Systematics: Rivulus marmoratus Poey 1880. Type locality, Cuba. Synonyms: R. ocellatus Hensel, R. heyei Nichols, R. marmoratus bonairensis Hoedeman. Sometimes referred to as "R. ocellatus marmoratus." Mitochondrial DNA studies suggest that populations from Southeast Brazil and Turks islands may be phyletically distinctive from those of Florida and Belize (Weibel et al., 1998 submitted), but most other populations are closely related. Closest congeneric relative apparently R. caudomarginatus Seegers from SE Brazil (Murphy & Collier, 1996). Genus Rivulus otherwise contains about 95 other described species from freshwater habitats in Central and South America, most poorly known and all of which presumably reproduce conventionally (see Huber, 1992 for review of literature). Order Atheriniformes, family Cyprinodontidae, subfamily Rivulinae (often treated as a distinct family, Rivulidae or Aplocheilidae).

Geographic range: Mainland: southeast coastal Brazil (south to at least Santos; rumored from Buenos Aires), northeast coastal Brazil (Huber, 1986), French Guiana, Venezuela (Taphorn, 1980), Nicaragua, Guatemala (D. Rosen, pers. comm), Belize (including barrier islands), Yucatan and southern Florida [on east coast north to about Vero Beach, with an apparently isolated population in Volusia country (D.S. Taylor, pers. comm.), on west coast north to Tampa Bay and possibly to Cedar Key]. Absence of mainland records between those cited (except Yucatan and Florida) likely reflects inadequate sampling rather than discontinuity of range. May occur essentially continuously along the coast from southeastern Brazil to the Yucatan. Islands: Florida keys to Key W., Tortugas, Bahamas (to at least the Turks and Caicos islands), Cuba, Isle of Pines, Sto. Domingo, Puerto Rico, St. Maarten, Grand Cayman, Jamaica, Curacao, Aruba, and Bonaire.

General description: Hermaphrodites easily recognized by characteristic mottled/marbled brownish color pattern, dark humeral splotch, caudal peduncle ocellus. Males (much rarer) have orange/pink body wash (sometimes quite conspicuous) with mottling, humeral splotch and caudal peduncle ocellus variably obsolescent or absent; single fins often dark-edged but may be clear. High lateral scale count (47 - 53) virtually diagnostic of species. Maximum size, about 75 mm total length (including tail); laboratory specimens grown to this length are usually 2 or 3 years old.

Habitat: Coastal mangrove forests, often in detritus or leaf-litter. Frequently in the borrows of land crabs Cardiosoma or Ucides spp (Taylor, 1988; Davis et al., 1990) and other holes or depressions. Usually in marine water, but sometimes in brackish water or hypersaline pools; apparently unknown from freshwater. Semiterrestrial, cryptic, fossorial. Long thought to be rare but often quite common in preferred habitats. Rather frequently overlooked by routine faunal surveys; difficult to collect by seining and apparently resistant to rotenone.

Ecology: Euryhaline and able to endure very abrupt and drastic salinity changes (including transitions between marine and freshwater in the laboratory). Chloride cells which may mediate euryhalinity discussed by King et. al. (1989). Thermal tolerance ranges not yet established., but breeding usually does not occur below 20 C. (Harrington, 1971; Lin & Dunson, 1995; B. Turner, unpubl.). Apparently intolerant of dissolved hydrogen sulfide (Abel et al., 1987). May be dependent upon aerial respiration (specimens in flooded traps, with no access to air, die). Integument has epidermal capillary net which may be adaptation for aerial respiration (Grizzle & Thiyagarajah, 1987).

Insectivorous/planktivorous; diet treated in detail by Taylor (1988-1996). Opportunistically cannibalistic and oophagous. See also Davis et al. (1990), Brockman (1975), Huehner et al. (1985), Davis et al. (1995) and Taylor et al. (1995) for general ecological observations. Population biology and dynamics almost unknown.

Frequently and routinely emergent; moves on moist substrates by jumping or "flipping." Biology of emergence, including duration/direction/significance of excursions, not completely known. Emergent individuals may be avoiding hydrogen sulfide/anoxia (Abel, 1981; 1984; Abel et al., 1987), but emergence of other Rivulus species associated with capture of terrestrial prey, especially ants (literature reviewed in Huber, 1992). Has been found in flooded tire tracks, also under and inside emergent rotting mangrove logs, in abandoned termite galleries (Belize, W.P. Davis, pers. comm.). Has also been found attached to mangrove leaves in canopy (Florida, R.W. Harrington, Jr. pers. comm.) and flipping along the ground in a field more than 100 m from water (Bimini, Bahamas, V. Walters pers. comm.). Can survive packed in moist leaves for at least 60 days (W.P. Davis, pers. comm.).

Usually not sympatric with other Rivulus species. However, normally occurs together with with R. caudomarginatus, the only other regularly marine species in the genus, in SE Brazil; adults of R. marmoratus found near surface, those of R. caudomarginatus found nearer the substrate, while juveniles and subadults of the two species do not appear to be spatially segregated (S. Hellner, pers. comm.; K. Lazara, pers. comm.). Range overlaps with that of R. tenuis (freshwater to brackish) along Belize coast (W. Davis, pers. comm.) and overlaps or directly adjoins that of R. cylindraceus (freshwater) near Havana (M. Smith, pers. comm.).

Reproductive biology: Generally thought to be synchronously hermaphroditic and obligately self-fertilizing. Gonad, an "ovotestis," is paired for much of its length, ovary-like in general appearance; oogenic and spermatogenic tissues not delineated by membranes (an "unrestricted" ovotestis), amount and location of testicular tissue variable, but latter usually along mesial edges of gonad and much less extensive than ovarian tissue (Soto, 1988). Mature spermatozoa found in ducts throughout organ. Testicular tissue not obviously zonal or ordered with respect to stage of spermatogenesis. Fertilization is essentially coincident with ovulation; both processes, as well as oviposition, occur with 24 hour rhythms (Harrington, 1963). About 80% of eggs are oviposited with 24 hrs of fertilization, but less than 1% are oviposited immediately; eggs are not oviposited in order of their fertilization (Harrington, 1963). Efficiency of self-fertilization generally very high, but can be reduced in some lines by photoperiod changes (Harrington, 1971), so that hermaphrodites emit some viable but unfertilized eggs. Endocrinology of ovotestis unknown. Cole & Noakes (1997) observed that the gonads of young laboratory-reared "adults" often contain seemingly mature ova but not testicular tissue; they argue that these are true females, and that the species may therefore be essentially a (diandric) protogynous hermaphrodite.

Age at sexual maturity often given as 120 days but not well established; specimens as young as 90 days (from hatching) can sometimes oviposit.

Embryonic stages delineated by McMillan (1979) and Koenig & Chasar (1984). Note: Harrington found developmental stages described for Fundulus heteroclitus to be useful guides for R. marmoratus. Koenig & Chassar (1984) also provide good description of standard method of laboratory culture in glass fingerbowls. Ova survive terrestrial stranding in drying substrate; stranded or buried developing embryos can enter diapause similar to that of annual killifish (Ritchie & Davis, 1986). Diapause especially evident at low temperatures in laboratory; embryos incubated below about 21 C frequently fail to hatch and require manual dechorionation.

Research history: Recognized as a distinct, valid species by Rivas (1945) after having been mistakenly hidden in the synonymy of R. cylindraceus, a Cuban species to which it is not closely related, by Jordan & Evermann (1896). Discovered in Florida by Harrington & Rivas (1958). Selfing hermaphroditism and environmental sex influences shortly thereafter discovered by the late R.W. Harrington, Jr., the most prominent student of the species. (Harrington, 1961-1975). Majority of published research done with Floridian clonal lines originally collected by Harrington (see below).

Lindsey & Harrington (1972) and Harrington & Crossman (1976) reported relationship between homozygosity in R. marmoratus and developmental instability of meristic features. Putative relationship subsequently shown to be questionable by Swain & Lindsey (1986); see also Swain (1987). Interpretation of experiments of this type complicated by strong "previous generation" effects (see Swain & Lindsey, 1986; Lin & Dunson, 1995).

Has been used in teratogen and toxicity screening (Koenig & McClean, 1980; Davis, 1984; 1988; Lin & Dunson, 1993; Park et al., 1990; 1992; 1993; 1994; Thiyagarajah & Grizzle. 1985; 1986), studies on carcino- and tumorigenesis (Grizzle & Thiyagarajah, 1988; Park & Kim, 1984; Courtney & Fournie, 1988; Couch, 1995; Thiyagarajah et al., 1995), macrophage induction (Vogelbein et al., 1987), and on mutation repair (Park & Yi, 1989). Other papers deal with potential suitabillity for mosquito control (Taylor et al., 1992), its skeletal development (Lee & Park, 1989), scale growth (Park & Lee, 1992), pseudobranch structure (King et al., 1993), development of photoreceptors (Ali et al., 1988a; 1988b; 1989), organ morphology and function (Thiyagarajah & Grizzle, 1987; 1995), the structure of certain oncogenes and their cellular homologs (Lee et al., 1994; 1995) and tissue specific oncogene expression (Goodwin & Grizzle, 1994a; 1994b).

Population genetics and clonality: Chromosomally uniform over vast range, 2N = 48 (Sola et al, 1997, see also Yang et al., 1987). Clonality and homozygosity in Floridian material originally established by fin and organ allografts (Kallman & Harrington, 1964; Harrington & Kallman, 1968) and subsequently with sensitive multilocus DNA fingerprinting techniques (Laughlin et al., 1995). Clonal pattern of inheritance confirmed by progeny testing of 29 field-caught hermaphrodites from various localities (Florida, Belize mainland, Bahamas, Brazil), by DNA fingerprinting. Initially, three histocompatibility clones recognized by allograft reactions, ("DS" and "NA" from near Vero Beach and "M" from near Miami). These clones not distinguishable by allozyme techniques (Massaro et al, 1975; Vrijenhoek, 1985, B. Turner, unpubl.) but readily resolved by DNA fingerprinting (Turner et al, 1990). Fingerprinting of other Floridian samples provides evidence of many clones, most at very low frequency and almost all separated by multiple mutations (Turner et al., 1992a); "sister" clones have not yet been detected. Clones cannot be distinguished by mtDNA control region sequences (B. Turner & M. Fisher, unpubl.) or by several variants of the "RADP" technique (B. Turner, unpubl.). Clonal composition, at least of the three S Florida populations repetitively sampled thus far, apparently turns over completely from year to year.

Differences detected among Harrington-Kallman clones in age at sexual maturity, extent of senescent gender inversion and efficiency of low temperature male induction (Harrington, 1971) and among seven other clones (FL to Brazil) in the low temperature induction of males and of morphological anomalies (Fisher, 1997). Differences also detected among five clones (FL to Belize) in reproductive "fitness values" and their responses to variation in salinity and food levels (Lin & Dunson, 1995). In aggregate, these differences suggest clonal divergence in ecologically relevant traits, but the extent of such divergence among sympatric clones is not yet known.

Gender ratios and outcrossing in natural populations: Most populations apparently consist of 100% hermaphrodites; no conventional ("gonochoristic") females have ever been collected from nature. Males usually extremely rare to nonexistent but fairly common on Belize keys (10 - 25% of catch in 1988, 1989, 1991, see Turner, et al., 1992b) and were transiently common on Curacao (Kristensen, 1970; see discussion in Davis et al., 1990). Belize males usually have no evidence of previous functional oogenesis, are fertile (by histology), and, in the laboratory, court hermaphrodites and females of other Rivulus species. Significant levels of outcrossing detected on one Belize key, Twin Cays, by progeny testing of field-caught hermaphrodites, using DNA fingerprinting; all hermaphrodites tested were heterozygous (Lubinski et al., 1995). This is the only record of field-caught heterozygotes in the species. Progeny testing thus far has detected only homozygotes in other populations. Outcrossing presumably occurs via male x hermaphrodite matings, but factors which regulate induction of males and behavioral receptivity and reduced self-fertilization by hermaphrodites are unknown. Outcrossing could also occur if the young "females" detected by Cole & Noakes (1997) occur in natural populations and mate with males. In either case, outcrossing appears to be rare, though it may be an episodic response to a particular combination of environmental circumstances. Its biological significance (if any) in this system is not yet clear.

Experimental gender transformations:. In 3 Floridian clones, primary males (males which were never hermaphrodites) are induced at high frequency by incubating developing embryos at temperatures between 18 and 20 degrees C. (Harrington, 1967). A specific "phenocritical" period for this induction has been delineated (Harrington, 1968). Adult, functional hermaphrodites undergo sex transformation ("sex succession" or "inversion") to secondary males in response to one or more short-day seasons; age threshold of response lowered if hermaphrodites are early-reared at high temperatures (Harrington, 1971). Some immature hermaphrodites precociously transform into secondary males if exposed to high temperatures (Harrington, 1975); this effect has not received detailed study. Induction efficiencies and thresholds for both primary and secondary males vary among the three clones used in above studies. Similar variation in low temperature primary male induction detected in seven other clones, ranging from Brazil to Florida (Fisher, 1997). Harrington (1975) claimed that primary and secondary males could be distinguished by the presence of residual oviducts and relict oocytes in the latter. This claim disputed by Soto (1988) and Soto et al., (1992) who showed known secondary males with no ovarian tissue remnants. Conspicuous orange/pink body coloration of males results from xanthophyll pigment very similar/identical to canthaxanthin; its intensity is correlated with testicular development or activity (Soto et al., 1994).

Experimental induction of developmental anomalies: Low temperature treatments of embryos also induce swim bladder malformations, opercular hyperplasia and vertebral kyphosis. The efficiency of this induction varies markedly among seven clones that span the range of the species (Fisher, 1997).

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Thiyagarajah, A. and J.M. Grizzle. 1986. Diethylnitrosamine-induced pancreatic neoplasms in the fish Rivulus ocellatus marmoratus. J. Nat. Cancer Inst. 77:141-147.

Thiyagarajah, A., J.M. Grizzle. 1987. Nuclear glycogen and lipid hepatocytes of a fish Rivulus ocellatus marmoratus. Can. J. Zool. 64(12): 2868-2870.

Thiyagarajah, A., and J.M. Grizzle. 1995 Morphology of liver, gall bladder, and pancreas in the self-fertilizing fish, Rivulus marmoratus (Cyprinodontiformes: Aplocheilidae). J. Alabama Acad. Sci. 66:126-136

Thiyagarajah, A., M. Ledet, J.M. Grizzle. 1995. Presence of a carcinoembryonic antigen in hepatic neoplasms of Rivulus.. Mar. Environ. Res. 39 (1-4): 279-281.

Turner, B.J., J.F. Elder, Jr. T.F. Laughlin & W.P. Davis. 1990. Genetic variation in clonal vertebrates detected by simple-sequence DNA fingerprinting. Proc. Nat. Acad. Sci U.S.A. 87: 5653-5657.

Turner, B.J., J.F. Elder, Jr., T.F. Laughlin, W.P. Davis & D.S. Taylor. 1992a. Extreme clonal diversity and divergence in populations of a selfing hermaphroditic fish. Proc. Natl. Acad. Sci. USA. 89(22): 10643-10647.

Turner, B.J., W.P. Davis & D.S. Taylor. 1992b. Abundant males in populations of a selfing hermaphrodite fish, Rivulus marmoratus, from some Belize cays. J. Fish Biol. 40: 307-310.

Vogelbein, W.K., J.W. Fournie & R.M. Overstreet. 1987. Sequential development and morphology of experimentally induced hepatic melano-macrophage centres in Rivulus marmoratus. J. Fish Biol. 31(Suppl. A): 145-154.

Vrijenhoek, R.C. 1985. Homozygosity and interstrain variation in the self-fertilizing hermaphroditic fish Rivulus marmoratus. J. Hered. 76: 82-84.

Weibel, A.C., T.E. Dowling & B.J. Turner. [1998]. The evolution of sexual reproduction in an hermaphroditic fish (Rivulus marmoratus): a phylogenetic and biogeographic study. [submitted].

Yang, S-H., G-S. Anh, Y-J. Kim & E-H. Park. 1987. Studies on chromosome and isozyme in hermaphroditic fish Rivulus marmoratus. Korean J. Genet. 8(4): 239.

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Papers on R. marmoratus not cited in above review:

Costa, W.J.E.M. 1994. Comments on the proposed conservation of the specific names Rivulus marmoratus. Bull. Zool. Nomenclature, 51(1): 44-46 ..

Goodwin, A.E. & J.M. Grizzle. 1994. Endogenous enzymes cause structural and chemical artifacts in methacrylate- and celloidin-embedded sections of unfixed freeze-dried tissues. J. Histochem. Cytochem. 42(1): 109-144.

Lazara, K.J. & Smith, M.L.. 1994. Comments on the proposed conservation of the specific name Rivulus marmoratus. Bull. Zool. Nomenclature, 51(1): 47-48.

Lee, J-S. 1994. Structure of the ras genes in the hermaphroditic fish Rivulus marmoratus Ph.D. diss. Hanyang University, Seoul, Korea. [Supervisors: E.-H. Park & J. Choe]

Thomerson, J.E. 1966. Rivulus marmoratus, a rare and unusual killifish from Florida. J. Amer. Killifish Assoc. 3: 48-51.

Webster, C.D. 1989. Nutritional value of brine shrimp nauplii for striped bass larvae. Ph.D. diss., Auburn University, Auburn, Alabama.(Chap. 4 is entitled: Growth response of the fish Rivulus ocellatus marmoratus fed brine shrimp nauplii from different sources with various fatty acid compositions).