Jellyfish 



39 



lection site, a significant difference (Kruskal-Wallis ANOVA on Ranks; H = 

 75.1, P < 0.01). Among sites outside the swarm location, there was also a trend 

 of increasing size with distance from the swarm; mean medusa diameter was 

 largest 350 m from the swarm, the location at which medusae were first observed 

 when entering Wolf Fork from the reservoir. Within the swarm there was a 

 bimodal size distribution of medusae concentrated in 9 - 13 mm and 15-18 mm 

 size classes, with more medusae in the smaller size classes. Away from the 

 swarm most medusae were in the larger size classes, but a semblance of the 

 bimodal pattern was apparent (Fig. 2). 



Fig. 3 Size distribution of C. sowerbyi medusae collected and 100 m from a 

 dense concentration (swarm) of medusa at Wolf Fork, Stonewall Jackson Lake, 

 West Virginia on 18 August and 12 September 1995. Otherwise as for Fig. 2. 



18 August 1995 



4 6 8 10 12 14 16 18 20 22 4 6 8 10 12 14 16 18 20 22 



Medusa diameter (mm) 



On September 12, I observed fewer medusae than on previous visits to 

 Wolf Fork. Water temperature was 25 C, and reservoir level had dropped about 

 1 m since the previous visit. The sky was overcast and light rain was falling, 

 whereas the sun shone on previous visits. Medusae were most abundant near the 

 18 August swarm site; at 100 m they were scarce, and were not observed else- 

 where. Mean diameter of medusae was the same at both locations (Fig. 3), 

 although more medusae from the 100 m site were in the largest size classes (20- 

 21 mm). Between August 18 and September 12, mean diameter of medusae at 

 the swarm location increased slightly more than 4 mm (Fig. 3). This is equiva- 



