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

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 digitate, 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%o to modified sea waters of 19-38%o. Most species altered their density within a few hours by osmoconforming to salinities ranging from 23-38%o, so that equilibrium buoyancy (either positive, neutral, or negative, according to species) was regained along with normal behavior. Even salinity differences of only l-2%o 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.

Fine-scale layer of hydromedusae is revealed by video plankton recorder (VPR) in a semi-enclosed bay in northern Norway

“…Many zooplankton taxa, including gelatinous zooplankton, show a general tendency to aggregate in association with temperature and salinity discontinuities (Arai 1992, or remain in convergent structures such as Langmuir cells (Hamner & Schneider 1986). At rest in surface waters, most hydromedusae species are negatively buoyant and sink slowly, and in order to remain at a specific depth they need to attain neutral buoyancy (Mills 1984). Hydromedusae are slow osmoconformers, and when they encounter abrupt salinity differences they may be constrained within these interfaces for hours until osmotic adjustment has occurred (Mills 1984, Mills & Vogt 1984.…”

“…At rest in surface waters, most hydromedusae species are negatively buoyant and sink slowly, and in order to remain at a specific depth they need to attain neutral buoyancy (Mills 1984). Hydromedusae are slow osmoconformers, and when they encounter abrupt salinity differences they may be constrained within these interfaces for hours until osmotic adjustment has occurred (Mills 1984, Mills & Vogt 1984. Buoyancy regulation in medusae can also be achieved by swimming (Arai 1973(Arai , 1976, actively excluding sulphate ions from the mesogloea in order to descend (Mackay 1969), or accumulating lipids in the gastrovascular tract to provide lift (Larson & Harbison 1989).…”

Visual pigment in the lens eyes of the box jellyfish Chiropsella bronzie

“…The edges of the top coverslip were sealed with nail varnish to prevent dehydration and movement of the tissue. Sea water was deemed an appropriate medium based on the isotonic nature of jellyfish (Mills 1984) and because electrophysiological recordings have been successfully carried out in sea water (Weber 1982;Garm et al 2007a). When using whole rhopalium preparations, the orientation of the rhopalium was carefully monitored before and after squashing, and the upper and lower lens eyes were identified using both orientation and morphology.…”

Intragel oxygen promotes hypoxia tolerance of scyphomedusae

“…The internal gel milieu is a dynamic environment. It accommodates buoyancy changes due to salinity shifts (Mills, 1984;Wright and Purcell, 1997), and gel likely provides important ions to musculature (Anderson and Schwab, 1981). We now know that gel also plays a key role in supporting oxygen delivery to tissues.…”