Barrel Amphipod : The Hollow Kindergarten

The delicate, translucent, gelatinous barrel of the salp's body contains within it, sitting inside the barrel, a squatter, Phronima sedentaria, an amphipod crustacean known as the barrel amphipod or the pram bug or variations on those names.

[tense music] The hollow kindergarten.

Phronima sedentaria is not the only species of amphipod crustacean you can find under such odd circumstances lodged inside a salp, but it is the most famous of them being the most abundant and found pretty much worldwide.

There's a terrific little video I'll put in the show notes at The Wild Episode, a perfect little transparent salp nestled amongst the seaweed on an Irish beach.

And inside it, moving, flexing its legs, a somewhat transparent, somewhat orangish amphipod, Phronima sedentaria, just sitting in there occupying most of the internal space of the barrel.

[mysterious music] Amphipods are kind of like shrimp, but generally a bit flattened from side to side, so taller than they are wide.

There are thousands of different species.

This is one of the major types of crustacean in the world.

And although the great majority are marine, you can find them in fresh water, things like various gammarus species that are familiar to any kids who go pond dipping.

And there are even some that are terrestrial, sand hoppers and lawn hoppers, all amphipods.

There are inevitably giant amphipods in the deep sea that get up to over 30 centimeters, a foot long, but most are small, some very small.

In the case of the barrel amphipod, males are only 1 or 2 centimeters long, less than an inch, while females can get to at least twice that, close to 2 inches.

They are what I'd call semi-transparent.

In life, they often have a sort of dull orange, browny-red dusting to their appearance, but it's not uniform or complete enough to prevent light passing through them.

There are one or two remarkable things about the anatomy of Phronima sedentaria that sort of relate to this semi-transparency, but we'll come back to that because the most extraordinary thing about the barrel amphipod is unavoidably its bizarre relationship with salps and, to be fair, sometimes other tunicates like pyrosomes or other somewhat gelatinous members of the oceanic animal community.