EVIDENCE THAT ION REGULATION IN HYDROMEDUSAE AND CTENOPHORES DOES NOT FACILITATE VERTICAL MIGRATION

Abstract

Medusae and ctenophores, like many types of gelatinous zooplankton, actively exclude sulphate ion from their mesogloeal body fluid and thus gain lift(buoyancy). It is hypothesized that vertically migrating species might show day/night variations in the rate of sulphate elimination to regulate buoyancy dynamically and that such changes are a major factor in vertical migration. To test this hypothesis, a series of laboratory experiments were conducted using medusae and ctenophores. Concentrations of radioactive sulphate were measured in equilibrium (uptake) experiments and concentrations of sodium, magnesium, potassium, and calcium ions were measured using atomic absorption spectrophotometry. No evidence of day/night (light/dark) changes in ion concentrations were found for the hydromedusae Aequorea victoria, Aglantha digitale, Gonionemus vertens, Mitrocoma cellularia, Phiaildium gregarium, Polyorchis penicillatus, Sarsia tubulosa, and Stomotoca atra or for the ctenophore Pleurobrachia bachei. It is concluded that changes in ionic regulation are not a major factor in diel vertical migration. It is thus hypothesized that for these animals vertical migration is accomplished solely by swimming.