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Density Is Altered in Hydromedusae and Ctenophores in Response to Changes in Salinity

Claudia E. Mills
Biological Bulletin
Vol. 166, No. 1 (Feb., 1984), pp. 206-215
DOI: 10.2307/1541442
Stable URL: http://www.jstor.org/stable/1541442
Page Count: 10
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Density Is Altered in Hydromedusae and Ctenophores in Response to Changes in Salinity
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Abstract

Laboratory experiments have determined the behavioral and gross physiological responses of hydromedusae and ctenophores subjected to sudden changes of salinities in the range that might be encountered in nature. Nine species of hydromedusae (Aequorea victoria, Aglantha digitale, Bougainvillia principis, Gonionemus vertens, Phialidium gregarium, Polyorchis penicillatus, Proboscidactyla flavicirrata, Sarsia tubulosa, Stomotoca atra) and two species of ctenophores (Bolinopsis infundibulum, Pleurobrachia bachei) were transferred from natural sea water of 30.5‰ to modified sea waters of 19-38‰. Most species altered their density within a few hours by osmoconforming to salinities ranging from 23-38‰, so that equilibrium buoyancy (either positive, neutral, or negative, according to species) was regained along with normal behavior. Even salinity differences of only 1-2‰ required 30-60 minutes adjustment time. Prior to regaining their equilibrium buoyancies, differences in relative density caused medusae and ctenophores to sink when introduced to low salinity water and to float in high salinity water. Hence, simple density differences combined with the natural intermittent swimming behavior of hydromedusae suggest that in many cases medusae may not actually be able to cross sudden density gradients. In the event that a medusa or ctenophore is moved into water of a different salinity, however, its ability to adjust will allow the animal to resume normal swimming and feeding activities within a short time.

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