Every individual is a hermaphrodite with both male and female genitals. When they have sex, they can simultaneously penetrate each other, with penises that extend to their whole body length. "They are relatively well-endowed, says Lange.
The penises are also forked. One branch ends in a cone-shaped structure called the penile bulb, which is ringed by small spines. It goes inside the partner's female genital opening, and delivers sperm. The other branch ends in a fiendish spine called the penile stylet. It stabs straight into the partner's forehead, and pumps fluid from the prostate gland. So, during sex, each slug gets a dose of sperm in the usual place, and an injection of prostate fluid just above its eyes. This goes on for just over 40 minutes.
"You may imagine I was quite excited and surprised to find out they reciprocally injected into their partners' head!" says Lange.
These Sea Slugs Penetrate Each Other In The Head During Sex: