The silversides or sand smelts are found in coastal areas of temperate to tropical seas and in fresh water. There are about 60 species (Nelson, 2006) with only one reported from the Caspian Sea and Iran. Most are small fishes with a maximum length of 60 cm.

These small, silvery fishes have a moderately elongate body; usually cycloid and moderately large scales; no lateral line but sometimes a pit on each scale; small teeth in the jaws and sometimes on the roof of the terminal, upwardly-directed mouth; wide gill openings with branchiostegal membranes free of the isthmus; 5-6 branchiostegal rays; gill rakers usually long and slender; two well-separated dorsal fins, the first with 3-10 unbranched but flexible rays, and the second with 1-2 unbranched and the rest branched rays; anal fin long; pectoral fins high on the flank; no pyloric caeca; egg membranes with filamentous outgrowths; back bluish to greenish with small melanophores but translucent; and a silvery stripe along the flank, often distinctively outlined with black.

Silversides can occur in vast schools in inshore waters and are an important item in the diet of other fishes. They have been used as bait but are too bony to be much used as food. Their food is plankton. Eggs are large and greenish. The sticky egg filaments entangle with plants, rocks or sand as anchors until hatching.

Genus Atherina
Linnaeus, 1758

Members of this speciose genus are found in fresh and brackish waters with a single representative in Iran. The record of Atherina hepsetus Linnaeus, 1758 in the Caspian Sea by Quignard and Pras in Whitehead et al. (1984-1986) is an error (Vasil'eva, 1994).

The body is compressed with a rounded belly, the mouth large and terminal, jaws large, reaching back to the anterior eye level or beyond, sides of the upper jaw are straight and premaxillaries protractile, the dentary bone has a high central portion, a preopercular notch is absent, rows of setiform teeth on the jaws, vomer and palatines, cycloid scales extending onto the head, short pectoral fins, vent nearer the pelvic fin origin than the anal fin, numerous gill rakers, 5-10 flexible rays in the first dorsal fin, and the second dorsal fin is similar in length to the anal fin with its origin above the anal fin.

Atherina boyeri
Risso, 1810

Common names

shisheh mahi (= glass fish), گل آذين ماهي (gol azin mahi, meaning flower decoration fish), atrinka (from the Russian).

[aterinka or xazar aterini in Azerbaijan; kaspi aterinasy in Turkmenian; Kaspiiskaya aterinka or Caspian silverside in Russian; Caspian sand smelt, Boyer's sand smelt, big-scale sand smelt].

Systematics

Atherina Boyeri was originally described from the sea shore and lower course of rivers in the Département du Var, France. Reshetnikov et al. (1997) give the date for this species as 1826. Atherina presbyter var. caspia Eichwald, 1831 described from "in Caspii maris littore australiore, sinu balchanensi" (i.e. southern shore of the Caspian Sea, Balkhan Bay) is a synonym.

Syntypes are in the Muséum national d'Histoire naturelle, Paris under MNHN A-4342 (2) and MNHN B-860 (1) (Eschmeyer's "Catalog of Fishes", downloaded 29 August 2007).

Formerly called Atherina mochon pontica natio caspia Eichwald, 1831 or Atherina mochon caspia in the Caspian Sea. Atherina mochon Cuvier, 1829 was described originally from Ivasa and Atherina presbyter pontica Eichwald, 1831 from near Odessa, Ukraine.

Bamber and Henderson (1985) demonstrated how the morphology of this species around Britain varies according to temperature and salinity during embryonic development. Isolation of populations results in local selection and random genetic drift and thus recognisable morphologies but these were not recognised as distinct. Kottelat (1997) reviews literature reports of variation in this species in the Mediterranean and Black seas and neighbouring fresh waters and concludes that lacustrine populations should be called A. boyeri while marine populations are A. mochon. The status of Caspian populations is not commented on.

The silverside in the Caspian Sea was referred to as Atherina boyeri caspia by Savenkova and Asanov (1991) and by Vasil'eva (1994; 1996b) and this seems a reasonable step as the population is isolated from other populations in the Atlantic Ocean and Mediterranean Sea. The subspecies is characterised by a reduced number of infraorbital bones (3 bones in Caspian Sea fish as opposed to 4 in Black Sea fish; apparently infraorbital bones 2 and 3 fuse), a reduced number of gill rakers (generally 19-27 as opposed to 27-37), and the form of the maxilla (the inferior margin in Caspian Sea fish is always smooth while in the Black Sea and Sea of Azov a "wing" is usually developed). However Kiener and Spillman (1972) allocated Caspian populations to this subspecies on the basis of larger size, hardly an adequate criterion. Vasil'eva (1994) makes the allocation based on a reduced number of gill rakers, reduction of the number of bones around the eye to three (preorbital, infraorbital and postorbital) and a smooth lower margin to the maxilla without a wing protuberance.

Dobrovolov and Ivanova (1999) studied two non-enzymatic proteins and 11 enzymes for putative Atherina boyeri and A. mochon pontica. They concluded that these are distinct species and indeed the Black Sea fish are a distinct species, A. pontica. They diverged 2.316MYA. These authors did not examine Caspian Sea material. Miller in Miller (2003) followed a conservative approach, regarding the various named and wide-ranging populations as representing a single polymorphic species.

Key characters

The two dorsal fins, cycloid scales, pectoral fin high on the flank and the vent remote from the anal fin are characteristic.

Morphology

First dorsal fin spines 5-10; second dorsal fin with 1-2 spines and 8-15 soft rays; anal fin with 1-2 spines and 9-18 soft rays; pectoral branched rays 10-17, usually 12-15; pelvic fin with 1 spine 5-7, usually 5, branched rays; lateral series scales 37-53, possibly to 61, see Vasil'eva (1994); total gill rakers 19-27 in the Caspian Sea, to 39 elsewhere, spinulose on the interior surface, and long and reaching about 7 rakers along the arch when appressed; and vertebrae 39-52. The anus is 4-5 scales in advance of the anal fin. Scales are higher than broad, with slight indentations on the otherwise straight dorsal and ventral margins, a rounded posterior margin, and a wavy to rounded anterior margin with a protuberant central point. There are no radii. Circuli are restricted to the anterior third of the scale with a central and vertical roughened area posterior to the circuli, presumably made up of fragmented circuli. The focus is central. The lower jaw symphysis fits into a notch in the upper jaws. The haemal arches of the anterior 4-7 caudal vertebrae are expanded around the swimbladder. Gut s-shaped. The chromosome number is 2n=48 (Klinkhardt et al., 1995).

Meristic values for Iranian specimens are: first dorsal fin spines 8(28), 9(5) or 10(1); second dorsal fin soft rays 11(10), 12(21), 13(2) or 14(1); anal fin soft rays 13(7), 14(15), 15(9) or 16(3); pectoral fin branched rays 12(1), 13(12), 14(14) or 15(6); pelvic fin branched rays 5 (34); caudal fin branched rays 13(1), 15(30), 16(2) or 17(1); lateral series scales 50(1), 51(10), 52(5), 53(3), 54(8), 55(3), 56(3) or 57(1); predorsal scales 17(1), 18(2), 19(12), 20(8), 21(3), 22(3), 23(1), 24(3) or 26(1); caudal peduncle scales 12(31), 13(2) or 14(1); transverse scales from anal fin antero-dorsally 10(2), 11(6), 12(23) or 13(3); total gill rakers 24(3), 25(4), 26(14), 27(8), 28(4) or 29(1); total vertebrae 45(1), 46(19), 47(7) or 48(7).

Sexual dimorphism

Females tend to be larger than males and there are some differences in morphometrics and meristics (Ghoorbanalidoost et al., 2003).

Colour

General colour is given in the family account. The lateral band is strongly developed and bright in Caspian Sea specimens. The belly is white, fins are pale to translucent grey. Lagoon specimens are brownish or grey-brown on the back. The peritoneum is brown to black, eggs being encased in a black peritoneum while the abdomen wall is a light brown.

Size

Reaches 11.8 cm and 17.6 g in the Atrak River (Savenkova and Asanaov, 1991), to 14.5 cm standard length in the Caspian Sea basin generally, and elsewhere to 19.0 cm standard length (Henderson and Bamber, 1987). Kiener and Spillman (1972) found a maximum of 15.4 cm in their Caspian Sea sample.

Distribution

Found from the shores of Europe to the Black and Caspian seas, and in the Uzboi Valley of Turkmenistan. In Iran, it is reported along the whole Caspian Sea coast, and from the the Safid River, Talar River, Anzali Mordab and Gorgan Bay (Abbasi et al., 1999; Holčík and Oláh, 1992; Roshan Tabari, 1997; Kiabi et al., 1999).

Zoogeography

This species probably entered the Caspian Sea from the Black Sea during Khvalyn transgression (10-70,000 years B.P.) via the Kumo-Manych Depression (Kosarev and Yablonskaya, 1994). Berg (1948-1949) contends that it entered the Caspian from the Black Sea in post-glacial times while most other Caspian fishes are relicts of earlier transgressions or migrants from northern waters.

Habitat

The Caspian silverside is a schooling fish found at depths exceeding 100 m but is concentrated at 10-20 m. Salinity tolerance is high, up to 77‰ (Miller in Miller, 2003), up to 60‰ in the Atrak River (Savenkova and Asanov, 1991). Reproduction can occur up to 42‰> while preferred levels are 3-12‰. A wide temperature range is tolerated, 0-31ºC (Miller in Miller, 2003). It is also found in lagoons and river mouths, and enters rivers to spawn, against currents up to 1.2 m/sec. Holčík and Oláh (1992) report its apparent recent occurrence in the Anzali Mordab in response to increased salinity there. It is also known from fresh water in Lenkoran.

Age and growth

Maturity is attained in the first year and life span varies from 1 to 5 years, 4-5 years usually in the Caspian Sea basin (Henderson and Bamber, 1987). Miller in Miller (2003) summarises growth and population dynamics from populations outside the Caspian Sea. Ghoorbanalidoost et al. (2003) found populations in the south Caspian Sea to be 73.82 mm long on average and 3.15 g in weight, with age  groups 1-3 years, average 1.7 years, total length and body weight relationship W = 0.00000615L^3.02 and a sex ratio of male:female = 0.47:0.53.

Food

Savenkova and Asanov (1991) report plankton, eggs and juvenile fishes to be food items in the Atrak River of Turkmenistan. Some populations also eat benthic organisms such as amphipods, worms and molluscs. Iranian specimens contained encysted cladocerans and beetles in their guts. It has a trophic plasticity, adapting to whatever conditions obtain (Miller in Miller, 2003).

Reproduction

Spawning in this species is intermittent and occurs along the coast from May to July, peaking in mid-May to mid-June (Savenkova and Asanov, 1991) or from April to August (Henderson and Bamber, 1987). Preserved Iranian fish samples have relatively large eggs from at April to September, e.g. 1.4 mm on 27 April, 1.6 mm on 14 May. Savenkova and Asanov (1991) also studied the annual spawning migration into the Atrak River (lower reaches in Turkmenistan, upper reaches shared with Iran). Fish are caught in a fish ladder 18-19 km from the sea and in the river. The first schools appear in the Atrek mouth as early as mid-February at water temperatures of 8-10°C but the mass migration takes place in mid to late March and the first half of April at 14-22°C. Spawning occurs in March, April and May but most intensively in April at flood water temperatures of 13.7-23.0°C. There is a larger migration in high-water years. Sex ratio is about 1:1. A female may be able to spawn 5-6 times in one season so that egg numbers and diameters vary within each individual for different generations of eggs. Egg deposition in the Caspian Sea is associated with the alga Cladophora to which the eggs are attached or entangled by long filaments (up to 15) and this plant is present in the Atrak. Egg diameters are up to 2.0 mm (3.5 mm in Europe, but possibly these are water hardened) and the light yellow eggs number up to 568 (Gon and Ben-Tuvia, 1983) although the Caspian Sea Biodiversity Database (www.caspianenvironment.org) gives 5500 eggs per female (presumably total seasonal fecundity). Only one gonad develops. Larvae are pelagic but may school close to shore.

Henderson and Bamber (1987) and Gon and Ben-Tuvia (1983) review the reproductive biology of this sand smelt. It can rapidly adjust its life history to a range of environments, from fresh water to coastal water.

Parasites and predators

The Caspian seal, Pusa caspica, is a predator on this species (Krylov, 1984) as are the larger fishes such as Stenodus leucichthys and Alosa saposchnikowii (Lönnberg, 1900a).

Economic importance

This species is food for a number of other fishes including such economic species as sturgeons, Sander and the predacious shads. It has been caught as a bycatch in kilka seine nets and used in fishmeal production. In Europe it has been sold fresh or canned.

Conservation

Kiabi et al. (1999) consider this species to be of least concern in the south Caspian Sea basin according to IUCN criteria. Criteria include abundance in numbers, widespread range (75% of water bodies), absent in other water bodies in Iran, and present outside the Caspian Sea basin.

Further work

The biology of this species in Iranian waters and its interrelationships with commercial species has not been investigated.

Sources

Some meristics are taken from Altun (1999) and Trabelsi et al. (2002).

Iranian material: CMNFI 1970-0507, 11, 22.2-56.3 mm standard length, Gilan, Caspian Sea at Hasan Kiadeh (37º24'N, 49º58'E); CMNFI 1970-0509, 5, 49.5-101.7 mm standard length, Gilan, pond at Hasan Kiadeh (37º24'N, 49º58'E); CMNFI 1970-0543A, 7, 52.9-83.1 mm standard length, Gilan, Caspian Sea at Hasan Kiadeh (37º24'N, 49º58'E); CMNFI 1970-0563, 18, 39.8-112.5 mm standard length, Gilan, Kazian Beach (ca. 37º29'N, ca. 49º29'E); CMNFI 1970-0581, 5, 47.6-54.8 mm standard length, Caspian Sea near Hasan Kiadeh (37º24'N, 49º58'E); CMNFI 1971-0343, 2, 65.9-72.5 mm standard length, Gilan, Langarud at Chamkhaleh (37º13'N, 50º16'E); CMNFI 1979-0081, 3, 74.1-77.3 mm standard length, Mazandaran, Caspian Sea, 3 km west of Chalus (36º41'N, 51º24'E); CMNFI 1980-0146, 4, 92.1-103.6 mm standard length, Mazandaran, Gorgan Mordab at Ashuradeh-ye Kuchak (36º50'N, 53º56'E); and CMNFI 1980-0160, 2, 49.2-73.8 mm standard length, Iran, Caspian Sea basin (no other locality data).

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