Sand goby, Pomatoschistus minutus. The black/blue spot on the first dorsal fin is at the upper edge of the fin. The common goby has a similar spot, but it is positioned lower on the fin membrane, close to the body.
31st January 2019 A large adult male sand goby, Pomatoschistus minutus, from Southampton Water. The images show the bright blue colouration of the anal fin which is characteristic of the males' breeding colours, and forms part of their mating ritual. As Blom, Wilson, Kvarnemo, Amorim and Svensson (2022) explain in "Male acoustic display in the sand goby – Essential cue in female choice, but unaffected by supplemental feeding": "Males develop a nuptial colouration with black pigmentation on pelvic, anal and tail fins. The anal fin has a clear blue colouration with a black trim, and there is a eye-spot on the first dorsal fin. This colouration is lacking in the cryptic female. Males build a nest by covering bivalve shells with sand and compete for mating opportunities with females, which in turn are the choosier sex. Male courtship behaviour normally starts with a visual display in which the male approaches the female with erect ornamented fins. Typically, the male swims back to the nest in a conspicuous manner, considered a lead display (‘lead swim’), and the female may choose to follow the courting male to his nest . The male might produce a series of sounds both when the female is outside the nest and when she has entered it". (full text available here).