The variety of fish and their biology is just mind blowing... during my fish hunting on the net I came across a little Killifish called the Mangrove Rivulus
This little 3inch - 7cm fishy shocked scientists when they kicked over a log and hundreds of these little guys fell out - on land!! And Alive!!! They have since discovered that they can spend up to sixty-six consecutive days out of water, which it typically spends inside fallen logs, breathing air through its skin. During this time its gills alter so that it can retain water and nutrients, while nitrogen waste is excreted through the skin. The change is reversed once they re-enter the water.
If that wasn't already amazing enough - there are no females - they are either male or hermaphroditic. Only about 5% of a population are born as males - after three or four years about 60% of the (self-fertilizing) hermaphrodites transform into secondary males by losing their female structure and function. The proportion of males depends on the environmental temperature, below 20° C the majority are males, above 25°C all are hermaphrodites. It is the only known naturally occurring, self-fertilizing vertebrate.