Under water to get out of the Rain - Part 4 - strange animal

Trevor Norton went caving in a tidal pool in Lanzarote.

I stripped off and walked into the water with reverence. The bottom was a tumble of boulders. From a thousand crevices long glaucous ribbons protruded. When I tried to touch them, they shot back into their hole with the sudden recoil of stretched elastic. What on earth were they?

I sneaked up on one and chopped with my diving knife before it vanished into its lair. The detached ribbon stayed alive for hours rhythmically extending and contracting. It was soft and frilled and had a groove all down one edge, and the tip was forked. It was distinctive, but was it a worm or what? As I stared, it contracted into a question mark.

I was not the first person to be puzzled by this creature. Long before, a biologist had examined such a piece and described it as a new species. What neither of us knew was that there was another bit. Hidden in the crevice sat the fat green plum of its body. The thin gelatinous concertina that I had captured was just its proboscis searching for food. It is called Bonellia and belongs to an obscure group I had never heard of.

Usually just the tube is visible.

 The plum is the female and she is seventy times bigger than the male, a tiny parasite that lives quietly among her folds. The larva is of undecided sexuality. If it encounters a female, it becomes a male and slips into the groove in her proboscis. In the absence of female company the larva becomes a female and waits for a male.