Sea Cucumbers, Urchins and Crinoids
Echinoderms are very popular host animals among crustaceans. On sea cucumbers you’ll find both the brilliantly coloured emperor shrimp as well as swimming crabs in various sizes. The tiniest ones are clawing onto the skin of the cucumber, while bigger ones sometimes seek refuge inside the anus of the echinoderm – have a look! If you’re lucky, you may even come across the pearlfish, which has chosen the same unusual living space.
Starfish and cushion stars also host shrimp and crabs, and the numerous warts and protrusions on the surface of the animals offers good protection – if a predator comes too close for comfort they quickly hide beneath the host.
A sea urchin offers even better protection, and among the lethal spines you can find black cardinalfish and the beautiful and rare Coleman shrimp, which usually lives in pairs. When the host animal moves about looking for food, the tiny commensal passengers enjoy the benefit of free locomotion. They are brought to new feeding grounds without having to spend the energy to get there.
Crinoids, often referred to as sea lilies or feather stars, are a true treasure-trove of commensal shrimp, crabs, squat lobsters and even tiny clingfish. Although attached to the reef with special feet called cirri, crinoids can move around and even swim.
They are experts at finding spots where the current brings in lots of food, and this makes them a prime piece of real estate for other animals. The shelter in between the arms or above the feet are excellent, and food is abundant.
Commensal crustaceans living on crinoids often have colouration perfectly adapted to match their host and thus become almost invisible to predators. This also suggests that their relationship is for life, and not just a one-night stand.
Commensal crustaceans living on crinoids often have colouration perfectly adapted
to match their host and thus become almost invisible to predators
ABOVE: Allogalathea elegan on a similarly-patterned crinoid OPPOSITE PAGE TOP: A pair of colourful emperor shrimps sitting on a sea cucumber OPPOSITE PAGE BOTTOM: A commensal shrimp, often found living on surfaces of starfish IMAGES: Christian Skauge