Halfbeak
2008/9 Schools Wikipedia Selection. Related subjects: Insects, Reptiles and Fish
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 A marine halfbeak, the ballyhoo, Hemiramphus brasiliensis
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Hemiramphinae Gill, 1859 |
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 name comes from the Greek hemi, meaning half, and rhamphos, meaning a beak or bill.
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. More recently, Hemiramphidae has been listed to include 12 genera and about 109 species, with the genus Oxyporhamphus moved to Exocoetidae.
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. It is believed by some authors that the recognition of Zenarchopterinae as a separate family (Zenarchopteridae) probably should be accepted.
Evolution
The fossil record of the halfbeaks extends into the Lower Tertiary. The earliest known halfbeak is Brachyrhamphus bolcensis from the Eocene at Monte Bolca, Italy. 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.
Phylogeny
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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. The halfbeaks and flyingfishes are considered to form one group, the superfamily Exocoetoidea, and the needlefishes and sauries another, the superfamily Scomberesocoidea.
On the other hand, recent studies have demonstrated that rather than forming a single monophyletic group (a clade), the halfbeak family actually includes a number of lineages ancestral to the flyingfishes and the needlefishes. In other words, as traditionally defined, the halfbeak family is paraphyletic.
Within the subfamily Hemiramphinae, the "flying halfbeak" genus Oxyporhamphus has proved to be particularly problematical; while morphologically closer to the flyingfishes, molecular evidence places it with Hemiramphus and Euleptorhamphus. Together, these three genera form the sister group to the flyingfish family. The other two hemiramphine genera Hyporhamphus and Arrhamphus form another clade of less clear placement.
Rather than being closely related to the flyingfishes, the subfamily Zenarchopterinae appears to be the sister group of the needlefishes and sauries. These is based on the pharyngeal jaw apparatus, sperm ultrastructure, and molecular evidence. However, this hypothesis has awkward implications for how the morphological evolution of the group is understood, because the fused pharyngeal plate has been considered reliably diagnostic of the halfbeak family. Furthermore, the existing theory that because juvenile needlefish pass through a developmental stage where the lower jaw is longer than the upper jaw (the so-called "halfbeak stage") the theory that halfbeaks are paedomorphic needlefish is untenable. In fact the unequal lengths of the upper and lower jaws of halfbeaks appears to be the basal condition, with needlefish being relatively derived in comparison.
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. A peculiarity shared by all halfbeaks that distinguishes them from the other fishes in the Beloniformes is that the third pair of upper pharyngeal bones are anklylosed (fused) into a plate. Halfbeaks are one of a number of fish families that lack a stomach, all of which possess a pharyngeal jaw apparatus (pharyngeal mill). Most species have an extended lower jaw, at least as juveniles, though this feature may be lost as the fish mature, as with Chriodorus, for example.
As is typical for surface dwelling, open water fish, most species are silvery, and darker above and lighter below, an example of countershading. The tip of the lower jaw is bright red or orange in most species. Small patches of colour, particularly among males, are only found on the fins and the tip of the beak.
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.
Sexual dimorphism
Sexual dimorphism is apparent in some species of halfbeak. Males of the ovovivaparous and vivaparous species all have a modified anal fin, the andropodium, similar to the gonopodium of poecilid livebearers, used to deliver sperm to the females. Although most of the egg laying species mate by shedding the milt externally, as is typical for bony fish, at least some egg-laying species practise internal fertilisation: male Zenarchopterus use a modified anal fin to direct sperm into the genital opening of the female prior to spawning.
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. By contrast, male Hemirhamphodon are larger than the females, and some species, such as Hemirhamphodon pogonognathus, also have a long beard-like tassle on the end of the beak.
Distribution
Halfbeaks are found in warm seas, predominantly at the surface, occurring in the Atlantic, Indian, and Pacific oceans. A small number are found in estuaries and some species, including all the species in the genera Dermogenys, Hemirhamphodon, Nomorhamphus, and Tondanichthys are confined to freshwaters. 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. Hemiramphus is a worldwide marine genus.
The freshwater halfbeaks all occur in freshwater or estuaries in the Indo-West Pacific region. There is a particularly high concentration of species on the island of Celebes.
Ecology and behaviour
Diet
Marine halfbeaks are omnivores feeding on algae; marine plants such as seagrasses; plankton; invertebrates such as pteropods and crustaceans; and smaller fishes. 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. The southern sea garfish Hyporhamphus melanochir for example is found in sheltered bays, coastal seas, estuaries around southern Australia in waters down to a depth of 20 m (66 ft). These fish form schools near the surface at night but swim closer to the sea floor during the day, particularly among beds of seagrasses. Genetic analysis of the different sub-populations of the eastern sea garfish Hyporhamphus melanochir in South Australian coastal waters reveals that there is a small but consistent migration of individuals between each sub-population, sufficient to keep them genetically homogenous.
Freshwater halfbeaks vary in social behaviour from open water schooling fish similar to the marine halfbeaks, as with species of Zenarchopterus, through to much more aggressive and combative fishes, as is best known with the "wrestling" halfbeaks of the genus Dermogenys. These non-schooling freshwater halfbeaks prefer to lurk among aquatic plants such as reeds, dead trees, and artificial structures of various types; from there they will wait for small prey animals to drift by or alight on the surface, and then the halfbeak will dart out of their hiding place to capture its prey. Notably, these freshwater halfbeaks feed extensively on female mosquitoes that are laying their eggs in the water, making them much better at mosquito control that species like guppies and mosquitofish that only take mosquito larvae.
Some marine halfbeaks, including Euleptorhamphus velox and Euleptorhamphus viridis, are know for their ability to jump out of the water and glide over the surface for considerable distances, and have consequently sometimes been called flying halfbeaks.
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. They are usually egg-layers and often produce relatively small numbers of fairly large eggs for fish of their size, typically in shallow coastal waters, such as the seagrass meadows of Florida Bay. 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. Hyporhamphus melanochir eggs are slightly larger, around 2.9 millimetres in diameter, and are unusually large when they hatch, being up to 8.5 millimetres in size.
Relatively little is known about the ecology of juvenile marine halfbeaks after hatching, though estuarine habitats seem to be favoured by at least some species. The southern sea garfish Hyporhamphus melanochir grows rapidly at first, attaining a length of up to 30 centimetres in the first three years, after which point growth slows down. This species lives for a maximum age of about 9 years, at which point the fish will be up to 40 centimetres and weigh about 0.35 kilogrammes.
Viviparity in the Zenarchopterinae
The freshwater halfbeaks of the genera Dermogenys, Hemirhamphodon, and Nomorhamphus are all livebearers, that is, they do not lay eggs but instead produce well-developed free-swimming young. However, there is a great deal of variation in the details. Meisner and Burns identified no fewer than five distinct modes of viviparity and ovovivparity in the freshwater halfbeaks:
- 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. Note that embryos eating eggs and other embryos has been observed in a few other fishes, most notably sharks (see article on oophagy). Example: Nomorhamphus ebrardtii.
As with other livebearing fish, freshwater halfbeaks produce small broods of large offspring compared with egg-laying species of similar size, with broods of around ten to twenty, 10–15 mm long offspring being typical.
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. They are utilized fresh, dried, smoked, or salted, and they are considered good eating. However, even where halfbeaks are targeted by fisheries, they tend to be of secondary importance compared with other edible fish species.
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. Male Dermogenys pusillius in particular fight vigorously and sometimes these battles end in injuries; this fish has therefore become known as the wrestling halfbeak and in some Asian countries fights between males are used for betting purposes in much the same way as the Siamese fighting fish.
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. Miscarriages are common, particularly if the females are stressed or shocked (for example, by being moved to another aquarium). Once the fry have been born, things get much simpler, as the baby halfbeaks are quite big and will eat newly hatched brine shrimps, small live foods such as daphnia, and powdered flake.
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