Gonorynchiformes (Milkfish and Relatives)
(Milkfish and relatives)
Number of families 3
Evolution and systematics
The placement of the order Gonorynchiformes within Actinopterygii (the bony fishes) has been problematic at best. This may have been due to the enormous amount of morphological variation among gonorynchiform subgroups. For example, individuals in the genus Phractolaemus are facultative air breathers and show many morphological specializations of the head, while members of genera Cromeria and Grasseichthys are paedomorphic, and show extreme cranial miniaturization. As a result, few researchers could believe that these genera could belong to the same taxonomic group. Pivotal studies by Rosen and Greenwood in 1970 and Fink and Fink in 1981, however, demonstrated that the Gonorynchiformes do form an evolutionary assemblage and belong to the superorder Ostariophysi, as the sister-group to those fishes with a functioning Weberian apparatus (e.g., carps, minnows, and catfishes). In the studies of Gayet in 1993 and Grande and Poyato-Ariza in 1999, the taxonomic composition and evolutionary relationships within this unusual group of fishes was investigated. These researchers corroborated previous studies in the placement of Gonorynchiformes within Ostariophysi, and Grande and Poyato-Ariza divided the group into three families: Chanidae, Gonorynchidae, and Kneriidae. Characteristics identifying gonorynchiform fishes include a unique arrangement of intermuscular bones, as well as a specialized articulation between the first neural arch of the vertebral column and the back of the skull.
The Gonorynchiformes is an ancient group, with a fossil record dating back to the early Cretaceous period (about 100 million years ago). Although controversial, the habitats, or paleoecologies, of the fossil forms were most likely subtropical. This is indicated by the plants and animals collected in these deposits, whose living relatives have subtropical habitats today. Because many fossil gonorynchiforms are endemic to specific geographic localities, an understanding of their distribution patterns can provide insight into the early history of Earth. For example, well-preserved fossil chanids have been collected from the Santana Formation of eastern Brazil and the bituminous shales of Equatorial Guinea, supporting a hypothesized physical connection between the continents of South America and Africa. This connection may have lasted until the early Tertiary period. Additional chanid fossils have been collected from marine and lacustrine deposits of Germany and Spain, while fossils belonging to the family Gonorynchidae have been found in early Cretaceous marine deposits of Israel and Lebanon and from freshwater deposits of North America. This interesting fossil-distribution pattern has lead researchers (such as Jerzmanska in 1977 and Gaudant in 1993) to hypothesize a possible Tethys Sea (an ancient sea that once separated northern Africa from Asia) origin for the Gonorynchiformes, with subsequent dispersal routes throughout the Pacific Ocean. This model, however, does not discount the possible Pangaean origin of the group as proposed by Patterson in 1975.
As of 1999, 20 nominal gonorynchiform genera (7 living and 13 fossil) and about 50 species had been described. These genera are grouped into three families. The first, the Chanidae, consists of one living representative, Chanos chanos, and five extinct forms. Chanids have similar body shapes and are identifiable by a distinctively shaped permaxilla, a notch in the anterior border of the dentary, and an anteroventral process of the hyomandibular, a bone that connects the jaws to the cranium. The second family, the Gonorynchidae, is represented by the marine Indo-Pacific form Gonorynchus, its extinct sister group Notogoneus, and four Cretaceous marine groups from the Middle East. These share multiple fusions of the caudal fin skeleton; there exists a fusion of hypurals 1 and 2, as well as the parhypural with preural centrum 1. In addition, all gonorynchids have a patch of conical teeth on the gill arches, indicating that they can crush and presumably eat crustaceans and organisms with hard shells and carapaces. The third family, the Kneriidae, consists of an interesting assemblage of morphologically diverse fishes: Phractolaemus, which breathes atmospheric air; Kneria, which sports an elaborate opercular structure on the side of its body that is used as an adhesive device; and Cromeria and Grasseichthys, two miniature paedomorphs thought at one time to be juveniles. All kneriids are endemic to Africa, they live in freshwater streams and rivers and have no known fossil record. They are grouped taxonomically by distinctive modifications of the back of the skull and anterior neural arches.
The Gonorynchiformes is a morphologically diverse assemblage of fishes, ranging in body shape from the silvery herringlike chanids, to the long and slender eel-like gonorynchids, to the tiny minnowlike kneriids. The fishes also vary in size. Gonorynchus is one of the larger genera, and can achieve a standard length of over 19.7 in (50 cm), while the miniature Grasseichthys achieves an adult body length of a mere 0.71 in (1.8 cm). With the exception of Kneria, no sexual dimorphism is evident. In this species, however, males sport a predominant an opercular apparatus, a suckerlike structure. This feature, although present in females, is rudimentary. Its apparent sexually dimorphic occurrence in males has led researchers to assume that its central role is in reproduction. It seems clear that this structure is an adhesive devise and that males can attach themselves to females. Males can also attach themselves to rocks or substrate if necessary.
Gonorynchiforms exhibit a widespread geographic distribution, with representatives found on virtually all continents except Antarctica. The milkfish (Chanos chanos) and species within the genus Gonorynchus inhabit waters of the Indian and Pacific Oceans. Unlike Chanos and Gonorynchus, fossil representatives are known from separate localities. Fossil chanids, for example, are restricted to Brazil, western Africa, and Europe. Fossil gonorynchids have a more complex distribution, with several members restricted to marine Cretaceous deposits of Lebanon and Israel, whereas Notogoneus is found in freshwater deposits of Europe, North America, Mexico, Asia, and Australia. The freshwater kneriids are restricted to specific river systems surrounding the Gulf of Guinea, and the central and southern parts of the Africa. Species of the genera Kneria and Parakneria are the most geographically widespread, with ranges overlapping throughout Zaire, Angola, Tanzania, Zambia, and the Congo Basin. The species Kneria auriculata reaches the southern tip of South Africa. Phractolaemus is found in the Niger and Congo River tributaries, whereas Grasseichthys is known from streams deep in the forests of Gabon and central Congo. Cromeria, the sister species to Grasseichthys, is found in tributaries and sandy river banks of the Nile and Niger Rivers.
Gonorynchiform species inhabit both marine and freshwater systems. As adults, milkfishes live in marine open-water habitats of the Indian and Pacific Oceans. The milkfish's diadromous nature enables it to breed in inshore waters, where it produces pelagic eggs. When the larvae reach about 0.4 in (1 cm), they enter brackish pools and creeks that have limited contact with the ocean. As mature fishes they return to the sea.
Gonorynchus species often live in coastal sandy habitats. They are nocturnal and remain buried in the sand during the day, thus their common name of sandfishes. Morphologically these fishes are well adapted for living in very dark, open, deep water on the continental shelf. They have a modified lateral line system that extends posterior to the hypural plate, and large eyes that are covered by transparent skin. Sandfishes have been recorded down to a depth of 525 ft (160 m) off Tasmania, and at depths of 340–2,225 ft (104–678 m) on the Chatham Rise and Challenger Plateau off New Zealand. Gonorynchus species are thought to breed in deep water. The young are transparent and have a long pelagic postlarval stage. Not until the fish reach a standard length of about 3.5 in (9 cm) do they become benthic. This long pelagic stage in their life cycle allows for the wide dispersal of juveniles.
Little is known about the habitat and ecology of the African kneriids. Phractolaemus is thought to inhabit quiet, shaded waters and to be an epiphytic feeder. It also has a gas bladder that is divided into many alveoli, enabling it to breathe atmospheric air. Like Gonorynchus, Cromeria is found near sandy riverbanks and apparently spends much of its time buried in the sand. The habitat of Cromeria is quite different from that of Grasseichthys, in that it is found further north in more arid environments. Grasseichthys inhabits forested areas farther south.
Chanos is a schooling species, both as a juvenile and as an adult. Collection data for Gonorynchus, however, suggests that this species is solitary. A solitary behavior is also inferred from collection data for Phractolaemus.
Feeding ecology and diet
Feeding ecology seems to be variable, in that only the gonorynchids have teeth and are known to eat crustaceans. Chanos, like Phractolaemus, has a well-developed epibranchial organ and consumes planktonic prey, most often plant material.
All gonorynchiform species are oviparous, i.e., fertilization and hatching of eggs occurs outside the body. A variety of egg types exists. Chanos and Gonorynchus produce pelagic eggs, whereas Kneria is thought to produce demersal eggs. Sexual dimorphism is clearly evident in Kneria and Phractolaemus. In all Kneria species, adult males develop a characteristic opercular apparatus on the side of the head. Male fishes have been observed swimming attached to females during courtship and mating. By doing this, the male is in close proximity to the female during egg production. In Phractolaemus, large thickened keratinized breeding tubercles form on the head and along the sides of adult males. Although the presence of breeding tubercles is characteristic of ostariophysans, tubercules are particularly well developed in Phractolaemus.
No gonorynchiform species is listed by the IUCN. However, the South African government has designated at least one Kneria species as endangered, and the specialized requirements and extremely limited ranges of other species of Kneria and Parakneria render them vulnerable to the degradation of their habitats by humans.
Significance to humans
The milkfish is commercially farmed in Southeast Asia. These fishes feature in an extensive aquiculture industry in the Philippines and in Indonesia, where the young are caught close to shore and then reared in coastal ponds. The milkfish is also the subject of a targeted fishery throughout its extensive range.
List of SpeciesMilkfish
Chanos chanos Forsskål, 1775, Red Sea.
other common names
English: Bangos; French: Chanos; German: Milchfisch; Spanish: Chano, sabalote.
Standard length of over 70.9 in (180 cm). Adults are silvery herringlike fishes with a forked tail, large eyes, pointed snout with terminal mouth, cycloid scales, and an epibranchial organ. The mouth is small and terminal. The jaws are toothless. The dorsal fin has 13–17 rays; the anal fin has 6–8; the pectoral fins 15–17, and the pelvic fins 10–11. Four or five branchiostegal rays are present on each side.
Throughout the Indian and Pacific Oceans.
Diadromous; adults occur in marine open waters, larvae inhabit brackish inland ponds.
Schooling fishes, both as juveniles and adults.
feeding ecology and diet
Larvae in coastal ponds consume diatoms and copepods. Adults have well-developed epibranchial organ used as an extension of the alimentary canal and may live on plant material.
Breeds in inshore waters and produces pelagic eggs. Larvae of about 0.4 in (1 cm) enter brackish waters and as young adults return to the sea.
significance to humans
Commercially raised for food in the Philippines and Indonesia and fished extensively throughout its range. Local fishermen use cormorants with rings around the birds' necks to fish for milk-fish. The rings prevent the birds from fully swallowing the fish.
other common names
English: Beaked sandfish, mousefish, sand eel; French: Caduchon; Spanish: Caduchón; Afrikaans: Spitsbek-sandvis.
Standard length 23.6 in (60 cm). Long slender fishes; mouth inferior, dorsal fin positioned posteriorly (predorsal length at least 70% that of standard length), swim bladder absent, lateral line extends to tip of caudal fin rays, number of lateral line scales ranges from 200–220, presence of median sensory barbel on ventral side of snout, presence of a clover-shaped barbel within the mouth extending from its roof, presence of ctenoid scales that cover the entire body. The young are transparent. Uniformly brown in color, with black patches at the tips of fins.
Indian and Pacific Oceans.
Marine environments, coastal sandy habitats as well as benthic waters, can reach over 1,970 ft (600 m) in depth.
Does not school. Young have long pelagic postlarval stage that may allow for a wide dispersal of juveniles. Nocturnal, and can be found buried in sand or mud during the day.
feeding ecology and diet
A benthic feeder of small decopod crustaceans.
Breeds in deep water. Young are pelagic, subadults of about 3.5 in (9 cm) become benthic.
significance to humans
No common name
Kneria wittei Poll, 1944, Congo River basin.
other common names
Standard length 3.4 in (8.6 cm). Small minnowlike fish. Males have large opercular apparatus, females have rudimentary opercular apparatus. There is a modification and expansion of the epicentral intermuscular bones in both sexes, cycloid scales, tail forked, and mouth terminal. The color pattern is darker on top, lighter on bottom, with darker stripe along side.
Rivers and tributaries throughout Zaire, Angola, Tanzania, Zambia (i.e., central and southern Africa).
Quiet pools, but most often fast-moving streams with waterfalls.
Schooling fishes, at least during mating periods.
feeding ecology and diet
Feeds mostly on algae and plant material. Like most gonorynchiforms, has an epibranchial organ which enables it to filter particulate matter from the water column.
Males attach themselves to females via the opercular apparatus during courtship, ensuring that the largest number of demersal eggs laid by the female will be fertilized.
Not listed by the IUCN. However, like all freshwater African gonorynchiforms, habitat destruction may eventually result in an endangered status.
significance to humans
Phractolaemus ansorgei Boulenger, 1901, Niger delta. Species of the genus Phractolaemus have traditionally been placed within their own family, Phractolaemidae, but according to current research, they are closely related to kneriids and have been placed in Kneriidae by Grande and Poyato-Ariza.
other common names
English: Hingemouth, snake mudhead; German: Afrikanischer Schlammfisch.
Standard length 6 in (15 cm). Elongate, cylindrical body with large cycloid scales. Head is small, broad, and strongly ossified; eyes are small and laterally positioned. Infraorbital bones two, three, and four are greatly enlarged. The mouth is highly projectile and capable of being thrust forward; at rest the mouth folds over into a depression on the upper surface of the snout. The mouth has no teeth except for a conical tooth on each dentry near the symphysis. There is a single narial opening preceded by a barbel. The opercular openings are narrow due to a sealing of the opercular boarder to the body wall. The interopercle is spinelike, and the preopercles are greatly enlarged, overlapping along the ventral midline of the body. There are six pelvic fin rays, six dorsal rays, and six anal fin rays. Three slender branchiostegal rays are evident. Unlike other gonorynchiform fishes, the swim bladder is divided into alveoli, which enables the fish to breathe atmospheric air. Body is uniformly gray above, light brown on the sides, pale ventrally with darkly colored fins.
Tropical Africa ranging through the lower Niger drainage and central Zaire basin.
Quiet, low-oxygenated muddy waters.
Little is known about the behavior of this species. Unlike other gonorynchiforms, it is able to breathe atmospheric air, an ability that has made this fish of interest to aquarists.
feeding ecology and diet
Feeds on small, mud-dwelling organisms. Based on the presence of a moderately developed epibranchial organ, some researchers believe this species is an epiphytic feeder.
Utilizes external fertilization. Exhibits clear sexual dimorphism, as males sport conspicuous whitish breeding tubercles on the head, along the lateral line, and on the caudal peduncle.
Not listed by the IUCN. However, continued habitat destruction will undoubtedly affect the population dynamics and future of this fish.
significance to humans
Not an economically important food fish, but has been imported into the United States as an aquarium fish.
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Terry Grande, PhD