TERRESTRIAL LIFE IN ISOPODS - EVOLUTIONARY LOSS OF GAS-EXCHANGE AND SURVIVAL CAPABILITY IN WATER

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 Linne, 1758, and Li gia pallasii Brandt, 1833, to compare the degree of vestigial adaptati on to aquatic existence versus adaptation to terrestrial existence. Mo st submerged A. vulgare, P. scaber, and O. asellus died by 18 h, wheth er in fresh water or isosmotic seawater. Ligia pallasii lived for almo st 2 d in fresh water and for at least 25 d in seawater (none died dur ing 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 semiterr estrial L. pallasii. Oxygen consumption of submerged A. vulgare (repre sentative of fully terrestrial forms) declined to 50% after 14 h and t o 17% by 24 h, by which time the animals were clinically dead. In comp arison, L. pallasii's aquatic oxygen consumption was maintained at nor mal or higher levels for at least 3 d. All species could survive sever al hours of immersion in fresh water, sufficient to withstand temporar y rain deluge. Ligia pallasii's superior capabilities to survive and r espire in seawater reflect its greater similarity to aquatic ancestors , and an evolutionary series showing progressive decline in survival a nd gas-exchange capability in water would be as follows: L. pallasii > O. asellus > P. scaber greater-than-or-equal-to A. vulgare.