Living on the Crinoid

There’s somethin’ wrong with the world today
I don’t know what it is
Something’s wrong with our eyes
We’re seeing things in a different way
And God knows it ain’t His
It sure ain’t no surprise

 We’re livin’ on the crinoid
We’re livin’ on the crinoid
We’re livin’ on the crinoid
We’re livin’ on the crinoid

– modified from Aerosmith

Recently I spent about 20 minutes observing one crinoid and its inhabitants. What is a crinoid you might ask? It’s an echonoderm. What’s an echinoderm you might ask? It’s an invertebrate (an animal without a backbone), belonging to the larger group of sea stars, sea urchins, and sea cucumbers. This animal does not have sturdy arms like sea stars, but fine, feathered ones, which it spreads out in the current to capture plankton.

Once in a while, you can find small crustaceans living on these crinoids. Once in a while, that is, in common diving locations. In Dauin, where I live and dive, I find that all the time. The crinoid I recently found harbored a crinoid squat lobster AND a crinoid shrimp. Both animals were painted in black and white, exactly like their crinoid host – a great example of co-evolution. I believe that the small crustaceans don’t benefit, but also don’t harm the crinoid. They in turn surely profit from the hiding place. This one-sided relationship is called commensalism.

It was hilarious to see how the squat lobster was struggling to hold on when the crinoid took some slow steps over the dark sand underwater in Dauin. It had to run from one arm to the next, and crawled from the arms’ underside to their side facing upwards. The crinoid was by no means sprinting, but it was fast enough to challenge the tiny lobster.

For the crinoid shrimp and the crinoid squat lobster, their whole world consists of the arms and the mouth disc of that crinoid. They will likely never set foot onto anything else, and all they see up close is the much larger invertebrate they inhabit. It’s like having a living, moving house which is painted like your skin; a living house which you never leave any time! What a life.