Far below the surface of the southeastern Pacific, researchers have recorded an unusual partnership between two very different deep-sea animals. Tiny shrimp-like crustaceans were seen gripping the bodies of giant sea spiders, apparently using the much larger creatures as moving platforms.

The observations, published in the journal Ecology, were made along the Chilean Margin and suggest that this behaviour may be more than an accidental encounter.

An Unexpected Encounter

The smaller animals were amphipods belonging to the species Mesopleustes abyssorum. Their much larger hosts were giant sea spiders, Colossendeis colossea.

Researchers led by Jorge Pérez-Schultheiss of Chile’s National Museum of Natural History observed the association while using the remotely operated vehicle SuBastian to explore the deep ocean floor.

The first encounter occurred inside a steep rocky submarine canyon at a depth of 3,155 metres. Two larger amphipods were attached to the sea spider’s sensory and reproductive appendages, while a smaller female clung farther back on one of its long walking legs.

The animals were not simply resting loosely on the host. They appeared to be firmly attached, suggesting that maintaining contact with the moving sea spider was important.

The Behaviour Appeared Again

A single sighting could easily have been dismissed as coincidence. Four days later, however, the researchers encountered the same type of relationship again.

This second observation took place approximately 225 kilometres from the first site and at a depth of 2,698 metres. Two large amphipods were attached to another giant sea spider.

The amphipods used long, claw-like structures at the ends of their limbs to maintain their grip. Seeing the behaviour twice, at widely separated locations, led the researchers to suggest that the association could be relatively common across this part of the deep southeastern Pacific.

Why Hitch A Ride?

Scientists do not yet know exactly what the amphipods gain from travelling on sea spiders, but several explanations are possible.

Transportation is one of the most obvious. Mesopleustes abyssorum are poor swimmers, so attaching themselves to a mobile animal could help them move across larger stretches of the seafloor than they could manage alone.

The sea spiders could also act as mobile meeting points. In a vast environment where individuals may be widely dispersed, hitching onto the same host could potentially make it easier for amphipods to encounter potential mates.

Another possibility is protection. Remaining attached to a much larger animal might make the amphipods harder for some predators to reach.

Food could also be involved. The deep sea is an environment where resources can be scarce and unevenly distributed. Travelling with a mobile host might help the small crustaceans reach areas containing more suspended or deposited food particles.

Deep-Sea Life Remains Mysterious

The findings are intriguing precisely because so little direct observation is possible at these depths. Much of what scientists know about deep-sea animals comes from specimens collected by nets or other sampling equipment, which can reveal anatomy and genetics but not necessarily natural behaviour.

Robotic vehicles such as SuBastian allow researchers to watch animals interacting in their normal environment, providing clues that would otherwise be impossible to detect.

The study does not yet establish whether the amphipods deliberately seek out sea spiders or whether the relationship provides a measurable survival advantage. The researchers say additional in-situ observations, population genetic work and studies of feeding ecology will be needed to distinguish between accidental contact, a dispersal strategy and a specialised adaptation to life on the deep seafloor.

For now, the sightings reveal how much remains hidden in the deep ocean. Even a tiny crustacean gripping the leg of a sea spider can expose an ecological relationship that scientists had never documented before.