Ecology of Brine Shrimp from Great Salt Lake, Utah, U.S.A. (Branchiopoda, Anostraca)

ECOLOGY OF BRINE SHRIMP FROM GREAT SALT LAKE, UTAH, U.S.A. (BRANCHIOPODA, ANOSTRACA)

ORLANDO CUELLAR Department of Biology, University of Utah, Salt Lake City, Utah 84112, U.S.A.

RESUMEN El ciclo estacional de Artemia del Gran Lago Salado (Utah, U.S.A.) fue estudiado durante 1978-1979, 1981 y 1985-1986. El ciclo empieza en la primavera con la salida del cascarón de los huevos invernales (quistes). La primera generación de hembras hecha una nidada a finales de mayo, produciendo la segunda generación de adultos a mediados de julio. El porcentaje sexual de los primeros adultos es casi igual. Empezando a mediados de julio, la segunda genera- ción de hembras forma el sexo dominante, sobrepasando a los machos de 66 a 91 % hasta mediados de agosto, después del cual las hembras declinan hasta 38 % en septiembre. Análisis estadístico del porcentaje sexual entre los camarones juveniles revela que las hembras sobre pasan a los machos significantemente, sugeriendo que las hembras adicionales fueron derivadas partenogenéticamente. Tales hembras puedan acelerar la producción de quistes para la siguiente primavera.

INTRODUCTION

The brine shrimp Artemia consists of several species') inhabiting saline lakes or coastal lagoons throughout the world (Clark & Bowen, 1976; Browne & Mac- Donald, 1982). Brine shrimp have been studied extensively in the laboratory concerning many aspects of their biology: reproduction (Browne et al., 1978), physiology (Engel & Angelovic, 1968; Herbst & Dana, 1980), growth (Gilchrist, 1960; Mason, 1963), nutrition (Reeve, 1963; D'Agostino, 1980), and karyology (Barigozzi, 1974). In comparison, relatively little is known about their natural ecology (Persoone & Sorgeloos, 1980; Brown & Mac- Donald, 1982; Dana, 1984; Lenz, 1984). In this regard, Brown & MacDonald (1982) state: "A list of Artemia references extends to over 3000 publications but only a small fraction are concerned with ecology or evolution." Despite the worldwide distribution of Artemia, most detailed ecological studies have been conducted in only two lakes, Mono Lake in California (Mason, 1967; Winkler, 1977; Lenz, 1980, 1984; Dana & Lenz, 1986), and the Great Salt Lake in Utah (Wirick, 1972; Gillespie & Stephens, 1977).

1) The Utah species is known as Artemiasalina (Linnaeus, 1758) but Verrill (1869) later des- cribed it as Artemia gracilis. 26

Aside from the unique ability of brine shrimp to inhabit highly saline condi- tions, an equally interesting feature of their ecology is the absence of males among Old World species. Approximately 70% of the populations studied so far reproduce exclusively by parthenogenesis. On the other hand, reproduction in the Western Hemisphere is strictly sexual (Browne & MacDonald, 1982). Given the almost identical physical environments of brine shrimp throughout the world, the lack of parthenogenetic populations in the Western Hemisphere poses a most intriguing biogeographic anomaly, which prompted Browne & MacDonald (1982) to query: "The accompanying question is why parthenogenesis does not similarly dominate in the New World." Yet, Jensen as early as 1918, and then Relyea in 1937 had reported that brine shrimp from the Great Salt Lake in Utah could reproduce by parthenogenesis. However, according to Bowen (1962), "These authors did not provide sufficiently detailed accounts of their experiments to replicate them." Attempts by Bowen (1962) to hatch the eggs of virgin shrimp from Utah failed, prompting her to refute the preliminary reports of Jensen and Relyea. Later, Flowers & Evans (1966) also mentioned parthenogenesis in the Utah shrimp: "It has been observed that males are scarce and that parthenogenetic develop- ment is common." A more recent study by Wirick (1972) on the ecology of shrimp from the Great Salt Lake revealed substantial seasonal variations in sex, females comprising as much as 73% of the adult population. It is now known that certain species of Artemia in the Old World consist of a mixture of parthenogenetic and sexual strains, with the parthenogenetic strains favored during the hottest part of the season when salinity is highest (Browne & Mac- Donald, 1982; Amat, 1983). The unusually high proportion of females reported by Wirick (1972) could be interpreted as evidence of parthenogenesis, but he did not suggest it, nor did he mention the dates when his samples were obtained. Beginning in 1978 I conducted a series of collections from the Great Salt Lake to corroborate the reported variations in sex, and to correlate such findings with the possibility of seasonal parthenogenesis in a New World Brine Shrimp. This paper reports the results of collections conducted during 1978- 1979, 1981 and 1985-1986.

MATERIALS AND METHODS

During 1978, 1979 and 1981 the shrimp were collected from the south shore of the lake by a small peninsula known as Black Rock located about 5 kilometers west of the Great Salt Lake Boat Harbor. All collection dates and number of shrimp counted per sample are listed in table I. The shrimp were collected with a 30 cm plankton net ( mm mesh), preserved in 10% aceto- formalin, and transferred to 50% ethanol. Only adult shrimp were enumerated in these samples. Males and females were sorted randomly until approximately 1000 individuals were counted per sample (table I). Mature