Echinolittorina Malaccana Research Articles

Exploring the adaptability of the secondary structure of mRNA to temperature in intertidal snails based on SHAPE experiment.

RNA-based thermal regulation mechanism is an important strategy for organisms to cope with temperature changes. Inhabiting on intertidal rocky shore, a key interface of the ocean, atmosphere and terrestrial environments, intertidal species have developed variable thermal adaptation mechanisms, however, adaptions at the RNA level remain largely uninvestigated. For investigating the relationship between mRNA structural stability and species distribution, in the present study, the secondary structures of cytosolic malate dehydrogenase (cMDH) mRNAs of Echinolittorina malaccana, E. radiata and Littorina brevicula were determined using the selective 2'-hydroxyl acylation analyzed by primer extension (SHAPE), and the change in folding free energy of formation (ΔGfold) were calculated. The results showed that ΔGfold increased as the temperature increased. The difference in ΔGfold (ΔΔGfold) between two specific temperatures (0° to 25°C, 0° to 37°C, and 0° to 25°C) among the three species were different, and the ΔΔGfold value of E. malaccana was significantly lower than those of E. radiata and L. brevicula. The number of stems of cMDH mRNA of the snails decreased with temperature increased, and the breakpoint temperature of E. malaccana was the highest among them. The number of loops was also reduced with temperature increased, while the length of the loop structure increased accordingly. Consequently, these structural changes can potentially affect the translational efficiency of mRNA. These results implied that there were interspecific differences in the thermal stability of RNA secondary structures in intertidal snails, and these differences may be related to their distribution.

Announcing the 2021 Journal of Experimental Biology Outstanding Paper Prize shortlist and winner.

Announcing the 2021 Journal of Experimental Biology Outstanding Paper Prize shortlist and winner.

Aestivation duration and plasticity in the rocky-shore littorinid snail, Echinolittorina malaccana

Aestivation duration and plasticity in the rocky-shore littorinid snail, <i>Echinolittorina malaccana</i>

Behavioral repertoire of high-shore littorinid snails reveals novel adaptations to an extreme environment.

Species that inhabit high‐shore environments on rocky shores survive prolonged periods of emersion and thermal stress. Using two Hong Kong high‐shore littorinids (Echinolittorina malaccana and E. radiata) as models, we examined their behavioral repertoire to survive these variable and extreme conditions. Environmental temperatures ranged from 4°C in the cool season to 55.5°C in the hot season, with strong seasonal and daily fluctuations. In the hot season, both species allocated >35% of their activity budgets to stress‐mitigating thermoregulatory behaviors (e.g. standing, towering) and relatively small proportions to foraging (<20%) and reproduction (<10%). In the assumedly benign cool season, greater proportions (>70%) of activity budgets were allocated to stress mitigation behaviors (crevice occupation, aggregation formation). Both species exhibited multifunctional behaviors that optimized time use during their tidally‐constrained activity window in the hot season. Females mated while foraging when awash by the rising tide, and some males crawled on top of females prior to ceasing movement to form 'towers', which have both thermoregulatory benefits and reduce searching time for mates during subsequent activity. The function of such behaviors varies in a state‐dependent manner, for example, the function of trail following changes over an activity cycle from mate searching on rising tides, to stress mitigation on falling tides (aiding aggregation formation), and to both functions through tower formation just before movement stops. Many of these behavioral responses are, therefore, multifunctional and can vary according to local conditions, allowing snails in this family to successfully colonize the extreme high‐shore environment.

An integrated approach to infer the mechanisms of mate choice for size

Size-assortative mating and sexual selection on size are common across species. Since both may be a result of mate choice, mate choice based on size should also be a widespread process. This behaviour is, however, rarely studied directly and thus the biological causes that determine size-based mate choice are poorly understood. To address this, we studied the size-based mate choice in an intertidal snail, Echinolittorina malaccana, that has been used as a model to understand this process. Previous studies, assuming a quantitative Gaussian mating preference function, have inferred that mate choice in this snail is caused by a size similarity mechanism (males prefer to mate with females slightly larger than themselves). To further test and quantify this proposed mechanism, we conducted mate choice experiments with alternative designs (single, male and multiple choice) in the laboratory and compared the results to mate choice data observed in natural populations. This integrated approach allowed us to elucidate the mechanism of mate choice by evaluating alternative mating models that best fitted the observed data of various designs. Results confirmed the similarity-based mechanism but showed deviations at extreme size classes. The single choice design indicated that mate choice was exercised during one-on-one male–female interactions, but the strength of mate choice increased with the presence of additional individuals (males in the male choice design, and both males and females in the multiple-choice design). Multiple-choice experiments are, therefore, the most valuable and useful design to infer how males choose mates in the wild, as they best mimic the natural scenario and the results are the most similar to those observed in natural populations. To elucidate the mechanisms causing this male choice for particular female sizes, the next steps are to identify the genetic basis as well as potential physiological benefits associated with choosing slightly larger females.

Temperature adaptations of the thermophilic snail Echinolittorina malaccana: insights from metabolomic analysis.

The periwinkle snail Echinolittorina malaccana, for which the upper lethal temperature is near 55°C, is one of the most heat-tolerant eukaryotes known. We conducted a multi-level investigation - including cardiac physiology, enzyme activity, and targeted and untargeted metabolomic analyses - that elucidated a spectrum of adaptations to extreme heat in this organism. All systems examined showed heat intensity-dependent responses. Under moderate heat stress (37-45°C), the snail depressed cardiac activity and entered a state of metabolic depression. The global metabolomic and enzymatic analyses revealed production of metabolites characteristic of oxygen-independent pathways of ATP generation (lactate and succinate) in the depressed metabolic state, which suggests that anaerobic metabolism was the main energy supply pathway under heat stress (37-52°C). The metabolomic analyses also revealed alterations in glycerophospholipid metabolism under extreme heat stress (52°C), which likely reflected adaptive changes to maintain membrane structure. Small-molecular-mass organic osmolytes (glycine betaine, choline and carnitine) showed complex changes in concentration that were consistent with a role of these protein-stabilizing solutes in protection of the proteome under heat stress. This thermophilic species can thus deploy a wide array of adaptive strategies to acclimatize to extremely high temperatures.

How extraordinary periwinkles withstand extreme heat

Life might seem to be less eventful for the tenacious creatures that carve out life on the rocky sea shore once the tide has receded, but looks can be deceptive. Once the tide has gone on sunny days, many molluscs are at risk of baking in the sun. But the heat holds little fear for one species making a living on the southern coast of China. ‘The rocky intertidal periwinkle snail, Echinolittorina malaccana, is one of the most heat-tolerant eukaryotes, with an upper lethal thermal temperature of near 55°C’, says Yun-wei Dong from Ocean University of China. So how do these masters of heat deal with wilting conditions? And how hot does it really get once the surf has departed? Intrigued by the molluscs’ robust reputation, Dong, with colleagues from Xiamen University and his own institution, set about finding out just how hot knobbly periwinkles get.First, Dong secured iButton thermometers alongside the hardy snails on their rocky shore homes for 3 months during the summer in 2014 and was impressed to see that the average temperature generally hovered around 32°C. However, the temperatures did creep much higher from time to time and on one occasion even exceeded an extraordinary 53°C. But what effect did these high temperatures have on the mini molluscs’ beating hearts?Warming the snails gradually from 25°C, the team discovered that the heat really set the periwinkles’ hearts racing, first hitting a high of ∼42 beats min–1 at 37°C before reaching a second high of ∼67 beats min–1 at 52°C, after which they began to fail. ‘According to the heart beat curve of E. malaccana, metabolic depression occurs from ∼37°C to ∼45°C’, says Dong, adding that this reduction in metabolic rate helps the snails to survive at intermediate temperatures.The team then tracked the compounds produced in the snails’ bodies as a result of their metabolism and realised that the hardy molluscs switched from running their metabolism on oxygen at lower temperatures to other sources of energy for anaerobic metabolism at temperatures above 37°C. ‘High temperature and hypoxia [low oxygen] commonly co-occur in intertidal molluscs’, says Dong, explaining that shifting to anaerobic metabolism as the temperature soars allows the periwinkles to continue meeting their energy demands as their oxygen supplies dwindle.In addition, the team noticed that the snails produced a range of compounds that protect them from the damaging effects of heat. They included molecules that mop up toxic forms of oxygen – reactive oxygen species – that are produced by stress, modified lipids that improve the stability of cell membranes at high temperatures and a range of small molecules that protect proteins from damage. ‘This thermophilic species thus can deploy a wide array of adaptive strategies to acclimatise to extremely high temperatures’, says Dong.Having revealed the physiological mechanisms that underpin the periwinkle's extraordinary resilience, Dong and his team are eager to discover how the modest mollusc's genome has adapted to permit them to endure such extreme temperatures. But he is also concerned about their future. ‘We will apply these physiological traits in a species distribution model for predicating species biogeographic pattern in the context of climate change’, says Dong.

Supratidal existence drives phenotypic divergence, but not speciation, in tropical rocky-shore snails

AbstractThe mechanisms underlying the evolutionary adaptation of animals that transcend the ecological barrier separating the intertidal and supratidal zones of rocky shores are poorly understood. Different wetting frequencies in these zones in tropical regions (daily vs. seasonally, respectively) impose different physical stressors, which should drive phenotypic variation and ultimately speciation in the animals that inhabit them. We studied morphological, physiological and genetic variation in a tropical high-shore gastropod that transcends these zones [Echinolittorina malaccana (Philippi, 1847)]. Variation in melanization, shell features and evaporative water loss was linked to regular seawater wetting, frequent activity and feeding, and solar exposure in intertidal snails, and to inactivity and prolonged aestivation in the shade during continuous air exposure in supratidal snails. Despite selective pressure for phenotypic divergence, and reproductive isolation of the populations in either zone, their mitochondrial COI gene sequences confirmed that they represent a single species. Speciation in our study case is probably constrained by the limitation on activity, mating and reproduction of supratidal snails, such that their populations can only be sustained through intertidal pelagic larval recruitment. Comparisons with other studies suggest that supratidal speciation and specialization for life in this zone probably require moderation of the abiotic (desiccative) conditions, to facilitate greater activity and interaction of animals during air exposure.

Shell standing in Planaxis sulcatus: convergence of heat avoidance behavior in rocky shore gastropods?

Shell standing behavior, which allows snails living in the hottest upper rocky shore zones to avoid overheating, is reported here for the first time in a nonlittorinid snail. This was observed in Planaxis sulcatus which cohabits with the littorinid Echinolittorina malaccana, suggesting convergence of this behavioral trait.

Differences in heat tolerance plasticity between supratidal and intertidal snails indicate complex responses to microhabitat temperature variation.

Tropical intertidal gastropods that experience extreme and highly variable daily temperatures have evolved significant and complex heat tolerance plasticity, comprising components that respond to different timescales of temperature variation. An earlier study showed different plasticity attributes in snails from differently-heated coastlines, suggesting lifelong irreversible responses that matched habitat thermal regimes. To determine whether heat tolerance plasticity varied at a finer, within-shore spatial scale, we compared the responses of supratidal (predominantly shade-dwelling) and intertidal (frequently solar-exposed) populations of the tropical thermophilic gastropod, Echinolittorina malaccana. Snails modified lethal temperature (LT50) under warm or cool laboratory acclimation, with the overall variation in LT50 being greater in the supratidal (56.0-58.0°C) than in the intertidal population (57.1-58.1°C). Similar maximum LT50s expressed by the populations after warm acclimation suggest a capacity limitation under these temperature conditons. The different minimum LT50s after cool acclimation corresponded with microhabitat temperature and field acclimatization of the snails. Different responses to the same laboratory acclimation treatment imply long-term (and possibly lifelong) thermal acclimatization, which could benefit sedentary organisms that are randomly recruited as larvae from a common thermally-stable aquatic environment to thermally-unpredictable intertidal microhabitats. These findings provide another example of thermal tolerance plasticity operating at microhabitat scales, suggesting the importance of considering microhabitat thermal responses when assessing broad-scale environmental change.

Non-reversible and Reversible Heat Tolerance Plasticity in Tropical Intertidal Animals: Responding to Habitat Temperature Heterogeneity.

The theory for thermal plasticity of tropical ectotherms has centered on terrestrial and open-water marine animals which experience reduced variation in diurnal and seasonal temperatures, conditions constraining plasticity selection. Tropical marine intertidal animals, however, experience complex habitat thermal heterogeneity, circumstances encouraging thermal plasticity selection. Using the tropical rocky-intertidal gastropod, Echinolittorina malaccana, we investigated heat tolerance plasticity in terms of laboratory acclimation and natural acclimatization of populations from thermally-dissimilar nearby shorelines. Laboratory treatments yielded similar capacities of snails from either population to acclimate their lethal thermal limit (LT50 variation was ∼2°C). However, the populations differed in the temperature range over which acclimatory adjustments could be made; LT50 plasticity occurred over a higher temperature range in the warm-shore snails compared to the cool-shore snails, giving an overall acclimation capacity for the populations combined of 2.9°C. In addition to confirming significant heat tolerance plasticity in tropical intertidal animals, these findings reveal two plasticity forms, reversible (laboratory acclimation) and non-reversible (population or shoreline specific) plasticity. The plasticity forms should account for different spatiotemporal scales of the environmental temperature variation; reversible plasticity for daily and tidal variations in microhabitat temperature and non-reversible plasticity for lifelong, shoreline temperature conditions. Non-reversible heat tolerance plasticity, likely established after larvae settle on the shore, should be energetically beneficial in preventing heat shock protein overexpression, but also should facilitate widespread colonization of coasts that support thermally-diverse shorelines. This first demonstration of different plasticity forms in benthic intertidal animals supports the hypothesis that habitat heterogeneity (irrespective of latitude) drives thermal plasticity selection. It further suggests that studies not making reference to different spatial scales of thermal heterogeneity, nor seeking how these may drive different thermal plasticity forms, risk misinterpreting ectothermic responses to environmental warming.

High thermal stress responses of Echinolittorina snails at their range edge predict population vulnerability to future warming

Populations at the edge of their species' distribution ranges are typically living at the physiological extreme of the environmental conditions they can tolerate. As a species' response to global change is likely to be largely determined by its physiological performance, subsequent changes in environmental conditions can profoundly influence populations at range edges, resulting in range extensions or retractions. To understand the differential physiological performance among populations at their distribution range edge and center, we measured levels of mRNA for heat shock protein 70 (hsp70) as an indicator of temperature sensitivity in two high-shore littorinid snails, Echinolittorina malaccana and E. radiata, between 1°N to 36°N along the NW Pacific coast. These Echinolittorina snails are extremely heat-tolerant and frequently experience environmental temperatures in excess of 55 °C when emersed. It was assumed that animals exhibiting high temperature sensitivity will synthesize higher levels of mRNA, which will thus lead to higher energetic costs for thermal defense. Populations showed significant geographic variation in temperature sensitivity along their range. Snails at the northern range edge of E. malaccana and southern range edge of E. radiata exhibited higher levels of hsp70 expression than individuals collected from populations at the center of their respective ranges. The high levels of hsp70 mRNA in populations at the edge of a species' distribution range may serve as an adaptive response to locally stressful thermal environments, suggesting populations at the edge of their distribution range are potentially more sensitive to future global warming.

Influence of shell morphometry, microstructure, and thermal conductivity on thermoregulation in two tropical intertidal snails

AbstractTropical intertidal organisms tolerate large fluctuations in temperature and high desiccation rates when exposed during low tide. In order to withstand the short‐term heat stress, intertidal organisms adopt behavioral responses to maximize their survival. Our previous research showed that tropical littorinids found at the upper and lower intertidal shores in Singapore exhibited different behavioral adaptations during low tide. Most of the upper‐shore Echinolittorina malaccana kept a flat orientation, with the aperture against the substrate and the long axis of the shell towards the sun, whereas a majority of the lower‐shore individuals of Echinolittorina vidua stood with the edge of the aperture perpendicular to the substrate on the rocky shore during low tide. This prompted analyses of the shells of these two species to determine whether the differences in the shell morphometry, microstructure, and thermal conductivity of shells of E. malaccana and E. vidua were associated with their respective behavioral responses to thermal stress. Analyses of shell morphometry and thermal conductivity showed that shells of E. malaccana were more likely to minimize heat gain, despite having a higher thermal conductivity on the outer surface, due to their light‐gray, elongated shell. By contrast, the dark‐colored, globose shells of E. vidua probably gain heat more readily through solar radiation. Scanning electron microscopy images of the shells of both littorinid species further revealed that they have cross‐lamellar structure; however, only individuals of E. vidua showed the presence of disjointed rod layers and a pigmented inner shell surface. Individuals of E. malaccana had a rough outer shell surface with holes that inter‐connect to form water‐trapping channels that probably aid cooling. Individuals of E. vidua, however, had a smooth outer surface with rows of kidney‐shaped depressions as microsculptures which probably help to stabilize shell shape. In both Echinolittorina species, behavioral responses were used to overcome thermal stress during low tide that was associated with shell morphometry and shell thermal conductivity. Such combined adaptations increase survivability of the littorinids at their respective tidal levels.

Comparative transcriptomics across populations offers new insights into the evolution of thermal resistance in marine snails

Adaptation to thermal conditions in intertidal ectotherms involves tolerating and surviving frequent and often extreme variations in environmental temperatures. Modulation of gene expression plays an important role in the adaptive evolution of thermal tolerance. To understand such patterns, we investigated the thermal tolerance among three Asian populations (from Xiamen, Hong Kong, and Singapore) of the marine littorinid snail Echinolittorina malaccana and examined gene expression profiles in these populations before, during, and after heat stress using an Illumina RNA-sequencing platform. Analysis of transcriptomic changes between different conditions revealed that a proportion of the differentially expressed genes showed similar expression profiles across all three populations, including many classic molecular chaperones such as HSP70 and HSP90 that may constitute the core of the thermal stress response machinery in marine snails. Meanwhile, population-specific transcriptomic responses to heat stress were also detected. A few genes in particular showed more analogous expression profiles in the more thermally tolerant Hong Kong and Singapore populations, and these genes are likely to contribute, at least in part, to the enhanced thermal tolerance of these populations. We argue that repression of a subset of metabolic genes including several cytochrome P450 gene family members, to minimize energy expenditure and control reactive oxygen species turnover, may underpin the enhanced thermal resistance in these two populations. As such, these findings offer new insights into how marine snails cope with thermal stress and their potential evolutionary trajectory toward adapting to a warming climate.

Fighting for mates: the importance of individual size in mating contests in rocky shore littorinids

Studies of mating contests have reported how traits (e.g. body size) related to resource-holding potential (RHP) and strategies to assess RHP and resource value influence contest outcome in many taxa but are rare in the Gastropoda. The influence of male size (as an index of RHP) and female size (as a measure of resource value) on contest outcome was investigated in two littorinid snails, Echinolittorina malaccana and E. radiata, in Hong Kong during May–June 2013. In these snails, contests between males take the form of a ‘challenger’ attempting to take over the copulation position occupied by a ‘defender’. Both challengers and defenders were, generally, smaller than the females in both species. In both species, the larger the challenger relative to the defender, the more likely he would replace the defender in the copulation position. The challengers were, however, more successful in E. radiata, as they generally challenged defenders that were smaller than themselves, suggesting an ability to detect rival size before entering into a contest in this species. When sizes of the contestants were similar, defenders were more likely to win contests in E. malaccana but not in E. radiata. Evidence for pure self-assessment of RHP and the ability to assess resource value in challengers was found in E. malaccana. Different fighting strategies appear to have evolved in these congeneric marine snail species, and decisions based on male and female sizes play an important role in determining male reproductive success.

De novo transcriptome sequencing of the snail Echinolittorina malaccana: identification of genes responsive to thermal stress and development of genetic markers for population studies.

Echinolittorina snails inhabit the upper intertidal rocky shore and face strong selection pressures from thermal extremes and fluctuations. Revealing the molecular processes of adaptive significance is greatly obstructed by the scarcity of genomic resource for these taxa. Here, we reported the first comprehensive transcriptome dataset for the genus Echinolittorina. Using Illumina HiSeq 2000 platform, about 52 M and 54 M paired-end clean reads were, respectively, generated for the control and heat-stressed libraries. Totally, 115,211 unique transcript fragments (unigenes) were assembled, with an average length of 453 bp and a N50 size of 492 bp. Approximately one third of the unigenes could be annotated according to their homology matches against the Nr, Swiss-Prot, COG, or KEGG databases, and they were found to represent 23,098 non-redundant genes. Gene expression comparison revealed that 1,267 and 6,663 annotated genes were, respectively, up- and downregulated with at least twofold changes upon heat stress. Gene Ontology and KEGG pathway analyses indicated that there were overrepresented amount of genes enriched in a broad spectrum of biological processes and pathways, including those associated with cytoskeleton organization, developmental regulation, signaling transduction, infection, and cardiac function. In addition, a transcriptome-wide search for polymorphic loci yielded a total of 11,228 simple sequence repeats (SSRs) from 9,938 unigenes and 138,631 single nucleotide polymorphism (SNP) and insertion/deletion (INDEL) sites among 22,770 unigenes. The large number of transcript sequences acquired, the biological pathways identified, and the candidate microsatellite and SNP/INDEL loci discovered in the study will serve as valuable resources for further investigations of genetic differentiation and thermal adaptation among populations.

The Biodiversity of Marine Gastropods of Thailand in the Late Decade.

This study is mainly based on the collection of marine gastropods along the east coast of the Gulf of Thailand which had been carried out along the coastline in 55 sites from the province of Chonburi to Trad during April 2005 – December 2009. As many habitats as possible were examined at each sites from sandy beaches, muddy sand, rocky shore, and coral reefs. A total of 306 species of gastropods were collected and had been classifi ed in53families 116genera.The most widespread species were Planaxis sulcatus (Planaxidae) and Polinices mammilla (Naticidae) found in 37 sites, followed by Echinolittorina malaccana (Littorinidae) in 35 sites. The highest diversity was 187 species in Trat whereas Koh Mark had the most abundance in this area. The lowest diversity was in Chanthaburi, 88 species, whereas Koh Nomsoa was the most abundant site.The diversity of gastropods in Chonburi was 152 species, whereas Koh Samaesarn and Koh Juang were the most abundant site. 137 species had been found in Rayong and Koh Munnai was the most abundant site. The data from this study had been compared with the resent studies in the late decade from the west coast of the Gulf of Thailand and Andaman Sea.The total gastropods in the late decade were 454 species 205 genera 69 families. (Keywords: marine gastropods, distribution, diversity, Gulf of Thailand)

Behaviour moderates climate warming vulnerability in high-rocky-shore snails: interactions of habitat use, energy consumption and environmental temperature

High-rocky-shore intertidal animals are predicted to be generally more vulnerable to climate warming than lower-shore species, because their thermal tolerances lie closer to maximum environmental temperatures (Te). However, this prediction is based on taxonomically and ecologically limited information. The present study investigated the effect of habitat use on climate warming vulnerability of the tropical high-shore snail, Echinolittorina malaccana (from Brunei Darussalam, 5°N), which aestivates in sun-exposed or shaded habitats. The thermal regimes of these habitats differed vastly, but snails showed similar daily energy consumption in either habitat, due to temperature-insensitive metabolism (TIM) between 35 and 46 °C in the sun-resting snails. However, maximum Te values in the shade and the sun were 35 and 46 °C, respectively, suggesting that sun-resting snails, which presently experience temperatures near the incipient lethal temperature range (46–56 °C), should be more threatened by further warming than shade-resting snails, which have an 11 °C ‘safety margin’. Thus, vulnerability of high-shore species to climate warming could be moderated by availability of shaded habitat, making predictions for these species more complex than previously realized.

Seasonal variation in utilization of biogenic microhabitats by littorinid snails on tropical rocky shores

Mobile species may actively seek refuge from stressful conditions in biogenic habitats on rocky shores. In Hong Kong, the upper intertidal zone is extremely stressful, especially in summer when organisms are emersed for long periods in hot desiccating conditions. As a result, many species migrate downshore between winter and summer to reduce these stressful conditions. The littorinids Echinolittorina malaccana and E. vidua, for example, are found on open rock surfaces high on the shore in winter but the majority migrate downshore in summer to the same tidal height as a common barnacle, Tetraclita japonica. In the laboratory, where environmental conditions could be controlled to approximate those occurring on the shore, we tested whether the downshore migration allowed littorinids to select barnacles as biogenic habitats to reduce stress and if this behaviour varied between seasons. In summer, littorinids demonstrated a strong active preference for the barnacles, which was not observed in the cool winter conditions, when animals were found on open rock surfaces even when barnacles were present. Littorinids, therefore, only actively select biogenic habitats during the summer in Hong Kong when they migrate downshore, suggesting that such habitats may play an important, temporal, role in mitigating environmental stress on tropical shores.

Boundary layer convective heating and thermoregulatory behaviour during aerial exposure in the rocky eulittoral fringe snail Echinolittorina malaccana

Many rocky intertidal snails behaviourally regulate their body temperatures under rapidly changing environmental temperatures. Thermoregulatory mechanisms vary among species relative to vertical position on the shore, the need to conserve water, and heating sources experienced. Because heat exchange through conduction or evaporation is limited in air-exposed, withdrawn eulittoral-fringe snails, studies on their thermoregulation have centered on the ways they counter direct solar irradiance on the shell (shell colour, etc.). However, these snails must also be significantly heated by convective flow from the boundary layer air (BLA) above the solar-heated rock surface (<4mm). Here we explore the contributions of direct and indirect solar influences on the heating of the eulittoral fringe, Echinolittorina malaccana, and assess the significance of a thermoregulatory behaviour that apparently alleviates convective heat gain. We studied shell attachment behaviour that results in the aperture being orientated perpendicularly to the rock surface (hereafter, standing). Using biomimetic models that were either exposed in the field directly to the sun or shaded (on sun exposed rocks) in standing or flat postures, we found that the sun's direct effect was relatively minor. The average thermal differential between the sunned models (direct and convective effects) and the shaded models (convective effect only) was maximally 2.1°C. Standing maximally lowered the average operative temperature (Tb) below that of flat snails by 6°C, by reducing convective heat gain from the boundary air. This potentially ensures the body temperature is kept below the critical thermal limit. Follow up laboratory experiments using live snails showed standing required a temperature gradient in the BLA, and was not observed when snails were heated in the absence of this gradient. Our findings link for the first time thermoregulatory behaviour and habitat BLA temperatures in rocky shore organisms. Furthermore, these results have implications for other small rock-dwelling ectotherms exposed to sunlight and having limited contact with the surface.