Scores of tiny shrimplike crustaceans make a nightly pilgrimage from underwater caves out to feed in the open sea. Bellies full, they find their way back, homing in on their specific cave over vast distances. (Vast for a shrimp.) How these millions of miniscule mysids find their way back home has been largely a mystery — until now.

Mysid shrimp (Hemimysis margalefi), nicknamed possum shrimp because the females carry their larvae in pouches, are about the size of an aspirin tablet and live in caves along the Mediterranean coast. Researchers found they use specialized receptors on their bodies to “smell” their back to their home caves at night.

The way mysids “smell” is different from the way humans experience odor through our noses. Instead, the shrimp sense a mix of chemicals dissolved in the water. This “chemical seascape” is made of compounds released by other creatures living in the home cave, especially sponges.

“We show for the first time that mysids can tell the water-borne odor bouquet — its so-called chemical seascape characteristic of their home cave apart from that of nearby caves,” lead researcher Thierry Pérez from the Mediterranean Institute of Biodiversity and Marine and Continental Ecology (IMBE) in Marseille, France, said in a statement. Their findings were published in the journal Frontiers in Marine Science.

To test this hypothesis, the research team devised an experiment using a Y-shaped tube that allowed water from two caves to flow into separate arms of the channel. The researchers tested the homing abilities of cave-dwelling mysid shrimp and compared them to a non-cave-dwelling species (Leptomysis spp.). Individual mysids were given the choice between water from different sources, including their home caves.

The results were clear: the mysids consistently preferred water from their home cave over that from other caves or the open sea. In fact, they spent up to 16 times longer in the water from their home cave than in other water. The non-cave-dwelling species had no preference.

Fernando Calderón Gutiérrez, a marine cave ecologist at Texas A&M University who was not involved in the study, said the research was “very well done” and “very simple in terms of the concept, and that’s one of the beauties of the work.” This approach could be used to understand homing abilities in other organisms, Gutiérrez said.

The ability to return to the safety of their cave in the dark of night is essential for their own survival and the broader ecosystem. Mysids play a vital role in the food chain, transporting nutrients from the sea into the dark cave environments and supporting a diverse array of sessile and cave-dwelling species.

Researchers studied mysids from three underwater caves in Calanques National Park near Marseille. The caves are 11-24 meters (36-79 feet) deep, and sponges dominate the cave floors. Researchers detected the presence of pollutants in the cave waters, highlighting the far-reaching impact of human activities on these underwater ecosystems.

Caves aren’t very resilient to disturbances. “The benthic [lower-level] community living there are regularly affected by disease outbreaks,” Pérez said. “Sponges are among the main victims.”

Because of their poor resilience to change, marine cave ecosystems, many of which harbor species found nowhere else on Earth, are considered key habitats by the European Habitat Directive, a set of measures to protect more than 1,000 species and 230 characteristic habitat types across Europe. A high number of caves “partly motivated the creation of the Calanques National Park” where this study took place, Pérez said

Climate change also harms marine life, including sponges, which can die when waters become too warm. Upcoming research shows changes in sponge biodiversity in less than five years in the study area, Pérez said: “Due to global [climate] change, mass mortality of sponges and corals are becoming more frequent.”

“If a very basic function driven by the interaction between sponges and mysids disappears because one or both are affected by environmental changes, the whole ecosystem functioning can collapse,” Pérez told Mongabay, “exactly like the end of pollination will lead to the end of humankind.”

Ultimately, Pérez said, this research shows that “interactions in the ocean are more complex than a food web.”

Banner image of the cave-dwelling mysid shrimp Hemimysis margalefi in the laboratory courtesy of Marie Derrien (CC BY SA 2.0).

Liz Kimbrough is a staff writer for Mongabay and holds a Ph.D. in ecology and evolutionary biology from Tulane University, where she studied the microbiomes of trees. View more of her reporting here.

In the Pacific depths, an underwater mountain hosts a bonanza of new species

Citation:

Derrien, M., Santonja, M., Greff, S., Figueres, S., Simmler, C., Chevaldonné, P., & Pérez, T. (2024). Circadian migrations of cave-dwelling crustaceans guided by their home chemical seascape. Frontiers in Marine Science, 11, 1448616. doi:10.3389/fmars.2024.1448616.

FEEDBACK: Use this form to send a message directly to the author of this post. If you want to post a public comment, you can do that at the bottom of the page.