gasterosteiform

▪ fish order

Introduction

      any member of the order Gasterosteiformes, a group of fishes characterized generally by soft fin rays, pelvic fins located on the abdomen, an air bladder without a duct to the gut, and a primitive kidney. Gill structures are somewhat degenerate. Most species have bony rings around the body or ganoid (i.e., thick, bony, enamelled, and diamond-shaped) plates rather than scales. Families within the order are Gasterosteidae (sticklebacks (stickleback)), Aulorhynchidae (tubesnout) (tube snout), Indostomidae (indostomid), Aulostomidae (trumpet fishes (trumpet fish)), Fistulariidae (cornetfishes (cornetfish)), Centriscidae (shrimpfishes (shrimpfish)), Macrorhamphosidae (snipefishes (snipefish)), Solenostomidae (ghost pipefishes (ghost pipefish)), and Syngnathidae (pipefishes and sea horses (sea horse)).

      Gasterosteiform fishes occur in both salt water and freshwater and are widely distributed. The smallest species are about three centimetres (about 1¹/₄ inches) long, the largest about 200 centimetres (about 80 inches). They are of limited economic importance, but many forms are popular aquarium fishes. Two families, Indostomidae and Aulorhynchidae, are represented by only one species each.

Natural history

Reproduction and life cycle

      Except for sticklebacks, pipefishes, and sea horses, little is known of the life cycles of Gasterosteiformes. The male stickleback builds nests of plant materials cemented together with mucus secretions. The usually drab body hues of the male change, reflecting red, which is sexually attractive to the female. Male pipefishes and sea horses brood the eggs, deposited by the female within the male's brooding pouch. The brooding organ of the male sea dragon is a specialized area of soft skin beneath the tail. In some ghost pipefishes, eggs are fastened to the female on filaments of skin in pouches formed by specialized fins on the ventral, or lower, side. The dwarf sea horse, Hippocampus zosterae, breeds nine months of the year; eggs hatch after 10 days into miniatures of the adult. They mature in two or three months and live less than a year. Tube snouts deposit eggs in cavities of ascidia (primitive colonial chordates) or in masses of algae bound with threads secreted in a manner similar to that of sticklebacks. Egg clusters, often multiple, are cared for by the male. Snipefish eggs are enveloped in a mucilaginous substance from which the larvae are freed as development proceeds. Cornetfishes lay free pelagic (i.e., drifting) eggs; thus, they do not receive parental care. The reproductive habits of shrimpfishes and trumpet fishes are unknown.

Ecology and behaviour

      For defense, most gasterosteiforms assume a vertical position among grasses, gorgonians (i.e., sea fans, a type of coral), and sea urchins. Such a posture serves to camouflage them; it also tends to present body spines or shields to predators normally oriented to the horizontal plane.

      In most families, locomotion is by means of the caudal, or tail, fin. Snipefishes swim forward or backward with equal ease on the vertical plane and do not seek shelter among marine growths. The caudal fin is absent in sea horses. The coiled tail of the sea horse is used for gripping seaweed and other plants or objects. Propulsion is by means of the dorsal fin (i.e., the large fin arising from the midline of the back). Tiny pectoral fins are used for steering. Vertical movement is by swimming. All fishes with an air bladder (swim bladder) use it to some degree for vertical motion. With little effort the sea horse rises or settles to another depth by changing the air volume within the bladder.

      With the exception of the snipefishes, most gasterosteiforms live among a wide variety of aquatic growths where they find food and safety and reproduce. Certain pipefishes and sticklebacks, in particular, are able to tolerate a wide range of salinity.

Form and function

      Sticklebacks are the most varied in form. The number of spines and bony plates is greatest in individuals living in the ocean. Each heavily armoured marine species is represented by half-mailed or naked (plateless) varieties in brackish or fresh waters. They are small scaleless fishes that grow to about 15 centimetres (six inches) in length. The short jaws are well armed with sharp teeth. The body is more nearly fusiform (i.e., tapered at both ends) than are those of other members of the order. Body plates may be absent or may vary in number. The soft dorsal fin is preceded by from two to 11 free spines, each connected to the dorsal surface by its own triangular membrane. Pelvic fins are thoracic (i.e., near the midsection) in position, each with a well-developed spine and one or two soft rays. The anal fin is preceded by a spine. The caudal fin is truncate (i.e., abbreviated).

      The body of the tube snout is elongated, slender, and cylindrical. It is tipped by a prolonged snout, the small, toothed mouth of which has a hinged upper jaw. The scaleless body is armoured with series of embedded bony plates. The first dorsal fin is represented by about 25 free spines; the rayed dorsal fin is far back on the body above the anal fin. Pectoral fins are broad and the caudal fin furcate (forked).

      In the indostomids the elongated body is covered with bony rings as in pipefishes and sea horses. The small mouth is at the tip of the snout. The teeth are minute, the gills rather lobe shaped, and the eyes large. The anterior (i.e., forward) dorsal fin consists of five isolated spines. Ventral fins (i.e., paired fins arising from the sides of the belly) bear no spine, have four rays each, and are located not far behind the pectorals. The anal fin is below the soft dorsal fin, and the rounded caudal fin has a short peduncle, or stem.

      Trumpet fishes, which seldom grow to more than 30 centimetres (one foot) in length, have an elongated, compressed, scaled body; the snout is prolonged into a rigid tubelike beak. The short, weak jaws have minute teeth. There are numerous dorsal spines. The ventral fins are abdominal; the caudal fin is truncate.

      Cornetfishes, which grow to more than 180 centimetres (six feet) in length, are similar in structure to trumpet fishes; however, there are no scales. Instead, bony plates are embedded in the skin. Dorsal spines are absent, and the ventral fins are located in the abdominal region; each has a spine and four rays. Four anterior vertebrae are elongated. The backbone extends through the forked caudal fin as a long central filament.

      Shrimpfishes, also known as razor fishes, are small, with toothless jaws at the end of a long snout. Scales are absent; the back is covered by transparent plates, forming the cuirass. Anteriorly the cuirass is affixed to the ribs; posteriorly it extends beyond the displaced dorsal, caudal, and anal fins. The body is compressed to a sharp edge ventrally; hence the name razor fish.

      In snipefishes the tubular snout has short jaws. The body form is variable, but all snipefishes tend to be short and deep and partly covered with the bony-plated cuirass, which is strengthened by its union with parts of the vertebrae. Areas lacking plates sometimes have scales. In addition to having several shorter spines, the dorsal fin has a very long, strong, serrated spine reaching nearly to the tip of the caudal fin. Each ventral fin has one spine and five rays. A long snout, and two posterior “handles” of spine and tail lends an appearance that is the basis for another common name, bellows fish.

      Ghost pipefishes have a tubular snout tipped with a small mouth; the short body has spinous dorsal and ventral fins. Gills are reduced to lobe-shaped tufts attached to rudimentary gill arches. Bony plates unite to form rings. This supporting external framework has reduced the need for well-developed musculature. Ventral fins and a pointed caudal fin are large in proportion to the body size.

      Pipefishes are long and slender. The axis of the head is in line with that of the body, and the long snout is tipped with a small mouth. Bony rings replace scales. The dorsal and pectoral fins are spineless, and ventral fins are absent. Generally, the caudal fin is rounded and reduced, but it is effective in moving the fish rapidly through the water. The slender posterior body portion, though not truly prehensile (i.e., capable of coiling and grasping), can be somewhat used in that manner. Sea horses are similar to pipefishes but differ in several important respects. The head is at an angle to the body proper. This, in addition to the shape of the head, creates a somewhat horselike appearance. The tail is prehensile and lacks a caudal fin.

Evolution and classification

Paleontology

      Gasterosteiformes appears to represent an early but highly specialized branch of the Acanthopterygii. Its evolution has been traced through limited paleontological data. Fossil sticklebacks occur in Miocene (7,000,000 to 26,000,000 years ago) strata and are moderately abundant in Tertiary (2,500,000 to 65,000,000 years ago) strata. Most of the order occurs in fossil remains in Eocene (38,000,000 to 54,000,000 years ago) and Oligocene (26,000,000 to 38,000,000 years ago) strata in the area of Monte Bolca near Verona, Italy. Deposits there (and in Sumatra) include an extinct family, Protosyngnathidae, related to the tube snouts. Trumpet-fish species, a closely related extinct scaleless family, Urosphenidae, and a small species of cornetfish have also been found there. Shrimpfishes, too, are within these strata, as well as in Oligocene deposits in various parts of Europe. Snipefishes are represented by an extinct genus, as are ghost pipefishes. Pipefishes occur in Miocene deposits in Sicily. Tertiary rocks contain Syngnathus and Calamostoma, pipefish forms that have a close relationship to true sea horses. Fossil sea horses are unknown.

Distinguishing taxonomic features

      The Gasterosteiformes are classified mainly on the basis of general body form, the structure and distribution of scales or body plates, fin form and position, and the structure of the skeleton and its individual parts.

Annotated classification

      The classification here is essentially that of P.H. Greenwood et al. (Bulletin of the American Museum of Natural History, no. 131, 1966).

Order Gasterosteiformes

 Eocene to Recent. Frequently with strong spines in dorsal and pelvic fins, spines absent in some; snout often elongated; body often with dermal plates; 9 families, marine and freshwater, widely distributed. Length about 3–200 cm.

      Suborder Gasterosteoidei

 

      Family Gasterosteidae (sticklebacks)

 Jaws short, armed with sharp teeth; body quite fusiform (tapered at both ends); body plates may be absent or may vary in number; body length to about 15 cm (6 in.); 11 species, fresh, brackish, and marine waters of Northern Hemisphere.

      Family Aulorhynchidae (tube snout)

 Body elongated, slender, and cylindrical; snout long, upper jaw hinged. One species, occurs in northeastern Pacific Ocean.

      Family Indostomidae (indostomid)

 Body elongated, covered with bony rings; teeth minute, gills lobe-shaped, eyes large. One species, Indostomus paradoxus, found in Lake Indawgyi in northern Burma.

      Suborder Aulostomoidei

 

      Family Aulostomidae (trumpet fishes)

 Body elongated and compressed sideways; jaws short and weak, teeth minute; dorsal spines numerous; length to about 30 cm (12 in.); about 4 species, tropical seas.

      Family Fistulariidae (cornetfishes)

 Similar in appearance to Aulostomidae; no scales, bony plates imbedded in skin; dorsal spines absent; backbone extends through caudal fin as a central filament. Grow to more than 180 cm (71 in.); about 4 species, tropical seas.

      Family Centriscidae (shrimpfishes)

 Body small, jaws toothless, scales absent, back covered by transparent plates; 4 species, shallow waters of Indian and Pacific Oceans.

      Suborder Syngnathoidei

 

      Family Macrorhamphosidae (snipefishes)

 Snout tubular, jaws short, body rather short and deep; in profile shaped like bellows; 11 species, temperate and tropical seas.

      Family Solenostomidae (ghost pipefishes)

 Snout tubular, mouth small; body short, with spiny dorsal and ventral fins; bony plates united to form body rings; about 5 species, tropical Indo-Pacific waters.

      Family Syngnathidae (pipefishes and sea horses)

 Pipefishes long and slender, snout tipped with small mouth; dorsal and pectoral fins spineless, ventral fins absent. Sea horses with head bent downward in horselike relation to body; tail prehensile; bony rings instead of scales; about 24 species of sea horses, widely distributed, marine; about 150 species of pipefishes widely distributed in shallow tropical seas.

Critical appraisal

      The American Fisheries Society (Special Publication No. 6, 1970) arranges the Gasterosteiformes as in Greenwood et. al., above, with the following exceptions; the taxonomic level of suborder is not used; family Gasterosteidae is expanded to include family Aulorhynchidae; family Centriscidae is expanded to include family Macrorhamphosidae.

      Of the several families considered in this article, the Indostomidae are the least known. The species of this family appear to be intermediate between Gasterosteidae and Aulostomidae.

      A tenth family, Pegasidae, is tentatively placed between Gasterosteidae and Syngnathidae. Some authorities list them separately as Pegasiformes. These small fishes are the so-called sea moths, found in Asiatic seas. The toothless mouth is not terminal but lies under the head and is overhung by a snout, or rostrum, often adorned with spines. The body is protected by knobby armoured plates, with the posterior portion rather elongated, square to rectangular in cross section and bearing a small dorsal fin. Spines along either side of this region may be absent or developed to varying degrees. Pectoral fins form expansive fans on either side. Ventral fins are reduced to a few fingerlike rays used for crawling on the bottom. The pegasids have no air bladder. Swimming ability for more than short distances is poor. Fossils of Pegasidae are unknown.

Warren Zeiller

Additional Reading

American Fisheries Society, A List of Common and Scientific Names of Fishes from the United States and Canada, rev. and enlarged, 4th ed. (1980); Leo Berg, Classification of Fishes Both Recent and Fossil (1947, reprinted 1980; originally published in Russian, 1947), English and Russian; W.A. Clemens and G.V. Wilby, Fishes of the Pacific Coast of Canada, 2nd ed. (1961), limited in breadth and depth due to few order representatives in that range; Earl S. Herald, Living Fishes of the World (1961, reprinted 1972); David Starr Jordan, Fishes, rev. ed. (1925), an excellent systematic work, although the nomenclature requires revision (college level); A.H. Leim and W.B. Scott, Fishes of the Atlantic Coast of Canada (1966), a work broader in scope than Clemens-Wilby (above); N. Tinbergen, “The Curious Behavior of the Stickleback,” Scient. Am., 187:22–26 (1952), a popularized segment of the author's 1951 Study of Instinct; Gilbert Whitley and Joyce Allan, The Sea-Horse and Its Relatives (1958), a brief, supposedly popularized account of order members in Australian waters—of amazing scope, interest, and depth.

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