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Species Name:    Rivulus marmoratus
Common Name:              (Mangrove Rivulus)

 

I.  TAXONOMY

Kingdom Phylum/Division: Class: Order: Family: Genus:
Animalia Chordata Actinopterygii Cyprinodontiformes Aplochilidae Rivulus
 
The hermaphroditic form of the mangrove rivulus, Rivulus marmoratus.  Photo 
courtesy of  D. Scott Taylor, Brevard 
County Mosquito Control District.    


 
Male form of R. marmoratus.  Males are extremely rare in Florida, with a total of only 
6 collected from the Everglades population.  Males have never been collected from the IRL.  Photo courtesy of  D. Scott Taylor, Brevard County Mosquito Control District.

 


Species Name:
 

Rivulus marmoratus Poey, 1880

Common Name:
Mangrove Rivulus

Species Description:
Rivulus marmoratus is a member of the Order Cyprinodontiformes which includes the rivuline fishes as well as the killifishes and live bearers. Species of the genus Rivulus often colonize marginal habitat or ephemeral habitats and are primarily found in Central and South America. R. marmoratus is the only member of the genus to both inhabit Florida, and to be wholly associated with marine habitats (Davis, Taylor and Turner 1995). Body color is variable and ranges from maroon (Harrington and Rivas 1958) to shades of brown, tan or cream (Thomerson 1966), with small spots covering the body. Body color is generally reflective of habitat (Thomerson 1966), with light colored fish typically inhabiting areas with light substrata and high turbidity; while those colored brown to maroon tend to inhabit areas with dark-bottoms. 

R. marmoratus is a self-fertilizing hermaphrodite that attains a total length of approximately 2 inches (~ 50 mm). Hermaphrodites of this species have a prominent caudal ocellus and a whitish border along the anal fin. Pure females, primary males and secondary males, have been observed in the laboratory, but only males and hermaphrodites have been observed in natural populations.


This species is able to live while emersed and can survive in moist detrital substrata during periods of drought or in response to highly anaerobic conditions (Abel et al. 1987; Davis, Taylor and Turner 1990, 1995). R. marmoratus has also been observed traversing the forest floor during the rainy season (Huehner et al. 1985).

Synonymy:
Has sometimes been synonymized with R. ocellatus (Seegers 1984), a Brazilian species.

Other Taxonomic Groupings:
Division Teleostei in the Class Actinopterygii is the newer taxonomic grouping for the ray-finned fishes (Compagno 1991, Nelson 1994, Bond 1996). The teleost fishes are further divided into subdivisions, with the Cyprinid fishes ordered as follows: Subdivison Euteleostei, Superorder Acanthopterygii, Series Atherinomorpha, Order Cyprinodontiformes.


II.  HABITAT AND DISTRIBUTION 
Regional Occurrence:
R. marmoratus has the widest range of any of the neotropical Aplochilidae (Taylor, Davis and Turner 1995), and its distribution closely parallels that of the red mangrove (Rhizophora mangle). Distributed from central Florida, through the Bahamas and the Caribbean to southern Brazil (Taylor, Davis and Turner 1995; Huber 1996), R. marmoratus commonly colonizes microhabitats in mangrove forests. Florida is the northern limit of this species’ range. (Taylor, Davis and Turner 1995).

IRL Distribution:
R. marmoratus is widely distributed but has been considered locally rare throughout Florida (Taylor, Davis and Turner 1995). However, Taylor (1992) suggested that this fish may be more abundant than has been assumed, but may be rarely seen because it inhabits marginal habitats and obscure areas. In the IRL and along the east coast of central Florida, this species prefers unimpounded, high marsh habitats inside the burrows of Cardisoma guanhumi, the great land crab. In the Everglades and throughout Florida’s west coast, these fish are generally found in stagnant and seasonal pools, as well as in mosquito ditches in mangrove habitats.

Taylor, Davis and Turner (1995) found that along the central regions of the IRL, Rivulus marmoratus is found primarily in the high marsh above the intertidal zone. These areas are flooded only seasonally, after very high tides, or by heavy rainfall. The tremendous structural and hydrological alteration of this prime habitat due to impoundment for mosquito control may have led to the extirpation of R. marmoratus from many of these areas. Taylor, Davis and Turner (1995) collected only 5 individuals from approximately 3000 hectares of impounded marsh, while over 600 individuals were collected from 1000 hectares of unimpounded marsh.


III. LIFE HISTORY AND POPULATION BIOLOGY
Age, Size, Lifespan:
R. marmoratus collected in the Indian River Lagoon ranged in length between 11- 37 mm (SL). Maximum size is approximately 50 mm.

Abundance:
Though it has a wide range and distribution, R. marmoratus has generally been considered locally rare. However, Taylor (1992) suggested that the species is likely more abundant than originally thought, but because it inhabits marginal habitats, it is rarely seen.

Locomotion:
Moves primarily by swimming; although it has been observed to "flip" out of water to escape predation. It has also been observed moving amphibiously between ponds or burrows.

Reproduction:
Natural populations of R. marmoratus are self-fertilizing hermaphrodites (Harrington and Rivas 1958, Thomerson 1965), the only natural example of cloning among vertebrates (Taylor, Davis and Turner 1995). The adult population in Florida consists almost solely (>99%) of arrays of homozygous clones (Davis, Taylor and Turner 1990). Davis, Taylor and Turner (1990) speculated that there may be a relationship between self fertilization in this species, and its ability to disperse widely and colonize marginal or seasonal habitat areas. The gonads of R. marmoratus are actually ovotestes that produce both eggs and sperm. DNA testing has shown that in spite of the fact that this species is clonal in nature, the genetic diversity within populations approaches that of sexually reproducing organisms, with the actual number of distinct clones being quite high.

Pure males are occasionally present in natural populations. In offshore cays in Belize, males may comprise 20-25% of the population.  Throughout Central and South America, males in highly colonized pools can average between 3-8% of the total population. The presence of heterozygotes in these populations provides evidence that genetic outcrossing does occasionally occur (Taylor, Davis and Turner 1990, 1995). In Florida, males are extremely rare, with a total of just 6 being collected from the west coast of Florida and the Everglades area (Davis, Taylor and Turner 1990). No males have been collected in the Indian River Lagoon. Taylor, Davis and Turner (1995) have hypothesized that the presence of pure male fish and heterozygotes in a population may be a response to some ecological stress or disturbance. They support this hypothesis by stating that male R. marmoratus can be easily produced in the laboratory by manipulating thermal conditions; and, that among populations sampled in Belize, more males and heterozygotes are collected in highly disturbed areas than from less disturbed areas.

Cole and Noakes (1997) studied gonadal development in laboratory reared specimens of R. marmoratus from Florida ancestors. All the fish examined in this study were less than 150 days old. This experimental population consisted of 63% pure females, 14% pure males, and 21% hermaphrodites. This study provided the first evidence that young adults of R. marmoratus sexually outcross. Since natural populations do not show this demographic, it was suggested that there may be an age-dependent shift in sex allocation from a female dominated population to a hermaphroditic population. This shift is somewhat similar to diandric protogynous hermaphroditism observed in other teleost fishes, particularly the Gobiidae. In this type of hermaphroditism, an individual can develop directly as a primary male; or, it can function first as a female, and later develop into a secondary male while still retaining female function (Cole and Noakes 1997). This study offers one possible explanation of how adult populations in Florida are overwhelmingly dominated by hermaphrodites. It remains for future study to determine whether the high genetic variability observed in natural populations is due to sexual reproduction by young adult fish, or to self fertilization by hermaphrodites.

Embryology:
Mature eggs measure over 1 mm in diameter. Eggs are laid near the surface of the water in various stages of development, with some being recently fertilized, and others already containing developing embryos (Thomerson 1966). Adults commonly catch and eat spawned eggs as they are released. The number of eggs released is positively correlated with body size of the adult and with season (Taylor 1992). Early in the reproductive season, R. marmoratus lays between 46 – 60 eggs. Later in the season, adult fish generally lay more than 75 eggs. There is some evidence that eggs may sometimes be aestivated before hatching, as there are a number of cases where juvenile R. marmoratus have been observed in isolated ponds and pools after prolonged periods of drought (Ritchie and Davis 1986, Davis, Taylor and Turner 1995).


IV.  PHYSICAL TOLERANCES
Temperature:
R. marmoratus has been collected in water temperatures ranging from 7 - 38° C (Taylor 1993). When subjected to laboratory temperatures below 9° C, R. marmoratus became moribund (Taylor 1992, 1993). Taylor (1993) observed a decline in the Indian River Lagoon population following a 1989 freeze event in which air temperatures in Fort Pierce reached –7.2° C (45 ° F), and water temperatures reached 11.1° C (52° F). While some thermal buffering by land crab burrows undoubtedly occurred, it apparently was insufficient to protect R. marmoratus from the extreme cold, as specimens of this fish were not collected again for 8-17 months following the freeze.

Salinity:
R. marmoratus is highly euryhaline (Taylor, Davis and Turner 1995) and has been collected from waters with salinities from 0 – 68 ppt.

Other Physical Tolerances: 
In addition to tolerating extremes in salinity and temperature, R. marmoratus also tolerates high sulfide levels. As such, it is generally found in areas depauperate of other fish species, such as crab burrows, stagnant pools and impounded ditches. Davis, Taylor and Turner (1990) observed that R. marmoratus inhabiting crab burrows often had a milky white coating on their body surfaces which sloughed off after capture. The authors interpreted this unusual character to an epidermal or mucous reaction to high sulfide levels in the burrow. Additionally, fish in these stagnant burrows that became caught in fully submerged traps were often observed to "drown.". It is thought that this response is another reaction to high sulfide levels within the burrows. It had been observed in the laboratory that when R. marmoratus is emersed, it tends to be extremely quiescent, not even ventilating its gills. At the time, it was thought that this was a voluntary response; but in light of the observed "drowning" observed in the burrows, the authors speculated that perhaps quiescence while emersed is a response to stagnant conditions, and is used when these fish are unable to emerge from burrows containing lethal hydrogen sulfide concentrations (Davis, Taylor and Turner 1990).

Abel et al. (1987) found that Rivulus marmoratus was fully capable of surviving longer than 30 days in moist detritus.


V.  COMMUNITY ECOLOGY
Trophic Mode:
R. marmoratus feeds on both terrestrial and aquatic animals (Taylor 1992). In one study, up to 40% of the fish collected had no food items present in the gut. This observation indicates that R. marmoratus may be an opportunistic feeder when confined in crab burrows, but probably feeds heavily when its marsh habitat becomes flooded (Taylor 1988, 1992). Small flying insects and ants comprised approximately 30% of the diet of R. marmoratus, while aquatic organisms such as polychaete worms, gastropods and mollusks accounted for approximately 70% of the diet (Taylor 1992). Fry of Rivulus marmoratus prey upon first instar larvae of Ades taeniorhynchus, adults fish prey on fourth instar Ades mosquitoes and larvae of Culex quinquefasciatus (Taylor, Ritchie and Johnson 1992).

Competitors:
R. marmoratus has few competitors due to its ability to inhabit marginal or ephemeral habitats.

Habitat:
In the Indian River Lagoon, R. marmoratus primarily inhabits unimpounded high marsh zones where great land crabs (Cardisoma guanhumi) have established burrows. The majority of fish are captured in shallow burrows less than 1 m in depth. Captures of 1 – 2 individuals are common, with as many as 26 fish being captured from a single burrow.

Activity Time:
R. marmoratus have been observed to be highly active at night around land crab burrows (Davis, Taylor and Turner 1990), but can be active throughout the day if water levels are high.

Associated Species:
Along the east coast of central Florida, Rivulus marmoratus inhabits the burrows of the great land crab, Cardisoma guanhumi. This association is thought to provide the fish with a thermal refuge when temperatures are low during the winter months (Taylor, Davis and Turner 1995), thus allowing this species to establish itself north of its thermal tolerance limits.


VI. SPECIAL STATUS
Special Status:
Rivulus marmoratus is a Species of Special Concern (SSC) in Florida.   In 1999, it was submitted by the National Marine Fisheries Service (NOAA) as a candidate species for protection under the Endangered Species Act.  As yet, it has not been officially listed as threatened or endangered.

Notes on Special Status:
Candidate species do not receive protection under the Endangered Species Act.  Available biological information for Rivulus marmoratus will be reviewed, and a further determination will be made to either remove the species from the candidate list, or to federally list it as threatened or endangered.  If it should become federally listed, it will then be afforded protected status, and a conservation and recovery proposal will be developed.

Economic Importance:
None

 

Report by:  K. Hill, Smithsonian Marine Station
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Page last updated: July 25,  2001