Pink Flatworm

You advance slowly along a rocky wall when a small mauve ribbon catches your eye. Barely a few centimetres, almost flat like a leaf, it glides between fixed organisms without seeming to disturb anything. You bring your lamp closer: its colour changes appearance and reveals a pink-lilac crossed by fine white lines.

This tiny worm is nonetheless a predator. In the northwestern Mediterranean, researchers showed that it could prefer certain small ascidians, notably Pycnoclavella communis. On the studied sites, it was even observed feeding on colonies that had become almost invisible during their rest phase, when only a discrete part remains on the substrate.

Under these conditions, the presence of the worm could sometimes betray that of its hidden prey. But the most remarkable aspect comes from its reproduction.

Prostheceraeus roseus is hermaphroditic and can inseminate its partner using a small stylet that pierces the tissues. Observations even show two individuals using their stylet at the same time.

Then the scene ends as discreetly as it began: each resumes its path on the rock, the body barely undulating, until the beam moves away and the pink merges once again into the deep hues. Under the lamp, it reveals itself; in the blue, it becomes almost a secret again.