Phyllosoma

A flat transparent disc with long jointed legs radiating from it, two stalked eyes and almost no visible body. The thing is as thin as a contact lens and in a torch beam it is often the eyes and the leg joints that register first, with the outline only appearing when the light crosses at an angle. It is confused with nothing much once seen, though small ones are passed over as debris.

It is the larval stage of the Achelata, the group that contains the spiny lobsters and the slipper lobsters. The adult is an armoured bottom-dweller; the larva spends months in open water, moulting repeatedly and growing through a long series of stages before it condenses into a swimming post-larva and goes down to settle. Which family it belongs to can sometimes be narrowed from the shape of the body and the antennae, but a photograph will not usually settle the species, and this entry is pitched at the infraorder for that reason.

Phyllosomas are frequently found riding jellyfish, salps and other gelatinous animals, gripping the bell with the hooked tips of the legs. The relationship appears to be both transport and food: the larva feeds on the host’s tissue as it travels. A phyllosoma on a jellyfish is the picture worth waiting for, and it is common enough that most blackwater divers eventually get one.

Lighting a transparent animal is the whole problem. Front light passes straight through and returns almost nothing; light angled from the side or slightly behind catches the leg edges and the gut and gives the shape something to sit on.