The toxic sea urchin Asthenosoma varium also known as an Indo Pacific Fire urchin, is an unlikely place for any animal to call home. Undoubtedly one of the most beautiful echinoderms in the sea, this brightly coloured and magnificent urchin packs a painful punch for all but a few specialized commensal species.
So what is in it for these specialized homemakers?
Those two essential commodities to us all, food and shelter. Making a home amongst the poisonous spines and pedicellaria is not an easy undertaking but it provides the ultimate in protection from predators who do not want to tangle with this painful urchin. Food is readily available as ocean plankton and detritus get caught up in the melee of spines.
Most well known of the homemakers is the Coleman shrimp, Periclimenes colemani, which is found exclusively on this host. This beautiful shrimp takes homemaking to the next stage by regularly mowing the lawn so to speak. To create a suitable abode the shrimp deftly clips away at the spines and tube feet on its chosen perch creating a flattened area to nestle within.
The Zebra crab or striped urchin crab, Zebrida adamsii, is another brightly painted habitant, although it is not exclusive to the fire urchin and can be found on a number of urchin hosts. whilst easy to spot when close to the urchin it"s camoflague is quite remarkable from a distance.
The commensal shrimp, Allopontonia iani, sometimes referred to, as the Penguin shrimp is another animal that takes advantage of the protection provided by the urchin. Almost comical in appearance it is a delight to behold and can sometimes be viewed on the underside of the urchin.
All three of these species are commensal to the urchin and it is not known if the urchin takes any benefit from their residency. It is assumed that they cause no harm to the urchin.
A more darker, by intent, resident is the parasitic snail, Luetzenia asthenosomae. This snail is a true parasite and feeds directly on the urchin. It uses its radula and trunk to pierce parts of the urchin and ingest it body fluids directly. Luckily, due to its small size this does not kill the urchin and it is common to see families of the snail living on the same urchin.
The Fire urchin is widespread in the Indo pacific and is normally found living in colonies on sandy rubble bottoms. Usually found at depths deeper than 25 meters it is known to move shallower during the daytime at some sites before returning to its more normal depths. Next time you see one remember to look more closely and you may be pleasantly surprised to witness its colourful and bizarre hitchhikers.