Figure 12.

Schematic outline of the light–cryptochrome photoreception/phototransduction system in sponges. It is established that sponges can react to exogenous and, very likely, also endogenous light via the cryptochrome photoreceptor. Spicules might act as waveguides for the transmission of photons. Bound to cell membranes and likely in concert with the G protein β (GNB-Trans), cryptochrome transforms photon signals into spin-correlated FADH-superoxide radical pairs (FADH^• and O₂^•−). This complex is linked with the nitric oxide synthase (NOS) system via the nitric oxide synthase–interacting protein (NOSIP) protein. Finally, it is proposed that a further signal transformation occurs in the NOSes under generation of electrical signals. The verification of the latter reaction, the generation of electrical signals, remains to be experimentally approached.