Sea Cucumber

Sea cucumbers, echinoderms from the class Holothuroidea, are very common in marine reef aquaria as they are prized for their unusual behaviour and appearance. Found on the sea floor worldwide, they have leathery skin and an elongated body containing a single, branched gonad, and they grow typically 10 to 30 centimetres (3.9 to 12 in) long. In the hobby, the sea cucumber consumes detritus that accumulates on the substrate (such as aragonite or sand).

Sea cucumbers can live without feeding for months, often up to half a year. If an aquarium system cannot support these creatures, the sea cucumber will shrink gradually as it digests its own body mass to survive, while starving. It would be wise to observe its proper feeding with signs of its feeding tentacles around its mouth to pick up and swallow sand from the bottom of the aquarium for planktons and decaying organic matter, and producing castings of excreted substrate.

Sea cucumbers can release highly toxic compounds, especially those known as “Sea Apples” which can be very lethal to the other tank inhabitants if they are injured. Be sure to to avoid possbilities of all powerheads and pumps to be accessible by them as they can squeeze themselves into spaces much smaller than their body. Remove any injured cucumbers immediately from the aquarium and perform a major water change, aided with activated carbon filtration to ensure the other inhabitants are saved.

Sea Slug

Sea slugs are mollusks, related to the shelled terrestrial snails that we are all too familiar with on land. They are distributed everywhere in both tropical and temperate seas, with the greatest numbers and the largest kinds found in tropical waters. Most sea slugs are rather small under 1 inch and some are as long as 12 inches, the largest which is found in the Great Barrier Reef of Australia.

Sea slugs display a great array of solid colors and patterns, and are also known as “butterlfies of the sea”. Most sea slugs have feathery structures (ceratia) on the back, very often in contrasting colours, and they have two pairs of tentacles on the head used for chemosensory and tactile reception, and have a small eye at the base of each tentacle. Being predominantly shell-less and daytime active organisms, creeping along the bottom or clinging onto submerged vegetation, usually in water just below the low tide line, these creatures have evolved some seriously potent and complex anti-predatory defenses.

Sea slugs graze on small sessile animals such as bryozoans, sponges, and coelenterates.  Some feed on sea anemones or corals, ingesting the stinging cells of their prey without discharging them and passing on from the slug’s digestive tract to the ceratia, where they are used for self defense. It is known that as the species are more colourful, the more toxic they are. The less toxic species tend to match the substrate they lie in. Most sea slugs are difficult or impossible to keep especially in small aquaria due to their specialised diet, delicate nature, and sensitive to acclimation. Stressed sea slugs releases toxic composition into the aquarium which may kill other organisms. These creatures are therefore not recommended for the inexperienced hobbyists.