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Sea slugs have a reputation for bizarre sex. For one, they’re hermaphrodites that penetrate each other at the same time. One species, post-coitus, amputates its own penis. (Don’t worry: it keeps two spares coiled up inside its body.) Despite knowing all this, Licia Sales, a zoologist at the University of São Paulo in Brazil, was still astonished when she discovered mating pairs of Phidiana lynceus sea slugs ripping off and eating each other’s fingerlike appendages, called cerata, during sex.

“At first glance, I didn’t understand if it was mating or agonistic behavior because the individuals were very aggressive,” Sales says.

Sales and her colleagues filmed five pairs of sea slugs under a microscope to get a close look at the behavior. They confirmed that the animals were trading sperm so mating was definitely part of the act. But they were also eating parts of each other. The scientists dissected the sea slugs, which are roughly 30 millimeters long, and counted the number of cerata that seven of them had swallowed. The mean number was 13.

The team isn’t sure why the sea slugs exhibit this behavior. They suspect it’s an example of traumatic mating, in which the attacker harms its partner to boost its own sperms’ chances of fertilization—a tactic that is also observed in other invertebrates. Male seed beetles, for example, have spikes on their penises that pierce a female’s reproductive tract making it less likely that she will couple with a rival in her injured state.

Traumatic mating often involves genitals, though, which is not the case for these sea slugs. “The damage is extra-genital, so it would not involve sperm displacement,” says José Eduardo Marian, a member of the team. Marian says harming non-genital body parts during sex is rare for hermaphrodites.

Instead, the researchers think cerata-eating sea slugs could be encouraging the spread of their own genes in a roundabout way. A sea slug that has just lost a lot of cerata would be scared of losing more, so it might avoid mating with another partner lest it become seriously injured. In this case, it’s more likely that the first partner’s genes will be passed down, but this would only be a short-term effect, since cerata regrow in about 24 days.

However, it’s also possible this nibbling could be mutually beneficial for the sea slugs, says Kyle Theodore Davis, an evolutionary biologist at the University of Miami in Florida who was not involved with the research. Davis thinks the behavior could provide new insight into hermaphroditism, in which partners may need incentives to participate in both male and female roles. “Perhaps offering up cerata for a partner’s consumption is a way to keep both parties interested thereby facilitating reciprocal sperm transfer,” he says.

Alternately, eating cerata could be a way for both mates to acquire new weapons. The fingerlike appendages, which play a role in digestion and gas exchange, also contain stinging cells used for defense that the sea slugs acquire by eating jellyfish-like prey.

According to Marian, since each sea slug is likely to have obtained stingers from different prey species, ingesting a partner’s cerata could be a way of diversifying their arsenal and therefore gaining protection against a wider range of predators. “There is still a lot to discover about these amazing animals,” he adds.