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

Abstract

The intertidal sea-urchin Echinometra lucunter (Linnaeus, 1758) has been submitted to diluted sea water (SW) of salinity 25, or concentrated sea water of salinity 45. In addition, ionic challenges have been offered, supplementing 25 SW with Mg2+, Ca2+ or K+, until the concentration of each of these ions would reach the level of full-strength 35 SW (control). Perivisceral coelomic fluid has been sampled after six hours in these treatments for measurements of osmolality and concentrations of Na+, Cl-, Mg2+, Ca2+, and K+. Urchins have been further observed until five days. SW concentration (45) lead urchins to death after two days of exposure, while urchins tolerated five days in 25 SW without any sign of distress. Urchins displayed osmoconformation and ion-conformation for NaCl, and occasional small gradients with respect to the water for Mg2+ (~6% in full-strength SW), and K+ (8.5% in 25 SW), after six hours. These results are consistent with data compiled from the literature, for echinoderms, which frequently show positive gradients (often higher than 25%) for the most relevant ions, between the coelomic fluid and external SW. Supplemented cations have shown mutual interference, mostly affecting their own coelomic fluid concentrations. Under the protocol used here, urchins of the species E. lucunter held gradients for Mg2+, Ca2+, and K+, but not for Na+ or Cl- or osmolality. They were also able to tolerate at least a 30% reduction in sea water salinity for five days, showing reasonable euryhalinity. However, when compared to other echinoderms, E. lucunter is not especially capable of maintaining large ionic gradients with respect to external SW

Echinoderm Responses to Variation in Salinity

“…Although body fluid osmolarity was not measured, it is safe to infer from the positive weight change and previous studies (Bishop et al, 1994;Freire et al, 2011;Santos et al, 2013;Stickle and Ahokas, 1974;Vidolin et al, 2007) that osmolarity in the perivisceral fluid and body tissues decreased in the hyposaline treatment group. As predicted, the mean weight change for the hyposalinity group was always greater than the control.…”

“…Similarly, in the brittle star Ophiocomina nigra, Na þ and Cl À were isosmotic in hyposaline water, but K þ levels were maintained at higher levels (Pagett, 1980a). The sea urchin, Echinometra lucuntur, 'held gradients for Mg 2þ , Ca 2þ , and K þ ' in the coelomic fluid, in both hypo-(20%) and hyper-(48%) conditions (Freire et al, 2011). Vidolin et al (2007) found that the echinoid, Lytechinus variegatus, displayed some ionic regulation of Na þ and K þ , and Santos et al (2013) demonstrated varying degrees of osmoconforming gradients in Na þ , Mg 2þ , and K þ for three species of echinoids (L. variegatus, E. lucuntur, and Arbacia lixula).…”

Early time course of variation in coelomic fluid ionic concentrations in sea urchins abruptly exposed to hypo‐ and hyper‐osmotic salinity challenges: Role of size and cross‐section area of test holes

“…However, upon a closer analysis, some gradients, although transient, are clearly revealed. Even under the typical salinity of their habitat (normal strength seawater), some occasional gradients are detected (Binyon, ; Diehl, ; Ferguson, ; Freire, Santos, & Vidolin, ; Russell, ; Santos, Castellano, & Freire, ; Stickle & Diehl, ; Vidolin, Santos‐Gouvea, & Freire, ). More frequently, ionic gradients of magnesium, potassium, and calcium are found in echinoderms, especially in dilute seawater.…”

“…More frequently, ionic gradients of magnesium, potassium, and calcium are found in echinoderms, especially in dilute seawater. Gradients of chloride and sodium also occur, albeit less frequently (Freire et al., ; Russell, ). Significant osmotic gradients are rarely present in these animals, but were already detected, especially when they were exposed to dilute seawater (Freire et al., ; Russell, ; Santos, Castellano, & Freire, ).…”

“…This study was conducted to deepen the analysis of the role of size and body permeability (i.e., cross section area of test holes) on the dissipation of ionic gradients in the first hours upon abrupt salinity challenges. The hypotheses are that: (1) the dissipation of the abrupt gradients created by sudden salinity changes will be faster in smaller species from stable habitats than in larger species from unstable habitats; (2) ionic gradients will not be the same for all ions; and (3) ionic gradients would be found more often under hypo‐ than hyper‐saline challenge, given previous results from the available literature (reviewed in Binyon, ; Diehl, ; Freire et al., ; and Russell, ).…”