Phronima in a Barrel

©

21 April 2010

Rainy day aboard the Tara. Long waves unfurl against the boat’s hull. Wind in the sails at 17 knots, we’re headed towards the archipelago of Saint Brandon.

Nets have been replaced by computers and books. Everyone has found a small spot inside Tara to keep dry, and work.

Ideal weather to talk about a representative of zooplankton that we’ve encountered several times this week: Phronima.

Immediately re-baptized “the alien”, the first Phronima we caught measured almost 10 centimetres. With a head in the shape of a Greek helmet, and big protuberant eyes, it looks like a frightening extraterrestrial. But Phronima is actually a rather common animal in the warm seas of the world — a crustacean with some very surprising domestic habits.

This creature is a hunter, capturing gelatinous animals among the plankton. Carefully cutting up the transparent cellulose envelope of its prey, Phronima recycles it, constructing a barrel as a shelter for itself. Each species of Phronima specializes in the capture of one type of animal. Some prefer medusa, others prefer salpes or siphonophores.

When it grows bigger, Phronima builds a new transparent tunica so as not to be short of space. It grips the barrel with its forelegs, and swims using rows of cilia hidden under its tail. Phronima doesn’t even have to leave its gelatinous shelter to hunt, nibbling its prey just outside the barrel, and pulling them inside when they’re sufficiently small.

Despite its frightening appearance, Phronima has maternal qualities rather unusual among crustaceans. The mother takes good care of her progeny: she lays eggs, then raises and feeds the larvae in her transparent incubator.

The specimen we photographed actually came out of its shelter – a rare image!

Sacha Bollet & Christian Sardet