High Intertidal Habitat Research Articles

Global diversification and evolutionary history of onchidiid slugs (Gastropoda, Pulmonata)

Many marine species are specialized to specific parts of a habitat. In a mangrove forest, for instance, species may be restricted to the mud surface, the roots and trunks of mangrove trees, or rotting logs, which can be regarded as distinct microhabitats. Shifts to new microhabitats may be an important driver of sympatric speciation. However, the evolutionary history of these shifts is still poorly understood in most groups of marine organisms, because it requires a well-supported phylogeny with relatively complete taxon sampling. Onchidiid slugs are an ideal case study for the evolutionary history of habitat and microhabitat shifts because onchidiid species are specialized to different tidal zones and microhabitats in mangrove forests and rocky shores, and the taxonomy of the family in the Indo-West Pacific has been recently revised in a series of monographs. Here, DNA sequences for onchidiid species from the North and East Pacific, the Caribbean, and the Atlantic are used to reconstruct phylogenetic relationships among Onchidella species, and are combined with new data for Indo-West Pacific species to reconstruct a global phylogeny of the family. The phylogenetic relationships of onchidiid slugs are reconstructed based on three mitochondrial markers (COI, 12S, 16S) and three nuclear markers (28S, ITS2, H3) and nearly complete taxon sampling (all 13 genera and 62 of the 67 species). The highly-supported phylogeny presented here suggests that ancestral onchidiids most likely lived in the rocky intertidal, and that a lineage restricted to the tropical Indo-West Pacific colonized new habitats, including mudflats, mangrove forests, and high-elevation rainforests. Many onchidiid species in the Indo-West Pacific diverged during the Miocene, around the same time that a high diversity of mangrove plants appears in the fossil record, while divergence among Onchidella species occurred earlier, likely beginning in the Eocene. It is demonstrated that ecological specialization to microhabitats underlies the divergence between onchidiid genera, as well as the diversification through sympatric speciation in the genera Wallaconchis and Platevindex. The geographic distributions of onchidiid species also indicate that allopatric speciation played a key role in the diversification of several genera, especially Onchidella and Peronia. The evolutionary history of several morphological traits (penial gland, rectal gland, dorsal eyes, intestinal loops) is examined in relation to habitat and microhabitat evolutionary transitions and suggests that the rectal gland of onchidiids is an adaptation to high intertidal and terrestrial habitats.

Characterization of the complete mitochondrial genome of the tidewater goby, Eucyclogobius newberryi (Gobiiformes; Gobiidae; Gobionellinae) and its phylogenetic implications

The tidewater goby, Eucyclogobius newberryi, lives exclusively along the coast of California, primarily in high intertidal habitat seasonally isolated from the full effect of the tide. Rapid declines in the number of populations led to its listing as an endangered species in 1994. Here, we first reported the complete mitochondrial genome of this species, which is 16,526 bp in length, containing 13 protein-coding genes, two rRNA genes, 22 tRNA genes, a putative control region and one origin of replication on the light-strand (OL). The overall base composition is 25.6, 28.3, 27.8 and 18.3% for A, T, C and G, respectively. The 13 PCGs encode 3801 amino acids in total, twelve of which use the initiation codon ATG except COI use GTG. The most frequently used amino acid is leucine (16.24%), while cysteine acid (0.77%) is the least frequently used one. Besides, a phylogenetic tree based on the maximum likelihood method was constructed to explore the taxonomic status of E. newberryi, which strongly supported it had a closest relationship with the genus Gillichthys. These results would provide comprehensive phylogenetic relationship within Gobionellinae and further conservation strategies for this species.

First record and conservation value of Periophthalmus malaccensis Eggert from Borneo, with ecological notes on other mudskippers (Teleostei: Gobiidae) in Brunei

The mudskipper Periophthalmus malaccensis is first recorded in Borneo, from two mangrove areas of Brunei Darussalam. The distribution and habitat types where this and other ecologically similar mudskippers (Periophthalmus and Periophthalmodon species) were found in Brunei are also described, from field surveys conducted in Bruneian coastal wetlands from 2013 to 2015. P. malaccensis has a relatively restricted geographic distribution, and probably got extinct in historical times in Singapore. In Brunei, this species occurs at low population density in high intertidal habitats, which are highly impacted by anthropogenic destruction and fragmentation. For these reasons, the conservation status of this species should be evaluated.

Virtual increase or latent loss? A reassessment of mangrove populations and their conservation in Guangdong, southern China

Contrary to the global trend, the area of mangrove in Guangdong Province, southern China, has been increasing over the last two decades. Currently, three exotic mangrove species have been introduced for large-scale afforestation since 1985. A reassessment of the overall status of the mangrove species, habitat change, population of introduced species, was conducted through a comprehensive literature review as well as field investigations covering 96 sites. The success of conservation efforts is also evaluated. Upstream and high intertidal habitats are more vulnerable than downstream and lower intertidal ones, with habitat alteration being the biggest threats. Five mangrove species have narrow distributional extents with small populations, which could incur regional extinction. With the introduced species having naturalized at 42 sites, their role in mangrove management needs to be reconsidered. These findings collectively suggest a need to manage latent species loss and habitat degradation beyond the apparent increase in mangrove area and protection.

A new species of Echinolittorina Habe, 1956 (Gastropoda: Littorinidae) from the Quaternary of Chile

We describe a new fossil littorinid species, Echinolittorina nielseni sp. nov., from the Quaternary Caldera Strata, Region de Atacama, northern Chile. Fossils of littorinids are globally rare because of their high-intertidal habitat on rocky shores. The new species has a large, broad shell with strong spiral ribs and an angled periphery, differing from the two living littorinids currently found along the coasts of mainland Chile and from all the extant species distributed in the southeastern Pacific. In comparison with the living Chilean Echinolittorina peruviana, the new species shows stronger ribs and more inflated whorls, but they share an unusual detail in the irregular arrangement of spiral sculpture. We hypothesize that the new species may be ancestral or sister to E. peruviana and discuss the adaptive significance of shell sculpture.

Invasion and morphological variation of the non-indigenous barnacle Chthamalus challengeri (Hoek, 1883) in Yangshan Port and its surrounding areas

Invasive species generally possess unique characteristics that allow them to survive the invasion process in order to establish and spread in new habitats. Successful invaders must resist both physical and physiological stresses associated with the changing environment. A common littoral barnacle, Chthamalus challengeri Hoek, 1883 (Crustacea, Cirripedia), which is native to Japan, South Korea and northern China, has become established in the high-littoral zone adjacent to Yangshan Port, Shanghai, China. A comparison of the morphology of Chthamlus species from Zhoushan archipelago with previous description indicates the occurrence of C. challengeri. The new immigrant becomes a dominant species in certain high-intertidal habitats of the adjacent area to of Yangshan Port. C. challengeri was found in part of sampling sites in Zhoushan in 2010; however, it dispersed to all the eleven sampling sites in 2012. Densities of C.challengeri had increased over 10 times in the last 2 years, with the highest mean value reaching 39533 ± 6243 ind. m−2 in the new habitat. The specific ratios of both operculum area (Sa) to base area (SA) and average height of parietal plates (H) to length of base (L) revealed that C. challengeri displays morphological changes to resist stronger currents in the new habitats for invasion.

Shallow gene pools in the high intertidal: extreme loss of genetic diversity in viviparous sea stars ( Parvulastra )

We document an extreme example of reproductive trait evolution that affects population genetic structure in sister species of Parvulastra cushion stars from Australia. Self-fertilization by hermaphroditic adults and brood protection of benthic larvae causes strong inbreeding and range-wide genetic poverty. Most samples were fixed for a single allele at nearly all nuclear loci; heterozygotes were extremely rare (0.18%); mitochondrial DNA sequences were more variable, but few populations shared haplotypes in common. Isolation-with-migration models suggest that these patterns are caused by population bottlenecks (relative to ancestral population size) and low gene flow. Loss of genetic diversity and low potential for dispersal between high-intertidal habitats may have dire consequences for extinction risk and potential for future adaptive evolution in response to climate and other selective agents.

A synthesis of variability in nearshore Alaskan marine populations

One of the primary goals of any monitoring program is to detect anthropogenic changes; however natural variability can be so high that it prevents detection of human-induced effects. This project synthesized existing data to identify patterns of temporal variation within nearshore marine habitats in the Gulf of Alaska and was motivated by the need for monitoring in this environment vulnerable to oil spills, such as the Exxon Valdez. I collected 786 time series that were greater than 2 years in length from unimpacted (control) sites for 226 species. Temporal variability (CV) varied widely from 1% to 447%, averaged 89%, and was not significantly different among marine populations of birds, mammals, fish, algae and invertebrates. Temporal variability of invertebrates and algae were not significantly different when abundance was measured as biomass, percent cover or density. Both invertebrates and algae showed higher variability in low intertidal habitat compared to high intertidal habitat. For invertebrates, populations on bedrock substrates had greater temporal variability than those on cobble or soft sediment substrates, while there was not a significant difference for algae on either cobble or bedrock substrates. Many of these results are surprising, given differences in movement patterns, survey methods, life histories, and so forth. These results suggest that no single group, measure or habitat will be better or worse for monitoring and detecting change. Understanding variability is a difficult task, but until we tackle it, we will likely not understand or have predictive capabilities in ecological populations and communities.

THE COMMUNITY STRUCTURE OF WESTERN ATLANTIC PATAGONIAN ROCKY SHORES

We examined the structure of rocky intertidal communities on the central Patagonian coast of Argentina. Extensive beds of the mussel Perumytilus purpuratus cover wave-exposed headlands from the low to extreme high intertidal (>95%), and a diverse assemblage of diminutive mobile invertebrates including limpets, starfish, and crabs live exclusively in the mussel bed matrix to avoid physical stress. On nearby wave-protected rocky shores, the high intertidal habitat is dominated by bare space (>85%) with mussels restricted to tide pools and crevices. Mussel beds cover the middle intertidal, while the low intertidal habitat is dominated by the erect coralline alga Corallina officanalis. These patterns are driven overwhelmingly by variation in extreme physical conditions. Desiccation stress generated by the dry southern trade winds is harsher than in any previously studied rocky intertidal system, including the Gulf of Panama, by >30% and is more severe on wave-protected than wave-exposed shores. Transplant experiments suggest that on wave-protected shores desiccation stress limits the upper distribution of mussels in the high intertidal and Corallina in the mid-intertidal, but at low intertidal elevations Corallina outcompetes mussels, restricting mussel distribution to mid-intertidal elevations. Transplant experiments also demonstrated that the coralline alga is precluded from wave-exposed shores by wave stress. Recovery from disturbance is unusually slow, ostensibly due to extreme physical stress. Consumer pressure is weak, with no common predaceous crabs or snails, and grazing by limpets showed limited control of community development, mostly by regulating ephemeral algae. Patagonian rocky shore communities are exposed to unusually harsh physical conditions and consequently are more strongly organized by physical stress than previously studied rocky intertidal communities.

Breathing Air in Air: In What Ways Might Extant Amphibious Fish Biology Relate to Prevailing Concepts about Early Tetrapods, the Evolution of Vertebrate Air Breathing, and the Vertebrate Land Transition?

The air-breathing fishes have heuristic importance as possible models for the Paleozoic evolution of vertebrate air breathing and the transition to land. A recent hypothesis about this transition suggests that the diverse assemblage of marine amphibious fishes occurring primarily in tropical, high intertidal zone habitats are analogs of early tetrapods and that the intertidal zone, not tropical freshwater lowlands, was the springboard habitat for the Devonian land transition by vertebrates. Here we argue that selection pressures imposed by life in the intertidal zone are insufficient to have resulted in the requisite aerial respiratory capacity or the degree of separation from water required for the vertebrate land transition. The extant marine amphibious fishes, which occur mainly on rocky shores or mudflats, have reached the limit of their niche expansion onto land and remain tied to water by respiratory structures that are less efficient in air and more vulnerable to desiccation than lungs. We further argue that evolutionary contingencies actuated by the Devonian origin of the tetrapods marked a critical point of divergence for a way of life in which selection pressures would operate on the physiology, morphology, and natural history of the different vertebrate groups. While chronically hypoxic and shallow water conditions in the habitats of some primitive bony fishes and some amphibians appear similar to the conditions that prevailed in the Devonian, markedly different selection pressures have operated on other amphibians and bony fishes over the 300 million years since the vertebrate land transition. For example, both egg development and larval metamorphosis in extant amphibians are geared mainly toward compensating for the uncertainty of habitat water quality or even the absence of water by minimizing the time required to develop there. In contrast, reproduction by most intertidal (and amphibious) fishes, all of which are teleosts, remains dependent on a planktonic larval phase and is characterized by specializations (brooding) that minimize overdispersal and maximize recruitment back to the littoral habitat.

The natural history, thermal physiology, and ecological impacts of intertidal mesopredators, Oedoparena spp. (Diptera: Dryomyzidae)

Abstract. The ecological roles of small (1–1000 mg) predators in benthic marine systems are poorly understood. We investigated the natural history and predatory impact of one group of such mesopredators—larvae of dipteran flies in the genus Oedoparena—which prey on intertidal barnacles. We 1) quantified patterns of larval Oedoparena distribution and abundance in the Northwest Straits of Washington State, USA, 2) determined larval physiological tolerance limits in the laboratory, and 3) conducted a manipulative field experiment to assess the role of microhabitat temperature on predation rates in Oedoparena. Members of Oedoparena in Washington are univoltine, with peak larval abundance in late spring and early summer. Infestation frequencies in the barnacles Balanus glandula and Chthamalus dalli were as high as 22% and 35%, respectively. In laboratory studies, larvae of O. glauca were able to tolerate temperatures up to 37°C; however, this temperature is often exceeded in high intertidal habitats. In a field manipulation using experimental shades, we demonstrate that the alleviation of physiological stress greatly increased the abundance of larvae of Oedoparena spp. As a result of increased larval densities under shades, adult B. glandula mortality increased from 5% to nearly 30%, and C. dalli mortality increased from less than 20% to over 60%. Because high intertidal barnacles serve as food and habitat for a diverse array of species, Oedoparena spp. have the potential to play a major role in structuring high intertidal communities, particularly in cooler microhabitats.

Recruitment in the intertidal limpet Lottia digitalis (Patellogastropoda: Lottiidae) may be driven by settlement cues associated with adult habitat

The ribbed limpet, Lottia digitalis, is found high in rocky intertidal habitat throughout its geographic range. In order to identify likely natural settlement locations for larvae of this species, laboratory-reared larvae were settled onto substrata collected from within and near an adult L. digitalis habitat. Of larvae exposed to rock chiseled from within high-intertidal adult habitat, 31.0% and 23.3% underwent metamorphosis during two separate experiments. Similarly, an unidentified filamentous green alga that was isolated from this rock induced metamorphosis in 26.6% and 8.7% of larvae during additional experiments. In contrast, larvae did not metamorphose upon bare rocks or upon rocks encrusted with a crustose corraline alga (CCA) that were collected from lower intertidal zones, nor did they metamorphose upon the macroalgae Ulva sp., Enteromorpha contorta, Alaria marginata, or Polysiphonia sp. The presence of mucus from adult conspecifics during these experiments did not enhance metamorphosis onto rock taken from adult habitat, but it did induce metamorphosis in an average of 13.5% and 7.0% of larvae introduced to the mid-intertidal bare rock and CCA substrata, respectively. Finally, 38.0% and 34.4% of larvae from two experiments underwent metamorphosis when exposed to the high-intertidal barnacle Pollicipes polymerus. These results suggest that larval settlement, rather than differential post-settlement mortality and/or migration, drives recruitment of L. digitalis in high-intertidal habitats.

Biogeography and microhabitat variation in temperate algal‐invertebrate symbioses: zooxanthellae and zoochlorellae in two Pacific intertidal sea anemones, Anthopleura elegantissima and A. xanthogrammica

Abstract. Temperate sea anemones in the genus Anthopleura are unique among cnidarians in harboring two phylogenetically distinct symbiotic algae, zooxanthellae (golden‐brown dinophytes, Symbiodinium) and zoochlorellae (green chlorophytes). To determine whether their physiological differences generate patterns in anemone habitat and biogeographic distribution, we sampled symbiotic algae in the small clonal A. elegantissima and the large solitary A. xanthogrammica at 8 field sites (and the other large solitary Anthopleura species at one site) spanning 18° of latitude along 2500 km of the Pacific coast of North America. We found that zoochlorellae predominate in low intertidal habitats and northerly latitudes and in A. xanthogrammica, while zooxanthellae constitute the majority of symbionts in high intertidal habitats and more southerly latitudes and in A. elegantissima. These data are consistent with published predictions based on photosynthetic efficiency of the two algae under varied temperature and light regimes in the laboratory. This anemone‐algal system provides a potential biological signal of benthic intertidal communities' responses to El Niño events and long‐term climate changes in the Pacific.

EXPERIMENTAL EVIDENCE FOR FACTORS MAINTAINING PLANT SPECIES DIVERSITY IN A NEW ENGLAND SALT MARSH

Understanding the processes that maintain species diversity patterns is vital for conserving and managing communities. In this paper, we examine the effect of plant species interactions on the maintenance of plant species number across intertidal zones characterized by different soil conditions in a New England salt marsh community. In this system, plant species number is low in the high intertidal and lower middle intertidal, and high in the upper middle intertidal. To understand the causes of this unimodal pattern of change in species diversity, we experimentally tested for the influence of competition, facilitation, and physical factors across this physical gradient. We established plots with and without plant neighbors, transplanted plant species into these plots, and measured mortality, leaf area, and flower number for a single growing season. We found that competition plays a more important role in the high intertidal habitat by decreasing leaf area and flower production and causing 100% mortality of one of the species tested. In contrast, physical factors are critical in the lower middle intertidal, causing 100% mortality of three of the four species tested, with or without neighbor interactions. In the upper middle intertidal, direct positive interactions were important to the higher species number. Three of the four species had 100% mortality without neighbors but minimal mortality or leaf area loss with neighbors. These positive effects were due to one particular facilitator species, Juncus gerardi L., which ameliorates the soil conditions that develop in its absence thus allowing other species to co-occur. Without Juncus, our data predict that plant species number would not be as high in the upper middle intertidal. We show that the rise in species number in the upper middle intertidal is dependent on three co-occurring conditions: the absence of a competitive dominant, less harsh physical conditions than the lower middle intertidal, and the presence of a facilitator species.

Endogenous tidal and semilunar moulting rhythms in early juvenile shore crabs Carcinus maenas: implications for adaptation to a high intertidal habitat

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 191:257-266 (1999) - doi:10.3354/meps191257 Endogenous tidal and semilunar moulting rhythms in early juvenile shore crabs Carcinus maenas: implications for adaptation to a high intertidal habitat Chaoshu Zeng*, Pere Abello**, Ernest Naylor School of Ocean Sciences, University of Wales-Bangor, Menai Bridge, Anglesey LL59 5EY, United Kingdom Present addresses: *Seikai National Fisheries Research Institute, 49 Kokubumachi, Nagasakishi, Nagasaki 850-0951, Japan E-mail: cszeng@snf.affrc.go.jp **Institut de Ciencies del Mar (CSIC), Passeig Joan de Borbo s/n, 08039 Barcelona, Spain ABSTRACT: Newly recruited early juveniles of the shore crab Carcinus maenas in North Wales are most abundant on the high intertidal of gravelly shores and, unlike adults, they do not undertake up-and-down shore migration with tides. Freshly collected first and second instar juvenile crabs showed persistent circatidal rhythms of moulting when maintained in constant laboratory conditions. Peaks of moulting occurred around expected times of high tides, with few crabs moulting at other times. The circatidal moulting patterns were similar in crabs collected at different stages of the neap-spring cycle. Daily monitoring of moulting in the laboratory of 23 batches of early crabs, collected from the high intertidal at 1 to 3 d intervals over 2 spring-neap cycles, further showed a marked circasemilunar moulting pattern superimposed on the tidal moulting rhythms. Significantly more crabs moulted within 24 h after collection when collected during spring tides than when collected at neaps. Moreover, the daily percentage moulting of the crabs on consecutive days after collection clearly followed the trend of predicted tidal height changes. Crabs collected on days of increasing tidal amplitude showed increasing moulting rates on the days after collection, whilst a decreasing trend of daily moulting rate was found if they were collected on days of decreasing tidal amplitude. For crabs collected at minimum neaps, when water did not reach the high intertidal even at high tides, virtually no moulting took place on the following days. Moulting at high tide, particularly during spring high tides, appears to be an adaptation to a high intertidal habitat which is only inundated at certain times during semilunar and tidal cycles. For C. maenas early juveniles, which remain in the high intertidal even when tides recede, anticipation of the rising and falling of tides through endogenous physiological programming to avoid ecdysis at the time exposed to air has clear adaptive value. The coupling of circatidal and circasemilunar moulting rhythms, and their endogenous control, reported in the present study appears to be the first demonstration of such a phenomenon in a crustacean. KEY WORDS: Tidal and semilunar moulting rhythms · Endogenous · Carcinus maenas early juvenile · High intertidal habitat · Adaptation Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 191. Publication date: December 30, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.

External carbonic anhydrase and affinity for inorganic carbon in intertidal macroalgae

Sixteen intertidal macroalga species from the Gibraltar Strait (Southern Spain) have been examined for both their external carbonic anhydrase activity (CA; 4.2.1.1.) and their affinity for inorganic carbon (C i). Information on the ability to use HCO 3 − was obtained by measuring the effect of increasing seawater pH on the photosynthesis and by examining the O 2 evolution curves vs. C i concentration. The species exhibited a range of characteristics of C i utilisation: the green macroalgae were saturated at seawater C i concentration with K m ranging from 250.0 to 493.3 mmol m −3 and the photosynthetic O 2 production, expressed as conductance for CO 2 ranging from 432.8 to 16.1×10 −6 m s −1. Only three out of the eleven tested red and brown macroalgae were saturated at C i concentration of seawater and for these algae HCO 3 − use was clearly indicated by the high initial slope of the O 2 evolution rate vs. C i curve. The most efficient HCO 3 − users were species from the high intertidal habitats and rockpools. Two methods were used to infer the presence of external CA: standard method (direct enzyme assay) and use of acetazolamide (AZ), a non-permeant inhibitor of CA, in assays of photosynthetic rate. Some discrepancies between the two methods were found and we obtained indications that the use of AZ could be a more sensitive method to detect external CA. It so, we suggest that external CA might be present in all the macroalgae examined. There appeared to be no correlation between the ability to use HCO 3 − and the presence of external CA. Therefore, the presence of external CA was not a strong indication for an efficient use of HCO 3 −, although some dependence between the requirement for external CA and habitats was found.

An Explanation of the High Strain Diversity of a Self‐Fertilizing Hermaphroditic Fish

The tropical and subtropical estuarine fish Rivulus marmoratus is known for its synchronous, self—fertilizing hermaphroditism. It matures within 3 mo and can produce eggs throughout the year. These features would allow a particular strain to quickly colonize refuge such as crab holes in the high intertidal zone habitat. However, a high strain diversity has been found in the field. In order to explain this observation, we examined the phenotypic responses of R. marmoratus to variation in biotic and abiotic environmental factors over two consecutive generation and employed a modified Euler—Lotka equation to estimate and compare the net reproductive rate, R, among strains. We included five strains of R. marmoratus and manipulated levels of temperature, salinity, and food for the parental generation and levels of food only for the second generation. Low temperature (19°C) inhibited sexual maturity, precluding any egg production during the entire 6—mo study. For individuals kept at high temperature (26°C), salinity and food had a interactive effect on time to maturity, body size at maturity , and number of eggs produced. Using multivariate analysis of variance, we found a significant two—way interactive effect (foods levels for the parental and the second generations) on the five reproductive traits studied in the second generation. When each trait was analyzed separately, we found significant three—way interactions for time to maturity and total length and maturity. We found that the fitness values for each strain varied in different environmental combinations (3 salinites X 3 incubation salinities for eggs X 2 food levels). However, four of five strains had similar overall fitness. Despite a lack of genetic variation within each strain, R. marmoratus demonstrates considerable phenotypic variation in response to environmental changes. This feature and genetic variation among strains may explain how this species can survive and even prosper in an extremely variable and stressful environment.

Osmoregulation and epithelial permeability in two euryhaline bivalve molluscs: Mya arenaria and Geukensia demissa

Mya arenaria (subclass Heterodonta) survives for at least 5 wk in media of 100–1000 mOsm; Geukensia demissa (subclass Pteriomorphia) survives in media of 50–1000 mOsm. Tissue hydration does not increase either in G. demissa or in M. arenaria acclimated to diluted seawater (SW). The concentrations of Na +, K +, Mg 2+, and Cl − in the blood of G. demissa conform to ambient over the entire salinity range. The concentration of Ca 2+ in the blood of animals acclimated to media of ≤200 mOsm is ≈2.5 mM. Blood concentrations of Na +, Mg 2+, and Cl − largely conform to ambient in M. arenaria. In M. arenaria acclimated to SW of >200 mOsm, blood Ca 2+ conforms to ambient, but is significantly hyperionic in animals acclimated to 50 mOsmSW. The diffusional water, Na +, and Cl − permeabilities of isolated mantle tissue from Mya acclimated to 1000 mOsm SW are, respectively, 3.8 × 10 −4, 5.7 × 10 −5, and 2.7 × 10 −5 cm·s −1. The permeabilities of the mantle of G. demissa acclimated to 1000 mOsm SW are: H 2O, 1.3 × 10 −4, Na +, 4.0 × 10 −5, Cl −, 2.9 × 10 −5 cm·s −1. There is no consistent trend of a decrease in epithelial permeability in animals acclimated to dilute SW. The hypercalcemia that occurs in Mya acclimated to dilute SW also occures in other brackish water heterodontid clams. This response does not occur in brackish water pteriomorph bivalves. The relatively low water permeability of Geukensia is an adaptation to the high intertidal habitats preferred by this species. Mya, which has a higher permeability to water, is protected from severe desiccation stress by its deep burrowing habit.

Oxygen uptake and heart rate of the clam Polymesoda caroliniana Bosc in air and in seawater

The rates of oxygen uptake and heart beat, and tissue levels of alanine and succinate of the marsh clam Polymesoda caroliniana Bosc were measured in animals exposed either to air and or isosmotic, aerated seawater (SW). The oxygen consumption rate was variable, the mean rate was approx. 380 μl/h/g dry weight in SW and in air. Animals exposed to air for 24 h did not accumulate an oxygen debt. The heart rate of animals with the valves ajar was 13 in SW and 11 in air. In clams exposed to air for 24 h, the levels of alanine and succinate in the gill were, respectively, 6.5 and 3.9 μmol/g wet weight. The alanine content of gills from animals maintained in SW was 5.6; succinate was 4.4 μmol/g wet weight. The adductor muscles of clams in SW contained 8.9 μmol/g wet weight alanine and 2.8 μmol/g succinate. The adductor muscles of animals exposed to air for 24 h contained 9.7 μmol/g alanine and 2.4 μmol/g succinate. Polymesoda caroliniana is capable of the same rate of O 2 uptake wether in air or submerged in SW. There is no marked bradycardia during emersion, and no evidence of the accumulation of metabolites indicative of anaerobic metabolism. While these response are typical of those found in other bivalve species that occur in high intertidal habitats, the air-breathing capacity of P. caroliniana is exceptional.

Intraspecific Competition and Facilitation in a Northern Acorn Barnacle Population

The role of density—dependent mortality in shaping adult populations of the acorn barnacle, Semibalanus balanoides, was studied on a sheltered New England shore and revealed that high recruitment densities can have both positive and negative effects on barnacle survival. Recruitment greatly exceeded the capacity of the space to support adults in the three years of this study. At low tidal heights, recruits grew rapidly, experienced severe crowding, and rarely survived to reproduce. At higher tidal heights, individuals grew more slowly and were less affected by crowding, and many (°10%) survived to reproduce. Recruit density was manipulated to examine crowding effects on survival. At low tidal heights, no recruits survived to reproduce at any density, due to intraspecific crowding, algal overgrowth, and predation. At intermediate tidal heights, lower recruitment decreased intraspecific crowding and increased survivorship, while at high tidal heights, lower recruitment decreased survivorship. In the high intertidal habitat, S. balanoides survivorship was higher on boulders than on rock cobbles due to thermal buffering, and intraspecific crowding decreased rock and barnacle temperatures and facilitated survival. Shading high intertidal cobbles from solar radiation decreased maximum rock and barnacle temperatures, dramatically increased survivorship and eliminated the survivorship advantage of high densities. High recruitment of S. balanoides can lead to massive density—dependent mortality in physically benign habitats, but in physically stressful habitats high recruitment density may buffer individuals from physical stress and facilitate survival. Variation in the nature of density—dependent interactions along environmental gradients may be important in generating pattern in many natural populations.