Echinoderms are restricted to the marine environment and are osmoconformer invertebrates. However, some species live in unstable environments. Especially those species, and those of larger body size, tend to show variable, albeit transient, ionic gradients between their coelomic fluid and external seawater. In order to further examine how sea urchin size relates to apparent ionic permeability of their body wall/epithelia, specimens of Echinometra lucunter, Lytechinus variegatus, Paracentrotus gaimardi, and Arbacia lixula-A. lixula of two distinct populations, Rio de Janeiro and Santa Catarina-were abruptly transferred from 35 psu to either 25 or 45 psu. Sodium, chloride, magnesium, and potassium concentrations were assayed in their coelomic fluids after 0, 1, 2, and 3 hr of exposure. Relative area of putative permeable (i.e., cross section areas of soft tissues, or test holes) surfaces (PPS) was estimated in empty tests as the sum of the peristomial area (oral hole in the empty test) and the total cross-section area of ambulacral holes, divided by the total volume (TV) of the test. L. variegatus and E. lucunter, the largest species, had PPS/TV values similar to that of the much smaller P. gaimardi. A. lixula was the "most putatively-permeable and conformer" among them all, especially urchins from the Santa Catarina population. Internal ionic levels equilibrated faster with external water in 45 than in 25, and differences among ions were observed. Body size is relevant, among many other factors, to aid conformers such as sea urchins to dwell in intertidal unstable habitats.
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