Sex, Fish, and Climate Change

This week I bring you a shocking finding: sex analysis is rare in marine science.

Okay, maybe you’re not shocked. Maybe, like 85% of studies on ocean acidification, you had never considered sex as a key variable in marine science. But the case study I’m highlighting this week argues that sex analysis is critical to understanding marine populations’ resilience—or vulnerability—to climate change.

In short: if you care about sea turtles, you should care about whether marine researchers study sex.

But first things first: what do marine biologists mean by “sex”? It’s one of those concepts most of us think we know, but probably have a hard time defining.

To understand what biologists mean when they say sex, we have to understand the two main ways that living things reproduce: sexually and asexually. In species that reproduce sexually, individuals combine their genetic material with another individual’s genetic material to make an offspring that is genetically unique. Humans reproduce sexually, as do cats, almost all fish, elephants, bees, and so on.

(The other option is asexual reproduction, in which one parent produces a genetically identical offspring. Starfish do this, as do bacteria and yeast. You can watch a pretty wild video of starfish dividing in half and growing new arms online.)

Within sexually reproducing species, biologists generally label the individuals that carry sperm “males” and the individuals that carry eggs “females.” This is what sex is in biology: a way of classifying individuals in sexually reproducing species.

But sex isn’t always just female and male. Some 94% of flowering plants have male and female parts simultaneously, a condition referred to as hermaphroditism in non-humans (in humans, having sperm and eggs is a variety of intersex).

Hermaphroditism is common in animals, too: 1 in 3 species of non-insect animals is hermaphroditic.

The king scallop has both female (orange) and male (cream) roe. CC0 1.0.

Hermaphroditism can work in a number of ways. For some species, male and female gametes (otherwise known as sperm and egg) coexist. The king scallop, for example, has both female (orange) and male (white) tissues at the same time—this makes them simultaneous hermaphrodites.

In other species, an individual might start life as one sex and then become another (maybe more than once!). Clownfish start life as male and then become female. Pacific cleaner shrimp, on the other hand, start as males and then become simultaneous hermaphrodites.

Sex isn’t just delightfully changeable for many marine creatures, it’s also not always determined by genes. In many species, whether an individual is female or male is decided by factors like age, environmental temperature, or even social status. The blue-headed wrasse—a small fish that lives on coral reefs in the Caribbean Sea—has a social mode of sex determination. The death of the largest male wrasse causes one of the females to become male to take his place.

So, what does all this have to do with climate change?

Species whose sex is determined by temperature can be especially vulnerable to global warming. Many fish, turtles, and all crocodiles fall into this category. The temperature of the environment in which marine turtles lay their eggs determines the sex of the hatchlings. The higher the temperature, the higher the portion of eggs that will hatch as female.

Recent work at the Great Barrier Reef off of Australia shows that rising temperatures are already putting green sea turtle populations at risk.

Green sea turtle hatchling. Image by Oregon State University, CC BY-SA 2.0.

Researchers from Australia and the US National Oceanic and Atmospheric Administration traced turtle populations at the Great Barrier Reef to the sites where they hatched. Sex isn’t easy to spot in turtles, so the researchers have developed new techniques to measure sex ratios in these populations. In other words, they had to make sex analysis a priority.

They found that over 99% of the young turtles from the warmer, more northern site were female, compared with 67% of the young turtles born at the cooler, more southern site. The warm temperatures on the northern beaches have produced almost all female turtles for the past twenty years.

The problem is obvious: if a population becomes entirely or nearly entirely female, it will be impossible for it to reproduce. The northern population is already at risk of extinction; the southern population could be heading in the same direction.

It’s not just sea turtles and rising temperatures, either. As human activity causes carbon levels in the atmosphere go up, the oceans dissolve some of this carbon dioxide, leading not, sadly, to soda water but to more acidic ocean water. Since the industrial revolution, surface ocean waters have gotten 30% more acidic.

Acidification is bad for many species, but it can have sex-specific results, too. Acidification results in higher ratios of males in zebrafish, for example, and fewer male oysters and chichlids (a family that includes tilapia and angelfish).

But the larger point here is that if researchers don’t account for sex in studying populations like these (like 85% of studies on acidification), they might miss how vulnerable these species are to warming environments.

You can see the way that failing to take sex into account can be misleading:

These two graphs are from a study of how female and male copepods (a kind of zooplankton) respond to elevated dissolved CO2, for example. In the first graph, you can see that high CO₂ has affected the respiration rate of both female and male copepods, though in different ways. The females’ respiration increased, while the males’ respiration decreased. In the second graph, you can see what happens if you pool data from females and males. While the range of respiration rates shrinks in high CO₂, the average appears not to change at all.

The data come from copepods, but the principle holds in many situations: you don’t know what you’re missing until you look.

So, there you have it: sex analysis is critical in marine research. And it’s not just an academic concern. Accurate information about populations and their vulnerability is important to managing marine resources, whether for commercial or conservation purposes.

I’ve said it once, but I’ll say it again—none of this changes without policy. In this case, funding agencies and journals can require sex analysis in marine science. In some cases, like the turtles, scientists will have to develop new ways of determining an organisms’ sex—but who’s going to do that unless researchers make it a priority?

Read about the full case study in the European Commission’s Gendered Innovations 2 Report (p82).