The remarkable looking animals to the right are Hooded Nudibranchs (Melibe leonina up to 17.5 cm). A nudibranch is a type of sea slug that has naked (“nudi”) gills (“branchs”).
Starting in the fall, around NE Vancouver Island, 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 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 Hooded Nudibranchs signal one another is by emitting a fruity scent (pheromone) 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 blog posting.
Typically in our area they lay their egg masses between January and April. Each ribbon of eggs is only about one centimetre wide and contains thousands of eggs.
Every dot is an egg capsule contains 15 to 25 eggs. After about 10 days, depending on temperature, 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 smaller plankton. After 1 to 2 months, they settle to the ocean bottom and change body shape and even digestive tract to become small Hooded Nudibranchs.
A few of my video clips of this species below.
For more information:
Lawrence, K. A. and Winsor H Watson. “Earth , Oceans , and Space ( EOS ) 10-1-2002 Swimming Behavior of the Nudibranch Melibe leonina.” (2017).
Newcomb, James M., et al. “Homology and Homoplasy of Swimming Behaviors and Neural Circuits in the Nudipleura (Mollusca, Gastropoda, Opisthobranchia).” Proceedings of the National Academy of Sciences of the United States of America, vol. 109, National Academy of Sciences, 2012, pp. 10669–76, http://www.jstor.org/stable/41601654.