I’ve been mucking around in mangrove swamps here on Abaco for the last five years. But just the other day I encountered a new critter, the Mangrove rivulus (Rivulus marmoratus), and what an interesting critter it is.

Mangrove rivulus are tough little (~3-5cm) fish. Their habitats are shallow, muddy flats where salinities and temperatures can reach extremes and dissolved oxygen is typically very low. But somehow these fish thrive in these harsh conditions. They can even live out of the water for up to 60 days! An interesting habitat association of these fish is that they are known to use land crab burrows. The pools of water at the bottom of these tunnels is enough to keep their gills wet enough to exchange gases.

In this photo you can see the spot on the base of the dorsal fin that can be used to help identify these fish. Apparently, only the hermaphrodites have the spot. Males do not.

Mangrove rivulus are also the only known self – fertilizing hermaphrodites! That’s right, most of the population contain male and female sex organs and they will self-fertilize! Some individuals are born males, and others will turn from hermaphrodites to male later in life, but there are no known full females in this species. That’s a pretty important adaptation when you are isolated in the bottom of a crab burrow!

These fish can be found throughout the Greater Caribbean region as well as coastal areas of Central and South America and South Florida. The primary habitat requirement is – you guessed it – mangrove swamps. Indeed, both of the fish I found over the past few days were in red mangrove swamps. One was found near Cherokee Sound and the other was a few miles north of Marsh Harbour. I’m surprised it took me so long to find this exceptional and interesting little fish, but I’m glad that I finally did.