Release of the dark-adapting hormone in the fiddler crab's far retinal layer triggered by methionine enkephalin and FMRFamide

Injection of the neuropeptides methionine enkephalin and FMRFamide into fiddler crabs results in dark adaptation of the distal retinal pigment.

• Both neuropeptides are associated with stimulating the release of a hormone that aids in dark adaptation of the retina.
• The effect on dark adaptation suggests an indirect action on the distal retinal pigment.
• The findings indicate a potential hormonal mechanism underlying retinal responses in fiddler crabs.

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