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  Evolution to land
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Desiccation resistance

  Topics regarding evolution to land in isopod crustaceans include desiccation resistance, considered here, and LIGIA A PROTOTYPAL LAND COLONISER, GAS EXCHANGE, MOULTING IN SEMITERRESTRIAL FORMS, REPRODUCTIVE MODIFICATIONS, and MODIFICATIONS IN DIGESTIVE PROCESSES, considered in other sections.
 
Research study 1
 

graph showing loss in live mass over time in 76% relative humidity for 3 species of terrestrial isopodsphotograph of isopod Ligia pallasii to show uropods
A study on 4 oniscid species in British Columbia, including the semiterrestrial Ligia pallasii, shows that overall resistance to drying at 76% RH follows the order L. pallasii < Oniscus asellus < Porcellio scaber < Armadillidium vulgare, which corresponds with the degree of exoskeleton permeability and matches the accepted order of increasing terrestriality. Providing that losses are less than about 25% over a period of a few hours, all species rehydrate quickly when returned to moist conditions. 

During feeding and other excursions Ligia pallasii commonly dips its uropods or last pair of appendages into puddles, with the tips pressed together.  The water is conducted by capillary action along the juxtaposed appendages into the gas-exchange areas and into the brood chambers of females. Carefoot et al. 1990 p. 157 In, The Biology of Terrestrial Isopods III (Juchault & Mocquard, eds.) Univ Poitiers Press.

 
Research study 2
 

graph showing levels of light, relative humidity, and temperature in the microhabitat of Ligia pallasii on Seppings Island, British ColumbiaThe sea slater Ligia pallasii in British Columbia occupies supralittoral rock-crevice or tumbled-rock habitats extending vertically only 1-2m above the high-water mark.  At one boulder/cobble site in Barkley Sound (Seppings Island) in situ measurements of temperature, light, and relative humidity at 1cm above the rock surface and within the boulder interstices show that microhabitat conditions are remarkably uniform. Thus, despite variable cloud cover and shading during one day in July, temperatures within crevices varied only from 15-16oC, and relative humidity from 99-100%.  It is likely that the first evolutionary “step” onto land by the ancestors of present-day ligiids was to a supralittoral habitat with similarly unvarying conditions of temperature and relative humidity.  Carefoot et al. 2000 Can J Zool 78: 588.

NOTE simultaneous data on temperature, light, and humidity obtained from miniature Stowaway® data-loggers

 
Research study 3
 

All isopods (and all crustaceans) rely mainly on the release of ammonia to rid the body of nitrogenous wastes.  However, whereas for marine isopods and other crustaceans this is in the form of ammonium ion NH4+, which is highly soluble in water, for all semiterrestrial (e.g., Ligia) and terrestrial isopods it is in the form of ammonia gas NH3, which requires no water for release.  In the semiterrestrial Ligia pallasii, over 90% of total nitrogenous waste is eliminated via ammonia gas.  The exact means of release is not known, but release along with urine from the paired maxillary glands on either side of the mouth is commonly suspected.  This unique excretory mode appears to be an evolutionary compromise.  Retention of ammoniotely via release of gaseous ammonia allows Ligia and terrestrial species to conserve energy, as it is a much less costly means of excreting waste nitrogen, and also avoids substantial water loss.  Carefoot et al. 1992 Comp Biochem Physiol 103A (3): 559; for original description of ammonia release in isopods, see Dresel & Moyle 1950 J Exp Biol 27: 210.

NOTE  other modes of nitrogenous release in terrestrial animals are ureotely (vertebrates, including humans) and uricotely (birds, insects, lizards, snails).  Urea is much less toxic than ammonia, but requires dissolution in water and release of liquid urine; uric acid is crystalline, non-toxic, and requires no water for its release.  The most metabolically costly to produce is uric acid, followed by urea, then ammonia

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