CREATURE FEATURE

Pram Bug

About the Pram Bug

Courtesy of the film Alien, we have an idea of what it’s like to be consumed from the inside out by a ferocious parasite. But truth is stranger than fiction when it comes to the phronima, a small, translucent crustacean that long ago abandoned the seafloor to haunt the ocean twilight zone.

A chance encounter with a phronima spells game over for a sinophore or salp floating innocently through the ocean. Using its crab-like front claws to skewer its victims, phronima chew out their gelatinous insides, leaving just enough tissue to provide structure and buoyancy. The parasitoid then climbs inside the hollow body of its victim and declares it home sweet home. Female phronima lay their eggs inside the barrel before climbing outside and pushing it around with their strong legs, earning it the nickname “pram bug”. Though it seems a bit creepy, taking over another species’ body turns out to be a good way for phronima to travel, meet potential mates, and raise children in a cold, dark environment.

At least one species, P. sedentaria, has the power to make itself invisible by manipulating special pigment-filled cells in its skin. But the phronima has even more superpowers up its sleeves. An anti-reflective bacterial coating in its body and legs dampens the power of other predators’ bioluminescent searchlights—like those adorning lanternfish—helping to camouflage the animal as it searches for its next victim.

Quick Facts

Common NamePram Bug
Scientific NamePhronima sp.   
Other NamesBarrel shrimp
Sizeroughly 2.5cm (1 inch)
Eats what?Salps, siphonophores, krill
Eats how?Chew and claw out a victim's gelatinous shell, then settle down to live inside it.
Is eaten by?Lanternfish