Every week on Facebook we see numerous ID posts about crabs…. Acro crabs, ‘Gorilla’ crabs, ‘Hairy’ crabs and others. Generally, crabs make their way into our systems as hitchhikers on liverock and/or corals.
What we know as Acro crabs are generally all quite safe commensal critters which live in and on branching SPS corals. There are several species across a few different genus’ But most acro crabs are from two main genus…Tetralia and Trapezia spp. They are quite easy to identify once you know what to look for. Each species has its preferred SPS coral and often has a pattern or colouring that matches it’s host. The crab literally guards its host against predators and picks it clean of detritus and debris.
Emerald crabs often get a mention as great for cleaning up nuisance algae, in particular…bubble algae (Valonia ventricosa). The bad news is that you will never see one here in Oz. The Emerald crab (Mithraculus sculptus) is an Atlantic Ocean species (Carribean and Gulf of Mexico) and Australian law prohibits the importation of inverts.
The infamous “Gorilla” crab….. This is a common name and is given loosely to a number of species because they appear to be hairy! The ‘hairs’ are actually sensory organs called Setae and all crabs (all crustaceans actually) have them. The setae are used for various purposes including…feeding filters, sensory organs, defensive tools etc. Decorator crabs use modified ones shaped like hooks to hold on to their decorations.
Most crabs in the aquarium (apart from the commensal SPS crabs) are no more than a nuisance or at the most, a bit destructive but among the pain-in-the-butt types you will occasionally find yourself with a member of a deadly crab family (yes, deadly).
Xanthid crabs often look just like your common, every day reef crab however, they contain lethal amounts of toxin very similar to Tetradotoxin. This is otherwise known as pufferfish toxin. It’s produced by a bacteria known as Vibrio which is very dangerous in its own right.
There are other families of crustaceans that we commonly see in the hobby such as squat lobsters, hermit crabs and so on but there are literally thousands of individual species which makes them well worth covering separately.
This literally means ‘Spiny Skin’ and the name belongs to a massive phylum (group) of animals with more than 7,000 species spread across many classes of critters such as the sea stars, urchins and sea cucumbers.
Echinoderms live in every ocean, at all depths, from very hot shallow tidal pools in Equatorial waters to the icy cold, deepest depths of the ocean, thousands of meters down.
Sea stars (Asteroidea) is, by far, the largest group with nearly 4000 species. They can be large, voracious predators (think-Crown of Thorns), sturdy and attractive Linckia stars, beautiful basket stars or microscopic hitch hiking brittle stars. We have come to recognise a number of seastars that are both beneficial and attracyive to keep in our tanks and in many cases, small ones hitch-hike in on liverock or coral. Most reefkeepers wil be familiar with the tiny waving legs of brittlestars poking out from holes in their liverock.
Urchins (Echinoidea)….tasty delicacies in Japan, tireless cleaners of liverock in our tanks. There are toxic varieties, amazing colours, cryptic sand-dollars and comical collectors. Urchins can and do have their place in reef tanks and aprt from a propensity to bulldoze unseccured corals, they will graze away at encrusting algaes to leave your rock clean.
Cucumbers (Holthuroidea)…. Not exactly a salad item but many are edible, some are stunning to look at while others are shocking to behold. Beautiful sea-apples, burnt sausages, curryfish and lollyfish all suggest something tasty but I don’t know so much…. There are a few well-known species of ‘cukes’ and their relatives that are kept nowadays such as Sea Apples, Tigertails, Burnt Sausage etc. Most Sea Cucumbers feed off the detritus and waste produced in our tanks either off the substrate or by filtering it from the water column. They can be a valuable and attractive (facinating) part of your clean-up crew but do some research and know what you need to keep them happy and healthy.
This massive group of marine Gastropods consists of almost 2,500 individual species that inhabit all oceans and seas from tropical to temperate to Arctic and Antarctic waters.
The name Nudibranch refers to ‘naked gills’ as Nudibranchs have their gills exposed on their backs. Each species of these amazing little guys is a highly evolved specialist in his native environment. They have developed intricate camouflage systems of not only colour adaptations but also cerata (appendages) that closely match various host corals.
The astounding colouration is vital in not only hiding the Nudibranch from predators but also as a warning sign indicating that they are unpleasant (at the very least) to eat. Some Nudibranchs deliberately consume the nematocysts of their prey corals and in turn, become toxic to predators. There are others that digest zooxanthellae and utilize it in the very same way that corals do….providing an alternate food source to the Nudi.
Be aware though, that not all (not many) Nudibranchs prey on corals…there are many that eat bryozoans and ascidians, sponge, algae, tunicates, hydroids and other micro-marinelife. They life fast and die young with most of their kind only living for a very short time. Apart from the pitfalls, many Nudibranchs make for excellent aquarium inhabitants if you are sure of and have access to its preferred food.
The bubble algae we see regularly in our tanks is Valonia ventricosa… All Valonia sp. (only 3 in the genus) are single-celled, multinucleate (each cell contains multiple nuclei) organisms and contrary to what is often said, none of them produces or contain spores.
As they are single cells (one of the largest on earth), any piece of the bubble or even the liquid in the tissue can and will produce a clone of the original. Any fish or invert eating a bubble will break the cell wall and release a cloud of trouble. The safest option is to remove the host rock from the tank and chip away the rock where the bubble is attached via its Rhizoid (root structure).