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Climate Change

Genetics aren’t the only factor in determining the sex of certain marine organisms

Paper:  Honeycutt, J. L., Deck, C. A., Miller, S. C., Severance, M. E., Atkins, E. B., Luckenbach, J. A., Buckel, J.A., Daniels, H.V. Rice, J.A., Borski, R.J., & Godwin, J. (2019). Warmer waters masculinize wild populations of a fish with temperature-dependent sex determination. Scientific reports9(1), 6527. https://doi.org/10.1038/s41598-019-42944-x

There are many environmental factors which affect the way marine organisms behave, and temperature is one of those very factors. While climate change has rapidly warmed Earth’s waters, causing certain species to move northward in search for colder ecoregions, shifting water temperatures can also have an effect on the sex of an organism. Yes, temperature can permanently determine whether an organism is male or female.

Researchers are seeing an over 90% female hatch rate in sea turtles in those areas that are warming the most. Photo Credit: Wikimedia Commons, Strobilomyces.

Sea turtles are an excellent example of critters whose sex is determined not by genetics, but by the temperature of their environment even before their eggs hatch. Warmer temperatures tend to hatch more female turtles, whereas colder temperatures tend to hatch more male turtles. Recently, air and water warming has caused a distinct increase in the temperatures of the sands sea turtles lay their eggs in, which correlates with a much higher female hatch rate. More females means less males to breed with; therefore, climate change’s warming is severely harming future sea turtle populations.

Fish join reptiles as organism similarly affected by climate variability. Research has already indicated that species like the Atlantic silverside show high rates of temperature dependent sex determination (TDSD). However more work is needed to determine how climate change’s warming effects can potentially impact these TDSD inclined species, to the point of increasing the vulnerability of certain commercially viable fishery stocks.

A southern flounder. Photo Credit: Flickr, NOAA Photo Library.

Honeycutt et al. (2019) examine the southern flounder, a benthic flat fish found in southeastern U.S. These fish are sexually dimorphic: their appearance or size differs based on their sex. Typically, female southern flounder grow faster and larger. Paired with management strategies which impose a minimum catch size, the southern flounder’s sexual dimorphism means that the commercial fishery is most dependent on females. Recently, however, North Carolina fisheries have been seeing a decline in juvenile and adult southern flounders, and in 2015 the species was classified as “near threatened” by the IUCN’s Red List (and remains so to this day).

A southern flounder’s sex isn’t just determined by temperature and environment, and (like many fish) combines with genetic determination. As a juvenile, an XY genotype denotes male, while an XX genotype denotes female. Based on environmental factors, an XX genotypic female may turn into an XX phenotypic male during a juvenile’s critical development stage. The factors which cause this change can be environmental, such as temperatures cooler or warmer than 23°C (~73°F), or even the color of their tanks if reared in captivity (blue backgrounds tend to induce this transformation). Because juvenile female southern flounders can eventually become male due to warming waters, climate change puts the fishery at even greater risk.

Temperature loggers were stationed at the following locations that had juvenile southern flounder habitats. Two of the locations, White Oak and Pierce Creek, were not included in final analyses. Photo Credit: Honeycutt et al. (2019).

Honeycutt et al. (2019) look at the sexes of juvenile southern flounder in three areas of North Carolina: the Pamlico River (the northernmost region and therefore the coolest on average), the Neuse River (an intermediate region), and south of New River (the southernmost region and therefore the warmest on average). For four years, temperature loggers observed daily temperatures in flounder nursery habitats. As expected, temperatures shifted both daily and annually. Throughout the study period, Honeycutt et al. (2019) found the most males south of New River, followed by Neuse River, and the Pamlico River. In 2017, researchers found 67% of the flounder sample in the Pamlico River to be male, 78% in the Neuse River, and 94% south of New River.

Honeycutt et al. (2019) then recreated these environments in a lab setting to determine whether or not rising temperatures were directly causing a shift in southern flounder sex. They created a 27°C (~80°F) constant condition (which would not mimic the daily and annual temperature shifts found in the field) that produced 98% male flounders. They then created three fluctuating conditions (which did mimic the daily and annual temperature shifts found in the field): 19°C (~66°F), 23°C (73°F), and 27°C (~80°F). These conditions produced 83%, 65%, and 100% male flounders respectively. 23°C, therefore, was the “sweet spot” which maintained the highest ratio of females.

The research team, through both their field and lab experiments, found that areas with the highest rate of male flounders also had the warmest waters, such as south of New River and the Neuse River. Natural daily and annual temperature fluctuations did not significantly impact male to female ratios, since even though the Pamlico and Neuse rivers both recorded lower water temperatures in 2016 and 2017 than previous years, they both still had higher proportions of males than previous years. Interestingly enough, the Pamlico River held the highest ratio of females, and Honeycutt et al. (2019) hypothesize this to be because its waters averaged 23°C – the temperature found to produce the most females in their lab experiment.

Ratios of males and females based on year and ecoregion. Pamlico River tended to have the most females, while south of New River tended to have the most males. Photo Credit: Honeycutt et al. (2019).

These results indicate that temperature could be the primary element influencing southern flounder sex ratios in the wild. Warmer nursey habitats from the southmost ecoregions produced significantly more males than those cooler habitats farther up north. Therefore, climate change paired with the southern flounder’s TDSD can drastically impact the health and viability of this important commercial fishery. With less females, not only will markets be forced to source lower amounts of fish, but the stock will have less reproductive potential as well. Combined with the other threats fish already face, such as overfishing, climate change’s influence on TDSD could be incredibly severe.

Honeycutt et al. (2019) end by concluding that as ocean temperatures are projected to keep increasing, factors like TDSD are integral to consider to manage, preserve, and protect marine organisms. Further work should examine how other critters manifest TDSD, and how climate change may be contributing to disproportionate, and potentially harmful, sex ratios within species.

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