Mangrove Killifish (Rivulus marmoratus Poey)
The mangrove killifish has to be one of the most unique animals in the world. This fish is the only known vertebrate that self fertilizes. In other words, the fish is a hermaphrodite (contains both male and female reproductive organs) and fertilizes its own eggs before laying tiny embryos in water.
As if this weren’t sufficient to place it on this blog, this fantastic little creature has another trick. It can climb trees. Yes, you read that right. When the pools around mangrove trees that support these fish dry up, the fish packs his/her/its bags and climbs up the trunks of mangrove trees and into channels in logs that were carved by insects. There, placed end to end, the fish curb their normally aggressive behavior and wait (up to several months!) for the pools to return.
Obviously, there is a problem with this strategy. Fish use gills to take in oxygen from water… so how do they survive for months in the logs of trees without water to breathe from? The fish alter their gills to retain water and nutrients, and reverse these changes once they return to the water.
killifish
This killifish video does not show them climbing trees, but it does show a brave killifish traveling overground from puddle to stream to escape predation. And it includes the phrase ‘his kaleidoscopic advances prove irresistable’ (or something like that), and it appears to be true.
The Journal of Animal Exceptionalism 