The sea is a mysterious place to those of us who only get to visit it for as long as our canned air lasts. In our short dives we often spot things which are puzzling or unexplained. One of the things we’d speculated about on more than one occasion was why sea urchins wear hats.

It is quite a common phenomenon to see them with bits of kelp stuck to their tops – and quite firmly held in place too! – We’d guessed that they might be camouflage, but the truth is a little more prosaic…

Kelp and other seaweeds are not terribly palatable, even to grazing animals such as the urchins. Once the fronds have been broken up, however, then they start to die, and are colonised by bacteria and fungi. As the fronds decompose the amount of protein increases, and they actually become more nutritious. Also, the defensive chemicals (such as phenols) that seaweeds use to deter grazing while they are alive start to disperse, so they become less toxic. As a consequence, it pays an urchin to hold onto a stash of seaweed fragments as a light snack, or possibly even to mature them a little for better eating later.

The urchin’s hat is, therefore, its equivalent of a lunchbox!

Thanks to Little et al. The Biology of Rocky Shores 2nd Ed. publisher OUP, 2009, p105-106 for the enlightenment! Also thanks to Rob for the photo.