Turd on a rock
Gobiisox nudus, also known as the chupador or tetard, a freshwater clingfish that reaches 6" long and is found on a couple Caribean islands, Central america, Columbia, and Venezuela. The pics are both by K.Thode.
The names derive from the fishs habitat in the Lacantún river of southern Chiapas state in Mexico. Only about 30 of the fish have been found since the 1990s and only one specimen was collected in a recent five-day expedition in 2005.