Every spring, young shrimp in the Mediterranean Sea turn from male to female—an important stage in their reproductive development. This change happens due to an abundance of a certain type of microalgae that the occasionally neon green-tinted shrimp rely on for their species’ survival. However, as ocean acidification intensifies, it could change the chemical makeup of the microalgae, potentially stunting the shrimp’s reproductive progress and threatening their existence, researchers report this week in PLOS ONE.
The shrimp, Hippolyte inermis Leach, dine on a specific kind of microalgae called Cocconeis scutellum parva, which flourishes in the seagrass meadows of the Mediterranean Sea, including acidified vents in the Bay of Naples. Eating the microalgae regulates the shrimp’s reproductive cycle.
Scientists have been fascinated by the sexual development of these odd little shrimp for years. Although Hippolyte inermis is considered a hermaphrodite like many other crustaceans, it is unusual in that it rapidly transitions from male to female without ever passing through an intermediate stage with attributes of both. This sex-reversal system has two distinct reproductive seasons. During the fall when Cocconeis microalgae is scarce, the majority of newborn shrimp are born male. After the spring, their male gonads age and drop off in a single molt and an ovary develops.
But younger shrimp who are born in the spring when microalgae are abundant can develop immediately into females by going through an even more rapid sex-reversal. Previous studies showed Cocconeis is responsible for this quick change. By releasing a still-unknown compound when eaten, Cocconeis kills the cells in the shrimp’s male sex gland, causing it to transition prematurely. This springtime switch helps restore balance after the population takes a hit in winter when predators, like black scorpionfish, devour the shrimp.
Lead author of the study Mirko Mutalipassi, a marine biotechnologist at the Stazione Zoologica Anton Dohrn in Naples, emphasizes that the shrimp’s dependence on the microalgae is so strong that their population growth syncs up with microalgae blooms.
“It’s really important for these shrimp,” Mutalipassi says. “This is the stabilizing factor for their natural population, because it allows the shrimp the ability to produce a lot of eggs and avoid being wiped out due to predation.”
The presence of such a strong plant-animal relationship in acidic conditions inspired Mutalipassi to use it as a tool for studying how increased ocean acidification will impact this ecosystem. “I’m really fascinated by co-evolution, both from a physiological point of view and a molecular point of view,” he says. “It’s a really interesting way to look at how two organisms interact with each other. It’s also a good model to study the effect of global changes on microalgae and invertebrates.”
Mutalipassi and his co-authors Valerio Zupo and Valerio Mazzella, both researchers at Stazione Zoologica, used the shrimp population as a probe to see what happens to the chemical composition of the microalgae as the ocean becomes more acidic. The research team grew Cocconeis at two different acidity levels: one at the present conditions, and one based on the predicted rise in ocean acidity over the next century as carbon dioxide levels increase. Afterward, they fed newborn shrimp one of the two groups of microalgae and observed whether they had different numbers of females, which would indicate a change in the microalgae’s compound that drives the shrimps' development.
The team’s results were surprising. Unlike some other microalgae that have failed to thrive under high CO2 levels, Cocconeis flourished, growing four times more cells under acidified conditions. This increase in growth implies that the microalgae could have a competitive advantage in acidified oceans of the future.
In contrast, the shrimp that were fed microalgae grown in higher levels of water acidification ended up with about half as many females as the shrimp that were fed the normal microalgae. Such a drastic difference suggests that the chemical compound that destroys the shrimp’s male sex glands may be changed by the acidified conditions, producing fewer females. In other words, Cocconeis thrives, but the shrimp suffer.
“This work is a neat example of researchers pushing beyond some of the basic questions of survival and growth of a single organism to also examine the relationships between species,” marine biologist Kaitlyn Lowder of the Scripps Institution of Oceanography at University of California San Diego says in an email. “To better understand what our marine ecosystems will look like in the future, it is incredibly important to look at the interaction between trophic levels, which can be difficult to do in a lab setting.”
Seemingly subtle changes like this that could trigger domino effects in an ecosystem are emblematic of the impact of climate change. As ocean acidification continues to disrupt the conditions of seawater, researchers are scrambling to learn how such changes might influence even the tiniest lifeforms on our planet.
Lowder, who was not involved in this study, argues that studying changes to the behaviors of organisms is crucial for gaining awareness about the changing environment. “It is only by pursuing these questions about the sexual transition of shrimp … that us scientists can get a better idea of what our oceans might look like in the future,” she says. “And importantly, [we can] have more stories about the potential impacts of ocean acidification to increase the public concern about this ongoing change to our oceans.”
Mutalipassi holds similar views, arguing that the chemical compound is really an “infochemical” for the environment—an underwater version of a canary in a coal mine.
“We now know that ocean acidification can disrupt a delicate ecological relationship that evolved over a million years,” Mutalipassi says. “This means that we have idiosyncratic consequences with the changes we are doing to our world.”
He also points out that the microalgae-shrimp relationship is only one of many that could be affected. “The impact of ocean acidification is bigger than what we see in the study,” he says. “We are just looking at a small piece of the puzzle.”