The Mangrove Rivulus is a small killifish (a family of numerous small, oviparous fishes much used as bait and in mosquito control) found in the tropical and subtropical portions of the western Atlantic/Caribbean Basin from central Florida south through the West Indies and Caribbean to Brazil. It is strictly a coastal species occurring in salt and brackish habitats and rarely in freshwater. Its range closely parallels that of red mangrove forests, which is its preferred habitat, where it occupies marginal and ephemeral (temporarily flooded) bodies of water such as stagnant, seasonal ponds, sloughs, and mosquito ditches. It also occurs in similar habitats away from the mangroves in elevated marsh habitats above the intertidal zone, especially in microhabitats such as the burrows of the great land crab. The crab burrows provide refuge for the Mangrove Rivulus during the dry season when seasonal pools of water dry up. Up to twenty-six small individuals have been found in a single crab burrow, but one or two per burrow is the usual number found.
Tolerating the extremes of temperature and salinity found in these habitats, the Mangrove Rivulus is one of the few fishes that can survive here. It can tolerate salinities from 0–68 parts per thousand and temperatures ranging from 7–38 degrees Celsius. It also can tolerate low oxygen levels and high sulfide levels that are produced in these habitats from decaying leaf litter, which would kill most fish species. The Mangrove Rivulus is also amphibious and frequently travels across land during rains to reach pools of water or crab burrows. It will also leave drying up bodies of water and waters that are too low in oxygen or too high in sulfides and bury itself in moist debris. It is capable of surviving out of water in this moist situation for up to 60 days. It is able to do this because of special epidermal capillaries that allow it to absorb oxygen and expel carbon dioxide through its skin.
Reproduction in the Mangrove Rivulus is very unusual. It is the only known vertebrate “hermaphrodite” that is self-fertilizing. Hermaphrodite means that it has both male and female reproductive parts. Both eggs and sperm are produced in reproductive organs called “ovotestes.” Each hermaphroditic individual fertilizes its eggs before laying them. The young are “clones,” meaning that they are genetically identical to the parent. This hermaphroditic mode of reproduction may be an adaptation to isolation in the crab burrows and other microhabitats these fish invade. Not all Mangrove Rivuli are hermaphrodites. There are two types of males that occur: primary males and secondary males. Primary males are hatched from fertilized eggs, but secondary males were once hermaphrodites that underwent a change and became males due to certain environmental conditions. True females have not been found in the wild but occur in captive-raised populations. Hermaphroditic individuals are identified by the presence of an eyespot at the upper base of the caudal fin and by a white band in the outer part of the anal fin. There is recent evidence that shows that the young adult fish are able to reproduce by normal sexual reproduction at first, but as they age, they become hermaphroditic. Eggs, at laying, vary in development from recently fertilized to ones containing already developing embryos. Mangrove Rivuli eggs are capable of surviving droughts in damp substrates or debris in a state of “embryonic diapause” where the embryo is in suspended animation. Eggs in this state are viable for several months and hatch soon after inundation by water.
God has implanted in the genes of His creatures enough genetic variability to allow for adaptation for survival under the most extreme circumstances. “It was the Maker of all things who ordained the wonderful adaptation of means to end, of supply to need. . . .
“He who studies most deeply into the mysteries of nature will realize most fully his own ignorance and weakness.” Education, 133.
David Arbour writes from his home in DeQueen, Arkansas. He may be contacted by e-mail at: landmarks@stepstolife.org.