The effects of tidal fluctuation of salinity on the perivisceral fluid composition of several echinoderms

Osmolality and ions of the perivisceral coelomic fluid of the intertidal sea urchin Echinometra lucunter (Echinodermata: Echinoidea) upon salinity and ionic challenges

“…The subtle, however significant signal of Mg 2+ and K + gradients (6-8%) maintained by E. lucunter may be related to the physiological relevance of both cations, and are well supported by the literature, that has quite often highlighted the maintenance of even much steeper gradients for these two cations, and also for Ca 2+ (STICKLE & AHOKAS 1974, STICKLE & DENOUX 1976, SHUMWAY 1977, PAGETT 1980, ROBERTSON 1980, DIEHL 1986, STICKLE & DIEHL 1987, BISHOP et al 1994, VIDOLIN et al 2007. Besides its general widespread action as a cofactor for various enzymes, and in several other intracellular roles (SARIS et al 2000), Mg 2+ is a major constituent of the sea urchin test, as aragonite (MgCO 3 ) (MILES et al 2007).…”

“…These three cations show higher positive gradients in echinoderm CFs, than sodium or chloride (Figs 7-12). In summary, different ions are treated differently, and different species of echinoderms respond differently to a same salinity challenge (e.g., ELLINGTON & LAWRENCE 1974, STICKLE & AHOKAS 1974, STICKLE & DENOUX 1976, SHUMWAY 1977, VIDOLIN et al 2007, BARKER & RUSSELL 2008. Most ionic gradients are indeed already dissipated after 6-8 hours in echinoderms (MADRID et al 1976, DIEHL 1986, STICKLE & DIEHL 1987, but there are species that maintain significant ionic differences, as clearly shown in Figs 7-12.…”

“…In other words, it takes longer for larger urchins to dissipate any gradient created upon a SW salinity change, in a so-called "damping effect" (STICKLE & AHOKAS 1974, STICKLE & DENOUX 1976, SHUMWAY 1977, HIMMELMAN et al 1984, DIEHL 1986, VIDOLIN et al 2007. Indeed, in the present study, when mortality was noted, smaller urchins died before the larger ones.…”

“…However, under a similar protocol, the sea-urchins L. variegatus and Arbacia lixula (Linnaeus, 1758) did not show evidence for the maintenance of Mg 2+ gradients (VIDOLIN et al 2007). Under a protocol of progressive but gradual salinity alteration, the sea-urchin S. droebachiensis has sustained a huge gradient for Mg 2+ (27 mM in SW of salinity 10, or 170%) when exposed to the simulation of a steep tidal cycle, essentially showing extracellular Mg 2+ regulation (STICKLE & AHOKAS 1974). Different rates of salinity changes (different experimental protocols) may account for these different results.…”

“…The basic rule is that when salinity is reduced, extracellular fluid (perivisceral coelomic fluid) ionic concentrations are "buffered", and positive gradients develop, with higher concentrations in the coelomic fluid. So, even with gradual dissipation of these gradients, the coelomic fluid is reported as hyper-ionic for at least some hours, in a way perfectly compatible with life under the rhythmic fluctuations of tidal cycles (STICKLE & AHOKAS 1974, STICKLE & DENOUX 1976, VIDOLIN et al 2007.…”