Which option given is unusual about the mangrove killifish?

The mangrove killifish (Rivulus marmoratus), found among the mangroves in Florida, Latin America, and the Caribbean, is a strange fish indeed.

For starters, this is the only vertebrate animal that is known to fertilize its own eggs. There are males and females in the species, but most of these little fish are hermaphrodites.

Mangrove killifish are able to alter their gills to be able to live out of the water. When the water around the mangroves dries up, these fish climb up into the trees and hide in logs until the water returns. Once it's safe, they change their gills back and venture back to the water.

When jumping on land, the mangrove rivulus does a "tail flip", flipping its head over its body towards the tail end. The rivulus' jumping technique gives it an ability to direct its jumps on land and to make relatively forceful jumps.