The reproductive behavior of most flatfishes is not known. Direct observations of courtship and mating have not been made for the majority of flatfish species. The sexes are separate, and individuals usually do not change sex during their lifetimes. Flatfishes have external fertilization. Where reproductive behavior has been observed, individual males and females may pair up during courtship and spawning. Sometimes the mating pair is joined by other males.
Most flatfishes spawn planktonic eggs that float freely in the water column. Some pleuronectid flatfishes, such as the winter flounder, lay eggs that are demersal and adhesive, such that after the female releases them, they remain on the bottom and stick to each other and to other items. Even among species with demersal, adhesive eggs, flatfishes do not construct nests during spawning, nor do they exhibit any type of parental care. Upon hatching, flatfish larvae are planktonic and usually are found in the water column far above the bottom. Larval stages vary in duration from a few days to a couple of months; the duration of larval stages is influenced greatly by ambient water temperatures. Following eye migration and metamorphosis, young flatfish settle out of the water column and assume a benthic lifestyle, with many species utilizing shallow-water habitats as nursery areas.
A strong seasonality in reproductive period has been noted for most temperate and boreal flatfishes, with most species having one spawning season per year. The timing of spawning seasons within the year varies by species and also by latitude. Some species spawn during periods of seasonally high temperatures, whereas others spawn during wintertime. Some warm temper-
ate species may have two spawning periods per year; for tropical and subtropical species, spawning periods may extend over several months. Spawning among temperate marine species corresponds to annual productivity cycles that are related to temperatures and photoperiods. In some tropical regions, spawning by some flatfishes also seems to correspond to seasonal monsoons, which influence productivity cycles.
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