk sac and the embryos have no connection to the maternal blood supply. Examples: Nomorhamphus megarrhamphus, Nomorhamphus weberi, and Nomorhamphus towoetii.
• Type 5: Fertilised eggs retained within the ovarian follicle only for the early stages of development, with the embryos later developing along the full length of the ovary. Superfetation does occur, and embryos of different ages can be found in the ovaries. The eggs are provided with a small yolk sac and the embryos only have a connection to the maternal blood supply for only part of their development. Late-stage embryos appear to eat eggs and small embryos in ovary.

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. One study of a bait fishery in Florida that targets Hemiramphus brasiliensis and Hemiramphus balao suggests that despite increases in the size of the fishery the population is stable and the annual catch is valued at around $500,000.

In the aquarium

Some of the smaller freshwater species are kept as aquarium fish in the ornamental fishkeeping hobby. Species of the genera Dermogenys and Nomorhamphus are quite commonly kept as aquarium fish; species of Hemirhamphodon and Zenarchopterus are rather less commonly seen. They are small and generally peaceful towards other species, although males can be aggressive to one another.

To be kept successfully, halfbeaks require an aquarium with plenty of space at the surface. Depth is not critical, so a tank that is wide is better than one that is deep. They are sensitive to low oxygen levels but are otherwise relatively hardy, with one important exception: they are intolerant to sudden changes in salinity, pH, hardness, or temperature. Consequently, they must be introduced to a new aquarium gently, and subsequent water changes are best small but frequent 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 disputer this and suggest that reports that these fish come from brackish water come from the misidentification of juvenile estuarine and marine halfbeaks as adult freshwater halbeaks.

Halfbeaks are nervous fish and shocks like sudden changes in illumination can cause them to swim around the tank frantically. They may hit themselves on the glass, injuring their beaks, or jump out of the tank completely. Injuries to the beak usually heal within a few weeks. They will eat insect larvae such as bloodworms readily, as well as crustacean eggs, shrimps, fruit flies, and even 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 a certain amount of green food probably does them good.

Halfbeaks will breed in captivity, but despite being livebearers they are not particularly easy to breed.

Conservation status

A small number of freshwater halfbeaks are listed in various categories on the IUCN Red List defining 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

Images of halfbeaks

See also

• Live-bearing aquarium fish
• USS Halfbeak (SS-352) American submarine named after these fish

References