Swelling tests in a series of isotonic and isoionic solutions of varying calcium-tosodium ratios were conducted on isolated dermal connective tissue of the holothurian Thyonella gemmata Verrill. The tissue swelled rapidly and attained a maximum volume increase of approximately 40 % when transferred from distilled water to NaCl solution; however, the volume did not change significantly in isotonic CaCl2 solution. At Ca2+/Na+ ratios ≤0.04 the tissue swelled at its maximum rate. The rate decreased with increasing calcium concentration, until at Ca2+/Na+ ≥0.40 no detectable swelling occurred. Similar results were obtained for Pentacta pygmaea Goldfuss. When tissues previously swollen in NaCl were placed in CaC2, the volume decreased significantly. Uniaxial tensile tests indicated that the elastic modulus of the tissue was much greater in Ca2+ solutions than in Na+ solutions. We hypothesize that dermal stiffness in holothurians is regulated by cation-sensitive crosslinks
