True sea slugs are divided into a number of taxonomic orders, however 5 are perhaps the most popular/dominant with regards to scuba divers and naturalists. To read our recent published work on sea slug diversity and importance on Koh Tao and Thailand, read the link here. The five major orders of Sea Slug to be found, particularly on Koh Tao are as follows:
Sap Sucking slugs that feed on macro algae. These slugs are diverse vegetarians and are famous for the ‘kleptoplasty’ ability that many species have which allows them to keep ingested chloroplasts active inside their own bodies. To translate, this means many species can effectively become ‘solar powered’ with minimal need to actively feed for varying lengths of time.
Naked Gill slugs are the most diverse and well known sea slugs in the marine environment. They are among the most colourful and charismatic of sea slugs and largely feed on sponges and other toxic/deterrent food sources that can be used in its own defences. The huge variety of shapes and sizes of nudibranchs make them among the most popular in the diving community.
Headshield/Tailed/Bubble shell slugs are largely carnivorous with several being omnivorous. Many species within this order are still in the process of losing their shell, a remnant of the snails they evolved from, with some that have retained shells large enough for the whole body of the animal and others with small, relatively useless shells. These slugs are also highly charismatic in colouration but are less diverse and often more cryptic than nudibranchs.
Side Gill slugs are actually closely related to nudibranchs, however many species look completely unlike their distant cousins. Some pleurobranchs are able to grow to significant sizes, often as large as a dinner plate. These slugs are typically less diverse than the other slug species discusses so far but nonetheless make up an important portion of the sea slug world.
Sea Hares are, regardless of what their name suggests, important herbivorous slugs. These animals have the ability to secrete toxins and deterrent fluids from their mantle and many species even have the ability to produce a noxious ink to repel predators. Like pleurobranchs, some species of sea hare can get extremely large, and can feed on large amounts of algal matter throughout their life times.