Elisabeth Hornung

Evolutionary adaptation of oniscidean isopods to terrestrial life: Structure, physiology and behavior

Terrestrial isopods (Oniscidea) are the most successful crustacean colonizers of land habitats. From an evolutionary point of view, they are excellent examples of model organisms that have adaptated to terrestrial life. The aquatic-terrestrial branching of the phylogenetic lines of the Oniscidea occurred in the marine littoral zone. The most oniscid species-rich areas are found in the circum-Mediterranean region. Studies on the morphology, physiology, ecology and biogeography of Oniscidea highlight the diversity of the group. They successfully colonized a wide range of terrestrial habitats by solving such ecological and physiological challenges as reproduction, respiration, excretion and protection against desiccation. During terrestrial adaptation, they evolved diverse morphological, ecological and behavioral traits. This review summarizes our present knowledge of some aspects of the morphology, physiology and behavior as it related to oniscidean adaptation to the terrestrial realm.

Breeding patterns in the oniscid isopod, Porcellio ficulneus Verh., at high temperature and under different photophases

Summary Porcellio ficulneus Verhoeff inhabits xeric Mediterranean grassland habitats. During winter vitellogenesis takes place and mancas are released in April. Under natural conditions some oocytes (about 20%) were lost. Both high temperature (25°C) at long or short photophases affected the reproductive pattern. Thus at 25°C and constant light vitellogenesis was markedly shorter than under field conditions. Oocytes matured sooner, but many were resorbed. The marsupium was formed earlier and contained fewer eggs. In some cases mancas that had developed could not emerge from the marsupium and died inside. In conclusion, P. ficulneus responds to warmer conditions by breeding earlier and reducing the number of offspring.

Oosorption and oocyte loss in a terrestrial isopod under stressful conditions

An increased number of resorbed oocytes was observed in ovaries of terrestrial isopods which were kept under different experimental temperature and photophase regimes compared with those observed in the natural population. Regardless of the nature of the stimulus: high or low temperature or long and short photophases, the female always responded through oosorption. In an iteroparous species such as Porcellio ficulneus B.-L., recruitment of resources for future utilization could be its main response to adverse ambient conditions.

SWITCHING LIFE‐HISTORY SENSITIVITIES TO STRESS IN SOIL INVERTEBRATES

Over the last decades ecologists have expressed a growing interest in studying the life-cycle responses of animals and plants to stress following global threats of climatic change and pollution. Subsequent reductions in population growth rate (λ) are the result of stress-affected changes in various vital rates. At present it is unknown whether a stressful environment increases the sensitivity of λ to changes in vital rates other than the ones which have been affected. In this paper we explore whether organisms exposed to anthropogenic pollutants are more sensitive to other environmentally induced life-cycle changes than prior to exposure. If so, this may enhance or reduce the direct effect of toxic substances at the population level. Since many soil organisms are continuously exposed to pollutants in the field we investigate the contributions of vital rates to λ under different pollutant concentrations. Soil invertebrates with divergent life cycles were selected (the mite Platynothrus peltifer and the iso...

Small scale gradient effects on isopods (Crustacea: Oniscidea) in karstic sinkholes

We studied abundance and diversity patterns of terrestrial isopod assemblages along a ‘micro-scale’ vertical gradient in sinkholes in the Aggtelek National Park, Hungary. Time restricted manual sampling yielded ten native species, including endemic and rare ones. Along the gradient we found no major differences in species richness and -composition, and abundance decreased from the bottoms to the upper zones of the sinkholes. Species specific habitat preference on a vertical gradient showed two distinct groups by indicator species analysis: occurrence of habitat “generalists” was irrespective of vertical zones while “specialists” were restricted to the bottoms of the dolines. The latter group is formed mainly by rare species. We found that both diversity and evenness of isopod assemblages were highest in the bottom zone. Our results draw the attention to the significance of such common, yet undiscovered surficial depressions that can provide shelters for rare and specialist species and can provide shelter for survival of populations under changing climatic conditions.

Does Shell Accumulation Matter in Micro-Scale Land Snail Surveys?

land snails are most often surveyed to generate inventories with sites often less than 1 ha in extent and sampling taking place in microhabitats within the site that researchers consider likely to harbor snails (cameron & Pokryszko, 2005; menez, 2001, 2007). the most cost effective approach to site-level sampling has been suggested as a combination of bulk sampling of litter/soil and time restricted search (emberton et al., 1996). this requires a single visit to each site, and the combination of methods enables the investigator to maximize the likelihood of finding both micro (< 5 mm) and macro (≥ 5 mm) snails (solymos et al., 2007). this approach often relies on dead shells as a probabilistic indicator that a given species is present (thurman et al., 2008) because several constraints (remote locations, investigator availability, weather conditions) may not enable surveys to be undertaken during the species’ activity periods. assessing species presence on the basis of dead shells can be realistic, given the rather sedentary and cryptic behavior of land snails, and in most cases gives comparable results at the site scale (rundell & cowie, 2003). recently, there is an increase in surveys that are addressing microhabitat-scale ecological differences regarding the relative abundances and species compositions of communities (e.g., under logs vs. leaf litter) within a site (Kappes et al., 2006; solymos & Pall-Gergely, 2007). taphonomic issues (e.g., differential preservation of shells in different microhabitats) and biases introduced by sampling methods with different selectivity to dead specimens might become increasingly important as spatial scale decreases. We carried out a field experiment in dolines (sinkholes, large karstic depressions) of the aggtelek karst area, hungary, to investigate malacoloGia, 2009, 51(2): 389−393

Diversity of terrestrial isopod species along a transect through northern Israel

The distribution of isopod species along a 70 km transect through northern Israel is described. The transect started from the Mediterranean seashore through Mt. Carmel and the Gilboa Mts. to the Jordan Valley. The habitats ranged from grassland, macqui to woodland in the Mediterranean region, onto grassland in the semi-arid region. Thirty isopod species were found, of these only one species was common to all the regions, and two species were found in four out of five regions. The largest number of species inhabited the mountainous habitats of Mt. Carmel and the Gilboa Mts: 20 and 14 species, respectively. Twelve species were found in the Coastal Plain; five of these were found exclusively there. Two of the four species found in the Jordan Valley were characteristic of that semi-arid region. Beta-diversity was found to be low in most cases and especially in the comparison between Mediterranean and semi-arid habitats.

Demography of some non-native isopods (Crustacea, Isopoda, Oniscidea) in a Mid-Atlantic forest, USA.

Abstract Introduced species dominate the terrestrial isopod fauna in most inland habitats of North America, including urban landscapes. These non-native species are often very abundant and thus potentially play a significant role in detritus processing. We monitored isopod assemblages in an urban forest for a year to examine the relationship between surface activity and abiotic environmental factors, and to analyze reproductive characteristics that might contribute to their successful establishment. Using pitfall trap samples we recorded five species, two of which, Trachelipus rathkii and Cylisticus convexus, were highly abundant. We determined size, sex and reproductive state of each individual. Surface activity of both species reflected variability in abiotic stress factors for isopods, such as soil moisture and soil temperature. Early spring the main trigger was soil temperature while later in the season increasing temperature and decreasing soil moisture jointly affected population dynamics. Activity significantly correlated with soil moisture. The temporal pattern of sex ratios supported the secondary sex ratio hypothesis. Males dominated the samples on the onset of the mating season in search of females. The pattern was reversed as females searched for suitable microsites for their offspring. Size independent fecundity decreased as conditions became more stressful late in the season.

Size dependent differences in litter consumption of isopods: preliminary results

Abstract A series of experiments were applied to test how leaf orientation within microcosms affect consumption rates (Experiment 1), and to discover intra-specific differences in leaf litter consumption (Experiment 2) of the common isopod species Porcellio scaber and Porcellionides pruinosus. A standardised microcosm setup was developed for feeding experiments to maintain standard conditions. A constant amount of freshly fallen black poplar litter was provided to three distinct size class (small, medium, large) of woodlice. We measured litter consumption after a fortnight. We maintained appr. constant isopod biomass for all treatments, and equal densities within each size class. We hypothesized that different size classes differ in their litter consumption, therefore such differences should occur even within populations of the species. We also hypothesized a marked difference in consumption rates for different leaf orientation within microcosms. Our results showed size-specific consumption patterns for Porcellio scaber: small adults showed the highest consumption rates (i.e. litter mass loss / isopod biomass) in high density microcosms, while medium-sized adults of lower densities ate the most litter in containers. Leaf orientation posed no significant effect on litter consumption.

Histological studies on the marsupium of two terrestrial isopods (Crustacea, Isopoda, Oniscidea)

Abstract The marsupium, a brood pouch in peracarid crustaceans (Crustacea, Malacostraca) has evolved in terrestrial environment for providing nutrition and optimal conditions for embryogenesis. In the present study we give details on the histology and ultrastructure of its constituting elements such as oostegites and cotyledons. Marsupia of two different eco-morphological types of woodlice, namely the non-conglobating species Trachelipus rathkii Brandt, 1833 and the conglobating species Cylisticus convexus De Geer, 1778 were investigated. Light microscopic (LM) studies showed some differences in the main structure of the two species’ brood pouch: in Trachelipus rathkii, a ‘clinger’ type woodlice, the oostegites bend outwards during brood incubation as growing offspring require more space, while in Cylisticus convexus, a ‘roller’ type isopod, the sternites arch into the body cavity to ensure space for developing offspring and still allowing conglobation of the gravid females. The quantitative analysis of the oostegites’ cuticle proved that the outer part is about 2.5 - 3 times thicker compared to the inner part in both species. Electron microscopic (TEM) examinations show only small histological differences in the oostegites and cotyledon structure of the two species. Cellular elements and moderately electron dense fleecy precipitate are found in the hemolymph space between the two cuticles of oostegites. The cells contain PAS positive polysaccharide areas. TEM studies revealed some differences in the cotyledon ultrastructure of the two species. Cotyledons of Trachelipus rathkii consist of cells with cristate mitochondria and granular endoplasmic reticulum with cisterns. Cotyledons of Cylisticus convexus consist of cells with densely cristate mitochondria and ribosomes attached to vesicular membrane structures. In both species cells with electron dense bodies were observed. We conclude that - besides the differences in marsupial shapes - the fine structure of the oostegites and cotyledons is hardly affected by the eco-morphological type, specifically the conglobating or non-conglobating character of the studied species.