Opsin evolution: key critters (cnidaria) Cnidaria .. 5+ opsins
established Biologists have belatedly realized that many
molecular and morphological innovations attributed to chordates
(or grudgingly to bilatera) actually track back much earlier to
the common ancestor with cnidaria (Eumetazoa) if not earlier
still to placazoa, sponges, and choanoflagellates. That's
certainly true of photoreception. Two cnidarian genome projects
have been more or less finished (Nematostella and Hydra) but
that selection needs to be seriously expanded. Hydra especially
repeats the whole mistake of sequencing a hugely derived genome
with relatively little applicability to Bilatera based on a
shallow 'model organism' approach (Hydra has only 66
publications in the last 30 years). A scientifically neutral
definition of eye needs to embrace the full variety of
photoreceptors, including those with fewer "features" than the
most complex. Probably the cutoff should be based on use of bona
fide opsins classifying to the root of the ence