You don’t even need to dive to see them: it’s enough to snorkel in front of Salaya Beach Houses, and you will see seastars, feather stars, sea cucumbers and, usually hiding in-between rock crevices during the day, sea urchins. All of these animals belong to the echinoderms, which translates as spiny-skined-animals.
Look at these sea stars we saw in a shallow sea-grass area on a recent trip to the neighboring island of Siquijor. What patterns! But echinoderm skins are not just pretty, but animals with a truly intriguing biology. A lot of their body functions are, true to their name, indeed centered in the skin.
Their skins are rough and sturdy, but don’t just have the name-giving spines (as sea urchins do), but a great variety of nobs, claspers and tiny tube feet. Intriguingly, these appendages all act in a semi-autonomous manner, and are controlled by tiny nervous systems of their own. It’s as if your fingers could move on their own, without any input from your brain. There is no alternative to this solution for the echinoderms: they don’t possess a brain! A set of clever, refined rules for the actions of each individual tube foot or spine makes sure they all work together for the benefit of the whole urchin.
Look at this short video about Salmacis, a sea urchin found in shallow waters all over the Philippines, you’ll be able to observe the tube feet in dome detail. The urchins can even grab a camera with these tube feet:
The tube feet are distributed all over the body of sea urchins, and in the crevices running along the bottom center of the arms in seastars. They are operated by muscles, like the appendages of all other animals as well, but only indirectly. The strength of the muscles is transferred to the tube feet via a hydraulic system, unique in the animal kingdom.
In sea urchins the tube feet are also known to be sensitive to light. In this way, they act together as one big eye. The resolution of this whole-body eye is not nearly as good as that of a human eye, but seemingly good enough for the vision needs of a sea urchin.
To sum it up, a sea urchin’s skin is covered with hundreds of little tube feet, which each act semi-independently, for the benefit of the whole animal. And these feet also double as an eye, covering the whole surface of the urchin. It does not get any more alien from us humans than that when it comes to animal bodies!