Kidnapper Crustaceans Use Tiny Mollusks as Unwitting Shields

Amphipods wear the so-called sea angels, which secrete chemicals that keep certain predators at bay, like backpacks

An amphipod with its victim clamped on its back. Charlotte Havermans

Deep within the Southern Ocean, tiny mollusks float through the water armed with a vital defense mechanism: they secrete potent chemicals that deter hungry predators. But sometimes, these little critters will be minding their own business when suddenly Crustacean kidnappers grab them and carry them off piggyback-style to use the small swimmers as living shields.

As Natasha Frost reports for Quartz, a new study published in the journal Marine Biodiversity has documented this fiendish behavior for the first time in the Southern Ocean. A team of researchers led by marine ecologist Charlotte Havermans of the Alfred Wegener Institute in Germany embarked on an expedition in 2016 to study amphipods of the suborder Hyperiidea, a type of shrimp-like crustacean that is a favorite snack of fish and sea birds. When the team examined amphipods pulled from four sampling sites, they noticed that a few of them had pteropods, a planktonic mollusk with a variety of idyllic nicknames—sea butterflies, sea angels, sea snails or sea slugs to name a few—clamped onto their backs.

The amphipods gripped their victims with two pairs of their legs and were not affected by the pteropods’ chemical secretions. Previous research has shown that one of the amphipods’ predators, the icefish, steered clear of individuals wearing sea snail knapsacks. This “tandem association” between amphipods and pteropods was first documented in 1990 around the coastal waters of Antarctica, but the behavior had never before been seen before in the open ocean.

Researchers collected 342 amphipods. Of these, only four were observed to be carrying sea snail abductees. Two of the kidnappers belonged to the species Hyperiella dilatata and carried a type of pteropod known as Clione limacina Antarctica; the other two were Hyperiella Antarctica amphipods and hauled the pteropod Spongiobranchaea australis. According to an Alfred Wegener Institute statement, the sample size is too small to say whether these are species-specific pairings, “where only a certain amphipod carries a certain pteropod species.”

Both male and female amphipods were observed with pteropods in tow, and one of the abducting females was carrying eggs. They kept a very firm grip on their unwitting protectors; the amphipods did not even relinquish their grasp after the pteropods died. What is less clear is whether the pteropods derive any benefit from the arrangement, though it certainly doesn’t seem that way. Unable to free themselves from their captors and hunt for food, the pteropods eventually die of starvation.

Though questions about these interlocked critters remain, the new study suggests that there is much to discover about associations between different sea creatures—particularly delicate ones like amphipods and pteropods, which often get crushed in sampling nets.

“In the future we will hopefully be able to use suitable underwater technologies with high-definition cameras to investigate even the smallest life forms in their habitat,” Havermans says in the statement. “This will provide insights into the numerous exciting mysteries of interspecific interactions, which have so far remained hidden for biologists—but which undoubtedly play an important role in predator-prey relationships in the ocean.”

Get the latest stories in your inbox every weekday.