This all started because I wanted to determine the amount of weight I’d need for a wetsuit that I hope to wear if we can ever travel to a warm water destination again. A dock dive was the perfect place for this so I could fiddle with weights without getting cold, but the short descent delivered more than I had bargained for.

Below our boat, old discarded dock floats were decorated with the eggs of Navanax inermis. Egg masses were draped on every available perch. The top of floats and the sea floor was punctuated by clusters of mating adults.

We tend to think of the predatory Navanax inermis as a typical nudibranch, but it’s actually in the group of headshield slugs, the cephalaspideans. Navanax anatomy differs from nudibranchs in that it has a vestigial internal shell and the head is broad and flat with sensory bristles on each side of the mouth, designed for swallowing prey whole.

Like other opisthobranchs, Navanax is hermaphroditic, having both male and female sex organs, but they’re located in different places. The penis is on the right side of the head while the genital opening is on the right side at the rear of the body. So the slug acting as male has to approach from behind.

Close-up of the head of one Navanax as it approaches the copulatory organ of the other.

It can get a little busy at the back end.

Sometimes, they can form a chain, where the one in the middle acts as male and female simultaneously.

Another mating chain of three Navanax inermis, with their parapodia (foot extensions) open.

Gould's bubble snails are common Navanax prey. Many of their empty shells were lying about.

• Fazinating (-) seahunt 18:57:41 08/07/20 (0)
• Re: Summer under the dock Halibug 07:49:14 08/07/20 (0)