by Mr. Fish

One of the more exciting things of tromping around in the muck of the everglades, besides running into the unexpected alligator that is, is the discovery of that rare fish for one’s aquatic collection.

This happened to me a short time ago when I brought in my seine net to discover a rather small, dark brown almost with a touch of maroon, killifish. I threw it into one of my plastic bags that I carry when I am on one of my fish collecting expeditions.

After placing the bag in the covered Styrofoam box in my canoe, I continued my seining of the area in the hopes of getting another specimen or two of this Killie.

To no avail, although for the next two hours I tried, did I land another one of these fish. If there were any more to be had in the area, they were obviously smart enough to stay away from my net.

I took the Killie home and placed it in an aquarium. An aquarium that I had done my best to make look like part of the everglades. I did leave out the alligators and the turtles, in case you were wondering.

After two days the Killie settled down and became less shy when ever I approached the aquarium. I was unsure of what species I had caught and so I started making some observations and took notes.

Although the color had faded when I first put the fish in the aquarium, it was now coming back full force. The overall color of the fish turned into a pastel maroon. A very prominent eye-spot is situated on the upper part of the caudal peduncle. The sides of the body were speckled with small black spots over a cross-hatched mesh background.

What I found out about this fish not only amazed me but I’m sure it will amaze you too. The fish turned out to be the Mangrove Rivulus, Rivulus marmoratus POEY 1880. As it turns out this is the only species of Rivulus found in North America. The genus Rivulus usually has been placed in the family Cyprinodontidae. It was recently transferred to the family Aplocheilidae, a group that traditionally has included only fish from the old world.

It resembles the killifish of the genus Fundulus in that it has a relatively long and slender body. The dorsal and anal fins are likewise situated on the hindmost third of the body. It has a rounded caudal fin.

My specimen turned out to be a female. Overall body coloration can be dark brown to an almost maroon in wild specimens. The sides of the body are speckled with small black spots against a reticulated background. A prominent eye-spot is situated on the upper part of the caudal peduncle base in hermaphrodites. What was that you say?

While maximum total length is listed at 60 millimeters or 2 1/4 inches. The average size is around 24 mm TL or one inch.

Males, which apparently have rarely been collected in Florida, lack the eye-spot, and in addition they have a red-orange cast to the flanks and fins.

Rivulus marmoratus is very closely related to Rivulus ocellatus HENSEL 1868. Actually some researchers feel that it is indeed the same fish. If this is true then its range is from southern Florida south throughout the West Indies to coastal areas of northern South America and south at least to Rio de Janeiro, Brazil. If the fish is indeed its own species or subspecies then it range is limited to southern Florida, Cuba, the Bahamas, Jamaica, and Yucatan. It has even been found on both Grand Cayman and Little Cayman islands.

In Florida, the mangrove Rivulus occurs in coastal localities on the Atlantic coast as far north as Indian River County and the Melbourne area in Brevard County. The species occurs in Dade County south throughout the Florida Keys and on the west coast almost to Ft. Myers in Collier County. It is reported to not exist between Indian River and Dade counties as human encroachment has wiped out its habitat.

Its habitat is varied. The fish has been taken from a wide range of saltiness, including fresh water, but most collections are in brackish water of 20-35 PPT. Florida collections of the Mangrove Rivulus have been taken in and around mangrove swamps and also high salt content marsh areas.

Specimens are often found in shallow stagnant water covered with a heavy leaf litter, in isolated drying pools, or in crab borrows. The fish will actually leave the drying waterhole by burrowing through leaf litter. One collector even reported a living specimen far from the water buried in damp peat on Marco Island, Florida. God only knows how he found that one.

Now for some more bizarreness! It is widely found to be common in the burrows of the great land crab Cardisoma guanhumi, along the Indian River lagoon. Up to 26 individuals have been gathered from a single crab burrow. Beds of saltwort (Batis maritima) and glasswort (Salicornia spp.) seem to be the favored sites for land crab burrows along the east coast, so that is where the Mangrove Rivulus can be found. Burrows of other crab species many also be utilized, particularly along the west Florida coast, where Cardisoma guanhumi is not common. Additional specimens have been found in south Florida and the Florida keys.

Now remember that funny word I used earlier on? Hermaphrodites! The most striking aspect of the Mangrove Rivulus is its unique mode of reproduction. In Florida, at least, its reproduction is as a synchronous self-fertilization, the ONLY such example known among vertebrate animals. To put that in English, it does not need to have a male around to fertilize its eggs. Because it has an ovotestis that simultaneously produces both eggs and sperm, the individual fertilizes its own eggs internally. As strange as this sounds, it has been conclusively proved by the laying of fertilized eggs by individual fish kept in total isolation from all other fish onward from the egg stage. Since both eggs and sperm are produced by one parent, the young are identical genetically to the parent. Captive raised specimens of the Mangrove Rivulus are in high demand for use in genetic, immunochemical, and bioassay research.

Male fish, known as gonochrists, are very rare in Florida but have been reported to be more common in the West Indies and Central America. Males are of two types, known as primary and secondary. Primary males develop directly from fertilized eggs, without going through the hermaphroditic phase. Primary males may also be produced from eggs incubated at low temperatures. Secondary males develop from hermaphroditic fish under certain ecological and experimental conditions. Normal two parent reproduction has not been demonstrated for the Florida population but has been reported from the West Indies; however, some researchers question these observations, so that the function of males in this species remains an intriguing and still unanswered question. Little is known of the reproductive life history of Rivulus marmoratus in the wild. Evidence of eggs in a mangrove forest substrate has been reported. The fish dwelling in crab burrows may deposit their eggs outside the burrow on the damp marsh surface, where they develop and hatch.

Now what do these little bizarre fish eat. Collector-researchers have found combinations of mosquito larvae and small insects. In addition, springtails, polychaetes, copepods, fish scales, and vegetable matter were found in some specimens. Small crabs and snails are also recorded among the food items eaten. There is evidence of an instance of cannibalism in individuals dwelling in the crab burrows. Rivulus marmoratus does poorly in captivity unless provided with live food. I found that feeding it mosquito larvae was the easiest thing to obtain and feed it.

I have not had any luck breeding my one lone killifish hermaphrodite. But I do plan on getting it some friends next time I see a crab burrow. I going to take a small dip net and get a few more. Wish me luck.