The remarkable looking animal to the right is the hooded nudibranch (Melibe leonina). A nudibranch is a type of sea slug that has naked (“nudi”) gills (“branchs”).

Hooded nudibranch (10 cm long). Note oral hood, sensory tentacles and naked gills (lobed structures on the left)

Since late fall, I have observed hooded nudibranchs gathering at one of my favourite dive sites. They come together in order to mate and it is awe-inspiring to see 100s of them clustered together,delicate and ghost-like, clinging to the thin strands of giant kelp.

Often, you can see them swimming on the surface and many people mistake them for jellyfish. It is indeed one of the most alien looking of the 200+ sea slug species of our area. The large disc-like head lets it feed on plankton and small crustaceans and the lobed structures on the animals’ backs are the naked gills.

Since sea slugs can only sense light and dark, the way hood nudibranchs “call” one another is to send out a fruity scent (pheromones) that attracts others of their kind. My personal experience after having picked up a dead hooded nudibranch on the beach, is that the smell is something like a mix of watermelon and grapefruit and the scent stayed on my hand for more than an hour.

After mating, both animals lay eggs (they are hermaphrodites) and then, they die. You can find additional information about why sea slugs are hermaphrodites at this past posting.

Yesterday, I found ample evidence that mating has been happening. I found many egg masses; images of which I have included at the link below. Each cluster of eggs is only about one centimeter wide and contains thousands of eggs. After about 10 days the eggs will hatch into larvae that will be part of the zooplankton soup of the ocean. The larva are called “veligers” and look very different from the adult hooded nudibranchs. They have a shell and a big flap on their head with which they swim and feed on plant plankton. After 1 to 2 months, they settle to the ocean bottom and change body shape and even digestive tract to become small hooded nudibranches.

Click here for my photos of hooded nudibranchs and their egg masses (includes images from yesterday’s dive).

Next week, the posting will include video of a swimming hooded nudibranch.