Fish live underwater, so it seems like it would be safe to assume that they always spawn underwater, too – but, as it turns out, there are exceptions to that rule.
California grunion are small, silvery fish, typically about five or six inches long when full-grown, that live along the Pacific coast between Punta Abreojos, Baja California Sur, Mexico and Point Conception, Calif. (they’re occasionally found as far north as Monterey, Calif.).
For a few nights during full moons and new moons (when high tides are particularly high), between March and September, sandy beaches along the coast can be overrun with the flipping and flopping of California grunion, starlight reflecting off their iridescent scales, as the females release their eggs under the cover and relative safety of wet sand and the males circle around to fertilize them. Their tasks complete, the adults ride the tide back underwater, and the eggs incubate under the sand for about ten days before they hatch and the larvae are washed out into the ocean to join their parents. (The California Department of Fish and Wildlife has a nice description of California grunion spawning behavior on their website.)
One spring, a team of researchers from Stony Brook University visited some of the sandy beaches where California grunion reproduce to collect fertilized eggs. The researchers were interested in investigating another unusual feature of the small fish – sex differentiation doesn’t occur until they are about two months old, and it appears to be determined, at least partially, by the environmental conditions the fish experience rather than completely by their genetics.
Those scientists recently reported the results of their study in the Journal of Experimental Marine Biology and Ecology. They raised larval California grunion at three different temperatures and under two different light regimes – one approximating longer day-lengths, such as would be experienced by the larval fish at mid-summer, and one approximating the shorter days late in the spawning season.
The researchers found that the environmental conditions that would indicate to the larval fish that they had been born earlier in the spawning season – colder temperatures and longer day-lengths – led to a higher proportion of female fish, presumably because the females born early in the spawning season gain an advantage over those born later. With more time to grow, the early-born females can maximize their size, and, eventually, their ability to reproduce – bigger females produce more eggs. (There doesn’t appear to be a similar size advantage for male California grunion.)
California grunion rely on at least two environmental cues – day-length as well as temperature – that indicate seasonality. As the scientists write, California grunion “appears to be the first documented case of a vertebrate with [environmental sex determination] that is cued by both temperature and photoperiod. Photoperiod may provide [California grunion] with a more reliable cue than temperature alone, given the small seasonal changes in temperature along the North American Pacific coast.”
On those summer nights when adult California grunion are racing up onto sandy beaches to spawn, the females laying the most eggs – the biggest females – may have been some of the earliest-born fish in previous years.