Owners of some short spined species of urchins such as tuxedos and pincushions may have noticed their spiky echinoderms occasionally sporting an assortment of rubble or coral. We often admired this remarkable phenomenon which looks quite interesting, but as we’ve previously documented this can pose a risk to corals in your reef tank if your urchin carries these polyps to somewhere they can sting other corals.
As aquarists we’ve been satisfied to attribute this behavior to urchins carrying corals and objects to one part camouflage, and another part self-defense. But scientists had to dive deeper into the urchin’s habit of carrying stuff around and developed some experiments to better understand why sea urchins do this in the first place.
As it turns out the topic of sea urchins carrying stuff is one of more than a handful of different research projects which have uncovered other proposed explanations for why sea urchins need more than their spines to get by. To our great surprise it turns out that some species such as the collector urchin which live in shallow tropical waters seem to carry bits of rubble around as a form of sunshade, protecting themselves from harsh ultraviolet light.
The green sea urchin which lives in wave-washed tidepools appears to carry bits of rubble around to help weigh themselves down against the force of waves in the intertidal zone. Another study of sea urchins from New Zealand seemed to indicate that these subtropical species preferred to carry around bits of rock that had their preferred algae growing on them like a kind of snack they can take with them.
And yet another study revealed that some antarctic species of sea urchins carry rubble to help defend themselves against predators who had adapted to deal with spines, but not a rubbley suit of armor. We were legitimately surprised to learn that there are at least half a dozen reasons for sea urchins to carry stuff around in the ocean and in our aquariums and we’ll give these invertebrates a little more credit for how they go about in our aquariums from now on.