Mucky Secrets - Part 19 - Sea Slugs Feeding & Mating - Lembeh Strait

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. All known nudibranchs are carnivores. The biggest family of nudibranchs, the chromodorodids, feed exclusively on sponges which are often concealed beneath sand or silt. Most sea slugs are quite specific in their choice of food and so they are often drawn towards the same place. This is also a good reason not to keep them in aquariums. This is a pleurobranch, a different type of sea slug, feeding on an ascidian or sea squirt, a type of tunicate. This slug's gill is on the right side of its body between its mantle and foot. Nembrotha nudibranchs also feed on ascidians. This lined Nembrotha is tackling 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 radular teeth enable the slug to deal with the tougher parts of the sea squirt's intestines. In a little over a minute, the meal is finished and the slug moves on. This Nembrotha of a very similar species makes its way through a colony of social ascidians and past a tiny relative. A liking for the same foodstuff increases the chances of encountering others of the same species, and moments later, this nudibranch had found itself 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. In many nudibranchs, the genitals are flush with the body or raised on a slight swelling. In this Nembrotha, they are at the end of a stalk which can be extended a long way to reach the partner. The penis, which is off to the side, is covered in tiny sharp barbs which lock it into the vagina, which is at the center 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. This is a mating pair of Hypselodorid nudibranchs. Their genitals are also covered in tiny spines that anchor them together during copulation. After fertilization, 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 behavior 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. This sap-sucking slug is not a nudibranch. It does not have gills as such.