From National Geographic:
New research published in Behavioral Ecology suggests that the small reef fish, no more than three inches long, may switch sex roles with their partner up to 20 times each day.
Chalk bass use a reproductive strategy known as “egg trading,” wherein they subdivide their daily egg clutch into “parcels” and alternate sex roles with their mating partner throughout a sequence of spawning bouts. More.
This suggests that what sex even is in the fish is different from what we find in mammals or birds.
See also: Can sex explain evolution?