Reproductive biology

Peculiar reproductive repertoires are numerous within the atheriniformes. Large eggs with adhesive filaments are characteristic of the group; the filaments are used to anchor the eggs to aquatic plants or other forms of substrate. Among the rainbowfishes, spawning occurs day after day for an extended period, each day of which the female attaches a small number of thread-bearing eggs, fertilized externally by the male, to underwater plants. Eggs typically hatch in seven to 18 days. Atheriniform larvae generally are large (0.16-0.35 in; 4-9 mm) at hatching, and although they still possess a yolk sac, they have open mouths and can begin feeding immediately.

A truly exceptional reproductive strategy has evolved in grunion (two species of atherinids). Leuresthes tenuis and L. sardina both time their spawning to take advantage of tidal effects. Grunion spawn during periods of only a few hours on only six nights a month, when tides are highest around the times of the full and new moons (semilunar tides). The fishes ride in with large waves and are left stranded on the sand as the water recedes. Once on the beach, the female burrows into the sand with her tail to lay the eggs, and an accompanying male deposits his milt simultaneously. The fertilized eggs are left buried in the sand a few inches below the surface; they hatch in about two weeks, when semilunar high tides return to uncover them. Possible advantages for the eggs are decreased predation and increased oxygen levels and incubation temperature.

Grunion Milt
Grunion emerge from the surf to spawn on the beach, laying their eggs soon after the full moon. (Illustration by Patricia Ferrer)
Grunion Egg Hatch

Phallostethids mating; male restrains female with anteriorly situated priapium—a mating organ. (Illustration by Patricia Ferrer)

Male phallostethids, with their bizarre gripping structures (described earlier), ram females, and the two become locked together physically by the priapium during copulation. Frantic bursts of swimming are required for the pair to disengage. Further description of this behavior is provided in the species account of Neostethis bicornis.

In a lineage of Menidia, sexual reproduction has been forgone entirely. The Menidia clarkhubbsi species complex from the Gulf of Mexico is made up of unisexual clonal lineages thought to be the result of hybridization events between M. beryllina and an as yet undetermined species, similar to M. peninsulae. The unisexuals produce eggs with a complete chromosomal complement that do not require fertilization but must still rely on sperm supplied by one of the bisexual species to stimulate embryo formation. Spawning itself is simple in Menidia. A single female—attended by several males— deposits eggs in aquatic vegetation, and the males leave milt; no courtship has been observed.

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