Deep-Sea Mystery: Why Are Amphipods Clinging to Giant Sea Spiders?

Scientists have captured a bizarre deep-sea interaction where tiny shrimp-like crustaceans were seen clinging tightly to giant sea spiders more than 3,000 meters (9,800 feet) below the surface of the Southeast Pacific.

The encounters, recorded by the remotely operated vehicle SuBastian during a 2024 expedition along Chile’s margin, show amphipods of the species Mesopleustes abyssorum attached to giant sea spiders identified as Colossendeis colossea.

The observations were detailed in a study published in the journal Ecology in September 2026.

Rather than simply landing on the much larger animals, the crustaceans appeared to maintain a firm grip on the sea spiders, raising the possibility that they were using the giants as a means of transportation.

Two encounters revealed the strange behavior

The first observation occurred on November 24, 2024, during the Chile Margin expedition. At a depth of 3,155 meters (10,351 feet), researchers spotted three M. abyssorum amphipods attached to a C. colossea.

Two larger amphipods were positioned toward the front of the sea spider, while a smaller female was attached farther back on one of its walking legs.

The larger pair were identified as an adult male and female, while the smaller female was carrying eggs. Just four days later, scientists encountered the behavior again about 225 kilometers (140 miles) away. At a depth of 2,698 meters (8,852 feet), another C. colossea was carrying two amphipods.

The two sightings at separate locations suggest the behavior may be more than a chance encounter, however it cannot establish that the behavior is routine

The amphipods appeared to be holding on

The researchers observed the amphipods using long, claw-like structures at the ends of their limbs to grip the sea spiders.

Their attachment was sufficiently strong that the animals remained attached even after collection, according to the research reported by Phys.org.

Scientists describe this type of transportation relationship as phoresis, in which one organism uses another organism as a means of getting from one place to another.

It does not necessarily equate to the host benefitting from the relationship.

The study’s authors wrote that an incidental interaction could not be ruled out, but said the apparent specificity and recurrence of the association suggest it could represent an underrecognized ecological strategy.

Why would a tiny crustacean need a sea spider ride?

The amphipod’s biology offers a possible clue to the unusual behavior. The amphipod is considered a relatively poor swimmer and does not have a planktonic larval stage that could allow young animals to disperse widely by drifting with ocean currents.

Yet records place the species across the Pacific, Atlantic, Indian and Southern oceans, at depths of roughly 690 to 3,500 meters.

A mobile sea spider could provide a way to cover territory that the amphipod would struggle to reach on its own. The sea spiders may be especially useful because members of Colossendeis have extremely long legs and can move along the seafloor.

Large sea spiders can also spread their legs and be transported by water movement, potentially increasing the distance covered by anything attached to them. The rides could help the amphipods reach new feeding grounds and potential mates.

The sea spiders may also become a meeting point

The sea spiders could offer the amphipods another advantage beyond transportation. Researchers noticed that the larger amphipods in both encounters included an adult male and female.

The additional amphipod seen during the first encounter was a female carrying eggs. The researchers have not observed mating on a sea spider making it a hypothesis rather than an established explanation.

These are not actually spiders

Despite their appearance, sea spiders are not true spiders. They belong to a separate group of marine arthropods called pycnogonids. Their bodies can be extremely small relative to their long legs, and some species have vital organs extending into their limbs.

The species involved here, Colossendeis colossea, is particularly notable. It is recognized as the largest known sea spider, with reported leg spans reaching about 70 centimeters (28 inches). That makes the size difference between the hosts and their tiny passengers striking.

The mystery goes back nearly a century

The behavior may not be entirely new to science. An association between Mesopleustes abyssorum and a sea spider in the genus Colossendeis was reported as early as 1927.

But that historical observation did not provide the kind of photographic or direct in-situ evidence now available from the SuBastian footage. There is also evidence from more recent deep-sea exploration that M. abyssorum can associate with other sea spiders.

A 2025 inventory of methane seeps off Costa Rica recorded the amphipod attached to Colossendeis macerrima at about 1,153 meters and noted that it remained attached after collection. The finding suggests the behavior may be a broader association between Mesopleustes amphipods and sea spiders, rather than an isolated encounter.

Partnership Remains Unexplained

For now, scientists cannot determine whether the amphipods intentionally seek out sea spiders or simply attach to them when the opportunity arises. Further observations will be needed to establish how common the association is and whether it helps the crustaceans travel, find mates or gain another advantage in the deep-sea environment.