Jellyfish 35 



mer where it has never before been observed or where it has not been observed 

 for many years (Slobodkin and Bossert 1991). Lentic systems in which the 

 medusae have been observed include reservoirs, natural lakes, ponds, quarries, 

 ornamental pools, and aquaria. 



Specific environmental factors associated with the formation of 

 medusae from polyps via asexual reproduction (budding) are poorly understood. 

 Factors suggested include increasing water temperature (McClary 1959) 

 increased alkalinity (Koryak and Clancy 1981, McCullough et al. 1981), 

 increasing dissolved CO2 (Acker and Muscat 1976), decreasing stream flow 



(Brussock et al. 1985), changing reservoir levels (Deacon and Haskell 1967) and 

 increasing supply of zooplankton (Lytle 1959), on which the medusae prey. 



Dispersal of C. sowerbyi among water bodies probably occurs via 

 polyps attached to aquatic plants or waterfowl, or in tanks used to transport fish 

 (Byers 1945, Bushnell and Porter 1967, Howmiller and Ludwig 1970). The 

 polyps can survive in moving water (Hutchinson 1967), so once the polyps enter 

 a river system, the medusae may eventually appear in downstream reservoirs 

 (e.g., Yeager 1987). 



Because the medusae occur unpredictably and the polyps are micro- 

 scopic and easily overlooked, the complete geographic distribution and ecology 

 of C. sowerbyi are not well known. Field studies have been mostly descriptive 

 (e.g., Garman 1916, Deevy and Brooks 1943, Dexter et al. 1949, Chadwick and 

 Houston 1953, Bushnell and Porter 1967, Koryak and Clancy 1981, McCullough 

 et al. 1981, Dodds and Hall 1984). Deacon and Haskell (1967) examined diel 

 activity patterns of medusae at Lake Mead, Nevada. Dodson and Cooper (1983) 

 examined trophic relationships of the medusae in the laboratory. Acker and 

 Muscat (1976) and DeVries (1992) reviewed the literature on the ecology of C. 

 sowerbyi. 



The purpose of this paper is to describe a swarm of C. sowerbyi 

 medusae I observed at Stonewall Jackson Lake, Lewis County, West Virginia, in 

 August and September 1995. My initial observations of the medusae at 

 Stonewall Jackson Lake suggested that the size distribution of medusae varied 

 with distance from the main concentration of medusae (swarm). I hypothesized 

 that the distribution of medusae resulted from dispersion from the apparent pop- 

 ulation center at the swarm, and I predicted that medusae collected away from 

 the main swarm location would be larger than medusae within the swarm 

 because more distant medusae would have had more time to grow. The null 

 hypothesis that medusae collected from all locations have the same size class dis- 

 tribution implies that the dense concentration of medusae at the swarm location 

 results primarily from aggregation due to water chemistry, temperature, food, 

 current, wind, or some other factor rather than from dispersion. I collected and 

 measured specimens to test this hypothesis. I also compared the mean size of 



