This week I attended a seminar by Professor Bernard Degnan from the University of Queensland on the early evolution of animal complexity. He discussed many genetic aspects of the evolution of multicellularity, the similarities between sponges and more complex organisms and the differences between sponges and their unicellular ancestors. The presentation was captivating (even if some of the genetic bits did go over my head) and I loved the passion he had for the topic.
But the thing that stuck with me is his argument that sponges have organs which they use to see. This absolutely goes against the grain of everything I’ve learnt about sponges in the last 20 years. No more believing that sponges are ‘simple’ (and on a side note: no more believing that just because it’s in a textbook means it’s infallible – this part of the scientific method has been drilled in to me intellectually, but this is my first experience of having my biological world rocked in reality).
Only a tiny part of the seminar related to sponge (Amphimedon queenslandica) response to light. Opsins (photo-sensitive proteins involved in light responses of all animals with nervous systems) are not found in sponges so how do sponges respond to light? During A. queenslandica larval development (cleavage) a cryptochrome gene is expressed before any cells become pigmented. Pigmented cells then form throughout the blastula and they migrate to form a pigment ring at the posterior pole of the larvae. This pigment ring is complemented by long cilia which move in response to blue-light stimulus (440nm) possibly using cryptochrome proteins. These long cilia dictate the swimming direction of the larvae and act as a rudder.
Pigment and cilia ring at A. queenslandica larval posterior pole
From 0-24 hours old the larvae are negatively phototactic (avoid light) and from 48 hours old they lose their ability to respond to light. This negative phototaxis early in the larvae’s life could be a response to an environmental cue (light) that indicates the presence (or absence) of an appropriate location for the larvae to settle and/or a timing cue for metamorphosis.
So the pigment ring structures in a sponge fit the minimum definition of an eye – pigment adjacent to photoreceptors. The evolution the sponge eye (albeit a neuron-less, opsin-less eye that loses function as the larvae develop) is a fascinating example of convergent evolution.
Rivera AS, Ozturk N, Fahey B, Plachetzki DC, Degnan BM, Sancar A and Oakley T (2012) Blue-light-receptive cryptochrome is expressed in a sponge eye lacking neurons and opsin. The Journal of Experimental Biology, 215, 1278-1286.