November 25, 2020

I, Science

The science magazine of Imperial College

On land, “slug” means slow, slimy, green and fond of cabbage, but it doesn’t have to be that way. From today, “slug” should make you think of beauty, carnivorous appetites and the surprising ingenuity of evolution. My first Seabeast of the Week is the nudibranch, a type of sea slug.

Thousands of nudibranch species are found in seas all over the world. Reef divers search them out to photograph and identify and sightings of the rarest and most striking species are something to be proud of. Marine biologists are fascinated by their unique forms and bizarre adaptations. Here is a picture of Pteraeolidia ianthina, the “blue dragon” from the Southwestern Pacific, which shows why.

Pteraeolidia ianthina, the blue dragon, by Jason Marks

“Nudibranch” means “naked-gill”, referring to the breathing apparatus nudibranchs have on the outside of their bodies. Pteraeolidia ianthina is covered in cerata (“horns”) with a large surface area, allowing the slug to get oxygen underwater. The slug is vividly coloured, like many nudibranchs, as a warning to predators. Such threats need backing up, and this slug is serious: the blue dragon can inflict a painful sting from the tips of its cerata. The source of this sting is the most interesting aspect of the nudibranchs, and more on that later.

Hermissenda crassicornis, by Matt Knoth

Here is another of my favourite nudibranchs, Hermissenda crassicornis. Nudibranchs have a lot in common with land slugs. All slugs and snails are molluscs, along with squid, octopuses, clams and plenty more. Their underside is a large “foot”, they have several pairs of tentacles on their head, and they lose their shell before adulthood.

Nudibranchs’ very basic eyes can see little more than light and dark and are not on the tops of stalks. Instead, the two things sticking up at the front of Hermissenda crassicornis (on the right) are its rhinophores, which have a strong sense of smell. Its oral tentacles, out to the sides, can taste, touch and smell.

Flabellina iodinea, the Spanish shawl, by Steven Thomas Snyder

Leminda millecra eggs, by Seascapeza

Like land slugs, nudibranchs are hermaphrodites with both sets of reproductive organs. When they mate, both slugs fertilise each other. You can see a pair of Spanish shawls, Flabellina iodinea, going at it on the right.

Nudibranchs lay their eggs as fine, often spiraled ribbons. See those of the frilled nudibranch (Leminda millecra) below right.

Nudibranchs are carnivores in that they eat other animals, going for ones that move even slower than they do. When hunting, they envelop prey in their mouth and use a rasping tongue-like organ called a radula to scrape away at its flesh. Here is a slightly unsettling video of one nudibranch devouring several of its smaller relatives. And here is a much nicer video of a lynx nudibranch “grazing” on flower-like animals called hydroids while perched on the back of an oyster.

Every nudibranch I’ve mentioned to you has cerata, because they are all “aeolids”. Others don’t, and most of those are “dorids”. These have gill plumes on their backs, which sometimes look like wings, other times like flowers. Despite lacking stingers, dorids are just as poisonous as aeolids and announce it with similarly vivid colours. Take a look at Hypselodoris apolegma (left) and Nembrotha chamberlaini (right) below.

Hypselodoris apolegma, by Chad Ordelheide

Nembrotha chamberlaini, by Alexander R Jenner







As I said, the source of nudibranchs’ stingers is the most interesting thing about them. How do they get them? They steal them! And this is not even the most impressive ability sea slugs can steal. But I’m not going to tell you how or from what until next week. Look out for the next Seabeast of the Week!

IMAGES: Wikipedia, wikimedia commons