A larval green spoonworm hatches from its egg, aimlessly drifting in the sea. A week later, it settles to the seabed to await its fate. Falling within reach of a female spoonworm’s roving proboscis, or tongue, the young worm comes under her spell and starts inching helplessly towards her, destined to become her sex slave.
Green spoonworms (Bonellia viridis) occur in the northeastern Atlantic Ocean and the Mediterranean Sea, at depths up to 50m. Females – whose soft bodies are around 8 cm long – live in abandoned burrows or gaps between stones. They don’t even leave home to feed, making use instead of their long mouthparts. Females can extend their tongues by up to two metres, vacuuming the sand around their burrow, flicking up any morsels of food.
The bright green colour of the spoonworm’s tongue comes from a chemical pigment called bonellin, which has many curious properties. It’s a potent antibiotic, and when activated by light it has also been found to kill cancer cells. Medical researchers hope to harness this unique ability for photodynamic therapy, whereby patients would be given an anti-tumour drug that is selectively activated within the body using light beams.
Bonellin is also the chemical that attracts young larval worms to adult females. Spoonworms aren’t born female or male, and their sex isn’t controlled by chromosomes. Instead, larvae that encounter a roaming tongue crawl towards the female, attaching themselves to her. They metamorphose into males, before crawling through her mouth into her uterus. Here they live out their lives as tiny sperm machines, less than 3 mm long. Larvae that settle out of reach of a female’s scavenging tongue turn into other females.
Sex determination “by the presence or absence of bonellin, is very interesting, because in most other organisms sexual differentiation is regulated by internal factors such as genes in chromosomes”, says Paavo Hynninen at the University of Helsinki in Finland. This weird behaviour “is thought to be an adaptation to always find a mate”, says Patrick Schembri, a biologist at the University of Malta. “Dwarf males live in a sort of semi-parasitic association with a female green spoonworm. In this way, a female always has males readily available to fertilise her eggs,” he explains.