Seaslugs - Part I. Headshield slugs and Sea hares.

Contributed by Ákos Lumnitzer

The official scientific classification system, which today is referred to as 'The Linnaean system of biological nomenclature', was devised by Swedish biologist Carolus Linnaeus and implemented during the 1750s. It is a universally accepted system used to classify all living animals into groups. The groups are used to hierarchically arrange organisms with the use of scientific keys into groups with similar characteristics, starting from the Phylum finally narrowing it down to the genus and species names that you may be remotely familiar with. Species is the basic classification unit used, and it refers to equivalent organisms able to produce fertile offspring within a genus, but can't interbreed with other species of the same genus.

Let's see an example; common in Sydney, Chromodoris splendida is a very attractive slug. Its name is comprised of the genus name Chromodoris, and the species name splendida. The genus is made up of many specimens that are very similar, but still have enough differences to warrant having other species names. Therefore Chromodoris splendida can only produce fertile offspring with another Chromodoris splendida, but not with Chromodoris tasmaniensis, even though they belong to the same genus and share very similar characteristics. Note also that the genus name always begins with a capital letter, but the species name always commences with a lower case character.

I have compiled a sequential account of the five main seaslug orders, supported by photographic records and some likely Sydney dive sites to encounter members of each order. The sites I have mentioned are only the ones I have personally dived and noted the presence of these slugs, however, as with any species of marine animal, it is highly likely to come across anything at your favourite dive anywhere in the world, provided the habitat and food source required by the particular animal you seek is present.

Order Cephalaspidea - Headshield slugs

The vast majority of headshield slugs still possess external shells, though generally they are unable to retreat into them fully, whereas others have reduced or internal shells. They all have well evolved headshield, which they use to dig under the substrate. These slugs tend to live under the sand, many are often seen at night, and crawl over the reef as well. The members of Chelidonura also have highly sensitive eyes on the anterior end of the head and bunched cilia around the mouth, which is used to track their prey's mucous trail. They possess no visible gills. A well-known Sydney Cephalaspid is the bubble shell, Bullina lineata.

Recommended dive sites in Sydney for Cephalaspids: Sydney Harbour, but I have seen many species at Shark point, Bare Island, Shiprock, Kurnell and Oak Park.

Order Anaspidea - Sea hares

Fishes tend to avoid Sea hares because they have a highly toxic compound called aplysiatoxin, which these slugs absorb from algae they feed on. The mantle of the slugs has many glands, one of which excretes a purple fluid-cloud when the animal is disturbed. The purple liquid is not thought to be poisonous, thus acts as a deterrent, but is an unused chemical from their diet. Some Sea hares are the largest opisthobranchs, growing to possibly more than 50 centimeters and over 4 kilograms in weight. Usually there are no visible gill plumes.