Survival and oxygen uptake in air and water were investigated in four species of terrestrial isopods, Armadillidium vulgare Latreille, 1804, Porcellio scaber Latreille, 1804, Oniscus asellus Linné, 1758, and Ligia pallasii Brandt, 1833, to compare the degree of vestigial adaptation to aquatic existence versus adaptation to terrestrial existence. Most submerged A. vulgare, P. scaber, and O. asellus died by 18 h, whether in fresh water or isosmotic seawater. Ligia pallasii lived for almost 2 d in fresh water and for at least 25 d in seawater (none died during this time). Oxygen uptake in water was 44 – 66% that in air for the three fully terrestrial species, and 160% that in air for the semiterrestrial L. pallasii. Oxygen consumption of submerged A. vulgare (representative of fully terrestrial forms) declined to 50% after 14 h and to 17% by 24 h, by which time the animals were clinically dead. In comparison, L. pallasii's aquatic oxygen consumption was maintained at normal or higher levels for at least 3 d. All species could survive several hours of immersion in fresh water, sufficient to withstand temporary rain deluge. Ligia pallasii's superior capabilities to survive and respire in seawater reflect its greater similarity to aquatic ancestors, and an evolutionary series showing progressive decline in survival and gas-exchange capability in water would be as follows: L. pallasii > O. asellus > P. scaber ≥ A. vulgare.
