Environmental Biology of Fishes Vol. 20, No.3, pp. 219-227,1987 <9 Dr W. Junk Publishers, Dordrecht. Feeding and reproductive biology of the currito, Hoplosternum littorale, in the Venezuelanllanos with commentson the possiblefunction of the enlarged male pectoral spines Kirk O. Winemiller Departmentof Zoology, The Universityof Texas,Austin, TX 78712,U.S.A Received 11.6.1986 Accepted 2.12.1986 Key words: Breeding phenology, Gonadal recrudescence,Sexual dimorphism, Food habits, Brood defense, Callichthyidae, Seasonality Synopsis For one year, Hoplosternum littorale were sampled monthly from Cano Maraca, a seasonalswamp-creek of the western llanos of Venezuela. H. littorale exhibited a more or less synchronized burst of reproductive activity that coincided with the onset of the rainy season. Spawning was preceded by gradual gonadal recrudescenceand a decline in visceral fat deposits during the 5 month dry season.The speciesis sexually dimorphic with adult males being of larger average size, attaining a larger maximum size, and possessing elongate, recurved pectoral spinesand fat deposits on the pectoral filJs just prior to a~d during the spawning season. Aquarium observations revealed that male H. littorale can use the enlarged pectoral spines as weapons during aggressiveattacks. This form of aggressivebehavior is most likely a component of the brood defense behavior of the species.Both immature and adult H. littorale were microphagousscavengers, taking a variety of food items from the soft mud substrate. Aquatic microcrustaceawere the major item in the diet of immature fish. During the late wet and dry seasons,61% of the stomachs sampled were empty and the intestines air- filled, indicating active use of the gut for aerial respiration. Introduction where research facilities operate (e.g. Greenwood 1974). Investigations of fish reproduction in tropi- Tropical freshwater habitats harbour the most di- cal lentic systems have demonstrated breeding verse and productive fish communities on earth, periodicity for several species,often in association yet relative to temperate systems, scant informa- with seasonalrainfall patterns (see Lowe-McCon- tion is currently available concerning their ecology ne111975,1979 for summary). Though rare, long- (for reviews see Roberts 1972, Lowe-McConnell term studies of reproduction by tropical fish species 1975). The reproductive biology of most tropical inhabiting relatively seasonal riverine environ- fish species is not well documented, particularly ments have revealed diverse tactics among sym- with respect to phenology (Schwassman 1978, patric taxa (Welcomme 1969, Kramer 1978, De Lowe-McConnell 1979). Most inferences con- Silva et al. 1985). cerning freshwater fish reproductive biology in the I collected fishes from a seasonalswamp'-creek in tropics are derived from studies of speciesused in the Venezuelan llanos over a period of twelve pond culture (e.g. Tilapia species, Lowe-McCon- months in 1984as a part of a comparative investiga- nell 1959) and species inhabiting African lakes tion of the trophic ecology of neotropical freshwa- 220 ter fish communities. Despite harsh conditions dur- current report were taken from a large, permanent ing the driest months (owing primarily to dissolved pool at the lower region of the estero. Figure 1 oxygen depletion), 35 specieswere common at the presents physiochemical data collected at this site.. site year-round. This report deals with the re- All measurements were made between 1500 and productive biology of the currito, Hoplosternum 1700h between the 22nd and 28th day of each littorale, in a seasonal tropical environment. H. month (except for December when the sampledate littorale is a member of the neotropical armoured was the 12th). Dissolved oxygen was determined catfish family Callichthyidae and servesas a popu- with a YSI oxygen meter, recalibratedprior to each lar food fish in the llanos. In addition to reproduc- sampling session.pH was determined with pH pa- tive phenology, the diet, unusual sexual dimor- per (MCB Reagents). Dissolved oxygen values are phism of the pectoral fins, seasonaldifferentiation missing for January and February becausea func- of the character, ~nd the probable function of the tional oxygen probe was not availaOte. All mea- enlarged male pectoral spine are documented surements were made at midpool in the deepest herein. region. The measurementsobtained from this pool are representative of values from two other loca- tions sampled within the estero. Methods Study site Data collection During 1984, fish collections were made at least Fisheswere collected by seine (3.2, 12.7mmmesh), once per month at several locations of the estero and dipnet and preserved immediately in 15% for- region of Cano Maraca in the state of Portuguesa, malin. Within one month, the specimens were Venezuela (8°52' 30"Lat.N; 69°27' 40" Long. transferred to 10% formalin and later to 45% iso- W). The region averages 1500m of rainfall per propanol. Each H. littorale specimen was mea- year. Cano Maraca is a second-order stream of the sured for standard length, dissected,and the entire Rio Apure-Rio Orinoco drainage system. The gut was removed. Contents from the anterior half 'estero' region is delimited by low-lying terrain that of the gut were examined under a dissectingmicro- experiences extensive sheet flooding during the scope, and sorted by functional category. The vol- wettest months (June-August). At this time, the ume of each category was determined by immer- estero is converted into a productive marsh with sion in graduated cylinders of various sizes. For abundant and diverse vegetation, the dominant volumes less than 0.005 ml, a value was estimated species being Eichornia spp., Heteranthera re- by spreading the item on a glassslide and compar- niforuris, Eleochoris interstincta, Paspalumrepens, ing it to a similarly spread substance known to and Cyperus sp. During the dry season(January- equal 0.005 mI. May), most of the estero dries up, and isolated The condition of the gonad was coded for each bodies of water remain in the scrub forest region of specimen basedon size and color, using the follow- the upper estero and main channel of the lower ing categories; clear = 1,translucent = 2, opaque= creek. The dry season pools were generally 3, small = 1, medium/small =2, medium = 3, me- blanketed by a layer of aquatic vegetation domi- dium/large = 4, large = 5. The gonad code for each nated by Pistia stratiotes, Ludwigia helminthor- individual was equal to (color code + size code) -7 2 rhiza, Salvinia spp., Hydrocotyl ranunculoides, and values ranged from 1 to 4. The same criteria and various duckweed species (Lemnaceae). The were used in coding male and female gonadswhile substrate of the estero and creek channel consisted taking into account intersex differences in the size, of a 10-15cm layer of soft mud and vegetative color, and texture of the fully mature gonads. The detritus over a clay foundation. fat content of the body cavity was coded using the All of the Hoplostemum specimensused for the following criteria: 1 = none, 1.5= tracesof fat in 221 Fig. 1. Seasonaltrends in dissolved oxygen concentration, water temperature, pH, maximum pool depth, and water current velocity measured at the pool where Hoplosternum littorale were collected during 1984(circles correspond to measurementsmade at the surface and triangles to measurementsmade at the pool bottom). the connective tissue around the viscera, 2 = small monthly collection were depositedin the Natural amounts around the viscera and traces in the con- History Collection of the Texas Memorial Mu- nective tissue of the coelomic cavity lining, 2.5 = seum, Austin, Texas. Additional voucher speci- moderate deposits around the viscera and a thin mens were depositedin the Museo de Historia layer on the coelomic cavity lining, 3 = large depos- Natural de UNELLEZ, Guanare,Portuguesa. its around the viscera and the coelomic cavity lin- ing, but not filling the coelom, 3.5 = very large deposits filling the coelom, but not producing a Results visible bulge, and 4 = the coelom packed with fat deposits, producing a visible bulge of the belly Breeding phenology and maturation region. Fecundity was estimated for 3 gravid fe- males by removing the ovaries from each, counting Hoplosternum littorale were collected every month the number of mature oocytes (defined as yolk- of the year except September. Most individuals laden oocytes having a diameter within 0.2 mm of were collected during the dry season(Fig. 2) ~hen that of the largest oocyte in the ovary) in a sample the local stock was concentrated at higher densities of ovarian tissue, weighing the sample, and weigh- in drying pools. Adult size classesof H. littorale ingthe ovary. were not collected during the peak wet period For selected specimens, the length of the pec- (June-Sept., Fig. 2) when the survivors from the toral spine was measuredin a straight line from the dry seasonpools were widely dispersedat low den- juncture of the spine with the outer body surface to sities in the flooded estero. The initiation of repro- its tip. The diameter of the spine was measured at duction by H. littorale coincided with the heavy the midpoint of its length (all measurementsper- rains that mark the onset of the rainy season in formed with calipers to the nearest O.Olmm). No early June. On June 13, several nests were ob- fewer than two H. littorale specimens from each served in the shallow, flooded estero region among 222 .!! 0 ..,~ :~ .., .~ ~ E- O E .§ 0~ Fig. 2. Monthly variation in the percentage of immature to total individuals (e), and the mean standard lengths (~) of Hoplosternum littorale at Cano Maraca. The vertical dashed line marks the abrupt onset of the rainy season. 4 4 3 " II 3 GO -- .., , If/~ 0 I XI u "T1 I Q '" 2 - -g 2 I c ). (") 0 I 0 t!) Co ~i ~ IX ! '~-6-~"" -:-.-+ - , , I I I I, I I , I I , - JFMAMJJASOND N= 8 13 16 20 34 7 6 3 - 2 7 22 XMa--OyumOla.062 1-021.631.691.69- - - - - - 0.85 (SO)0.070_220.120-300.08 - - - - - - 0.07 Fig.