The Abbreviated Larval Development of Caridina Singhalensis Ortmann, 1894 (Decapoda, Atyidae)

Abstract

The highly abbreviated development of Caridina singhalensis, endemic to Sri Lanka, is described. The peculiar morphological features found in the larvae are compared to those of the two other examples of direct development described in the Atyidae. The marked morphological similarities among the reduced larval phases, independently evolved in C. singhalensis and the Japanese species (C. brevirostris and C. denticulata ishigakiensis), allow the identification of a syndrome associated with direct development in the genus. Caridina singhalensis Ortmann, 1894, is endemic to the highlands of Sri Lanka where its present distribution is restricted to a six mile stretch of the headwaters of the Belihul Oya (6?48'N, 80?48'E). This species lays very large eggs compared to other caridinids and undergoes a direct development. While a number of descriptions of larval development are available for the genus Caridina (Babu, 1963; Benzie, 1982; Chinnaya, 1974; Daday, 1907; Glaister, 1976; Lakshmi, 1975; Nair, 1949; Pillai, 1975; Shen, 1939; Shokita, 1973a, 1976, and 1981), only two examples of direct development have been reported previously-C. brevirostris (Shokita, 1973a) and C. denticulata (Shen, 1939; Mizue and Iwamoto, 1961; Shokita, 1976). (The status of C. denticulata subspecies is not clear and the larval description of Shen (1939) may refer to a species different from those described by the other authors. If the specific difference between these forms is upheld then the number of direct developments described in the Atyidae previous to the present paper would be three. Details are given in the discussion.) The development of C. singhalensis is of interest not only as the third direct development reported for the Atyidae but also because some of the morphological changes associated with extremely abbreviated development have progressed further in this species compared to those described to date. In addition the first stage larva shares a number of peculiar features with C. brevirostris and C. denticulata, although clearly of separate origin, and the description of this form allows the identification of a syndrome associated with direct development in the Atyidae. The present paper describes the larval development of C. singhalensis and discusses these features. Ovigerous females were collected from the Belihul Oya at Black Bridge on Horton Plains (1 inch Haputale Map Sheet M, grid ref. 204179) in February/March 1980 and transported to laboratories in Peradeniya University, Kandy. Each female was placed in a small 10 ml container and checks were made several times daily to establish the presence of larvae. While a number of eggs hatched successfully, none of the larvae moulted to stage II and all died within a few hours of hatching. Mortality was in all probability due to high temperatures in the laboratory. No controlled temperature facilities were available and ambient temperature was approximately 30?C, far in excess of the temperatures of approximately 17-20?C to which the animals are exposed naturally in the highlands. Samples of larvae from field populations at Horton Plains, where C. singhalensis is the only prawn, were therefore obtained and moult stages inferred from size and morphological differences. Individual larvae were preserved in 70% alcohol and later dissected in polyvinyl lactophenol, stained with lignin pink. Drawings were made with the aid of a camera lucida and measurements made using a compound microscope and ocular graticule. Carapace measurements were made from the antero