Sight for the blind?

Larvae across many phyla have distinct eye spots or eyes. Even 'primative' sponge larvae have a pigmented ring of cells that can sense light. This may not be surprising since light is pervasive in most environments and serves as an important cue for feeding and predator avoidance, amongst other things. (For additional gorgeous pictures and information about marine larvae check out the Friday Harbor Invertebrate Embryology Course Blog.

Yet, sea urchin larvae (below) seem to lack any discernible light sensing organs. There are pigment cells (red) scattered across the body and concentrated often at the tips of the arms.

 

However, a light sensing function has not (yet?) been assigned to these cells or regions. So why might sea urchin larvae have lost their ability to sense light?  Or do they have a way to sense light, possibly a novel way, that is yet to be discovered? 

Adult sea urchins do it in a strange, unique way (Ullrich-Luter et al 2011). Why not larvae too?  The entire adult sea urchin serves as a type of eye. Visual opsins are found in the tube feet all around the urchin, but there is no associated pigment. The test (shell) of the sea urchin serves as a 'shading pigment' to provide directional information about the ambient light. 

So what do sea urchin larvae do to sense light? There are projects going on around the world that are starting to shed light on this question.  We've got a handful of genes that specifying putative (novel?) light-sensing cells. It will be a collaborative, international effort... and I hope a fruitful one.

Ullrich-Luter ME, Dupont S, Arboleda E, Hausen H, and Arnone MI. 2011. Unique system of photoreceptors in sea urchin tube feet. Proceedings of the National Academy of Sciences USA. 108(20):8367-72.