Ionic Composition and Ion Provisioning in Marsupial Fluid of Terrestrial Isopods (Isopoda, Oniscidea)

Abstract

Abstract The marsupium of the primarily terrestrial isopod sub-order Oniscidea is modified from the basal marine design and represents one of the key evolutionary innovations enabling the invasion of land habitats. In the Ligiidae, the marsupial chamber is provisioned with water from the external environment via a capillary channel formed from the apposed 6th and 7th pereiopods. The other oniscidean families possess a 'closed' marsupium, isolated from the pleural water-conducting system and provisioned with water and ions by sternal cotyledons. How these different modes of fluid provisioning influence the ionic composition of marsupial fluid and its osmotic and ionic relationship to the haemolymph remains essentially unstudied. We analysed the ionic composition (Na+, K+, Ca2+, Cl–, pH) of marsupial fluid and haemolymph in 4 species of Oniscidea: Ligia occidentalis (Ligiidae), Ligidium lapetum (Ligiidae), Alloniscus perconvexus (Alloniscidae), and Armadillidium vulgare (Armadillidiidae). In all four species, ion concentrations show a broad similarity between marsupial fluid and haemolymph, but with elevated potassium concentrations in the 'closed'-marsupium species. The sternal integument of Armadillidium vulgare is permeable to inulin and the volume of the combined marsupial fluid and haemolymph is not significantly enlarged compared to the haemolymph of non-gravid females. Accordingly, the 'closed' marsupium of A. vulgare can be viewed as a functional subdivision of the haemolymph space. In Ligidium lapetum, marsupial water is derived from external freshwater sources, but marsupial fluid [Na+] and [Cl–]are actually significantly higher than haemolymph values. This species thus possesses an effective means of ion provisioning, paralleling that seen in the 'closed' marsupial species.