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Hemolymph homeostasis in relation to diel feeding activity and microclimate in the prototypal land isopod Ligia pallasii Thomas H. Carefoot, Jonathan Wright, Steven C. Pennings, Andreas Ziegler, Martin Zimmer, Roger F. Uglow, Angela R. Danford, and Jean Paul Danko
Abstract: Twenty-four-hour variability in hemolymph osmolality was assessed in a field population of the semiterrestrial isopod Ligia pallasii and related to feeding activity and microenvironmental conditions of relative humidity (RH) and temperature in the species’ habitats. Hemolymphs were also analyzed for levels of ammonia and glucose as indicators of digestive and absorptive activities. Comparative data on hemolymph osmolality and microhabitat conditions were also obtained for several ligiid species in Hawai′i. Late-morning peaks in hemolymph osmolality in L. pallasii were coincidental with peaks in ammonia and glucose and with predicted times of digestion/absorption based on observed feeding activity in the population, suggesting that the elevations in osmolality were due to saltloading from the species’ seaweed diet. Uniformity of microenvironmental conditions of RH and temperature in Ligia’s supralittoral habitat, whether temperate or tropical, and regardless of time of day or localized weather conditions, suggests that dietary salt-loading is likely the greatest challenge to osmotic homeostasis in ligiids. This feature and other aspects of hemolymph osmolality in ligiids are discussed in the context of colonization of land by prototypal terrestrial isopods. Résumé : La variabilité de l’osmolalité de l’hémolymphe au cours d’un période de 24 h a été évaluée chez une population naturelle de l’isopode semi-terrestre Ligia pallasii et reliée à l’activité alimentaire et aux conditions microclimatiques d’humidité relative (RH) et de température dans les habitats fréquentés par l’organisme. Les concentrations d’ammoniaque et de glucose de l’hémolymphe ont été évaluées également comme indicateurs des activités de digestion et d’absorption. Des données comparatives sur l’osmolalité de l’hémolymphe et sur les conditions dans les microhabitats ont également été obtenues pour plusieurs espèces de ligiidés d’Hawaï. Les valeurs maximales de l’osmolalité enregistrées tard le matin chez L. pallasii coïncident avec les concentrations maximales de glucose et d’ammoniaque et avec les moments de digestion/absorption prévus en fonction des périodes d’activité alimentaire observées chez la population, ce qui semble indiquer que l’augmentation de l’osmolalité est reliée à l’absorption du sel des algues consommées par l’espèce. L’uniformité des conditions microclimatiques de température et d’humidité relative dans l’habitat supra-littoral de Ligia, que ce soit en zone tempérée ou tropicale, et quelles que soient l’heure du jour et les conditions climatiques locales, indique que l’absorption de sel dans la nourriture constitue le plus grand défi au maintien de l’homéostase osmotique chez les ligiidés. Cette propriété, de même que d’autres aspects de l’osmolalité de l’hémolymphe des ligiidés sont examinés dans le cadre de la colonisation du milieu terrestre par les prototypes des isopodes terrestres. [Traduit par la Rédaction]
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Introduction Isopods of the suborder Oniscidea are highly successful colonizers of land, with over 4000 species inhabiting such diverse habitats as marine supralittoral shores, savannahs, marshland, forests, and deserts. Known as wood lice or sow bugs, they represent the largest terrestrial radiation within
the Crustacea and are the only ones to have colonized xeric biotopes. The section Diplocheta, consisting of the extant family Ligiidae, represents the most basal lineage within the Oniscidea (Erhardt 1997) and includes the genera Ligia and Ligidium. Approximately 30 species of Ligia are distributed worldwide, but are mostly restricted to supralittoral habitats extending vertically only a few metres above the high-water
Received June 22, 1999. Accepted October 19, 1999. T.H. Carefoot1 and J.P. Danko. Department of Zoology, The University of British Columbia, Vancouver BC V6T 1Z4, Canada. J. Wright. Department of Biology, Pomona College, Claremont, CA 91711, U.S.A. S.C. Pennings. Marine Institute, University of Georgia, Sapelo Island, GA 31327, U.S.A. A. Ziegler. Sektion Elektronenmikroskopie, Universität Ulm, D 89069 Ulm, Germany. M. Zimmer. Institut für Neurobiologie, Zoologie und Didaktik der Biologie, Heinrich-Heine-Universität, Universitätsstraße 1, D 40225 Düsseldorf, Germany. R.F. Uglow and A.R. Danford. Department of Biological Sciences, University of Hull, Cottingham Road, Hull HU6 7RX, England. 1
Author to whom all correspondence should be addressed (e-mail:
[email protected]).
Can. J. Zool. 78: 588–595 (2000)
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mark. Here they feed on seaweeds cast up by the tides or on encrusting diatoms. Species of Ligia possess such primitive features as undifferentiated saclike pleopods for gas exchange, an open or “amphibious-type” marsupium for incubation of eggs, and an open-type water-conducting system (Hoese 1984). In these features, as well as others that reflect a blend of aquatic and terrestrial adaptive traits, Ligia exemplifies the prototypal land isopod. While reproduction, life cycle, feeding biology, and activity patterns of Ligia species are fairly well studied, less is known about their physiology, particularly in the ways in which emancipation from the sea has been facilitated or constrained (for a review see Carefoot 1993). A case in point is the extent to which species employ homeostatic control of hemolymph osmolality, and the extent to which this depends on the availability of both salts and water. Although a number of studies have investigated osmoregulation in Ligia spp., these have with one exception used animals submerged in seawater, despite the normal habitat of the genus being air. These studies, using otherwise morphologically and behaviourally similar ligiid species, indicate a remarkably variable, species-dependent osmoregulatory capability ranging from hyperosmoregulation only (down to 25% normal seawater concentration) to hyposmoregulation only (up to about 175%) to both (Parry 1953; Todd 1963; Wilson 1970; Wright et al. 1997). In the only study to date assessing osmoregulation during dehydration in air, Wright et al. (1997) showed that Ligia exotica could hyporegulate until lethal desiccation (21% mass loss or an estimated 35% hemolymph-volume loss) occurred. Potential challenges to osmotic homeostasis in Ligia spp. are evaporative water loss and dietary salt-loading, the latter being especially exacerbating, owing to the high salt content of its seaweed foods. Evaporative loss in the terrestrial Crinocheta (a large grouping representing such familiar forms as oniscids, porcellionids, and armadillidids) is countered by a remarkable ability to absorb water vapour from the air in relative humidities (RH) above 86–90%, depending upon species (Wright and Machin 1990, 1993a, 1993b). This feature is absent in ligiids. The mechanism by which ligiids accommodate dietary salts is not known, but could include intra- or extra-cellular sequestration of ions, rectal or maxillary gland excretion, or secretion into and subsequent loss through the pleon fluid, which bathes the gas-exchange surfaces of the pleopods (Wright et al. 1997). The Pacific-coast sea slater, Ligia pallasii, represents an ideal study organism for the investigation of osmoregulatory processes amongst primitive oniscideans. It is a large species (to 6 cm length, 2 g live mass) that ranges from the Aleutian Islands to northern California and inhabits rock crevices in vertical cliffs or tumbled rock. Its vertical range is limited to a few metres above high-tide mark. Although it makes daily feeding excursions to the strand line and upper intertidal region when the tide is out, it does not normally enter the sea. Like many other ligiids, L. pallasii is nocturnal, spending its daylight hours in rock crevices and emerging at or just before dusk to commence nighttime feeding and other activities. In the present study we investigated diel variation in hemolymph osmolality in a population of L. pallasii for which daily activity cycles are known (Carefoot et al. 1998)
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and related this pattern to variation in microclimate and to changes in levels of hemolymph ammonia and glucose associated with feeding. For comparative purposes we also did one-time samplings of hemolymph from three Ligia species in Hawai′i, L. exotica, L. hawaiensis, and L. perkinsi, and recorded daily changes in microenvironmental conditions of temperature and RH in the habitats of the last two. Of these three species, the first has a wide distribution throughout the Pacific basin, while the others are endemic to Hawai′i. Ligia exotica and L. hawaiensis have similar ecological requirements to L. pallasii, while L. perkinsi lives amongst moss on trees and on cliff overhangs in areas of freshwater seepage. Its severence of a connection to the sea, considered requisite for survival in other Ligia species, is remarkable. Since only 6 of 30 or so Ligia species in the world are known to live away from the sea (S. Taiti, personal communication; Schmalfuss 1978), its presence in Hawai′i provided an opportunity for a unique comparison.
Materials and methods Hemolymph osmolality and ammonia and glucose levels in field animals Beginning at 12:00 on 18 July 1998, we collected 20 adult L. pallasii of mixed sexes at 4-h intervals over a 24-h period from beneath cobble boulders on Seppings Island, B.C., near the site of the Bamfield Marine Station, and promptly sampled for hemolymph. The cobble habitat consisted of 20–30 cm diameter granitic rocks adjacent to cliff faces. Hemolymph samples were obtained by inserting 0.8 mm O.D. glass capillary tubes through the intersegmental cuticle between the third and fourth tergites and drawing in 8–10 µL. Each sample was then expelled into a known mass and volume of deionized water in a preweighed Eppendorf tube, which was later reweighed to determine the precise volume of hemolymph collected. After sampling, animals were sexed, weighed, assessed as to moult stage, and returned to a different area of the shore so that they were not re-collected. To compare these results with those from other Ligia species, we also sampled hemolymphs from L. exotica collected from pilings around the Honolulu docks, from L. hawaiensis collected from the mud surface in a marsh dominated by Batis maritima/ Bulbostylus capillaris (sedge) in the Waipahu region of Pearl Harbor, and from L. perkinsi collected from moss on cliff overhangs in the Pali escarpment region of Oahu. All species except L. perkinsi were collected during daylight hours. We also collected single water samples from the immediate seawater habitat of each species or, in the case of L. perkinsi, from seepage water from the cliff face. Hemolymph osmolalities (expressed in milliosmoles per kilogram or mosm) were measured using a Wescor 5100 vapor-pressure osmometer. For L. pallasii we were particularly interested in temporal variation in hemolymph osmolality over the 24-h period, but data were also analyzed for effects of size, sex, and moult stage. Hemolymph ammonia concentrations in the diluted samples were determined as total ammonia (NH3 + NH4+) using the flowinjection / gas-diffusion method of Hunter and Uglow (1993) with a detection limit of
