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  Evolution to land
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  Gas exchange
  Topics regarding evolution to land in isopod crustaceans include gas exchange, considered here, and LIGIA A PROTOTYPAL LAND COLONISER, DESICCATION RESISTANCE, MOULTING IN SEMITERRESTRIAL FORMS, REPRODUCTIVE MODIFICATIONS, and MODIFICATIONS IN DIGESTIVE PROCESSES, considered in other sections.
 
Research study 1
 

photograph of abdomen region of isopod Ligia pallasiiThe branchial chamber in an isopod consists of five pairs of pleopods1.  In marine forms the pleopods are double, with hemolymph-filled sacs that act as simple "diffusion gills" and outer flattened parts that act as protection. In terrestrial forms, including the semiterrestrial Ligia, each pleopod is also double, with an innermost endopod and an outermost protective exopod. The exopods fit snugly together to form a protective cover for the branchial chamber (see photo of abdomen on Left).  In all terrestrial genera, the endopods are similarly structured, photo/drawing composite of exopod morphology of 4 species of oniscid isopodsfleshy, hemolymph-filled sacs, while the exopods, in addition to fulfilling a protective role, are variously modified for gas exchange.

Note in the diagram2 on the Right an apparent evolutionary3 transition from sac-like structures in Ligia, to corrugated lung-like cavities in Oniscus, to a gradation of air-filled invaginations called “pseudotracheae” in Porcellio and Armadillidium.  This increasing specialisation is reflected in an evolution to more terrestrial habitats occupied, from splash-zone marine habitats for ligiids, to wet-forest habitats for oniscids, and to more arid habitats for porcellionids and armadillidids.  Carefoot & Taylor 1995 p. 47 In, Terrestrial Isopod Biology (Alikhan, Ed.)  AA Balkema, Rotterdam.

NOTE1  lit. “swim legs” G., referring to the fact the these abdominal appendages are used for swimming in various crustaceans, including aquatic isopods

NOTE2 in addition to the evolutionary progression they represent, the 3 terrestrial species shown here are selected because, along with L. pallasii, they can readily be collected within a few hundred meters of one another where human habitations are found close to rocky shorelines

NOTE3 although some authors believe that the different modes of gas exchange in oniscids have evolved independently, it is convenient for our purposes to consider them as an evolutionary transition as shown here

 
Research study 2
 

graph showing survival of 4 species of oniscid isopods in water isosmotic with their hemolymph
While Ligia pallasii tend not to spend time in seawater, there may be occasions when they enter this environment. For example, when foraging for seaweeds or during exposure to storm surge in their supralittoral habitat, they are undoubtedly occasionally washed by waves into the sea. Ligia may also enter shallow tidepools voluntarily or, more precipitously, to tumble or jump from rocks and cliff edges when in escape flight. Experiments at Bamfield Marine Sciences Centre, British Columbia show that L. pallasii is able to survive indefinitely in seawater, with oxygen consumption being maintained at a high level.  More specialised terrestrial forms such as the pillbug Armadillidium vulgare and other common woodlice species are incapable of long-duration aquatic gas exchange and die in less than a day when immersed.  Carefoot & Taylor 1995 p. 47 In, Terrestrial Isopod Biology (Alikhan, ed.)  AA Balkema, Rotterdam.

NOTE tests are done at 15oC in 100% seawater for L. pallasii and, for the other oniscid species, in seawater diluted to match the species' hemolymph osmolality (varies from about 60-65% seawater)

 
Research study 3
 

A comparison at the University of British Columbia of aquatic oxygen uptake of the semiterrestrial Ligia pallasii and the 3 common terrestrial species featured above is also revealing, not just for reason that the level of underwater gas-exchange proficiency tends to reflect the degree of terrestriality, but also for the fact that there is at least some retention in terrestrial isopods of an ancestral ability for underwater oxygen uptake.  Moreover, all species can 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 for 4 common west-coast oniscid species would be as follows: L. pallasii > O. asellus > P. scaber > A. vulgare.  Taylor & Carefoot 1993 Can J Zool 71: 1378.

NOTE  the authors use seawater isosmotic with the internal body fluids of each species to reduce possibly confounding effects of osmotic stress

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