Differences in ion regulation in the sea urchins Lytechinus variegatus and Arbacia lixula (Echinodermata: Echinoidea)

The regular sea urchin Lylechinus van. egatus, a species previously reported from areas of reduced salinities, and Arbacia lixula, a species unreported from diluted waters, were submitted to seawater dilution or seawater dilution in magnesium- supplemented waters. Seawater (35 psu) was either proportionally diluted with filtered dechlorinated tap water (30 psu, 25 psu), or diluted and supplemented with magnesium as MgCl(2) (30+Mg, 25+Mg), up to full-strength seawater Mg(2+) levels (35 psu, similar to 54 mM Mg(2+)). Magnesium supplementation was intended.to verify the interfering effect of magnesium on osmo-ionic concentrations of the coelomic fluid (CF) of two ecologically distinct species of sea urchins. After 6 h in control (35 psu) or experimental seawater, CF samples were withdrawn by puncturing through the peristomial membrane. Coclomic fluid osmolality ([Osm]), and concentrations of ([Na(+)]), ([Cl(-)]), ([Mg(2+)]) and ([K(+)]) were measured for both species. Under all conditions, L. variegalus displayed higher CF osmolality, [Na(+)], and [K(+)] values than the water (and A. lixula). Comparatively, L. varlegalus is designated as a'hyper-conformer', while A. lixula is an 'Iso -conformer'. The CF [Mg(2+)] showed no evidence of being controlled by either species. Mg(2+) Supplementation in diluted seawater affected Mg(2+) and Cl(-) levels only. Na(+) appears to be taken up actively by L. variegatus. rendering its CF mostly hyper-ionic for Na(+) (and hyperosmotic) relative to external seawater. The different gradients observed with the different ions suggest selective permeabilities or ion regulation by L. variegatus.