tetraodontiform

▪ fish order

Introduction

      any member of the order Tetraodontiformes, a group of primarily tropical marine fishes that evolved from the Perciformes (the typical advanced spiny-rayed fishes) during the Eocene Period of the Cenozoic Era, about 50,000,000 years ago. Included are the triggerfishes, puffers (puffer), and porcupine fishes (porcupine fish).

      The approximately 320 species of modern tetraodontiforms are notable for a high degree of diversity in anatomical structure and way of life. The great diversity evident among the 11 families of the order is also seen within some families, but not in others. Members of the deepwater, bottom-dwelling Triacanthodidae, the most primitive family, for example, range from relatively normal configurations to weirdly specialized forms with extremely long tubular snouts; the shallow-water members of the Triacanthidae, closely related and derived from the Triacanthodidae, are of rather uniform configuration. Likewise, the balistids are rather uniform in body plan; but monacanthids, which evidently evolved from them, include a series of species ranging from the normal to the exceedingly elongated and highly specialized.

General features

      The tetraodontiforms make up about 5 percent of the tropical marine fishes of the world. Most species range in size from about eight to 60 centimetres (three to 24 inches) in length, but one ocean sunfish reaches more than three metres (11 feet). They are often strikingly patterned or gaudily coloured. With the exception of the relatively deepwater Triacanthodidae and Triodontidae, the members of this order are usually found in waters less than about 65 metres (200 feet) in depth and are especially prominent around coral or rocky reefs and on open sand and grass flats.

      Many species, especially of puffer fishes (Tetraodontidae), have poisonous flesh, at least during certain seasons of the year, but most of the highly poisonous (poison) substance (tetraodontoxin) responsible for the numerous annual fatalities in Indo-Pacific regions is contained in the viscera. The flesh of the poisonous species can be safely eaten only when the freshly caught specimen has been carefully cleaned and washed in the exacting manner of fugu (or puffer fish) chefs in Japan. But the majority of tetraodontiforms are palatable, and in numerous tropical regions the flesh of various triggerfishes and trunkfishes is highly esteemed. Other than as food in tropical coastal areas, man makes little direct use of tetraodontiforms, except for the dried bodies of the hard-cased boxfishes and the spine-studded, inflated puffers as curios. In fact, the order Tetraodontiformes contains so many strangely specialized species that the group has intrigued mankind from early times; a 1st-century Roman author, Pliny the Younger, for example, discussed puffer fishes and ocean sunfishes in his Naturalis Historiae. While most adult tetraodontiforms have thick, spiny skins or other defensive mechanisms that protect them from most predacious fishes, the young, relatively defenseless stages are eaten in great quantities by certain game fishes—dolphin, marlin and other billfishes, tunas, and various jacks.

Natural history

Feeding habits

      As one would suspect from their usually well-developed and massive dentition, often with the teeth fused together in a parrotlike beak, most tetraodontiforms feed on hard-shelled crustaceans, mollusks, and echinoderms. But some, with massive, crushing jaws and teeth, such as the ocean sunfishes, often feed extensively on such soft-bodied invertebrates as jellyfishes (medusae). Some, such as boxfishes (boxfish), blow a jet of water out of their mouths onto sand bottoms to expose burrowing invertebrates; others (such as some triggerfishes (triggerfish)) specialize in eating spiny sea urchins or even clams and oysters. A few species, especially the long-snouted Triacanthodidae, have reduced or even rudimentary teeth, some apparently feeding on the scales of other bottom fishes. Other species probably feed on soft-bodied invertebrates, probing with the snout into holes in the bottom or into recesses in outcroppings to obtain food unavailable to less specialized fishes. Although many species have specialized feeding habits, the order as a whole can be considered as comprising opportunistic predators on invertebrates.

Locomotion

      Most tetraodontiforms swim by the rather unusual method of rapid undulations or complex scullings of the soft dorsal and anal fins (in the midline of the back and underside, respectively); the powerful caudal fin (except in the Molidae) is reserved for rapid bursts of speed. The paired pectoral fins (just behind the gills) are in an almost constant state of rapid vibration, which gives a delicacy of control to their movements that is unusual even among fishes.

Activity cycle

      Those tetraodontiforms for which data are available are diurnal, feeding or otherwise active during daylight but quiescent at night, often retiring to holes or crevices in coral or rocky reefs to sleep. When disturbed during the day, as by a potential predator, some species take rapid flight; others dive into reef crevices. Other species avoid the attention of predators by remarkable colorations or patternings that permit them to blend into the environment, which may be anything from a coral reef to a bed of bottom sea grass. One relatively defenseless species (a filefish), moreover, is an excellent mimic in body form and bright coloration of a spiny-skinned, inflatable, and perhaps poisonous puffer.

Form and function

      The Tetraodontiformes are distinguished externally by a small gill opening restricted to a relatively short slit on the side of the head and a small mouth, usually equipped with massive teeth. The scales of the body are typically highly modified into overlapping (in triacanthoids and balistoids) and even sutured (in ostraciontoids) plates or into sharp, projecting spines (in tetraodontoids and diodontids); in some cases, the skin itself may be thickened and hardened by deep layers of connective tissues (molids). There are no anal fin spines, and the dorsal fin spines are either absent or present only in reduced number (never more than six). The pelvic fin, which in the perciforms has one spine and five soft rays, in tetraodontiforms is either absent or reduced to no more than one spine and two small soft rays. The skeleton of tetraodontiforms is notable for a reduced number of bones, a number of the separate bony elements of the ancestral perciforms having been lost through the processes of reduction, consolidation, fusion, or failure to develop. The hallmark of the evolution and diversification of the tetraodontiforms, in fact, has been the reductive tendencies in some parts—number of skeletal elements, number of fin spines, size of mouth and gill opening, and number of teeth—with the simultaneous elaborative tendencies in other systems—scale and skin development, inflation apparatus, size of teeth and fusion with jawbones, and poisonous flesh.

Classification

Annotated classification

      The Tetraodontiformes are classified as follows, with only the most obvious external differences that distinguish the groups mentioned.

Order Tetraodontiformes (Plectognathi)

 Small mouth and gill openings; reduced dorsal and pelvic fin spines; no anal fin spines; skin usually tough or spiny.

      Suborder Balistoidei (Sclerodermi)

 Teeth separate, discrete, individual units.

      Superfamily Triacanthoidea

 Six dorsal spines and a large pelvic spine.

      Family Triacanthodidae (spikefishes)

 The most primitive members of the order; deepwater species with a truncated or rounded tail; deep caudal peduncle (the region between the end of the anal fin and the front of the tail); nonstreamlined body; soft dorsal and anal fins of about same length along their bases; Indo-Pacific and Caribbean.

      Family Triacanthidae (triple spines)

 Shallow-water derivatives of the spikefishes; deeply forked caudal fin; slender caudal peduncle; body relatively streamlined for rapid swimming; soft dorsal fin base much longer than anal fin base; Indo-Pacific, sometimes found in estuaries.

      Superfamily Balistoidea

 Two or 3 dorsal spines, the 2nd spine serving to lock the 1st in an erected position; pelvic spine rudimentary or absent.

      Family Balistidae (triggerfishes)

 Three dorsal spines; 8 outer teeth in each jaw; worldwide.

      Family Monacanthidae (filefishes)

 Two dorsal spines; 6 or fewer outer teeth in each jaw; worldwide.

      Superfamily Ostraciontoidea

 No dorsal spines, body encased in a turtle-like cuirass (carapace) of sutured, platelike scales.

      Family Aracanidae (keeled boxfishes)

 Carapace open behind the dorsal and anal fins and bearing a ventral keel; usually in deeper water than the ostraciontids; Indo-Pacific.

      Family Ostraciontidae (boxfishes, trunkfishes, cowfishes)

 Carapace closed behind anal and usually behind dorsal fin, no ventral keel; worldwide.

      Suborder Tetradontoidei (Gymnodontes)

 Teeth more or less fused to the jawbones, forming a parrot-like beak.

      Superfamily Triodontoidea

 Three tooth plates, 2 in upper and 1 in lower jaw.

      Family Triodontidae (pursefish)

 Most primitive member of the suborder, the only species to retain even the pelvic bone of the pelvic fin apparatus (completely lost by all other members of suborder); body somewhat elongate; 1 species; deep water; tropical Indo-Pacific.

      Superfamily Tetraodontoidea

 Four tooth plates, 2 in each jaw; the skin bearing small erectile spines.

      Family Tetraodontidae (puffer fishes)

 A large number of species, differing from the sharp-nosed puffers mainly in osteological structure, but always having a prominent nasal apparatus; worldwide.

      Family Canthigasteridae (sharp-nosed puffer fishes)

 Single, inconspicuous nostril on each side of head; snout more laterally compressed than in the tetraodontids; worldwide.

      Superfamily Diodontoidea

 Two tooth plates, 1 in each jaw; the skin bearing huge spines; caudal fin normal.

      Family Diodontidae (porcupine fishes and burrfishes)

 Characteristics of superfamily. Spines erectile (porcupine fishes) or fixed (burrfishes); worldwide.

      Superfamily Moloidea

 Two tooth plates, 1 in each jaw. Skin relatively smooth but often exceptionally thick; caudal fin highly modified or absent; swim bladder absent.

      Family Molidae (ocean sunfishes)

 Three species, 2 of which reach enormous size, 1 up to 3.3 metres (11 feet) in length and 1,900 kilograms (4,000 pounds) in weight; tropical and subtropical oceans worldwide.

Critical appraisal

      The classification of the order is still in a state of flux, and some authorities are willing to recognize as families only those that are listed above as superfamilies. More important, the ostaciontoids are often recognized as a third suborder of Tetraodontiformes rather than, as above, a superfamily of the suborder Balistoidei. Only recently has the ordinal term Tetraodontiformes come into use; the group still is often called the Plectognathi and its two suborders the Sclerodermi and Gymnodontes.

James Chase Tyler

Additional Reading

J.E. Böhlke and C.C.G. Chaplin, Fishes of the Bahamas and Adjacent Tropical Waters, 2nd ed. (1992), includes an excellent combination of scientific and popular accounts of the Caribbean tetraodontiforms, with all of the species well illustrated. Bruce W. Halstead, Poisonous and Venomous Marine Animals of the World, vol. 2, Vertebrates, 2nd rev. ed. (1988), contains a comprehensive review of the poisonous properties of the tetraodontiforms, with numerous illustrations of poisonous species. James C. Tyler, A Monograph on Plectognath Fishes of the Superfamily Triacanthoidea (1968), is a technical monograph on the two most generalized families of tetraodontiforms but also includes general accounts of the way of life, the distribution, and the relationships of these families, as well as an extensive bibliography on related articles.

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