Trace metal concentrations in two New Zealand talitrid amphipods: effects of gender and reproductive state and implications for biomonitoring

This is the first in-depth investigation of whether the gender or reproductive state of talitrid amphipods affects the bioaccumulation of trace metals. Concentrations of copper, zinc, and cadmium were measured in the beach flea Transorchestia chiliensis (Milne-Edwards) and the sand hopper Talorchestia quoyana (Milne-Edwards) (Amphipoda: Talitridae) from sites in and near the Avon-Heathcote Estuary, Christchurch, New Zealand. For T. chiliensis, the whole body trace metals concentrations (μg g⁻¹) were generally similar for nonbrooding, brooding, and brooding females that had the embryos removed. Where there were differences between female groups (3 out of 15 samples), concentrations in nonbrooding females were below those for brooding females. The trace metal concentrations of separated embryos did not follow those of their mothers. The body zinc concentration was similar for males and females. For copper and cadmium, body concentrations for females were higher than males at the two most contaminated sites. Cadmium body concentrations were similar between sites, and the lowest concentrations were from amphipods from one of the Estuary sites rather than the reference site. In T. quoyana, the trace metal concentrations in nonbrooding female and male sand hoppers were similar for copper and zinc, but cadmium concentrations were higher in nonbrooding females than in males. Copper and zinc concentrations within amphipod body tissues did not reflect those in the sediment or their food. The implications of these results are discussed in relation to previous studies and the use of beach fleas and sand hoppers as metal biomonitors. The beach flea T. chiliensis is recommended as a suitable trace metal biomonitor in New Zealand coastal waters with the potential to be affected by anthropogenic trace metal contamination.     

Some factors affecting survival and oxygen uptake in a subtropical beach flea

The survival and oxygen uptake of the supralittoral amphipod Chroestia lota Marsden & Fenwick were investigated in humid air and sea water between 15 and 35°C. Seven-day exposure experiments were made on three size groups of amphipods at 6 constant temperatures (15, 20, 25, 30, 35, 40 °C) and three cyclic temperatures (15–25, 20–30, 25–35°C) in air and in sea water at 34 and 17%. salinity. Neither size, treatment nor temperature affected survival between 15 and 30°C. Mortality increased > 30°C with large individuals being consistently less tolerant than medium and small amphipods. While amphipods exposed to cyclic temperatures during submersion had reduced survival compared with constant temperatures, those individuals exposed to cyclic conditions in humid air showed the greatest resistance. Oxygen uptake of Chroestia increased with dry body wt and, over the range 15–35°C, this semi-terrestrial beach flea could maintain its aerial VO₂ following submersion. Oxygen uptake increased directly in proportion to gill area and the weight specific gill area was low, consistent with the need to reduce desiccation. It is suggested that total gill area does not limit oxygen uptake in Chroestia and that cutaneous respiration may be important especially in aquatic conditions.     

Thaumamermis zealandica n sp (Mermithidae: Nematoda) parasitising the intertidal marine amphipod Talorchestia quoyana (Talitridae: Amphipoda) in New Zealand,with a summary of mermithids infecting amphipods

A new nematode, Thaumamermis zealandica n. sp. (Mermithidae; Nematoda), is described parasitising the intertidal marine amphipod Talorchestia quoyana Milne-Edwards (Talitridae) from the coast of South Island, New Zealand. The new species is characterised by the degree of dimorphism of the spicules, the arrangement of the genital papillae, the shape and length of the vagina and the anteriorly-placed amphids. This is the first known marine host of a member of the family Mermithidae. Published records of mermithid nematodes from amphipods are presented.     

Desiccation sensitivity and tolerance in the moss <Emphasis Type="Italic">Physcomitrella patens</Emphasis>: assessing limits and damage

The moss Physcomitrella patens is becoming the model of choice for functional genomic studies at the cellular level. Studies report that Physcomitrella survives moderate osmotic and salt stress, and that desiccation tolerance can be induced by exogenous ABA. Our goal was to quantify the extent of dehydration tolerance in wild type moss and to examine the nature of cellular damage caused by desiccation. We exposed Physcomitrella to humidities that generate water potentials from −4 (97% RH) to −273 MPa (13% RH) and monitored water loss until equilibrium. Water contents were measured on a dry matter basis to determine the extent of dehydration because fresh weights (FW) were found to be variable and, therefore, unreliable. We measured electrolyte leakage from rehydrating moss, assessed overall regrowth, and imaged cells to evaluate their response to drying and rehydration. Physcomitrella did not routinely survive water potentials <−13 MPa. Upon rehydration, moss dried to water contents >0.4 g g dm⁻¹ maintained levels of leakage similar to those of hydrated controls. Moss dried to lower water contents leaked extensively, suggesting that plasma membranes were damaged. Moss protonemal cells were shrunken and their walls twisted, even at −13 MPa. Moss cells rehydrated after drying to −273 MPa failed to re-expand completely, again indicating membrane damage. ABA treatment elicited tolerance of desiccation to at least −273 MPa and limited membrane damage. Results of this work will form the basis for ongoing studies on the functional genomics of desiccation tolerance at the cellular level.     

Kinetics of biofilm formation and desiccation survival of Listeria monocytogenes in single and dual species biofilms with Pseudomonas fluorescens,Serratia proteamaculans or Shewanella baltica on food-grade stainless steel surfaces

This study investigated the dynamics of static biofilm formation (100% RH, 15 °C, 48–72 h) and desiccation survival (43% RH, 15 °C, 21 days) of Listeria monocytogenes, in dual species biofilms with the common spoilage bacteria, Pseudomonas fluorescens, Serratia proteamaculans and Shewanella baltica, on the surface of food grade stainless steel. The Gram-negative bacteria reduced the maximum biofilm population of L. monocytogenes in dual species biofilms and increased its inactivation during desiccation. However, due to the higher desiccation resistance of Listeria relative to P. fluorescens and S. baltica, the pathogen survived in greater final numbers. In contrast, S. proteamaculans outcompeted the pathogen during the biofilm formation and exhibited similar desiccation survival, causing the N_{21 days} of Serratia to be ca 3 Log₁₀(CFU cm^?2) greater than that of Listeria in the dual species biofilm. Microscopy revealed biofilm morphologies with variable amounts of exopolymeric substance and the presence of separate microcolonies. Under these simulated food plant conditions, the fate of L. monocytogenes during formation of mixed biofilms and desiccation depended on the implicit characteristics of the co-cultured bacterium.     

Heat production inLittorina saxatilis Olivi andLittorina neritoides L. (gastropoda: Prosobranchia) during an experimental exposure to air

The adaptation of littorinid molluscs to prolonged aerial exposure was investigated by the determination of heat production.Littorina saxatilis, inhabiting the upper eulittoral, reached a maximum metabolic activity during submersion (heat production: 3.26×10⁻³J s⁻¹ (g_adw)⁻¹. On the first three days of desiccation, the heat production was continuously reduced to 40% of the submersed value. A prolonged aerial exposure was lethal for this species. In the supralittoralL. neritoides, three stages of energy metabolism could be observed: An intermediate heat production during submersion (1.97×10⁻³Js⁻¹ (g_adw)⁻¹), an increased metabolism during the first hour of aerial exposure (heat production 204% of submersed value), and a minimal metabolism (39% of the submersed value and 19% of maximum value) during the following days and weeks of desiccation. Recovery depended on water salinity;L. saxatilis proved to be less euryhaline thanL. neritoides. Thus, the metabolic adaptations correlate with the level of littoral habitat; inactivity combined with a drastically reduced energy consumption is a metabolically economic way to survive in periodically dry environments.     

Parasitism and the burrowing depth of the beach hopper Talorchestia quoyana (Amphipoda: Talitridae)

Talitrid amphipods spend their days burrowed in sand to avoid predators as well as desiccation and heat stress, although other factors may influence burrowing depth. We investigated the potential role of mermithid nematode parasites in determining burrowing depth in the amphipod Talorchestia quoyana. Mermithids grow as parasites inside amphipods until they reach adulthood, when they must emerge from their host into moist sand to complete their life cycle and reproduce. When allowed to burrow to a depth of their choice in experimental situations, large amphipods burrowed deeper than small ones. In addition, deep-burrowing amphipods were more likely to be infected by mermithid nematodes, and harboured longer worms, on average, than amphipods that burrowed close to the sand surface. This last result is not an artefact of the larger size of deep-burrowing amphipods: the increase in worm length with increasing depth was found after statistical correction for host size. In other words, amphipods that burrowed deeper harboured longer worms than expected based on their body size, whereas those that stayed near the surface of the sand column harboured worms shorter than one would expect based on host size. This implies that the greater burrowing depth of infected amphipods is a consequence, and not a cause, of infection. These results emphasize the importance of parasitism as a determinant of the small-scale spatial distribution of their hosts.     

Acclimation and endogenous abscisic acid in the moss Physcomitrella patens during acquisition of desiccation tolerance

The moss Physcomitrella patens has been used as a model organism to study the induction of desiccation tolerance (DT), but links between dehydration rate, the accumulation of endogenous abscisic acid (ABA) and DT remain unclear. In this study, we show that prolonged acclimation of P. patens at 89% relative humidity (RH) [?16 MPa] can induce tolerance of desiccation at 33% RH (?153 MPa) in both protonema and gametophore stages. During acclimation, significant endogenous ABA accumulation occurred after 1 day in gametophores and after 2 days in protonemata. Physcomitrella patens expressing the ABA‐inducible EARLY METHIONINE promoter fused to a cyan fluorescent protein (CFP) reporter gene revealed a mostly uniform distribution of the CFP increasing throughout the tissues during acclimation. DT was measured by day 6 of acclimation in gametophores, but not until 9 days of acclimation for protonemata. These results suggest that endogenous ABA accumulating when moss cells experience moderate water loss requires sufficient time to induce the changes that permit cells to survive more severe desiccation. These results provide insight for ongoing studies of how acclimation induces metabolic changes to enable DT in P. patens.     

Subterranean termite open-air foraging and tolerance to desiccation: Comparative water relation of two sympatric Macrotermes spp. (Blattodea: Termitidae)

The foraging patterns of termites are strongly related to physiological limits in overcoming desiccation stress. In this study, we examined moisture preferences and physiological characteristics of Macrotermes carbonarius (Hagen) and M. gilvus (Hagen) as both exhibit conspicuous patterns of foraging activity. Despite both species showing no significant differences in calculated cuticular permeability, and percentage of total body water, they differed greatly in rate of water loss and surface area to volume ratio. For example, M. carbonarius which had a lower surface area to volume ratio (29.26-53.66) showed lower rate of water loss and percentage of total body water loss. This also resulted in higher LT₅₀ when exposed to extreme conditions (≈ 2% RH). However, contrasting observations were made in M. gilvus that has smaller size with higher surface area to volume ratio of 40.28-69.75. It is likely that the standard equation for calculating insect surface areas is inadequate for these termite species. The trend was further supported by the result of a moisture preference bioassay that indicated M. carbonarius had a broader range of moisture preference (between 5% and 20%) than M. gilvus which had a relatively narrow moisture preference (only 20%). These results explain why M. carbonarius can tolerate desiccation stress for a longer period foraging above-ground in the open air; while M. gilvus only forages below ground or concealed within foraging mud tubes.     

Foliar abscisic acid content underlies genotypic variation in stomatal responsiveness after growth at high relative air humidity

Background and Aims

Stomata formed at high relative air humidity (RH) respond less to abscisic acid (ABA), an effect that varies widely between cultivars. This study tested the hypotheses that this genotypic variation in stomatal responsiveness originates from differential impairment in intermediates of the ABA signalling pathway during closure and differences in leaf ABA concentration during growth.

Methods

Stomatal anatomical features and stomatal responsiveness to desiccation, feeding with ABA, three transduction elements of its signalling pathway (H₂O₂, NO, Ca²⁺) and elicitors of these elements were determined in four rose cultivars grown at moderate (60 %) and high (90 %) RH. Leaf ABA concentration was assessed throughout the photoperiod and following mild desiccation (10 % leaf weight loss).

Key Results

Stomatal responsiveness to desiccation and ABA feeding was little affected by high RH in two cultivars, whereas it was considerably attenuated in two other cultivars (thus termed sensitive). Leaf ABA concentration was lower in plants grown at high RH, an effect that was more pronounced in the sensitive cultivars. Mild desiccation triggered an increase in leaf ABA concentration and equalized differences between leaves grown at moderate and high RH. High RH impaired stomatal responses to all transduction elements, but cultivar differences were not observed.

Conclusions

High RH resulted in decreased leaf ABA concentration during growth as a result of lack of water deficit, since desiccation induced ABA accumulation. Sensitive cultivars underwent a larger decrease in leaf ABA concentration rather than having a higher ABA concentration threshold for inducing stomatal functioning. However, cultivar differences in stomatal closure following ABA feeding were not apparent in response to H₂O₂ and downstream elements, indicating that signalling events prior to H₂O₂ generation are involved in the observed genotypic variation.     

Distinct contractile and cytoskeletal protein patterns in the Antarctic midge are elicited by desiccation and rehydration

Desiccation presents a major challenge for the Antarctic midge, Belgica antarctica. In this study, we use proteomic profiling to evaluate protein changes in the larvae elicited by dehydration and rehydration. Larvae were desiccated at 75% relative humidity (RH) for 12 h to achieve a body water loss of 35%, approximately half of the water that can be lost before the larvae succumb to dehydration. To evaluate the rehydration response, larvae were first desiccated, then rehydrated for 6 h at 100% RH and then in water for 6 h. Controls were held continuously at 100% RH. Protein analysis was performed using 2‐DE and nanoscale capillary LC/MS/MS. Twenty‐four identified proteins changed in abundance in response to desiccation: 16 were more abundant and 8 were less abundant; 84% of these proteins were contractile or cytoskeletal proteins. Thirteen rehydration‐regulated proteins were identified: 8 were more abundant and 5 were less abundant, and 69% of these proteins were also contractile or cytoskeletal proteins. Additional proteins responsive to desiccation and rehydration were involved in functions including stress responses, energy metabolism, protein synthesis, glucogenesis and membrane transport. We conclude that the major protein responses elicited by both desiccation and rehydration are linked to body contraction and cytoskeleton rearrangements.     

Effect of desiccation to low moisture content on germination,synchronization of root emergence,and plantlet regeneration of black spruce somatic embryos

A desiccation protocol was developed to evaluate the effect of different levels of desiccation on germination and plantlet regeneration of black spruce somatic embryos. Large desiccation chambers (80 l) with four liters of saturated salt solutions provided constant relative humidities (RH) of 63, 79, 88, and 97% (± 2%). Under these conditions, an embryo mass of 10 mg always dried fast even at 97% RH. In contrast, an embryo mass of 80 mg generated different kinetics of water loss, from fast drying at 63% RH to slow drying at 97% RH. Drying rates similar to those obtained with 80 mg embryos were also generated by combining 40 mg embryos with 40 mg water. The effects of drying rate and embryo MC on germination rate, root elongation, and plantlet regeneration were examined. A fast drying rate to 4–5% embryo MC, obtained under 63% RH, was detrimental to germination and plantlet development. However slower drying rates, obtained under 79–97% RH and generating 7–19% MC in the embryos, gave developmental responses similar to the control. Synchronization of root emergence was improved only for embryos desiccated to approx. 16% MC under 97% RH. The optimal desiccation protocol using large desiccation chamber at 97% RH and a constant embryo mass of 40 mg embryos plus 40 mg water was applied to five genotypes of black spruce. For all genotypes, desiccated embryos gave plantlet regeneration rates similar to the control undesiccated embryos. This revised version was published online in June 2006 with corrections to the Cover Date.     

Thermal tolerance,evaporative water loss,air-water oxygen consumption and zonation of intertidal prosobranchs: a new synthesis

Duration of emergence increases with tidal height on rocky shores therefore, emergence adaptations in intertidal species such as littorine and other prosobranch gastropods have been considered correlated with zonation patterns; temperature tolerance, desiccation resistance and aerial respiration rate all commonly assumed to increase progressively with increasing zonation level. Such direct correlations are rarely observed in nature. Maximal aerial gas exchange occurs in mid-shore, not high shore species. Temperature tolerance and desiccation resistance do not increase directly with shore height. Thus, hypotheses regarding physiological correlates of zonation require revaluation. A new hypothesis is presented that the high tide mark presents a single major physiological barrier on rocky shores. Above it, snails experience prolonged emergence and extensive desiccation; below it, predictable submergence and rehydration with each tidal cycle. Thus, desiccation stress is minimal below the high tide mark and maximal above it. Therefore, species restricted below high tide (the eulittoral zone) should display markedly different adaptive strategies to emergence than those above it (the eulittoral fringe). A review of the literature indicated that adaptations in eulittoral species are dominated by those allowing maintenance of activity and foraging in air including: evaporative cooling; low thermal tolerance; elevated aerial O₂ uptake rates; and high capacity for radiant heat absorption. Such adaptations exacerbate evaporative water loss. In contrast, species restricted to the eulittoral fringe display adaptive strategies that minimize desiccation and prolong survival of emergence including: foot withdrawal, preventing heat conduction from the substratum; aestivation in air; elevated thermal tolerance reducing necessity for evaporative cooling; position maintenance by cementation to the substratum and increased capacity for heat dissipation. In order to test of this hypothesis the upper thermal limits, tissue and substratum temperatures on emergence in direct sunlight and evaporative water loss and tissue temperatures on emergence in 40 °C were evaluated for specimens of six species of eulittoral and eulittoral fringe gastropods from a granite shore on Princess Royal Harbour near Albany, Western Australia. The results were consistant with adaptation to the proposed desiccation barrier at high tide. The eulittoral species, Austrocochlea constricta, Austrocochlea concamerata, Nerita atramentosa and Lepsiella vinosa, displayed adaptations dominated by maintenance of activity and foraging during emergence while the eulittoral fringe littorine species, Bembicium vittatum and Nodilittorina unifasciata displayed adaptations dominated by minization of activity and evaporative water loss during emergence. The evolution of adaptations allowing tolerance of prolonged desiccation have allowed littorine species to dominate high intertidal rocky shore gastropod faunas throughout the world's oceans.     

Membrane Organization of the Desiccation-Tolerant Moss Tortula ruralis in Dehydrated States

Membrane organization of the desiccation tolerant moss Tortula ruralis was studied in several intensely dehydrated states (75% relative humidity [RH], 90% RH, plasmolysis in molar salt, freezing to −20°C) by ³¹P nuclear magnetic resonance and ultrastructural analyses. Both methods revealed that even at 75% RH (−400 bars), the moss cellular membranes retained extended phospholipid bilayers. Ultrastructural analyses of the fully hydrated moss showed an extensive proliferation of membrane vesicles in the endoplasmic reticulum. During dehydration, these vesicles form layers of membrane under the plasmalemma and in some cases appear to fuse with the surface membrane. This suggests that these vesicles may serve as a reservoir of membranes to accommodate for membrane surface area changes during desiccation and subsequent rehydration.     

Smaller stomata require less severe leaf drying to close: A case study in Rosa hydrida

Stomata formed at high relative air humidity (RH) close less as leaf dries; an effect that varies depending on the genotype. We here quantified the contribution of each stomatal response characteristic to the higher water loss of high RH-grown plants, and assessed the relationship between response characteristics and intraspecific variation in stomatal size. Stomatal size (length multiplied by width), density and responsiveness to desiccation, as well as pore dimensions were analyzed in ten rose cultivars grown at moderate (60%) or high (85%) RH. Leaf morphological components and transpiration at growth conditions were also assessed. High growth RH resulted in thinner (11%) leaves with larger area. A strong positive genetic correlation of daytime and nighttime transpiration at either RH was observed. Stomatal size determined pore area (r = 0.7) and varied by a factor of two, as a result of proportional changes in length and width. Size and density of stomata were not related. Following desiccation, high RH resulted in a significantly lower (6–19%) decline of transpiration in three cultivars, whereas the relative water content (RWC) of high RH-expanded leaflets was lower (29–297%) in seven cultivars. The lower RWC of these leaflets was caused by (a) higher (33–72%) stable transpiration and/or (b) lower (12–143%) RWC at which this stable transpiration occurred, depending on the cultivar. Stomatal size was significantly correlated with both characteristics (r = 0.5 and -0.7, respectively). These results indicate that stomatal size explains much of the intraspecific variation in the regulation of transpiration upon water deprivation on rose.     

Desiccation tolerance in Physcomitrella patens: Rate of dehydration and the involvement of endogenous abscisic acid (ABA)

The moss Physcomitrella patens , a model system for basal land plants, tolerates several abiotic stresses, including dehydration. We previously reported that Physcomitrella patens survives equilibrium dehydration to ?13 MPa in a closed system at 91% RH. Tolerance of desiccation to water potentials below ?100 MPa was only achieved by pretreatment with exogenous abscisic acid (ABA). We report here that gametophores, but not protonemata, can survive desiccation below ?100 MPa after a gradual drying regime in an open system, without exogenous ABA. In contrast, faster equilibrium drying at 90% RH for 3–5 days did not induce desiccation tolerance in either tissue. Endogenous ABA accumulated in protonemata and gametophores under both drying regimes, so did not correlate directly with desiccation tolerance. Gametophores of a Ppabi3a/b/c triple knock out transgenic line also survived the gradual dehydration regime, despite impaired ABA signaling. Our results suggest that the initial drying rate, and not the amount of endogenous ABA, may be critical in the acquisition of desiccation tolerance. Results from this work will provide insight into ongoing studies to uncover the role of ABA in the dehydration response and the underlying mechanisms of desiccation tolerance in this bryophyte.     

Desiccation tolerance and drought acclimation in the Antarctic collembolan Cryptopygus antarcticus

The availability of water is recognized as the most important determinant of the distribution and activity of terrestrial organisms within the maritime Antarctic. Within this environment, arthropods may be challenged by drought stress during both the austral summer, due to increased temperature, wind, insolation, and extended periods of reduced precipitation, and the winter, as a result of vapor pressure gradients between the surrounding icy environment and the body fluids. The purpose of the present study was to assess the desiccation tolerance of the Antarctic springtail, Cryptopygus antarcticus, under ecologically-relevant conditions characteristic of both summer and winter along the Antarctic Peninsula. In addition, this study examined the physiological changes and effects of mild drought acclimation on the subsequent desiccation tolerance of C. antarcticus. The collembolans possessed little resistance to water loss under dry air, as the rate of water loss was >20% h(-1) at 0% relative humidity (RH) and 4 degrees C. Even under ecologically-relevant desiccating conditions, the springtails lost water at all relative humidities below saturation (100% RH). However, slow dehydration at high RH dramatically increased the desiccation tolerance of C. antarcticus, as the springtails tolerated a greater loss of body water. Relative to animals maintained at 100% RH, a mild drought acclimation at 98.2% RH significantly increased subsequent desiccation tolerance. Drought acclimation was accompanied by the synthesis and accumulation of several sugars and polyols that could function to stabilize membranes and proteins during dehydration. Drought acclimation may permit C. antarcticus to maintain activity and thereby allow sufficient time to utilize behavioral strategies to reduce water loss during periods of reduced moisture availability. The springtails were also susceptible to desiccation at subzero temperatures in equilibrium with the vapor pressure of ice; they lost approximately 40% of their total body water over 28 d when cooled to -3.0 degrees C. The concentration of solutes in the remaining body fluids as a result of dehydration, together with the synthesis of several osmolytes, dramatically increased the body fluid osmotic pressure. This increase corresponded to a depression of the melting point to approximately -2.2 degrees C, and may therefore allow C. antarcticus to survive much of the Antarctic winter in a cryoprotectively dehydrated state.     

A comprehensive analysis of the physiological and anatomical components involved in higher water loss rates after leaf development at high humidity

To better understand the poor regulation of water loss after leaf development at high relative air humidity (RH), the relative importance of the physiological and anatomical components was analyzed focusing on cultivars with a contrasting sensitivity to elevated RH. The stomatal responsiveness to three closing stimuli (desiccation, abscisic acid feeding, light/dark transition), as well as several stomatal features (density, index, size and pore dimensions) and the cuticular transpiration rate (CTR) were determined in four rose cultivars, grown under moderate (60%) and high (95%) RH. Moreover, the effects of changes in stomatal density and pore dimensions on the stomatal conductance (g_s) were quantified using a modified version of the Brown and Escombe equation. Higher water loss, as a result of plant growth at high RH, was primarily caused by an increase in residual g_s, and to a lesser extent due to higher CTR. It was estimated that in leaflets subjected to desiccation the enhanced g_s in high RH- as compared to moderate RH-grown plants was mostly due to poor stomatal functionality and to a lesser extent the combined result of higher stomatal density and longer pore length. It is concluded that the reduced degree and, specially, the reduced rate of stomatal closure are the primary causes of the large genotypic variation in the control of water loss in high RH-grown plants. Furthermore, it was found that although changes in stomatal length have no influence on stomatal functionality, changed anatomical features per se represent a significant and direct contribution to the increased water loss.     

On some ecological aspects of the lugworm <Emphasis Type="Italic">Abarenicola affinis chiliensis</Emphasis> Wells, 1963 (Polychaeta: Scolecida: Arenicolidae) from shallow soft bottoms of northern Chile

We conducted an ecological study of a population of the polychaete Abarenicola affinis chiliensis in the shallow subtidal zone of Caleta Guardiamarina Riquelme in northern Chile. The study area is characterized by a hypoxic bottom consisting of fine sands and a silt-clay mixture, with high values of total organic matter content (TOM > 20%). During the two study periods (spring 1996 and 1997), the numerical contribution of A. affinis chiliensis to the total macrofauna was greater than 80%. A similar pattern was recorded for biomass, with A. affinis chiliensis surpassing 95% of the total macrofauna biomass. Statistical analyses did not detect significant differences in environmental and biotic variables between years (with the exception of salinity, which increased slightly), indicating temporal stability of these variables over the study period. Our results do not support the hypothesis of Wells that seawater temperatures act as a physical barrier impeding the dispersal of Abarenicola species. This hypothesis should be reexamined for some representatives of Abarenicola on the Pacific coast of South America.     

Gill remodelling during terrestrial acclimation in the amphibious fish Polypterus senegalus

Fishes are effectively weightless in water due to the buoyant support of the environment, but amphibious fishes must cope with increased effective weight when on land. Delicate structures such as gills are especially vulnerable to collapse and loss of surface area out of water. We tested the ‘structural support’ hypothesis that amphibious Polypterus senegalus solve this problem using phenotypically plastic changes that provide mechanical support and increase stiffness at the level of the gill lamellae, the filaments, and the whole arches. After 7 d in terrestrial conditions, enlargement of an inter-lamellar cell mass filled the water channels between gill lamellae, possibly to provide structural support and/or reduce evaporative water loss. Similar gill remodelling has been described in several other actinopterygian fishes, suggesting this may be an ancestral trait. There was no change in the mechanical properties or collagen composition of filaments or arches after 7 days out of water, but 8 months of terrestrial acclimation caused a reduction in gill arch length and mineralized bone volume. Thus, rather than increasing the size and stiffness of the gill skeleton, P. senegalus may instead reduce investment in supportive gill tissue while on land. These results are strikingly similar to the evolutionary trend of gill loss that occurred during the tetrapod invasion of land, raising the possibility that genetic assimilation of gill plasticity was an underlying mechanism.