Mangrove Rivulus

Ever hear the term “A fish out of water”? Meet the amazing Kryptolebias marmoratus, better known as ‘rivulus’

Although uncommon within coastal south and central Florida, the Neotropical killifish Kryptolebias marmoratus is widely distributed. The majority of these tiny fish are gray-brown hermaphrodites, while the rare male has a bright red-orange tint. What's a hermaphrodite? In the case of a mangrove rivulus, the ovulated eggs are self-fertilized resulting in homozygous clones. This convenient feature allows the rivulus to perpetuate when males are scarce.

A dedicated group of scientists from diverse locations has spent the last couple of decades studying this interesting creature, braving the mud and mosquitoes in mangroves from Central America to Florida.

Rivulus is probably best known as a self-fertilizing hermaphrodite. But this little (max size: 2.5”) fish is also becoming ‘famous’ for being semi-terrestrial:

Rivulus live in the nasty water of crab burrows and fetid mangrove pools, and when conditions get too bad, they leave the water and hide in damp locations, like piles of leaves or under errant coconut husks.

In fact, in the lab, D. Scott Taylor found that they could live 66 days out of the water in damp mud: “This is probably the coolest fish around”, Taylor says. “Not only do they have a very bizarre sex life, but they really don’t meet standard behavioral criteria for fishes.”

The definition of ‘fish’ was really stretched when the scientists found that rivulus will also move into decaying logs when their pools start to dry. In both Belize and Florida, the scientists examined dead mangrove logs at the edges of drying pools. The logs had been ‘galleried’ by beetle larvae and termites, leaving a honeycomb of narrow cavities inside. It seems that our intrepid fish will wiggle inside the moist chambers of the logs as the pools start to dry, waiting out the dry-spell, packed together like peas in a pod.

And a bunch of them do this: inside one log (5 ft long X 4” diameter) in the Florida Keys, over 100 were found. When the log was carefully broken apart, they came flipping out like so many grasshoppers! Not very characteristic for a fish, would you say?

“This is a classic example of how nuances of natural history can be easily overlooked. In-depth study is required at every level of any organism to dissect out the details”, Taylor said.

Indeed: this may be the first known case of fish living in trees!

Available Documents

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  • American Naturalist - February 2008
  • Meet The Mangrove Rivulus

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