|
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
Submit additional information, photos or comments to:
irl_webmaster@si.edu
Page last updated: July 25, 2001
|
