Osmeriformes
(Smelts, galaxiids, and relatives)
Class: Actinopterygii
Order: Osmeriformes
Number of families: 9
Evolution and systematics
Fossil Osmeriformes include Speirsaenigma lindoei, the oldest fossil osmerid from the Paleocene formations of Alberta, Canada, and a presumed close relative of the ayu (Plecoglossus altivelis; suborder Osmeroidei), Enoplophthalmus (Osmeroidei) from Europe, and possibly the family Pattersonellidae within the Argentinoidei.
The systematics of the "lower" euteleosts, particularly the Protacanthopterygii (the pikes, salmons, and smelts), has been in a constant state of flux since about the middle of the 1970s. Many phylogenies developed since that time considered that the Protacanthopterygii included three main lineages, one that included the pikes, pickerels, and mudminnows (Esociformes), another including the salmons, trouts, chars, graylings, and whitefishes (Salmoniformes), and a third comprising the freshwater and diadromous smelts, the galaxiids, the southern graylings, the marine argentines, deepsea smelts, barreleyes, and slickheads (Osmeriformes). The ancestor of the remaining teleosts, the "higher teleosts," or the Neoteleosti, is thought to be either the common ancestor of the Protacanthopterygii and the Neoteleosti, or to have risen within the Protacanthopterygii (Johnson and Patterson, 1996). However, many researchers did not necessarily formally recognize any of the above groups at the order level. Furthermore, the precise relationships of the various members of these groups has been a source of constant debate among ichthyologists researching the "lower" euteleosts (Johnson and Patterson [1996], as well as Rosen [1974], Fink [1984], Nelson [1994], and Begle [1991 and 1992]), who base their research primarily on anatomical and morphological data, and the most extensive study utilizing molecular data of Waters et al. (2000).
In light of the lack of consensus among these studies, it seems prudent, for the time being at least, to retain the traditional groupings/orders of the Esociformes, Salmoniformes, and Osmeriformes, while modifying them slightly to take into account the thorough studies of Johnson and Patterson (1996) and Waters et al. (2000). Such a scheme has been used in this chapter. However, it is likely that, with the collection of additional molecular data from a suite of taxa that represents all the major groupings, a further revision to the classification of the "lower" euteleostean fishes will occur.
If the classification given above is accepted, the Osmeriformes contains two suborders, the Osmeroidei (the smelts, galaxiids, etc.), which includes four families and approximately 70 species, and the Argentinoidei (the argentines, deepsea smelts, etc.), which includes five families and approximately 170 species. Just as it is likely that changes to the classification of this group will occur with the collection of more data, so it is also likely that the number of species will increase.
The nine families of Osmeriformes are: smelts (Osmeridae); salamanderfishes (Lepidogalaxiidae); New Zealand smelts or southern graylings (Retropinnidae); galaxiids or southern minnows (Galaxiidae); Argentines or herring smelts (Argentinidae); barreleyes or spookfishes (Opisthoproctidae); deepsea smelts (Microstomatidae); tube shoulders (Platytroctidae); and deepsea slickheads or slickheads (Alepocephalidae).
Physical characteristics
Morphological characteristics that are unique to, and present in, all members of the Osmeriformes have not yet been determined, and this is reflected in the placement of the group and its constituent members at different taxonomic levels by many researchers, including Fink (1984), Eschmeyer (1990), Johnson and Patterson (1996), and Helfman et al. (1997).
It is often stated that members of this group are generally silvery and elongate fishes. However, this probably reflects the fact that most authors are more familiar with the Northern Hemisphere species of the family Osmeridae (smelts) than with other groups. For example, even though members of the Southern Hemisphere families Galaxiidae and Lepidogalaxiidae are generally elongate, only a few have a silvery belly and none could be considered silvery overall, the majority having a brownish or olive base color overlaid with stripes, spots, or mottling of various colors. In addition, although many Argentinidae possess silvery sides, many species found in the other families of the suborder Argentinoidei, and in particular those that inhabit the deep oceans, are black or brown in color. In regard to shape, most members of the order are elongate, but the Opisthoproctidae (barreleyes) are often deep bodied and laterally compressed.
Members of the Osmeriformes are usually very small to small fishes, ranging in size from 1.2 to 27.5 in (3–70 cm);0.0016 oz to 3 lb (0.22 g–1.3 kg). The maximum length recorded for the eastern Australian galaxiid (Galaxiella pusilla) is only 1.5 in (3.9 cm), and the mean length of males and females at maturity is only 0.9 and 1.1 in (2.4 and 2.8 cm), respectively. However, some representatives of the argentines (Argentinidae) and slickheads (Alepocephalidae) may reach considerably larger sizes than the diminutive eastern Australian galaxiid, including the slickhead (Alepocephalus rostratus) of the western Mediterranean and eastern Atlantic, which attains a length of at least 27.5 in (70 cm).
There are several physical characteristics of the major groups of Osmeriformes that help to distinguish them. Helfman noted that Osmeriformes have a single dorsal fin made up of soft rays, have abdominal pelvic fins, often have an adipose fin, and generally have a toothless maxilla. The smelts (super-family Osmeroidea) are typical osmeriforms, in that they are elongate fishes with bright silvery scales, and have a dorsal fin situated toward the center of body, as well an adipose fin and a forked caudal fin. In most species, fingerlike filaments rising from the pyloric region of the stomach (pyloric ceca) are present, these structures aid digestion. Although most osmeroids are covered with bright silvery scales, others, such as the ayu (subfamily Plecoglossinae) are olive dorsolaterally and white ventrally. An adhesive membrane that acts as an anchoring mechanism surrounds the eggs of all members of the Osmeroida, and several species smell of cucumber instead of fish.
In contrast to the predominantly silvery Osmeroidea, fishes in the super-family Galaxioidea (galaxiids and relatives) often have a brown-to-olive base color overlaid with stripes, bands, or spots of various colors. Although members of the Retropinnidae and some species of the Galaxiidae, i.e., the single species Lovettia sealii (Tasmanian whitebait), and both species of Aplochiton, have an adipose fin and a deeply forked caudal fin, the other galaxiids and the salamanderfish (Lepidogalaxias salamandroides) have no adipose fin and a truncate or rounded caudal. Pyloric ceca are present in both species of Aplochiton and some other members of the Galaxiidae. The majority of the Galaxiidae are scaleless.
Like the Osmeroidea, most members of the super-family Argentinoidea (argentines, deepsea smelts, and relatives) are generally silvery, and have a dorsal fin situated toward the middle of the body and a forked caudal fin. Although some species within the Microstomatidae and the Bathylagidae (deepsea smelts) possess an adipose fin, others do not. Most members of this group have small mouths with no teeth on the upper jaw. As is the case with many fishes that are nocturnal or inhabit deep waters, many members of the super-families Argentinoidea and Alepocephaloidea (slickheads and relatives) have very large eyes, an adaptation that maximizes the collection of the little light available in poorly lit waters.
Unlike the other marine, and often deep-sea, members of the Osmeriformes, such as the Argentinoidea, the Alepocephaloidea are generally black/brown in color, have a dorsal fin situated toward the back of the body, do not possess an adipose fin, and generally have large mouths that, except for the three species of Leptochilichthyes, have teeth on the upper jaw.
Sexual dimorphism has been recorded in some members of the osmeriforms. For example, female salamanderfish attain lengths of over 2.7 in (7 cm), but the males are much smaller and have never been recorded at lengths greater than 2 in (5 cm). Furthermore, the males develop a series of enlarged scales, or a scale sheath, around their anal fin, which is absent in females. Males of many osmerids develop tubercles during the breeding season. In addition to exhibiting sexual dimorphism with regard to differences in size, some osmeriforms develop nuptial coloration. For example, during the breeding season, some galaxiids become particularly brightly colored. The black-stripe minnow (Galaxiella nigrostriata) develops a bright orange to vermillion lateral stripe, which is often highlighted by the development of dark brown to black stripes on either side.
Distribution
The first suborder of the Osmeriformes, the Osmeroidei (smelts, galaxiids, etc.), is restricted to latitudes north and south of the tropics. Within this suborder, those taxa that constitute the super-family Osmeroidea are, with the notable exception of some members of the tribe Salangini (icefishes and noodle-fishes), restricted to the cooler regions of the Northern Hemisphere. The Osmeroidea have a wide distribution, occurring in the Arctic, Atlantic and Pacific Oceans, as well as the rivers that drain into these oceans. However, they are absent from the Indian Ocean and its drainages, presumably a reflection of the fact that the vast majority of species comprising this super-family occur in cool temperate waters. They are also absent from the Mediterranean and its drainages. The tribe Osmerini (smelts) has a circumpolar distribution, with representatives in the Arctic, Atlantic, and particularly the Pacific Oceans, but the ranges of the Plecoglossinae (the ayu, or sweetfish) and the Salangini are far more restricted; both these groups are found only in the coastal waters and drainages of the eastern Pacific. However, while the Plecoglossinae and the Salangini do not occur in the Arctic or Atlantic Oceans or their drainages, their extension into the lower latitudes is far greater than any other member of the Osmeroidei. For example, the Plecoglossinae, represented solely by the ayu, has a range that extends from northern China to Korea and Taiwan in the south; the Salangini, represented by 11 species, has a range that that has a similar northern extent of its range but extends into Vietnam.
In contrast to the Northern Hemisphere Osmeroidea, the three families in the super-family Galaxioidea are found only in the Southern Hemisphere. The most widespread family is the Galaxiidae, representatives of which are found in Australia (where 22 of the 35 species occur), Lord Howe Island, New Caledonia, Tasmania, Chile, Argentina, and the tip of South Africa. Two of the remaining three families, the Retropinnidae and Prototroctidae, are found in Australia and New Zealand. Two species of the Retropinnidae, the Australian smelt (Retropinna semoni) and the Tasmanian smelt (Retropinna tasmanica), together have an extensive range that encompasses much of subtropical and temperate eastern Australia. The cucumberfish (Retropinna retropinna) and Stokell's smelt (Stokellia anisodon) are found in New Zealand. The Prototroctidae are represented in Australia by a single species, the Australian grayling, found in the coastal streams of New South Wales, Victoria, and Tasmania. The New Zealand grayling (Prototroctes oxyrhynchus) was last recorded in the 1920s. The third family of Southern Hemisphere Osmeroidei, Lepidogalaxiidae, consists of the single species, the salamanderfish, which is confined to the acid/peat flat region of the southwestern tip of Western Australia.
In contrast to the suborder Osmeroidei, whose members occur in fresh or coastal marine waters and essentially have a holarctic distribution, the obligate marine members of the suborder Argentinoidei can be found in all ocean waters from the Arctic in the north, through the Atlantic, Indian, and Pacific Oceans (including tropical and equatorial regions), to the southern ocean around Antarctica in the south. Species of the family Opisthoproctidae, subfamily Microstomatinae, are found in tropical to temperate regions of the Atlantic, Indian, and Pacific Oceans. Fishes in the subfamily Bathylaginae are represented in the subarctic to Antarctic, Atlantic, Indian, and Pacific Oceans; members of the family Argentinidae are found throughout Atlantic, Indian, and Pacific waters; and members of the families Platytroctidae and Alepocephalidae are found in all oceans.
Habitat
Osmeriforms are found in most aquatic habitats, in fresh waters in torrential mountain streams, slow-flowing rivers, ponds, lakes, and even pools that become completely dry for several months of the year, to highly variable salinity of estuaries, out into shallow inshore marine habitats, and on to the meso-and bathypelagic regions of the open ocean (deeper than 9,840 ft/3,000 m). Thus, with the exception of the freshwater systems of the lower latitudes of the Americas, Africa, the Indian subcontinent, and Australia, members of this order may be found in marine and freshwater environments off or in all of the continents. Furthermore, in the more temperate regions of the Southern Hemisphere, they are likely to be encountered in any aquatic habitat that fish biologists sample.
For example, in the fresh waters of Australia, species such as the Australian grayling and the climbing galaxias (Galaxias brevipinnis) tend to be restricted to the upper reaches of river systems, where they are found in shady areas over a rocky substrate in clear waters of moderate-to fast-flowing tributary streams. Other species, such as the barred galaxias (Galaxias fuscus), although also restricted to the headwaters of river systems, appear to prefer the slower-flowing pools below riffles and rapids. Further downstream, species such as the Australian smelt and the trout minnow (Galaxias truttaceus) tend to be found in the still to slow-flowing backwaters of streams and rivers, and to lakes and swamps, where they are found amid vegetation, logs, rocks, and other structure around the margins. Other species of Southern Hemisphere osmeriforms, such as swan galaxias (Galaxias fontanus), Clarence galaxias (Galaxias johnstoni), and Pedder galaxias (Galaxias pedderensis) are most commonly found in lakes and pools; Tasmanian mudfish (Galaxias cleaveri) and swamp galaxias (Galaxias parvus) occur in muddy marshes and swamps; and black-stripe minnow and salamanderfish inhabit small tannin-stained and acidic pools that often become completely dry in summer. Other Southern Hemisphere forms, such as the western minnow (Galaxias occidentalis), are found in all the above habitats, as well as in many of the brackish, salt-affected, streams, rivers, and lakes that are unfortunately becoming more common in southwestern Australia. Some populations of trout minnow and common jollytail (Galaxias maculatus) spend their adult lives in rivers and lakes, but move down into the brackish areas of estuaries to spawn. The larvae then move out to sea, where they spend the first year of life before moving back into rivers as juveniles.
In the Northern Hemisphere, most osmeriforms in fresh waters are usually anadromous forms on their annual spawning run, however, some species, such as the rainbow smelt (Osmerus mordax) of North America and ayu of Asia, have both anadromous or amphidromous and freshwater populations. Freshwater populations of both these species tend to spend their adult life in lakes.
As with the freshwater osmeriforms, the marine forms also occur in a wide range of marine habitats. The majority of the anadromous, amphidromous, and strictly marine Osmeroidea, such as the capelin (Mallotus villosus), are found in the shallow coastal waters out to the continental shelf, while members of the super-family Argentinoidei are found in these waters and also out over the continental shelf and into the open ocean. Although representatives of the Argentinoidei occur in all major habitats of the open ocean, only one species of the family Alepocephalidae, Microphotolepis schmidti, is known to inhabit the epipelagic zone (the top 656 ft/200 m of the water column). However, the Argentinoidei is well represented in the deeper zones of the open oceans. For example, in the Western Central Pacific Ocean (based on the FAO designation), Nansenia ardesiaca, Nansenia pelagica, and Xenophthalmichthys (subfamily Microstomatinae); Bathylagus nigrigenys and Dolicholagus longirostris (longsnout blackmelt, subfamily Bathylaginae); and Dolichopteryx longipes, Opisthoproctus grimaldii, Opisthoproctus soleatus, and Winteria telescopa (family Opisthoproctidae), are meso-to bathypelagic, meaning that they live in open waters above the bottom at depths of 656–13,120 ft (200–4,000 m). In the same region, Argentina spp. and Glossanodon struhsakeri (family Argentinidae) are benthopelagic, living on and just above the bottom, on the outer continental shelf and slopes to depths of about 4,590 ft (1,400 m). Approximately 59 species of the family Alepocephalidae and approximately 21 species of the family Platytroctidae are found above the continental shelf and ocean ridges, and are considered to be engybenthic, as they inhabit a layer of water that extends from just above the bottom to several feet above it. Although most of the species of the families Alepocephalidae and Platytroctidae are known from depths of 2,625–8,200 ft (800–2,500 m), a few species of the former are inhabitants of the abyssal depths, having been caught from more than 16,400 ft (5,000 m).
Not only are the osmeriforms found in a diverse array of habitats, they also live in some of the most inhospitable environments in which fishes are found. For example, fishes that live in the deep oceans, such as members of the suborder Argentinoidei, live in one of the most hostile environments on Earth, where little, if any, light visible to the human eye penetrates below 656–2,625 ft (200–800 m). Thus, deepsea fishes, depending on the amount of light entering their domain, must either have eyes that maximize light collection or rely on other senses in such a dark environment. Furthermore, such a lack of light in the ocean depths has two major effects; first, due to the lack of the warming light, temperatures in the bathypelagic zone are 35.6–41°F (2–5°C); second, as no light enters this deep zone, photosynthesis cannot occur and phytoplankton cannot survive, thus productivity is dependent on the "rain" of dead organisms and feces from the euphotic zone (the zone of light penetration) and food is consequently scarce. Third, low productivity is one thing, but the massive volume of the ocean deep means that the little food available is spread very thinly. If these factors were not enough to create a harsh environment, the added difficulty of living in deep water, where pressure increases by one atmosphere for every increase in depth of 33 ft (10 m), cannot be underestimated. Not only are chemical reactions affected by increases in pressure, but the gas solubility in the blood and tissues and the difficulty of secreting gas into the swim bladder are also increased.
In addition to the severe environmental conditions of deepsea environments, other osmeriforms, such as salamanderfish and black-stripe minnow, live in small, tannin-stained and acidic pools that often become completely dry in summer, habitats which are equally extreme.
Behavior
Published works describing the behavior of osmeriform species are scarce. The few that do exist generally describe the spawning migrations of the freshwater, anadromous, and coastal forms, or the migrations of juveniles of the amphidromous forms back into the rivers. For example, with the onset of winter rains, many galaxiids move upstream into tributaries and feeder streams where they spawn amongst flooded vegetation (western minnow), or on rocks (marbled galaxias, Galaxias olidus). During these upstream migrations, some species, such as climbing galaxias and western minnows, can "climb" obstacles as high as 33 ft (10 m) by jumping and wriggling in an eel-like fashion, using their pelvic and pectoral fins to lever themselves forward and upward. This is quite a feat, considering that individuals of these species are rarely larger than 6 in (15 cm). Species that have amphidromous populations, such as the common jollytail, undertake migrations. The adults move downstream to spawn in estuaries, the larvae that result then move out to sea before returning to the rivers in large schools of so-called whitebait. The spawning migrations of the amphidromous, anadromous, and coastal forms are described in the species descriptions of the ayu, eulachon, and the capelin, respectively.
Virtually nothing is known about the behavior of the deepsea osmeriforms. However, two behavior patterns are likely in at least some species. First, most mesopelagic fishes undergo vertical feeding migrations, in which they move to the epipelagic zone at night to feed on the abundant zooplankton found in this region. Such migrations have been recorded for at least one species within the Argentinoidea, the long-snout blacksmelt (Dolicholagus longirostris), so it is likely that other species also undertake such daily migrations. Second, many species of deepsea fishes have bioluminescent light-emitting organs called photophores, which are thought to be used for attracting mates (by flashing in particular pulse patterns), for attracting prey items, and for "hiding" the fishes from predators. The latter mechanism is thought to work in those species that live in regions into which some light just penetrates, whereby rows of photophores along the ventral surface reduce the chance that the fish is silhouetted when viewed from below.
Feeding ecology and diet
Although some representatives from most groups, including the capelin and the slender argentine (Microstoma microstoma), prey predominantly on small benthic and pelagic arthropods, others, such as the surf smelt (Hypomesus pretiosus) and European smelt (Osmerus eperlanus), have much wider diets, ingesting arthropods, fishes, mollusks, and worms. The Agassiz's slickhead (Alepocephalus agassizii) feeds on crustaceans, worms, echinoderms, and comb jellies. Other species of osmeriforms are, or become, specialist feeders. The mirrorbelly (Opisthoproctus grimaldii) is reported to feed on the stinging cells and tentacles of jellyfishes, and others, such as the ayu and black-stripe minnow, both exhibit marked shifts in diet at the end of their larval/juvenile stages.
Osmeriforms are preyed on by a wide variety of predators. For example, sharks, as well as a variety of bony fishes, seals, birds, and dolphins, have been reported to prey on the eulachon (Thaleichthys pacificus); bony fishes and squids prey on the argentines.
Reproductive biology
Fishes in the suborder Osmeroidei, such as the surf smelt and capelin, show a diverse array of breeding strategies, undergoing extensive migrations from coastal seas to surf beaches and estuaries, where they often form massive spawning aggregations, males pressing against females until they release their eggs, the males then releasing their sperm. The eggs are then fertilized and wave action buries them below the sand. Other members of the Osmeroidei, such as the eulachon and ayu, move up into rivers to spawn. Freshwater populations of Northern Hemisphere osmeroids tend to undergo similar spawning migrations, and may spawn in habitats similar to those of the migratory forms, such as the ayu. They may also spawn in different habitats. The freshwater forms of the European smelt tend to spawn in lakes, the anadromous forms spawn in rivers.
Within the Southern Hemisphere osmeroids, the Tasmanian smelt is anadromous, spending most of its life in the sea before returning to estuaries and rivers to spawn. In other species, such as the common jollytail, most populations are amphidromous. The adults of this species live in fresh water, but during autumn move downstream to spawn in estuaries. Spawning takes place on spring tides amid inundated vegetation, the eggs developing out of water and hatching two weeks later on the next spring tide. The larvae are then washed out to sea, where they feed and develop into juveniles before returning to the rivers to complete their life cycle. In purely freshwater forms such as the western minnow, migrations into tributaries or into the shallows of lakes are usual.
Within the Argentinoidei, stout blacksmelt (Pseudobathylagus milleri), and the longsnout blacksmelt produce planktonic eggs and larvae, and it is likely that other members of this suborder do likewise. Little other data are available on the reproductive behavior in the deepsea osmeriforms.
The age at first maturity, whether the adults survive to spawn in more than one year, and the spawning pattern, spawning period, and number and size of eggs vary greatly within the order. For example, anadromous forms of the European smelt do not spawn until their third or fourth year, when during a short breeding season of one to three months they produce 8,000 to 50,000 small (0.02–0.03 in/0.06–0.09 cm) demersal eggs. Adults usually die after spawning. Other species, such as the mudminnow (Galaxiella munda), which becomes mature at the end of its first year of life, has a spawning period that extends for three to four months, during which they produce several small batches (37–92) of smallish (0.06–0.05 in/0.09–0.14 cm) demersal eggs. Most mudminnow die within one or two months of spawning. Unlike the above two species, the western minnow spawns at the end of its first year and can live to spawn a further four times.
The salamanderfish is unique in the Osmeriformes in that, rather than broadcasting its eggs and sperm, it undergoes internal fertilization.
There are no reports of parental care in the Osmeriformes.
Conservation status
Eight species of Osmeriformes are listed by the IUCN. One species is classified as Extinct, one species as Endangered, one species as Critically Endangered, three species as Vulnerable, and two species as Data deficient. In addition, many Australian forms are considered by the Australian Society for Fish Biology to be data deficient to endangered, largely because of land clearing and the subsequent degradation of freshwater habitats. Many deepsea forms are small and rare, and thus are likely to be overlooked by conservationists.
Significance to humans
Due to their generally small size and/or inaccessibility (in deepsea forms), most Osmeriformes are of little importance to humans, either commercially or recreationally. However, in the Northern Hemisphere, eulachon, capelin, European smelt, and ayu form the basis of important local commercial and recreational fisheries during their spawning migrations, and in the Southern Hemisphere, the juveniles of the anadromous species, such as the Tasmanian whitebait and common jollytail, form the basis of important commercial whitebait fisheries in New Zealand and Chile. In Australia, massive falls in whitebait capture during the 1970s led to the closure of the fishery. However, it has since been reopened as a tightly regulated recreational fishery.
Species accounts
California slickheadGreater argentine
Western minnow
Black-stripe minnow
Salamanderfish
California smoothtongue
Barreleye
Capelin
Ayu
Eulachon
Australian smelt
Resources
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Sazonov, Y. I. "Platytroctidae." In Living Marine Resources of the Western Central Pacific. Vol. 3, Batoid Fishes, Chimaeras and Bony Fishes, Part 1 (Elopidae to Linophrynidae), edited by K.E. Carpenter and V. H. Niem. Rome: FAO, 1999.
Sazonov, Y. I., and D. F. Markle. "Alepocephalidae." In Living Marine Resources of the Western Central Pacific. Vol. 3, Batoid Fishes, Chimaeras and Bony Fishes, Part 1 (Elopidae to Linophrynidae), edited by K. E. Carpenter and V. H. Niem. Rome: FAO, 1999.
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[Article by: Howard Stamper Gill, PhD]
