STUDIES ON (, ) OF SRI LANKA. IV. SOME ASPECTS OF THE POPULATION ECOLOGY OF THE ENDEMIC FRESHWATER SHRIMP CARIDINA PRISTIS ROUX, 1931

BY

K. H. G. M. DE SILVA Department of Zoology, University of Peradeniya, Peradeniya, Sri Lanka

RÉSUMÉ Une population de Caridinapristis dans un petit cours d'eau a été échantillonnée chaque mois pendant deux ans. On a constaté que la reproduction était continue, mais avec une saison de ponte maximale pendant et immédiatement après la haute saison des pluies, ceci entrainant un recrutement maximal de la population. Une corrélation négative significative a été observée entre la densité de la population et la proportion des femelles ovigères présentes. Les mâles semblent croître plus lentement que les femelles, les tailles maximales atteintes par les mâles et les femelles étant respectivement de 18 et 19 mm. La durée de vie probable de la crevette ne dépasse pas un an. Le rapport mâle/femelle de la population était de 0,84, mais ce rapport était plus proche de 1,0 dans les classes de petite taille, et supérieur à 1,0 dans celles de taille moyenne. Les femelles développaient de multiples pontes et le nombre d'oeufs produit croissait avec la taille des ces femelles, s'échelonnant de 7 à 28. Les relations entre la longueur du corps, la longueur de la carapace et le poids sont présentées, ainsi que d'autres données démographiques.

INTRODUCTION

Atyid shrimps are common in the tropical and subtropical inland waters and form an important component of the littoral fauna. However, the population ecology of atyids is little known. The few works dealing with aspects of popula- tion ecology of atyids are Kamita (1956, 1956a, 1957b, 1958, 1959), and Shokita (1979) on several species of Caridina and Paratya compressa (De Haan, 1844) in Japan, Williams (1977) on Kemp, 1917, in southern , and Hart (1981) on C. nilotica (P. Roux, 1833) in South Africa. The present work examines the seasonal changes in abundance and popula- tion structure of Caridina pristis J. Roux, 1931, in a small stream. This small shrimp is endemic to Sri Lanka and is found in slow flowing streams between 300 and 1000 m altitude (De Silva, 1983). Two subspecies, namely, C. p. pristis J. Roux, 1931, and C. p. cruszi De Silva, 1982, have been described from dif- ferent parts of the island (De Silva, 1982). This study deals with the former subspecies. 226

MATERIALS AND METHODS - Habitat. The shrimps were collected from a small, permanent stream (07 ° 15' 30 "N 80 ° 35' 45 "E) situated within the campus of Peradeniya Univer- sity. The stream is approximately 0.5 km long, 25-50 cm wide at most places and 10-30 cm deep, but overflows this channel in the rainy season. The bottom is usually covered by mud and leaf litter, and the shrimps were often seen resting and feeding on the decaying leaves. Previously a tributary of the river Mahaweli, the stream was diverted in 1950 into an artificial pond. C. pristis was found along the length of the stream but not in the pond. Methods. - Monthly samples were collected from the stream over 2 years from May 1979 to April 1981. Ten locations were selected at regular intervals along the length of the stream. An area of 1.0 x 0.5 m2 was sampled at each station using a hand net with a 1.0 mm mesh size. Routine sampling was car- ried out by netting for 30 seconds because test sampling indicated that drawing the net repeatedly for about 30 seconds was sufficient to collect almost all the shrimps in a sampling area. The same collections were put into polythene bags with water and were taken to the laboratory, 200 m from the stream. The shrimps were picked from the debris in the laboratory and preserved in 70% ethyl alcohol. The body length of each shrimp, taken as the distance from the posterior margin of the orbit to the end of the telson excluding setae, was measured to the nearest 0.1 mm under a dissecting microscope. Carapace length, from the posterior margin of the orbit to the posterior margin of the carapace, was also measured to the nearest 0.1 mm in a subsample of shrimps from each monthly collection. In order to obtain length/weight relationships, a sample of shrimps was selected to cover the length range, and each shrimp was measured to 0.1 mm and was then weighed to the nearest milligram after drying with filter paper. The same individuals were also used to obtain individual dry weights to the nearest 0.1 mg after drying to constant weight at 60 ° C . The sex was determined by examining the second pleopod for appendix masculina. The eggs were removed from each ovigerous female and counted. The length, breadth and developmental stage of each egg were noted. During the first six months of the study, water temperature, pH, and dissolved oxygen concentration were measured at the time of sampling at all ten stations using portable electronic meters (W.P.A. (U.K.) E1 Environmen- tal Multiprobe and WTW (West Germany) Model 56 oxygen meter). As the measurements from the ten stations were similar, data were obtained subse- quently from only the sixth station, which was located near the mid-point of the length of the stream. Mean monthly water temperatures were calculated from weekly maximum and minimum temperatures recorded at Station 6. Mean monthly air temperatures and monthly rainfall were calculated from the daily data obtained from the Meteorological Office at the Central Research Institute of Agriculture, Gannoruwa, situated about 2 km from the stream.