Climate Change: The Ocean’s “Mood Killer”

by Jessica Schiff
figures by Rebecca Senft

Under the waves and in the ocean, climate change is like a persistent third wheel. Akin to that friend who always insists on tagging along on your dates, his/her/their presence makes it difficult for you and your significant other to have alone time. In the ocean, climate change continually prevents the seduction and reproductive success of all kinds of marine creatures – ultimately impacting ecosystem health, coastal economies, recreational activities, and food security.

Why does climate change impact marine reproduction?

Reproductive strategies in the ocean rely heavily on the conditions of the surrounding environment. Many marine animals like sea urchins, corals, and clown fish rely on water conditions for reproduction because they reproduce externally, meaning the eggs are fertilized outside of the animals’ body in the water. These animals depend on specific cues and conditions in their environment to signal mating season. Currents, temperature, salinity, light, and acidity are all factors that dictate when and how successfully different marine organisms reproduce – and this is where climate change makes romance difficult. Climate change is directly correlated with increasing levels of carbon dioxide, a potent greenhouse gas, in the Earth’s atmosphere. Increasing levels of carbon dioxide has in turn been linked to warmer and more acidic oceans, disrupted nutrient and chemical cycles, as well as altered oceanic currents. Changing even one of these factors can “kill the mood” under the sea and have disastrous consequences on food webs and the environment.

**Figure 1: A “boiling” ocean.** Climate change is linked to warmer oceans, which are unsuitable for fish because they are very sensitive to changes in temperature. Fish are cold-blooded so an increase in ocean temperatures will increase metabolism, driving the demand for more oxygen. However, warm water holds less dissolved oxygen, meaning that there is less oxygen available to meet this greater demand. This can lead to increased mortality and decreased reproductive capacity.

How do increased ocean temperatures impact fish reproduction?

Many fish, including salmon, bluefin tuna, and swordfish, rely heavily on temperature to signal spawning, the release of eggs and sperm into the water (Figure 1). Studies have shown that increasing ocean temperatures signal earlier reproductive development in spring-spawning species and shorten the spawning period, leading to less opportunity for reproduction. For autumn-spawning species, warmer temperatures delay spawning and reproductive development because they require cooler temperatures to signal spawning.

Fish are sensitive to increased water temperatures because warmer temperatures result in thermal inhibition of reproductive processes. Higher temperatures directly impact the functions of the hypothalamo–pituitary–gonadal (HPG) axis glands by affecting reproductive hormone production, structure, and function, resulting in the suppression of hormones and reproductive signals. The temperature threshold at which these effects occur varies between species, with arctic species being affected at much lower temperatures than tropical species. Increasing ocean temperatures also impact reproduction because many fish species migrate for reproduction and rely on environmental cues, such as temperature, to signal migration. Changing water temperatures can result in earlier migration or delayed migration, and that, coupled with shifting spawning seasons, could cause fish to reproduce in areas unsuitable for their young, resulting in population crashes.

How does increased acidity impact fish reproduction?

Acidity also dictates reproduction. Oceans are a natural sink for carbon dioxide, absorbing much of the gas from our atmosphere. However, chemical reactions between water and carbon dioxide produce an acid, increasing ocean acidity. As a result, marine organisms may struggle to demonstrate their “flirting skills.” This is because acidity can impact the shape and function of pheromones – the “love potion” of the ocean – that are used to attract mates. A common marine invertebrate, the slipper snail, relies heavily on the transmission of pheromones to attract mates and reproduce (Figure 2).

Slipper snails are sequential hermaphrodites, meaning that a snail first develops as a male and then at a later point will develop into a female, depending on the presence of pheromones. If pheromones are absent, the male slipper snail will quickly transition to a female. Once female, the snail releases pheromones that attract other juvenile males who stack upon her shell. The now-female snail continues to release hormones that prevent the male snails from transitioning to female themselves. However, when the snail stack has enough males, chemical signals induce the male closest to the bottom female to transition from male to female in order to take advantage of the excess of males. This unique reproductive strategy depends significantly on chemical messengers and is highly sensitive to changes in acidity that occur as more carbon dioxide is pumped into our atmosphere and subsequently dissolves into the oceans.

**Figure 2: The slipper snail’s “love potion”.** Slipper snails rely heavily on chemical signals, pheromones, to attract mates and to determine when a snail will transition from one sex to the other. Climate change is linked to warmer and more acidic oceans, both of which are conditions that can interfere with chemical signals in the water.

Pheromones are exquisitely sensitive to any changes in acidity. An increase in acidity could alter the structure of the pheromone, preventing it from being properly received and “read” by another snail¹. In addition to changing the structure of the pheromone itself, a more acidic environment could also change the structure of the snail’s pheromone receptor sites, making them unable to receive the chemical message and interpret the chemical messages¹. If the slipper snails are unable to properly send chemical messages, or to read them, female-male sex ratios would be thrown off, resulting in a catastrophic population collapse. Other invertebrates, such as barnacles, are also highly sensitive to changes in acidity. Laboratory studies have shown that both field-collected and lab-raised barnacles fail to produce viable embryos and larvae in the projected acidity of oceans in the future.

The consequences of climate change on reproduction and the world

The impacts of climate change on the ocean are manifold and extensive, portending severe consequences for life in the seas. Marine organisms’ reproductive strategies are intimately tied to water conditions. Mate attraction, migration signals to spawning sites, and suitable conditions for fertilization and larval development are just some of the critical facets of marine life cycles adversely impacted by climate change. Declining population sizes of various marine organisms also disrupt food webs and ocean health, and threaten economies, livelihoods, and human food security.

Around the world, over 3 billion people rely on the oceans for food and 870 million people rely on fisheries for livelihoods (Figure 3); that number is projected to increase with a growing human population. Fish populations are already strained by the increasing demand for seafood. As climate change progresses and reduces the reproductive success of marine organisms, their populations will struggle to maintain a healthy size, risking the collapse of entire food webs and ecosystems. If small invertebrates cannot reproduce because their pheromones are structurally altered by acidity, then small fish will struggle to find enough food to eat, and other larger fish (and even whales!), marine mammals, and people will also suffer from lower supply. Population balances will change dramatically, and oceans will no longer be able to support a healthy diversity of organisms.

**Figure 3: Sustainable fishing.** In addition to all the marine creatures that call the ocean their home, billions of people in the world rely on healthy oceans for food and millions of people rely on the ocean for their livelihoods. The health of the ocean and marine animals directly impact coastal economies, food security, livelihoods and ecosystem health.

The impacts of climate change as a disruptive third wheel in ocean romance reach further than sea level rising and warmer waters. Adverse changes in the reproductive success of marine organisms threaten the health of entire ocean ecosystems, and in turn, the livelihoods of millions of people too.

Jessica Schiff is a Master of Science student in Environmental Health at the Harvard T.H. Chan School of Public Health.

Rebecca Senft is a fifth-year Program in Neuroscience Ph.D. student at Harvard University who studies the circuitry and function of serotonin neurons in the mouse.

For More Information:

To learn how to make ocean friendly, sustainable choices when eating seafood check out the Monterey Bay Aquarium Seafood Watch
Check out this TEDx Talk by Triona McGrath, a chemical oceanographer, to learn more about how climate change impacts ocean acidification
Read Sex in the Sea: Our intimate connection with sex-changing fish, romantic lobsters, kinky squid, and other salty erotica of the deep by Marah J. Hardt to learn more about unique marine reproductive strategies

Reference:

Hardt, M. J., & Chimovitz, M. (2017). Sex in the sea: Our intimate connection with sex-changing fish, romantic lobsters, kinky squid, and other salty erotica of the deep (pp. 42-45). New York: St. Martin’s Griffin.