Aerial and aquatic respiration in littoral Oniscidea (Isopoda) from Southern California, USA

Abstract

Abstract The large radiation of terrestrial isopods (suborder Oniscidea) includes several families that are represented primarily in marine-littoral or riparian habitats. Among these are members of Ligiidae and Tylidae as well as several basal families within the section Crinocheta. Structural and physiological evidence supports a marine-littoral ancestry of the Oniscidea. We examined aerial and aquatic respiration (measured as VCO2) in six species of marine-littoral Oniscidea representing five families, as well as one riparian and one endogean species. Complimentary data were collected for immersion tolerance and whole-animal permeability in air, and structural specialization of the respiratory pleopods was examined using SEM. Ligia occidentalis Dana, 1853 (marine, littoral) and Ligidium lapetum Mulaik & Mulaik, 1942 (riparian) showed similar VCO2 in air and water. VCO2 in air for the other species was significantly higher than in water. Compared across species, aerial VCO2 scaled with mass in accordance with Kleiber’s law (β = 0.774) while aquatic VCO2 increased in approximate proportion to mass (β = 0.957). At least some specimens of the six marine-littoral species survived over 24 h immersion. Ligidium lapetum and the endogean trichoniscid Brackenridgia heroldi (Arcangeli, 1932) also tolerated prolonged immersion in freshwater but did not survive beyond 5–6 h, probably due to limited capacity for hyper-regulation. The upper shore sand-burrowers, Tylos punctatus Holmes & Gay, 1909 and Alloniscus perconvexus Dana, 1856 had the lowest permeability among the study species and are the only representatives with elaborated pleopodal respiratory fields (Alloniscus) and lungs (Tylos). The ventral lung spiracles of T. punctatus are surrounded by an extensive cuticle meshwork and we propose that this functions as a plastron field to enhance aquatic gas exchange. Collectively, the results show that littoral species tolerate significant periods of immersion, allowing them to withstand habitat inundation during spring high tides, storm swells and, in riparian species, rainstorms and snowmelt.