‘The Heart Urchin Pea Crab’
Dissodactylus primitivus on Meoma ventricosa
Music, Video, and Aquarium
2010 Coral Morphologic

Barely 7mm in size, the aptly named heart urchin pea crab (Dissodactylus primitivus) lives its entire life as a passenger upon the slow-moving red heart urchin (Meoma ventricosa). It is an example of the unusual life that can be found by looking in unexpected places on Floridian coral reefs. The red heart urchin is an unusual member of the echinoderm clan (e.g. urchins, sea stars, sand dollars, sea cucumbers) that spends most of its time burrowing in the sand. It sifts through the grains of sand searching for organic detritus that constitutes its diet. Likewise, the heart urchin pea crab lives a well-protected life (usually below the sand) amongst the spines of this fist-sized urchin. While most crabs move swiftly, this pea crab moves slowly in order to navigate through the corridors of spines, even spending time inside the urchin’s mouth. It is likely that the crab feeds upon some of the food that would otherwise be consumed by the urchin. This commensal relationship appears mildly parasitic, as the urchin doesn’t seem to gain any sort of direct benefit from the crab living amongst its spines. Frequently, several heart urchin pea crabs will live communally without any noticeable negative impact to their host urchin’s health.

If you look closely, you’ll notice the rhythmic working of its gills and circulatory system within the heart urchin pea crab’s translucent, eggshell exoskeleton.