Rare fossil captures ancient shrimp hiding inside a clam

About 100 million years ago, when dinosaurs roamed the land, three little shrimp went house hunting. Perhaps seeking shelter from predators, far from protective coral reefs, they chose a giant clam—not the largest on the block, but cosy at about 10 inches wide.

They settled in only to be quickly flooded with silt and mud. Their intended shelter suddenly became their tomb. And there they sat until 2016, when an Australian farmer came along and found them. The fossilised clam containing the three shrimp, each about 1.2 inches long, is now housed in Australia’s Kronosaurus Korner museum.

Recently described in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, the fossil represents the oldest example of a shrimp using another creature (or creature’s home) for shelter—something living shrimp still do today. This behaviour, known as inquilinism, is seen in many animals, both on land and in the oceans.

Because the fossilised shrimp were preserved whole, they were likely alive in the shell when they were rapidly inundated with mud, perhaps during an earthquake or intense storm. Had they washed into the shell after they died, they would not have been intact. “Shrimp are fairly delicate,” says René Fraaije, director of the Oertijdmuseum natural history museum in the Netherlands, who was not part of the study. “If you find complete specimens, with the carapace and tail and legs attached to each other, it must have been a living animal.”

The shrimp could have scurried into the clam to nest or moult, but there isn’t evidence of those behaviours. They might have been seeking shelter from the storm that ultimately buried them, but nailing down a short sequence of events like that is nearly impossible “without a time machine,” says Russell Bicknell, a palaeontologist with the University of New England in Australia and lead author of the new study.

A likely explanation, Bicknell says, is that the shrimp were following a basic survival instinct: hiding from predators. “Shrimp weren’t anywhere near the apex of the food chain,” he says. “Almost anything, except things like the bivalves that are filter-feeding, could have had a chomp on these little guys.”

The specimen is the latest addition to a slowly growing list of fossilised critters borrowing shelter from other animals, and it tells biologists that some shrimp have been opting for inquilinism for at least 100 million years.

“This is a big finding, a big discovery,” says Ninon Robin, a palaeontologist at the Royal Belgian Institute of Natural Sciences who was not involved in the study. “It’s quite rare to find specimens like this that are in association. It’s a lot of luck.”

A polite squatter

On the spectrum of organism partnerships, inquilinism falls squarely between symbiosis, where both organisms benefit, and parasitism, where one organism benefits at another’s expense. If the host organism is still alive—as is sometimes the case, like tiny pea crabs nestling inside mussels—it doesn’t stand to benefit, but it’s not really bothered either. The squatter gets a little safety without having to give anything in return.

A classic inquiline is the hermit crab. It does not build its own shell as it grows, but rather relies on the castaways of other, shell-producing animals such as snails. The hermit crab must use another’s shell to survive, but for other inquilines, including shrimp, it’s more a matter of convenience.

Inquilinism “started pretty early on” in the history of animal life, says University of Alabama palaeontologist Adiël Klompmaker, one of the study’s authors. The oldest known animal likely evolved more than 541 million years ago, although some evidence suggests the first animals emerged much earlier; animals with shells likely appeared shortly after.

Soon after some animals evolved shells, Klompmaker says, other animals started using them to hide. The oldest plausible fossil evidence of inquilinism is a set of trilobites, an extinct group of marine arthropods, found inside the shells of nautiloids, a group of cephalopods that date back to the Ordovician period, about 485 to 444 million years ago. An assortment of ocean dwellers has also been found in ammonites, which are extinct mollusks with distinctive spiral shells that could reach nearly six feet in diameter.

“Large nautiloids, large ammonites, those sorts of things offer a lot more protection because you can go farther inside,” Bicknell says. But in a pinch, a clam shell might have had to do.

Hiding from danger

In addition to the three shrimp tucked in a clam, palaeontologists found another, larger clam hosting about 30 little fossilised fish in the same geologic formation. The clam with fish has not yet been described in detail in a scientific paper, but having two well-preserved specimens of small organisms living in the same kind of clam strongly suggests that the critters moved in as a response to environmental threat, Bicknell says.

If these critters were seeking safety from predators, as Bicknell believes, they may not have had anywhere else to hide. There’s no evidence of a coral reef in the area, which likely would have provided better hiding spots for shrimp and other animals near the bottom of the food chain.

“There are tons of dangers on the ocean floor,” Klompmaker says. Away from the reefs, with a dearth of options for hiding from predators, even a drafty bivalve might have looked appealing to a shrimp.

If the shrimp did indeed move into the clam to hide from predators or environmental upheaval, the fossil preserves early evidence of the animals learning to live on the seafloor. “From the very beginning, they have been adapting to this very specific ecology,” Robin says. “That was the only way for them to thrive.”

Bicknell is enthusiastic about the finding. “I just love that we have this these chance fossils preserved, these needle-in-a-haystack things,” he says. “They’re almost time capsules … that give us a really nice insight into how members of an extinct ecosystem interacted with each other.”