Atherinopsids

Atherinopsids, or neotropical silversides, belong to the order Atheriniformes and family Atherinopidae. There are about 104 species within thirteen genera found in euryhaline, marine, and freshwater habitats distributed throughout the tropical and temperate waters of the Western Hemisphere. Three well-known atherinopsid fishes are the Atlantic silverside (Menidia menidia), California grunion (Leuresthes tenuis), and Gulf grunion (L. sardina). There are three species in Arkansas: the brook silverside (Labidesthes sicculus), golden silverside (L. vanhyningi), and Mississippi silverside (Menidia audens).

Prior to the Neogene Period of the late Tertiary Period (66 to 2.5 million years ago), there is no fossil record of atherinopsids in North America. However, on San Francisco Bay, important prehistoric intertidal fisheries were especially well documented for atherinopsids.

Silversides are small elongated schooling fishes with two widely separated dorsal fins. The first consists of flexible spines, and the second has one spine followed by soft rays. The anal fin has one spine on the leading edge followed by soft rays. The pectoral fin tends to sit high on the sides of the body, and there is no lateral line. On the flank or lateral sides is a broad, silvery band or stripe. They have cycloid scaled heads, and, although the swim bladder is present, the pneumatic duct connection with the pharynx is absent. The mouth position is terminal, and the jaws are produced into a short beak.

Silversides are of no particular commercial value except as baitfish. These fishes are important forage fishes for other commercial species, however. Because they are mostly surface dwelling, they are noted for their interest in surface objects, running into and making short jumps out of the water over floating objects.

One common silverside species in Arkansas is the brook silverside. It is an extremely slim and elongated translucent fish with a long, beak-shaped pointed snout. It has a maximum total length of about 102 mm (4 in.). Scales in the lateral series number 74 to 94, pectoral fin rays number 12 to 15, the pelvic fin has one spine and five rays, and the anal fin possesses one spine with 21 to 27 rays. Dorsolateral coloration ranges from translucent green to pallid yellow. Labidesthes sicculus can be differentiated from the similar golden silverside (L. vanhyningi) by the presence of an anterolateral process of the post temporal that is longer than it is wide (versus wider than long in the golden silverside), a ratio of thoracic length to abdominal length greater than two (versus less than two), and a mid-lateral stripe that is narrow in front of the first dorsal fin (versus expanding in front of the first dorsal fin).

Populations of L. sicculus range from drainages in the upper Mississippi River and the Great Lakes/St. Lawrence drainages in the north to the Atlantic slope, from the Santee River south to the Florida Everglades, the Gulf of Mexico, and westward to Texas. It is the most widespread freshwater atherinopsid in North America, and, in Arkansas, L. sicculus is found statewide. This fish is an open-water species that occupies freshwater lakes, ponds, quiet pools, and small rivers, usually with aquatic vegetation. The environment ranges from cool to warm water with variability in levels of gradient and vegetation. However, L. sicculus prefers clear water with low substrate concentrations and is vulnerable to turbid waters.

Labidesthes sicculus feeds on microcrustaceans, chironomids, zooplankton, insect larvae, terrestrial spiders, and winged insects. In one study, they were reported to be highly specialized feeders with cladocerans composing the majority of their diet, followed by small flying insects and midge larvae. The young feed almost exclusively on microcrustaceans. Because the brook silverside is on the lower end of the food chain, it is preyed on by many larger fish.

In Arkansas, the spawning season of L. sicculus occurs during late spring and early summer. Parents gather in pools over aquatic vegetation or gravel beds. Eggs have adhesive anchoring filaments that attach to underwater vegetation, logs, or rocks. Fry (about 5.5 mm [0.2 in.] in length) hatch in about a week, and growth is extremely rapid (up to 1 mm/day). After the eggs hatch, the fry immediately swim away from the shore into deeper water but still stay just below the surface and begin to congregate in large schools. Brook silversides quickly reach a maximum size of about 80 mm (3.1 in.) and reach sexual maturity by their first summer. Life span is only about one year.

The golden or stout silverside (L. vanhyningi) is a recently re-described species (formerly within L. sicculus) that is widely distributed in the southeastern United States from the Gulf of Mexico drainages in the Neches River in Texas east around the southern tip of peninsular Florida and north to the Pee Dee River drainages of North Carolina. It is native to the majority of this distribution but has also been stocked outside of its natural range as forage food for sport fishes. In Arkansas, golden silversides are found in sporadic localities of the Ouachita, Red, and White river drainages. The ecology and natural history of L. vanhyningi is similar to L. sicculus.

The Mississippi silverside (M. audens) is native to middle North America and has been introduced into various parts of California. They are widespread in freshwater environments from the lower Mississippi River basin northward to southeastern Missouri and western Kentucky, and westward to Oklahoma. They also occur in the Pearl River drainage, Mississippi and Louisiana, and Pickwick Reservoir and the upper Tombigbee River in Alabama. Mississippi silversides were introduced in 1967 into Clear Lake and the Blue Lakes of California to help control dipterans such as the Clear Lake gnat (Chaoborus asticopus) and midges. The following year, they were introduced into the lakes and reservoirs of Alameda and Santa Clara counties. Since then, they have been found in the San Francisco Bay and Central Valley and have also become widespread in central California. In some areas, they are the most abundant fish. In Arkansas, M. audens inhabits large rivers of the state, including the Arkansas, Mississippi, Ouachita, Red, and the lower White. The Arkansas Game and Fish Commission also stocked M. audens in some large reservoirs, and it was probably introduced accidentally in the early 1960s in Lake Chicot (Chicot County).

Mississippi silversides are quite elongate, with their lengths up to six to seven times their depth. They have large eyes, a considerably upturned mouth, and a head noticeably flattened on top. Of the two widely separated dorsal fins, the anterior fin is small and has four to five weak spines, while the posterior fin is larger, with one spine and eight or nine rays. The lengthy anal fin is somewhat sickle-shaped, has one spine, and contains 15 to 20 rays (vs. 20 to 26 in L. sicculus). They are metallic silvery on the sides, and the dorsum is somewhat yellowish with a translucent greenish venter. Its head region is similar to L. sicculus, but the Mississippi silverside has different scale counts and possesses a shorter, rounded snout (versus a long, pointed snout in L. sicculus). Like other North American silversides, these are small fish, with a maximum total length of 125 mm (5.5 in.), but most adults are less than 100 mm (3.9 in.).

Mississippi silversides primarily feed on zooplankton, aquatic insects (midge and mayfly larvae), amphipods, copepods, and fallen terrestrial insects. By congregating and moving through waters in enormous schools, they are capable of reducing populations of preferred small arthropods and crustaceans. In turn, they are important forage species for a variety of predatory fishes and various birds. These silversides congregate on sandbars or over gravel bottoms with overhead canopy where available, but then migrate out to open water in search of additional food, which increases predation risk. This fish is a significant forage species for predatory fishes such as white and yellow basses (Morone spp.) and largemouth bass (Micropterus salmoides).

In Arkansas, M. audens spawns from late March or April through July. Large females are capable of producing up to 2,000 eggs per day during their three-month reproductive period. As in Labidesthes, eggs of M. audens have anchor filaments that are often found on algal growth on brush stems. Eggs hatch in about three weeks, and the young grow to about 60 mm (2.3 in.) by their first winter and 100 mm (4.0 in.) by the end of their second summer. Life span is relatively short, with adults dying after their second summer.

Several parasites have been reported from these silversides from various parts of their range. In Arkansas, a recent study reported parasites from L. sicculus and L. vanhyningi from the Ouachita and Red River drainage basins of Arkansas. Found were Dermocystidium sp., Proteocephalus sp., and Ergasilus funduli. However, as of 2019, no parasites have been reported from M. audens from the state.

For additional information:
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Chris T. McAllister
Eastern Oklahoma State College