Density regulation inSarsia tubulosa (Hydrozoa)

The majority of wild-caughtSarsia tubulosa M. Sars medusae are less dense than the surrounding water. The bell ofS. tubulosa is the buoyant structure; the tentacles and manubrium sink if cut off from the bell.S. tubulosa individuals placed in dilute seawater sink initially but recover positive or neutral buoyancy and normal activity within a couple of hours. In all cases observed animals were able to achieve positive buoyancy in seawater of 20.25 S and some individuals were able to adjust to lower salinities. In most cases where positive buoyancy was not attained within two hours the animal did not achieve positive buoyancy within twelve hours and died within that period. While the mechanism of regulation is not known, ionic pumping, possibly involving the extrusion of sulphate ion, has been suggested to be responsible for the buoyancy of mesoglea in other jellyfishes.