(Milkfish and relatives)
Class: Actinopterygii
Order: Gonorynchiformes
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.
Physical characteristics
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.
Distribution
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.
Habitat
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.
Behavior
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.
Reproductive biology
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.
Conservation status
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.
Species accounts
MilkfishSandfish
Kneria wittei
African mudfish
Resources
Books:Aizawa, M. "Gonorynchidae." In Fishes Collected by the Shinkai Maru Around New Zealand, edited by K. Amaoka. Tokyo: Japan Marine Fishery Resource Research Center, 1990.
Grande, T. "Distribution Patterns and Historical Biogeography of Gonorynchiform Fishes (Teleostei: Ostariophysi)." In Mesozoic Fishes: Systematics and the Fossil Record: Proceedings of the International Meeting, Buckow, 1997, edited by G. Arratia and H. P. Schultze. Munich: Pfeil, 1999.
Roberts, C. D., and T. C. Grande. "The Sandfish Gonorynchus forsteri (Gonorynchidae), from Bathyal Depths off New Caledonia, with Notes on New Zealand Specimens." In Proceedings of the 5th Indo-Pacific Fish Conference, edited by B. Séret and J.-Y. Sire. Paris: Société Française d'Ichtyologie; Institut de Recherche pour le Développement, 1999.
Periodicals:Bertmar, G., B. G. Kapoor, and R. V. Miller. "Epibranchial Organs in Lower Teleostean Fishes: An Example of Structural Adaptation." Trop. Atlan. Biol. Lab., Bureau of Comm. Fish. 76 (1969): 149.
Cope, E. D. "On Two New Forms of Polydont and Gonorhynchid Fishes from the Eocene of the Rocky Mountains." Memoirs of the National Academy of Sciences 3(1885): 161–165.
Fink, S. V., and W. L. Fink. "Interrelationships of Ostariophysan Fishes (Teleostei)." Zoological Journal of the Linnean Society 72, no.4 (1981): 297–353.
Forsskål, P. "Descriptones animalium, avium, amphibiorum, piscium, insectorum, vermium: quae in itinere orientali observait." Postmortem auctoris edidit Carsten Niebuhr 20, no. 34 (1777): 1–164.
Gaudant, J. "The Eocene Freshwater Fish Fauna of Europe: From Paleobiogeography to Paleoclimatology." Kaupia 3(1993): 231–244.
Gayet, M. "Relations Phylogénétiques de Gonorhynchiformes (Ostariophysi)." Belgian Journal of Zoology 123, no. 2 (1993): 165–192.
Grande, T. "Revision of the Genus Gonorynchys Scopoli, 1777 (Teleostei: Ostariophysi)." Copeia 2 (1999): 453–469.
Grande, T., and F. J.Poyato-Ariza. "Phylogenetic Relationships of Fossil and Recent Gonorynchiform Fishes (Teleostei: Ostariophysi)." Zoological Journal of the Linnean Society 125(1999): 197–238.
Grande, T., and B. Young. "Morphological Development of the Opercular Apparatus in Kneria wittei (Ostariophysi: Gonorychiformes) with Comments on Its Possible Function." Acta Zoologica 78, no. 2 (1997): 145–162.
Jerzmanska, A. "Süßwässerfische des alteren Tertiärs von Europe." In Eozäne Wirbeltier des Geiselatles, edited by H.W. Matthes and B. Thaler. (1977): 67–76.
Linnaeus, C. "Systema Naturae." Laurentii Salvii 12, no.1 (1766): 528.
Morioka, S., A. Ohno, H. Kohno, and Y. Taki. "Recruitment and Survival of Milkfish Chanos chanos." Japanese Journal of Ichthyology 40, no. 2 (1993): 247–260.
Patterson, C. "The Distribution of Mesozoic Fishes." Mèmoires du Muséum national d'Histoire naturelle, Paris 88, sèr. A(1975): 156–173.
Poll, M. "Descriptions de poissons nouveaux recueillis dans la region d'Albertville (Congo Belge) par le Dr. G. Pojer." Bulletin du Musée Royal d'Histoire naturelle de Belgique 20, no. 3 (1944): 1–12.
Rosen, D. E., and P. H. Greenwood. "Origin of the Weberian Apparatus and the Relationships of the Ostariophysan and Gonorynchiform Fishes." American Museum Novitates 2428(1970): 1–25.
Seegers, L., "Revision of the Kneriidae of Tanzania with Description of Three New Kneria Species (Teleostei: Gonorynchiformes)." Ichthyol. Explor. Freshwaters 6, no.2(1995): 97–128.
[Article by: Terry Grande, PhD]
