Many animals possess an extraordinary homing ability, enabling them to navigate vast distances with remarkable accuracy. From migratory birds like the Arctic tern, which travels thousands of miles across the globe, to salmon that return to the exact river of their birth to spawn, these creatures use a combination of environmental cues.

Among such creatures is a group of inconspicuous but equally impressive possum shrimps.

Homing possum shrimps

Possum shrimps — named thusly for the pouches in which females carry their larvae — are well known for their environmental sensibility. They’re sometimes used as bioindicators to monitor water quality and are even used in labs to test for pesticides and other toxins. Yet some possum shrimps have another remarkable ability: to find their way back to their specific sea cave.

The researchers looked at a species called Hemimysis margalefi, an understudied species of possum shrimp. The species lives in caves in the northwestern Mediterranean Sea. Millions of individuals form swarms in an individual cave and researchers think they remain “loyal” to the same cave throughout their lifetime.

Here’s the kicker though: at dusk, they move hundreds of meters into open water to feed on algae and zooplankton. Now, a few hundred meters may not seem that much to you (if you’re a good swimmer, at least) but if you’re a tiny shrimp, it’s huge. At dawn, the shrimps find their way back to their cave — but how do they do it?

How do they do it?

To get to the bottom of this question, divers sampled seawater from three caves in Calanques National Park off southern France. The three caves (‘Fauconnière’, ‘3PP’, and ‘Jarre’) are several kilometers apart, and between 11 and 24 meters underwater.

In addition to the water samples, the researchers also gathered hundreds of adult H. margalefi from the Fauconnière and Jarre caves. They also as well as other possum shrimps that don’t live in underwater caves but in shallow waters.

They then conducted an experimental trial in which single shrimps were placed in a Y-shaped channel. Each arm of the Y was connected to a 10-liter tank filled with seawater from one of the caves. The goal was to see whether the shrimp would prefer the seawater from their own cave and to see how quickly they made their decision. Between trials, the channel was emptied and rinsed with control water, after which the tanks were swapped between the arms.

Overall, 286 individuals were tested this way. Out of them, 230 were H. margalefi and 56 were in the control group. Trials were conducted in the morning and afternoon, to test for any effects of the time of day on the shrimps’ preferences but this didn’t seem to have an effect.

Overall, H. margalefi strongly preferred water from their own cave. They quickly made a decision and spent way more time in the seawater from their own cave. They treated water from the other cave just like regular ocean water — uninteresting.

The smell of home

“Here 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,” said Dr Thierry Pérez, CNRS research director at the marine research station of Endoume near Marseille in France, and the lead author of a new study in Frontiers in Marine Science.

Researchers used a high-tech method called UHPLC-HR-MS, which combines advanced liquid chromatography and mass spectrometry. Using this they compared the small molecules (metabolites) found in different underwater caves. They discovered that the underwater environment comprised mainly of natural compounds like peptides, fatty acids, steroids, and alkaloids, along with pollutants introduced by humans. The chemical makeup of water from the Jarre cave was very different from that of the Fauconnière cave, while the 3PP cave had a composition that was in between the two.

The researchers believe that stationary organisms like sponges, which are abundant in these caves and known to produce many specialized chemicals, are major contributors to the unique chemical environment of each cave. The shrimps then use this chemical “fingerprint” to trace their own cave.

“We know that food availability in marine cave ecosystems largely depends on the daily migrations of zooplankton such as mysids. Our results suggest that any change in water quality or sessile fauna inside caves can alter their chemical seascape, with a likely negative impact on the functioning of the whole ecosystem,” said Pérez.

This is significant because it highlights the interconnectedness of these ecosystems. It suggests that damage of the sponges and corals would also ripple down to other species.

“This is concerning, because due to global change, mass mortality of sponges and corals are becoming more frequent.”

“We are currently following up on our results by trying to correlate the chemical seascapes from different caves with the biodiversity of sessile organisms living in them, focusing on the role of metabolites from sponges and corals.”

The study was published in Frontiers in Marine Science. 10.3389/fmars.2024.1448616