Image: Fluorescing Goatfish
A 'fluorescing' Bluestriped Goatfish, Upeneichthys lineatus, photographed at night at a depth of 5 m, Camp Cove, Sydney Harbour, New South Wales, 27 March 2013.
The photograph was taken using two Seacam flashes with Nightsea BE3 Exciter filters (to get the near-UV light). A yellow filter on the camera lens cut back the non-fluorescent blue light reaching the sensor.
Thank you to Ulricke Siebeck who wrote: "In the case of fluorescence something in the skin (recently termed ‘fluorophores’) is illuminated with short wavelength light, which it absorbs and then re-emits at longer wavelengths. The resulting effect is a bit like the effect you get when you wear a white t-shirt under black light illumination. This ‘glowing’ only happens if the right stimulating light is used – which in nature only happens during the day. During the day the emitted signal is swamped by the ambient illumination (with the exception of perhaps red at certain depth) so I don’t think it can be detected then. The image you were sent was taken at night when the photographer was able to restrict the illumination to the stimulating wavelengths and he probably took the photo through a filter which removed the stimulating wavelength and therefore only transmits the emitted longer wavelengths."