The sea slug Chromodoris reticulata has a disposable penis, reports AFP.
The red-and-white slug is a hermaphrodite, performing both sexual roles during copulation. Mating Chromodoris reticulatae both give and receive sperm, and store sperm from multiple partners, choosing to either fertilize its eggs or digest the sperm. The thumb-sized, shell-less mollusc lives in Southeast Asia.
"No other animal is known to repeatedly copulate using such 'disposible penes'," a new study in the Royal Society journal Biology Letters. The scientists went on to describe the slug's behavior as "extremely peculiar."
Copulation can last between dozens of seconds and a few minutes, and after, both slugs will discard their penes. The Osaka City University team, led by Ayami Sekizawa, observed an internal spiral structure inside the slugs that grows into the replacement penis, they believe. "We propose that the tissue at the spiral part of the penis is compressed and undifferentiated, gradually differentiating into the 'next penis'," they wrote in the study.
Once the penis is discarded, the slug wastes no time growing a new one. "It may need approximately a day for the spiral structure to be ready for copulation."
As if self-severing its penis wasn't enough, the researchers also found that the slug's penis was covered in spines. The Osaka team suggests that these spines help to remove stored sperm from previous sexual partners (similar to the human penis). These spines also might explain the penis' detachment after copulation: they face backward, making it difficult to remove.
"Chromodoris reticulata may compensate for the short-term cost of decreased reporductive opportunities caused by the loss of a penis with the reproductive advantage gained by sperm displacement," said the study.
Many other nudibranches detach parts of their body, often to distract predators, allowing them to escape. And octopi are known to detach their reproductive organs after copulation, but so far, Chromodoris reticulata is the only observed species to not only remove its penis, but also to grow it back. The scientists observed one of the slugs detach and grow back its penis three times.
They do not know if there is a limit to how many times the slugs can detach and regrow the sex organ.
