Michel Roggo / Rudd (Scardinius erythrophthalmus) close up of anal fin, Europe, Captive

Alex Mustard / African pompano (Alectis ciliaris) juvenile with long trailing dorsal and anal fins, swimming in mid-water at night. In-camera double exposure. Anilao, Batangas marine protected area, Luzon, Philippin...

Hiroya Minakuchi / Bottlenose Dolphin (Tursiops truncatus) female underside showing urogenital, anal and mammary slits with ventral fin, caught along the coast of Japan, courtesy of Taiji Whale Museum

Jane Burton / African mouthbrooder {Haplochromis burtoni} male enticing female onto flat stone by spreading his anal fin and displaying his egg spots, captive, from Lake Tanganyika, Tanzania, sequence 2/7

Jane Burton / African mouthbrooder {Haplochromis burtoni} female trying to pick up dummy-eggs on male's anal fin as he sheds his milt and fertilises the eggs that she is carrying in her mouth, captive, from Lake Ta...

Jane Burton / African mouthbrooder {Haplochromis burtoni} female trying to pick up dummy-eggs on male's anal fin as he sheds his milt and fertilises the eggs that she is carrying in her mouth, captive, from Lake Ta...

Joel Sartore / Photo Ark / Glass knifefish (Eigenmannia virescens) swimming into frame, Fin-Tastic Fish, Nebraska. Captive.