Article: Brown, E. E., et al. (2014). “Temperature and photoperiod effects on sex determination in a fish.” Journal of Experimental Marine Biology and Ecology 461(0): 39-43.
It’s pretty simple for us primates, our sex is determined by genetics with no influence from the environment. But many other organisms, like some fish, reptiles, and invertebrates have what is called environmental sex determination (ESD) in which the sex of the offspring is not determined by genetics but rather environmental cues. Having ESD is advantageous in certain situations, and would evolve over genetic sex determination (GSD), when the environment is patchy and if the environment favors higher fitness of one sex over another. Even within a species, both ESD and GSD can be present depending on where the population is located and how patchy or consistent the environment is. ESD can provide boosts in male or female fitness that often helps populations during breeding periods. For example, Atlantic silversides (Menidia menidia) have higher numbers of females born in colder temperatures at the start of the breeding season. Female silversides need to be of a certain size and fitness to be able to breed; if more females are born in the beginning of the mating season, then more females will be sexually mature by the time the next breeding season comes around.
Temperature is the most well-known, and well-studied, ESD cue. But there are plenty of places around the world that don’t exhibit large variation in temperature, and as a result, temperature isn’t an influencing ESD cue. Recently, researchers began investigating a close relative of the Atlantic silverside, the California grunion (Leuresthes tenuis) (Fig 1 and Fig 2). The typical habitat of the grunion doesn’t experience high variability in temperature during the breeding season, but does experience large changes in the photoperiod, or the duration of sunlight during the day. Researchers set out to see how both temperature and photoperiod play a role in sex determination of the California grunion.
California grunion are found in the eastern Pacific, ranging from Mexico to California. They breed from March to September and reach sexual maturity after one year. Researchers at Stony Brook University collected fertilized eggs of L. tenuis from two breeding populations (one from California, the other from Mexico) and brought them back to the laboratory. Fertilized eggs hatched and larvae were separated into different temperature and photoperiod treatments. Three temperatures (17°, 21°, and 25°C) were tested with a photoperiod of 15 hours of light, 9 hours of dark. One temperature (21°C) was tested with a shorter photoperiod of 12 hours of light and 12 hours of dark. All fish were grown until researchers were able to determine the sex.
Overall it was found that temperature played a significant role in the sex ratio of larval grunion. At a lower temperature (17°C) there is a significantly larger proportion of females than at 21° and 25°, with 25° having the lowest proportion of females (Fig 3). But the importance of this study lies in how photoperiod impacts sex ratios. It was shown that a longer photoperiod led to a significantly higher proportion of females (Fig 4). The shorter photoperiod led to the largest proportion of males than any other treatment.
Photoperiod has been shown as an important ESD cue for some invertebrate species, like shrimp, but this study provides the first evidence in vertebrates. Photoperiod likely plays a role similar to temperature, as an indicator of season and of proximity to breeding. Sex is only determined in this fish species at a certain size (20-25mm); this size is reached after about 2 months. Because this species begins to spawn in March, the fish reach 20-25mm around June, when the photoperiod is increasing. But fish spawned later in the summer are reaching this size while the photoperiod starts to decrease. Photoperiod and temperature go hand-in-hand in determining the sex ratios of larval grunion. Colder temperatures with increasing photoperiod yield more females, while warmer temperatures and decreasing photoperiod yield more males. This strategy exists in this species for similar reasons as the silverside; female size and health are of the utmost importance for a successful breeding season, so if more females are born at the beginning of the breeding season (colder temperatures with increasing photoperiod) then more females will be ready to breed at the start of next year’s breeding season. This study helps us better understand sex determination and how different strategies evolve based on the environment in certain ecosystems.
Photoperiod may end up being a very important environmental cue as our planet continues to be impacted by climate change. Photoperiod is not going to change with increasing carbon dioxide, but temperature will, meaning photoperiod may be relied upon by many organisms in the future.