Observations on the Ecology of the Freshwater Jellyfish in Lake Mead, Nevada

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Abstract
Observations and collections of Craspedacusta sowerbyi Lankester were made at about 1-week intervals from 25 Oct. 1962 to 25 Jan. 1963 in Lake Mead, Nevada. The medusa population was most concentrated in deeper water in the morning. Vertical distribution became nearly uniform by midafternoon. Numbers of medusae at the surace were very low at night and increased rapidly after the sun struck the water surface. Laboratory observations indicate an attraction to light as distinct from a stimulation to activity by light. Females were always more abundant than males, but the number of males was highest on 2 Dec. 1962. Medusae first appeared in the lake in June 1962. The population became abundant in late Sept. and disappeared at the end of Jan. 1963. Above-average water temperatures and a rapid rise in water level to an unusually high level are correlated with the development of the medusa population. Complete disappearance of males and decline of the population correlate with water temperatures below 60 F.

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