Photographer: Paul Osmond
(see this users gallery)
A bright red male Spinecheek Anemonefish (Premnas biaculeatus) within his host sea anemone on a coral reef within the Banda Sea in Indonesia.
The female of this species is larger, and generally display a much muted red coloration than the male. They are also tremendously territorial and their aggressive displays are well worth observing.
The smaller male, as can be seen here, are much brighter and much smaller than the large female.