Oeco1ogia (1993) 95: 266-276 Oet:a/ogia @ Springer-Verlag 1993
Seasonality of reproduction by livebearing fishes in tropical rainforest streams
Kirk O. Winemiller Department of Wildlife and FisheriesSciences, Texas A&M University, CollegeStation, TX 77843-2258,USA
Received:26 August 1992/ Accepted: 29 March 1993 Abstract. Reproductive ecology, population structure, Most populations show some degree of seasonality in and diets of three common livebearing poeciliid fishes reproduction, particularly in temporally variable en- (Alfaro cultratus, Phallichthys amates, Poecilia gilli) vironments at intermediate or high altitudes or latitudes. from rainforest streams in Costa Rica were investigated In tropical regions characterized by strong wet-dry sea- over ten continuous months. The region experienceslittle sonality, reproduction is also often markedly seasonal annual temperature variation, and although monthly (Janzen 1967, 1979; Lowe-McConnell 1979; Winemiller rainfall is continuous each year, two brief dry seasons 1989). Organisms in temporally variable habitats may typically occur. Monthly changes in indices of ovarian allocate annual reproductive effort in either of two man- condition, percentages of females with developing em- ners: (1) production of large clutches during a brief and bryos, and population size structure revealed that re- presumably optimal period, or (2) production of multiple productive output by females of all three speciesvaried clutches, each consisting of a few offspring over short seasonally. Based on testicular condition, males were intervals during a more extended favorable period. reproductively active year-round, however the mean Among freshwater fishes, large speciesliving at higher gonadal index for males of two algivorous speciesshow- latitudes tend to be associated with the former strategy ed low levels of seasonal cycling that largely coincided (Mahon 1984; Wootton 1984; Winemiller and Rose with female variation in reproductive effort. All three 1992),whereas small speciesfrom either tropical or tem- specieshad seasonaldifferences in the female size-brood perate regions may exhibit either strategy (Hubbs and size relationship, whereby larger females tended to carry Strawn 1957; Kramer 1978; Constantz 1979; Hubbs more embryos during ~hewet season.Several important 1982,Heins and Rabito 1986; Burt et al. 1987; Winemil- adult and neonate food resourcesare more available in ler 1989; Winemiller and Rose 1992). In environments the flooded forest during the wet season,which is also the characterizedby relatively small annual variation, organ- period when conspecifics and predators are at their isms may have extended and relatively asynchronous or lowest per-area densities. Three hypotheses are discuss- continuous reproduction. An inverse trend has been ed: (I) brood size in relation to conspecific density- noted for the relationship betweenlatitude and spawning mating frequency, (2) reproductive allocation in response seasonduration and multiple clutching among speciesof to variation in adult food resources,and (3) selection for both marine (Johannes 1978) and freshwater fishes greater reproductive effort during conditions optimal for (Hubbs 1985; Gotelli and Pyron 1991; Winemiller and juvenile growth and survival. Data for Alfaro were con- Rose 1992). sistent with the latter two hypotheses.In Phallichthys and Fishesof the neotropical family Poeciliidae are almost Poecilia, diets were poorer during wet seasons,indicating exclusivley ovoviviparous and viviparous (Thibault and that reproductive effort does not coincide with availabil- Schultz 1978; Constantz 1989). The family shows large ity of adult food resources, and that selection probably interspecific variation in life-history characteristics such favors greater reproductive effort during periods optimal as egg size, brood size, interbrood interval, and age at for juvenile growth and survival. maturity (Reznick and Miles 1989). In addition, several poeciliid specieshave large within-species variation in Key words: Costa Rica - Poeciliid fishes Reproductive life-history traits (Hester 1964; Constantz 1979; Reznick seasonality 1982; Stearns 1983; Bums 1985; Snelson 1984, 1985; Trexler 1985; Andersson 1986; Travis et al. 1989; Meffe 1990). Becausepoeciliids are relatively small and uncon- strained by special spawning or larval rearing habitats by virtue of internal gestation, continuous annual reproduc- 267 tion should be a viable strategy for poeciliids in habitats characterized by low environmental seasonality. Species of the genus Gambusiahave extended summer spawning seasons in temperate North America (Hughes 1985; Meffe 1985; Botsford et al. 1987; Reznick and Braun 1987; Yan 1987). Even within the temperate zone where annual reproduction remains markedly seasonal, Gam- busia reproductive periods tend to be of longer duration in more constant spring-fed habitats (Meffe 1985; Yan 1987). In Florida, two seasonal peaks in reproduction, early and late summer, were documented for Poecilia latipinna, and no reproduction occurred between November and March (Snelson 1984).Only a few studies J F M A M J J A s D have monitored reproduction by natural poeciliid popu- lations in tropical settings. Near the Tropic of Cancer in Mexico, three sympatric species of Poecilia (2 sexual speciesand 1 triploid unisexual species)mate year round but have different periods of peak reproductive output (Balsano et al. 1981). Becausemales do not discriminate well betweenunisexual femalesand bisexual conspecifics, it was hypothesized that asynchrony in reproductive ef- fort might facilitate interspecific coexistence.In Central America, Turner (1938) observed continuous reproduc- tion in Brachyrhaphis episcopi. Morris and Ryan (1992) documented continuous reproduction by three Xiphorus species in Central Mexican streams. Their preliminary data indicated that females of equivalent size may produce fewer offspring during the summer-fall rainy seasonwhen mountain streamsare turbid from flooding. Winemiller (1989) collected gravid female guppies, Poecilia reticulata, throughout the year in seasonalwet- dry habitats in Venezuela. In Trinidad, Reznick (1989) During 1985,two wet and two dry seasonswere recognized (Fig. 1). documented lower reproductive effort by guppies from Although actual datesmay vary by as much as a month and a half, two wet seasonsamples in comparison with fish from two this bimodal rainfall distribution is typically observedeach year. dry seasonsamples. Wet seasonsoccurred from late May until mid-September(referred This study reports findings from a ten month inves- to here as the "primary wet season")and from early November through March (secondarywet season).Dry seasonsoccurred from tigation of poeciliid population ecology in a Central March until early May (primary dry season),and during October American tropical rainforest. Data obtained from three (secondarydry season). locally abundant poeciliids demonstrate large intraspe- Most of the western area of the park is coveredby pristine, cific variation in adult body sizeand seasonalpatterns of lowland tropical rainforest, but at leastfour other vegetativezones gonadal condition, brood size, and population size struc- are recognizedwithin the reserve:cativo swampforest, low marsh ture. Available evidence indicates that reproductive ef- woodland, herbaceousmarshland, and halophytic coastal vegeta- tion. Many of the low marshwoodlands are dominatedby the palm fort in these speciesis greater during wet periods and Raphiataedigera. Apart from the hills of the Lomasde Sierpein the corresponds with changesin aquatic habitat dimensions, park's westernsector, most of the region is coastalplain lying only fish densities, the availability of juvenile food resources, severalmeters above sea level. Sedimentsof the plain are of alluvial and for at least one species,the availability of adult food origin, formed by a coalescenceof river deltas. A distinctive net- resources. work of narrow lagoons lies parallel to the coast (for additional descriptionsof regional aquatic habitats seeWinemiller and Leslie 1992). Materials and methods B. Sampling and field measurements A. Study region Fishes were sampled each month from two streams in Tortuguero Fish populations were investigated in two streams located within National Park between February and December, 1985. A descrip- Tortuguero National Park in the Caribbean coastal lowlands of tion of methods used for assessment of habitat characteristics at Costa Rica's Limon province between 10° 20' and 10° 35' lat. N. sampling locations appears in Winemiller and Leslie (1992). The The region averages about 5.0 m of rainfall per year with two larger habitat, Cano Agua Fria Viejo (AF), is a sluggish braid of principal rainy seasons from July to August and November to the Rio Tortuguero. Some pools of AF exceed 3.0 m depth, but January (Fig. 1). Generally, no month receives less than 50 mm most stream reaches range between 0.5 and 2.0 m. AF exhibits little precipitation, and temperature shows little seasonal variation or no water current during dry periods, and salinity measurements around annual means ranging between 23-26° C (Hirth 1963). in Cano Agua Fria Viejo never reached 0.1 ppt during the 1985 During exceptional years, rainfall may excede 7.0 m at Tortuguero. study period. The substrate of AF consists of sand and patches of 268 coarse detritus, the latter being most common near the stream organ) were recorded as transitional/maturing. Femaleswithout margins under floating sedgesand associated aquatic plants. Dense dark pigmentation(gravid spot) near the baseof the anal fin were mats of floating aquatic plants (Eichhomia, Hydrocotyl, Salvinia, recorded as immature, whereas females with gravid spots and Azolla) sometimes cover the entire surface of stream segments stage-1ovaries were consideredmature body size class with im- during dry periods and are removed by faster currents during wetter mature ovaries.Many of the femalesin the latter group werepro- periods. During extended wet periods, large areas of the forest bably in a transitional stageof maturation. Relativeamounts of vis- bordering AF become flooded with standing water and are col- ceral fat depositsin males,females, and immatureswere estimated onized by several fish species, including poeciliids. following Winemiller's (1989) scalewhich rangesfrom 1.0 for no The other site, Quebrada (QB), is a small creek at the park's observablefat deposits within the coelomic cavity to 4.0 for a northern boundary on the barrier island that separates the sea and coelom filled with fat depositsthat produce a visible bulge in the the coastal lagoon, Laguna Tortuguero. Because QB has a neglig- belly region (scaleincremental units = 0.5, resulting in 7 possible ible gradient and drains direcly into the lagoon, its lower reaches values). are under tidal influence during much of the year. Salinity fluctua- Diametersof the largest oocytes from stage4.0 ovaries were tions were small « 1.0 ppt) in the stream reach adjacent to the measuredthrough the long axis using a dissectingmicroscope fitted lagoon, and salinity measurements from the upper reaches of QB with an occular micrometer.The stageof embryonicdevelopment and nearby forest pools never exceeded 0.1 ppt during 1985. The was classifiedaccording to the following criteria: (I) recently fer- substrate at QB was essentially the same as AF. Water current tilized, or embryoappears as a thin opaquewhite streak; (2) uneyed velocity was relatively slow (0-0.20 ms -1) on a year-round basis. embryo; (3) early eyedembryo; (4) intermediateeyed embryo; and Like AF, the forest adjacent to QB is covered with shallow standing (5) late eyed embryo=embrY9s approximately one week or less water during the wettest months. away from parturition. This index compareswith the index de- Fishes were collected using dipnets (1.0; 3.2 mm mesh) and velopedby Reznick (1981)for Gambusiain the following manner: seines(6.2 x 1.9 m, 4.7 mm mesh; 2.5 x 1.9 m, 3.2 mm mesh) at both I = Reznick0 and RIO, 2 = early portion ofR 20, 3=laterportion sites. A castnet (1.9 m dia., 12 mm mesh) was also used to capture of R 20 plus R 30, 4= R 40, and 5 = R 50. The number of embryos some of the larger Poecilia gilli and other large species from the in each ovary and, when present,the occurrenceof multiple em- deeper pools (> 1.5 m) at AF. Monthly field sampling did not target bryonic stageswithin an ovary were recorded.Number of develop- any particular fish species or size class; rather, the principal goal ing embryoswas usedas the estimateof brood size. was attainment of a large random sample of all species that ap- Stomach contents were analyzed following the methods in proximated the natural relative proportions of species at each site. Winemiller (1990).Most animal food itemswere identified to order, Each monthly sampling effort lasted from 0.5 to 1 day (QB) or from and plant and detritus categories were grouped according to 3 to 5 days (AF), and each day's effort was continued until an hour broader taxonomic and functional categories(e.g., spores,small of collecting produced no additional rare species for the sample seeds,vegetative detritus, diatoms, filamentousalgae). For Alfaro, (further details given in Winemiller 1990). The capture methods contents from the anterior half of the gut were identified, sorted, were effective for adult and larger immature size classes of all and quantifiedvolumetrically by water immersion.For detritivore/ poeciliid species, but the smallest immature size classes were un- algivores,Phallichthys and Poecilia,a small sampleof the contents derrepresented. Fishes were preserved in 10% formalin and later from the anteriormostgut was removedand placedon a wet mount transferred to 45 % isopropanol. Voucher specimens and records of slide and examinedunder a light microscope.Percentages of iden- sample sites and dates are on file at the Texas Natural History tified items were visually estimatedand the total volume of each Collection of the Texas Memorial Museum, Austin, Texas.~ consumeditem wascalculated based on the measuredvolume of the entire gut and contents(only for the region containingfood). Diets are comparedas percentvolumes of items consumed. C. Measurements,indices, diet analyis
All poeciliidswere measured for standardlength (SL) using vernier D. Data analysis calipers.For eachmonthly sampleand eachof the two study sites, 20-30 individual specimenswere selectedfor dissectionand exam- Seasonalvariation in speciesrelative abundancesin the two local ination of gonadsand stomachcontents. The selectionof specimens fish assemblageswas estimatedfrom monthly field samples.Stan- for dissectionsubsamples was based on size,and soughtto represent dard lengthdistributions of matureindividuals (femaleswith gravid the approximate size distribution in the larger sample.Whenever spots,males with fully formed gonopodia),distributions of relative fewer than 30 specimenswere collectedduring a particular month gonad condition for mature sizeclasses, the percentageof mature at a site, all of the specimenswere dissected. femaleswith embryos, and the numbers of juveniles in samples The condition of male and femalegonads was coded basedon (juveniles= SLs< the minimum SL for confirmed mature in- the relative size,texture (smooth,fine grainy, coarsegranular), and dividuals)were examined in relation to temporalvariation. Because color (clear, transluscentopaque) criteria of Winemiller (1989),in some monthly samplescontained fewer than 30 individuals of a which fully immatureand completelyregressed gonads are recorded given species/sexand the two streamsexhibited similar temporal as 1.0and ripe gonadsare codedas 4.0 (for males-large testeswith changesin habitat conditions (e.g.,habitat and food availability in a milky or creamy white appeareance;for females-large ovaries responseto rainfall), life-history analyseswere performedon com- with large vitellogenicoocytes). Most gonadsat stages1.5 through bined AF and QB monthly samplesin order to increasesample sizes 3.5 appearedto be in the processesof maturation; howeverregres- for comparisons.The distributions of body lengthsduring the wet sing gonads and post-parturition ovaries would fall within this (March-May, Oct.) and dry (June-Sept.,Nov.-Dec.) seasonsdid rangeas well. Briefly, the index is computedas follows for females: not differ significanty betweenthe two streamsin four out of six (size[l-small; 2-medium small; 3-medium; 4-medium large; 5- comparisons(i.e., 3 speciesx 2 seasons;ANOV A, P> 0.05). In large])+ (color/texture[l-clearor translucentyellow; 2-opaqueyel- addition, the distributions of gonadal indices were comparedbe- low with visible oocytes;3-opaque yellow with large mature oocy- tween sitesby species,sex, and season(yielding 12 comparisons), tes])/2. Gonad size criteria for males were the sameas those for and noneof the 12gonadal comparisons were significantly different females,and testiscolor/texture criteria were I-clear or translucent, (Kolmogorov-Smirnovtest, P> 0.05). 2-opaquewhite, 3-smoothmilky white. Gonadsof femalescontain- Relationshipsbetween body length and brood size were ex- ing embryosat any developmentalstage were recorded as condition plored using linear regression(least squaresmethod basedon In- 5.0. The gonadindex scalecontains 7 possiblevalues for malesand transformeddata) and relationshipsamong body length, gonadal 8 possiblevalues for females.Males that possessedincompletely condition, and visceralfat depositswere examined with Spearman's formed gonopodia (anal fin modified to serveas an intromittent correlation coefficient(rJ. For eachspecies, the minimum length at 269
maturity, maximum body length, brood size, mature oocyte dia- meter,relationship between female size and fecundity,and seasonal- diet shifts were determined.The effectof seasonon brood sizewas examinedby comparing slopesand interceptsof the female body size-broodsize regressionsfor each season.Relative consumption of different food categoriesduring different seasonswas compared using Chi Square(class entries are the volumetric proportions).
Results
A. Species relative abundances
Excluding consideration of the lower stream reach adja- cent to the lagoon, a total of 21 specieswas collected from the small creek, QB. Only three poeciliid species co iO ~ Alfaro cultratus, Phallichthys amates, and Poecilia gilli, ~ were continuously present during the entire 10 month "> ~ sampling period. Total numbers of individuals collected E- from QB during the study were 630 ( Alfaro), 533 (Phal- o Q; lichthys) , and 322 (Poecilia). Approximate numerical .0 E relative abundances based on the entire fish community ~ and 10 month sample period were 0.19 for Alfaro cul- z fratus, 0.16 for Phallichthys amates,and 0.11 for Poecilia gilli. Sample relative abundances of Alfaro change changed little with season(0.18 wet, 0.21 dry), whereas relative abundances changed approximately two-fold in 30., Phallichthys (0.11 wet, 0.21 dry) and Poecilia (0.15 wet, 10 Malas I Poecilia In 25- 11m Females I 0.07 dry) at QB. During the wet seasons,all three poec- ;; I ~ iliids were commonly captured from both stream channel "C "> 20- and flooded forest pools. :c E. Of the 57 fish speciescollected at AF, five were poec- - 15" iliids (Alfaro, Belonesoxbelizanus, Phallichthys, Poecilia, 0 ; Brachyrhaphisparismina). Total numbers of individuals .c 10" E '-C collected from AF during the study were 165 (Alfaro), ~ z I 142 (Belonesox), 51 (Brachyrhaphis) , 451 ( Phallich- ~ II I,," thys), and 313 (Poecilia). The numerical relative abun- I 0 II .,... l"" dances(estimated over 10 mo) were 0.12 for Phallichthys, ~45.0 50.0 55.0 60.0 65.0 70.0 75.0 >75.0 0.105 for Poecilia, 0.02 for Alfaro, 0.02 for Belonesox, and 0.005 for Brachyrhapis. Belonesox and Brachy- Standard Length (mm) rhaphis sampleswere too small for analysis of reproduc- Fig. 2. Frequencydistributions of sizeintervals of mature male and tive seasonality. The relative abundances of the three female Alfaro cultratus, Phallichthys amates, and Poecilia gilli common poeciliids changed little with season (Phallich- (speciesdata combinedfor all months and both streams) thys- 0.10 wet, 0.13 dry; Poecilia- 0.10 wet, 0.11 dry; Alfaro- 0.01 wet, 0.03 dry). AF poeciliids also dispersed into the forested floodplain during the wet seasons,how- Table 1. Life history data recordedfor three common poeciliidsat Tortuguero, Costa Rica. All units are in mm standardlength (SL); ever some individuals remained in the main stream chan- ranges are for smallest to largest SLs recorded for mature in- nel where they were frequenly observed and captured. dividuals; NAdultsis the total number of mature individuals exam- ined
B. Temporal pattern of reproductive indice. Smallest Largest Largest Rangesm-lg NAdults mature immature mature and population structure Alfaro The three common speciesdisplay a considerable range Males 24.7 31.8 50.3 25.6 150 of values betweenminimum and maximum adult sizesof Females 27.8 37.4 65.5 37.7 212 each sex (Table 1, Fig. 2), and the range was greater in females than males. Size frequency distributions were Phallichthy, unimodal for both mature males and females of all three Males 19.6 32.8 38.0 18.4 193 Females 21.6 35.9 55.0 33.4 328 species (Fig. 2). There were no statistically significant relationships between between body length and gonadal Poecilia index for males of any of the species(r. ranging from 0.02 Males 39.8 52.0 71.9 32.1 to 0.26, df range = 44-70, P> 0.05). Females had statis- Females 37.3 57.6 93.2 55.9 ~ 270
MAMJJASOND MAMJJASOND
Month Month Fig. 3. Mean gonadal index values(with standarddeviation bars) Fig. 4. Percentagesof pregnantfemales and the numberof juveniles for monthly samplesof mature male and female Alfaro cultratus, in monthly samplesof three live-bearing poeciliid fishes(species Phallichthysamates, and Poeciliagilli. Valuesfor malesmay range data grouped acrossstreams). Juveniles are definedas individuals betweenI (no evidenceof gonadalrecrudescence) and 4 (maximum < SL of the smallestmature male of the species(values appear in gonadaldevelopment) for malesand betweenI (no recrudescence) Table 1) and 5 (pregnant)for females(species data combinedfor all months and both streams) tically significant but low correlations for body length occurence of juveniles coincided with the second reduc- with gondal index (Alfaro r.=0.25, df= 130, P<0.05; tion in ovarian and embryo indicies. Phallichthys r.=0.43, df= 124, P 5'°1 Alfaro e 0 >- 0 .c 4.0 E 1-0-- Fornal.. I w 1--. - Moles I '0 3.0 a; .c E ~ ~ ~ 2.0j ...J . 1.0 i- e, , , . , M M D 3.2 3.4 3.6 3.8 4.0 4.: Ln (Female SL (mm) 5.0, °hallichthys -;; 0 >- .Q E w 4~o.1 '0 . . ~ 0 0 0 ; .0 E ~ 3.0' ~ .08 . c -' 2.0I ...... 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 Ln (Female SL (mm» 6.0" PoBcilia 0 ~ .c 5.0. E w 4.0 '0 Q; .c 3. E :. ~ 2 I: ..J . .. . 1.0I , ...... MAMJJASOND 3.9 4.0 4.1 4.2 4.3 4.4 4.5 4.6 Month Ln (Female SL (mm» Fig. 5. Monthly variation in the meanvisceral fat index in male and Fig. 6. Relationships between female body length and the number femalepoeciliids. (bars equal one standarddeviation, samplesizes of embryos for three poeciliids. Filled circles correspondto wet are the sameas in Fig. 3) season samples, open circles are from dry season samples. Correla- tions between In(standard length) and In(number of embryos) were as follows: Alfaro, wet, r2 = 0.77, F 1,42= 137.49, P C m u ~ ~ Detr. Diat. Filam. Micro.Aquat. Terr Delr. Dial. Filam. Micro. Aqual. Terr 100 ] Poecille, AF m Wet (N=82) 0 Dry (N=84) 80 ~ E ~ Fig. 7. Volumetric percentagesof aggregatediet ""0 > items consumedby three poeciliids at Quebrada (Q) and Agua Fria (AF) during the wet and C ~ dry seasons(Detr. = detritus,Diat. = diatoms, " ~ Filam.= filamentousalgae, Micro. =microcrus- Q. tacea,Aquat.=aquatic insects,Terr.=terrestrial arthropods. Samplesizes for eachseason appearin legendboxes Detr. Diat. Filam. Micro. Aquat. Terr. female-biased(ratio.s: 1.0), exceptfor two instancesin Table2. Seasonalcomparisons of the averagenumber of developing which samplesizes were very small (n< 10). embryosin ovariesof femalepoeciliids broken down by different minimum body length criteria (samplesizes in parentheses) Season Alfaro ( Season-length-fecundityrelationships All females Females>45 mm Females>55mm All three specieshad positive slopes in the relationship Dry 18.9(29) 26.0 (16) 26.9 (9) between female size and number of embryos (Fig. 6). Wet 27.2 (44) 44.3 (21) 66.7 (6) Slopes were significanly greater for females collected during the wet season compared with the dry season (Alfaro, t=3.44, df=53, p x=43), and 7 to 210 (Poecilia, x=75). The mean stan- during the rainy seasonfloods at both study sites. Even dard lengths of females containing embryos were though the total numbers of individual fishescould actu- 45.6 mm (Alfaro), 42.5 mm (Phallichthys) , and 69.7 mm ally increaseduring the wet seasons(via higher reproduc- ( Poecilia). Maximum diameters of mature oocytes were tive effort in females), densities on a per-area basis are 3.75 mm (Alfaro), 2.10 mm (Phallichthys) , and 3.00mm greatly reduced within the expanded floodplain habitat. (Poecilia) , and oocyte diameter was unrelated to female The available evidence for the three poeciliids sup- size in each case (P> 0.05). ports a hypothesis of year-round reproduction by these populations. However, there was also good evidencethat the condition of female gonads and proportions of gravid D. Seasonaldiet shifts femalesof the three speciespeaked just prior to or during Volumetric analysis of stomach contents revealed three the early phase of the two wet seasonsat Tortuguero feeding guilds among the five poeciliid speciescaptured (Figs. 3, 4). Males exhibited relatively little seasonal at the two study sites: algivorejdetritivores (Phallichthys, variation in mean testicular condition, although Phal- Poecilia) , terrestrial arthropod feeders (Alfaro, Brachy- lichthys and Poecilia showed low amplitude cycles that rhaphis) , and a piscivore (Belonesox). Phallichthys and were either in phase with, or slighly ahead of, those Poecilia each consumed significantly greater fractions of exhibited by conspecific females (Fig. 3). In all three detritus during the wet seasonsand greater fractions of species,the averageclutch size was larger during the wet diatoms during the dry seasonsat both the QB (Pallich- season,primarily becausethe largest females tended to thys X2=20.14, df=I, P Synchrony of reproductive effort with optimal conditions for juvenile growth and survival. With regard to physio- logical response to environmental cues, longer photoperiod was shown to increase the production of yolked eggsand embryos and increasegonadal indices of male Poeciliopsis gracilis at -140 N in El Salvador (Burns 1985). Photoperiod varies only slighly at Tor- tuguero ( - 80 N) but potentially could provide a cue for predicting future ecological conditions (Schwassman 1978). If the Tortuguero poeciliids possessphysiological mechanisms for adjusting reproductive output in res- ponse to perceived environmental cues, this would imply either that the physiological responsesare adaptive in the References current environmental setting, or that they are neutral or AnderssonM (1986) Geographicvariation and aspectsof the life maladaptive and derived from the ancestoral condition history of Belonesoxbelizanus Kner (Pisces:Poeciliidae) from and may have been adaptive in a historical setting. CentralAmerica. M.S. Thesis,Northern Illinois Univ., DeKalb, IL, USA Regardless of the proximate mechanism that may Balsano JS, Kucharski K, Randle EJ, Rasch EM, Monaco PJ yield greater reproductive effort during the wet season, (1981)Reduction of competition betweenbisexual and unisex- this strategy should be adaptive with respect to juvenile ual femalesof Poeciliain northeasternMexico. Env BioI Fish predation mortality. 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