Some physiological and ecological aspects of the penetration into water of reduced salinity by certain members of the Ophiuroidea

Abstract

The problems of penetration into brackish water by echinoderms are reviewed. A comparison is made between Loch Etive and the Baltic Sea with respect to their individual hydrographies. Experiments indicate that ophiuroids behave as osmo conformers and that while Ophiocomina nigra is stenohaline, Ophiura albida is potentially euryhaline. It is argued that the perivisceral fluid of ophiuroids is similar, ionically and functionally, to the ambulacral fluids of asteroids and echinoids. Salinity tolerance experiments confirm that O. albida is potentially euryhaline. Differences in tolerance between O. albida and Amphiura chiajei are related to their modes of life. Evidence suggests that acclimatisation by O. albida can extend the tolerance to diluted sea water. It is shown that O. albida, from Loch Etive, can be adapted to full strength sea water rapidly and without subsequent morphological deleterious effects. Possibilities resulting in the presence and maintenance of O. albida at the head of Loch Etive are discussed. It is considered that an increase in the percentage of skeleton in ophiuroids is important in the development of tolerance to reduced salinity. A proposal that O. albida from the Oresund may represent a more advanced stage than O. albida from Loch Etive in adaptation to brackish water is made. Evidence suggests that smaller animals have higher Sr/Ca atom ratios than larger individuals of the same species. This may be manifested by a faster growth rate in young animals or by the physical constraints of the incorporation of the strontium ion into the calcite lattice. Data from O. albida (Loch Etive) demonstrates that there may be some concentrating mechanism in these ophiuroids. Discrimination ratios between the Sr/Ca atom ratio in ophiuroids and the ambient medium are calculated. In an elevated ambient strontium level ophiuroids may take up the 'extra' available strontium to different degrees which may partly reflect the original ambient strontium concentration to which the particular species are adapted.