Part 19 of my documentary, “Mucky Secrets”, about the fascinating marine creatures of the Lembeh Strait in Indonesia.
In this video we study how sea slugs (including nudibranchs) feed and mate.
All known nudibranchs are carnivores. The biggest family of nudibranchs, the chromodoridids, feed exclusively on sponges.
Most sea slugs have a ribbon-like tongue covered in microscopic teeth called a radula to help them consume their prey. The form of the radula varies greatly and is important as a basis for taxonomic classification.
We see a pleurobranch, Pleurobranchus forskalii, a different type of sea slug, feeding on an ascidian, or “sea squirt”, a type of tunicate.
Nembrotha nudibranchs also feed on ascidians. We see a Nembrotha lineolata feeding on a blue club tunicate. The ascidian feeds by filtering plankton from the water with its delicate, blue, sieve-like interior enclosed in a clear outer sac, its tunic. The sea slug everts its proboscis, its oral tube, out of its mouth and, with ruthless efficiency, sucks this fleshy interior right through the tunic. The radula teeth enable the slug to deal with the tougher parts of the sea squirt’s intestines.
Most sea slugs are quite specific in their choice of food, and so they are often drawn towards the same place. This increases the chances of encountering others of the same species and finding a mate. As they have no vision, nudibranchs locate each other initially through smell then touch.
During copulation, they line up their genitals which are on the right side of their body. All sea slugs are hermaphrodites and contain both male and female reproductive systems. During mating, each nudibranch receives sperm from the other.
We see a pair of Nembrotha purpureolineata nudibranchs mating. The penis, which is off to the side, is covered in tiny, sharp barbs which lock it into the vagina, which is at the centre of the stalk. The male organs often mature before the female ones. Small nudibranchs with an immature female reproductive system can store the sperm they receive until they start producing fertile eggs.
We also encounter a mating pair of Hypselodoris bullocki nudibranchs. Their genitals are also covered in tiny spines that anchor them together during copulation.
After fertilisation, a mucus-bound ribbon of eggs is laid in a spiral, often on or near the species’ food source. Most egg masses are toxic to predators and are abandoned by the parent.
Hypselodorid nudibranchs often follow each other around, top to tail. The reason for this ‘trailing’, or “tailgating” behaviour is a mystery. It’s thought to be a prelude to mating, but in some cases the trailing slug might simply be getting an easy ride in the search for food.