Maintenance of ionic gradients and tissue hydration in the intertidal sea cucumberHolothuria griseaunder hypo- and hyper-salinity challenges

Abstract

Echinoderms are exclusively marine, osmoconformer invertebrates. Their distribution patterns are strongly influenced by salinity. Nevertheless, several species of the phylum inhabit the challenging intertidal zone, characterized by steep and fast salinity fluctuations. This study evaluated the response of coelomic fluid ionic concentrations (sodium, chloride, magnesium and potassium) of the intertidal sea cucumberHolothuria griseato hypo- and hypersaline challenges. A stepwise protocol was performed for the whole animal exposure to both anisosmotic conditions: from 35 to 15 psu along 8 h, and from 35 to 45 psu along 6 h, to simulate intertidal conditions. Tissue water regulation by the longitudinal muscle, oesophagus and posterior intestine was also evaluated, upon hypo- and hyper-osmotic shocks of 20 and 50% of intensity with respect to the isosmotic control. Ionic gradients were observed between coelomic fluid and external water, mainly for potassium and magnesium, but also sodium, and in a greater extent under hyposaline conditions than under hypersaline exposure. Consistently,H. griseashows retracted tube feet in 15 psu, but a more relaxed appearance and exposed tube feet in 45 psu. In addition,H. griseashowed greater capacity for tissue water maintenance during hyper- than in hyposmotic conditions.Holothuria griseashows an avoidance behaviour in low salinity (thus sustaining ionic gradients), preventing its tissues from intense swelling. This strategy allows it to dwell in the intertidal region.