Cheating the locals: invasive mussels steal and benefit from the cooling effect of indigenous mussels

The indigenous South African mussel Perna perna gapes during periods of aerial exposure to maintain aerobic respiration. This behaviour has no effect on the body temperatures of isolated individuals, but when surrounded by conspecifics, beneficial cooling effects of gaping emerge. It is uncertain, however, whether the presence of the invasive mussel Mytilus galloprovincialis limits the ability of P. perna for collective thermoregulation. We investigated whether varying densities of P. perna and M. galloprovincialis influences the thermal properties of both natural and artificial mussel beds during periods of emersion. Using infrared thermography, body temperatures of P. perna within mixed artificial beds were shown to increase faster and reach higher temperatures than individuals in conspecific beds, indicating that the presence of M. galloprovincialis limits the group cooling effects of gaping. In contrast, body temperatures of M. galloprovincialis within mixed artificial mussel beds increased slower and exhibited lower temperatures than for individuals in beds comprised entirely of M. galloprovincialis. Interestingly, differences in bed temperatures and heating rates were largely dependent on the size of mussels, with beds comprised of larger individuals experiencing less thermal stress irrespective of species composition. The small-scale patterns of thermal stress detected within manipulated beds were not observed within naturally occurring mixed mussel beds. We propose that small-scale differences in topography, size-structure, mussel bed size and the presence of organisms encrusting the mussel shells mask the effects of gaping behaviour within natural mussel beds. Nevertheless, the results from our manipulative experiment indicate that the invasive species M. galloprovincialis steals thermal properties as well as resources from the indigenous mussel P. perna. This may have significant implications for predicting how the co-existence of these two species may change as global temperatures continue to rise

Manipulating thermal stress on rocky shores to predict patterns of recruitment of marine invertebrates under a changing climate

For rocky intertidal organisms, temperature is often considered the most influential factor governing early survival and growth. Nevertheless, our review of the literature revealed that few studies have manipulated temperatures in the field to test for effects on these critical early life history processes. Here, we present the results from a novel manipulation of substratum temperature using settlement plates of different colour (black, grey and white) and infrared measurements of temperature to test hypotheses that temperature influences the early survival and growth of recent settlers of the intertidal barnacle Tesseropora rosea. Mean surface temperatures of black and grey plates were as great as 5.8°C (on average 2.2°C) and 4.8°C (on average 1.6°C) hotter than white plates across the sampling period, respectively. Cooler, white plates had significantly greater settlement and early growth than hotter, black plates, but differences in plate temperature did not significantly influence early survival or recruitment, though patterns were consistent with thermal variability. Comparisons between grey coloured natural rock and plates indicate that grey plates thermally mimic natural rock. Nevertheless, on average, more than twice as many larvae settled on plates than on natural rock, but early post-settlement survival on natural rock was double that on plates, suggesting that this artificial surface may not adequately capture the natural variability in early life history processes. Regardless, our simple and repeatable thermal manipulation represents a useful tool for experimentally investigating the effects of temperature on recruitment processes and simulating future temperature variability associated with climate change

Temperature variability at the larval scale affects early survival and growth of an intertidal barnacle

For rocky intertidal invertebrates, the transition from pelagic larva to benthic settler represents a critical life-history stage characterised by high mortality. This mortality has been attributed to biotic factors such as predation or individual larval quality, as well as to abiotic factors such as thermal or desiccation stresses. Surprisingly little is known about how temperature varies at very fine spatial scales relevant to newly settled larvae. We used infrared (IR) imagery to determine (1) whether in situ rocky substrates during aerial exposure exhibit repeatable fine-scale (1 mm) temperature variation at the larval scale, and (2) whether the presence of adult conspecifics ameliorates effects of substratum temperature and promotes early growth and survival of settlers. We tracked the settlement and early survival of larvae to determine whether fine-scale variation in temperature influences early life history processes of the intertidal barnacle Tesseropora rosea. Larval settlement did not vary with fine-scale variation in rock temperature, but early post-settlement growth and survival were both inversely related to temperature. Furthermore, we found that rock temperatures decreased significantly with increasing proximity to adult T. rosea and that larvae that settled within 15 mm of adults survived better than those that settled within 16 to 30 mm, highlighting positive effects of gregarious settlement. This is partially explained by conspecific adults shading rock and reducing rock temperatures. We present the first use of IR technology to test for variation in rock temperature at a scale relevant to individual larvae, demonstrating that such fine-scale variation in thermal stress impacts the early-life history stages of a benthic marine invertebrate

Supply-side biogeography: Geographic patterns of settlement and early mortality for a barnacle approaching its range limit

Species range limits are often associated with reduced adult densities, and this may reflect the failure of a particular life-history stage. For benthic marine invertebrates, settlement is a time of great mortality that strongly influences adult population structure, at least at local spatial scales. In south-eastern Australia we determined that adult abundance of the intertidal barnacle Tesseropora rosea declines over a 450 km region of rocky shore from the middle to the southern limit of its range, and we tested the hypothesis that this biogeographic pattern reflects variations in the production, settlement, or early post-settlement mortality of larvae or adult mortality. Sampling at 2 sites on 11 rocky shores in this region over 2 yr revealed that none of the life-history stages or demographic processes displayed a latitudinal gradient or a clear decline towards the south, and settlement and adult mortality were highly variable among locations. Indeed local variation in early lifehistory processes and adult mortality appears to dictate regional variability and observed latitudinal patterns of adult abundance of T. rosea, but longer term studies spanning at least a decade may determine if storage from one strong year in recruitment can set patterns of adult abundance

Facing the heat: does desiccation and thermal stress explain patterns of orientation in an intertidal invertebrate?

This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. A key challenge for ecologists is to quantify, explain and predict the ecology and behaviour of animals from knowledge of their basic physiology. Compared to our knowledge of many other types of distribution and behaviour, and how these are linked to individual function, we have a poor level of understanding of the causal basis for orientation behaviours. Most explanations for patterns of animal orientation assume that animals will modify their exposure to environmental factors by altering their orientation. We used a keystone grazer on rocky shores, the limpet Cellana tramoserica, to test this idea. Manipulative experiments were done to evaluate whether orientation during emersion affected limpet desiccation or body temperature. Body temperature was determined from infrared thermography, a technique that minimises disturbance to the test organism. No causal relationships were found between orientation and (i) level of desiccation and (ii) their body temperature. These results add to the growing knowledge that responses to desiccation and thermal stress may be less important in modifying the behaviour of intertidal organisms than previously supposed and that thermoregulation does not always reflect patterns of animal orientation.Much of what we understand about orientation comes from studies of animals able to modify orientation over very short time scales. Our data suggests that for animals whose location is less flexible, orientation decisions may have less to do with responses to environmental factors and more to do with structural habitat properties or intrinsic individual attributes. Therefore we suggest future studies into processes affecting orientation must include organisms with differing levels of behavioural plasticity

Testing the intermittent upwelling hypothesis: intercontinental comparisons of barnacle recruitment between South Africa and Australia

Recent debates have arisen as to whether productivity and subsequent ecological processes increase linearly with increasing intensity of upwelling, or whether productivity responds more favourably to upwelling of intermediate magnitude, as predicted by the intermittent upwelling hypothesis (IUH). Most studies on the topic take place within eastern boundary systems, where the intensity and frequency of upwelling are high. Here, we test the generality of the IUH towards the other end of the upwelling spectrum, within two regions located at similar latitudes along western boundary currents of two continents. We measured barnacle recruitment and colonisation, which we expected to be linked positively to productivity, across eight rocky shores along the east coasts of South Africa and Australia selected to capture a range of upwelling regimes. Based on Bakun Upwelling Indices (BUI), the four South African sites experienced persistent to intermittent levels of upwelling, whilst the four sites along the east coast of Australia were predominantly downwelling sites with occasional upwelling events. Satellite chlorophyll a concentrations ([Chl-a]) also showed a marked difference between the two continents, with 2–3 times higher concentrations in South Africa than Australia. In situ sea temperature measurements revealed slightly different oceanographic patterns, which were nonetheless compatible with both BUI and [Chl-a] measurements. Barnacle recruitment was typically greater within South Africa and but was generally found to vary unimodally with mean BUI (i.e. being greater at sites that experienced moderate upwelling conditions) and to increase linearly with increasing upwelling frequency between January and April when barnacle larvae are known to be most abundant in the water column. Viewed in isolation, our data provide moderate support for the IUH. But when placed in a broader context, with our eight study locations representing just one end of the upwelling continuum, they provide strong evidence for the IUH

Using biomimetic loggers to measure interspecific and microhabitat variation in body temperatures of rocky intertidal invertebrates

Until recently, marine scientists have relied heavily on satellite sea surface temperatures and terrestrial weather stations as indicators of the way in which the thermal environment, and hence the body temperatures of organisms, vary over spatial and temporal scales. We designed biomimetic temperature loggers for three species of rocky intertidal invertebrates to determine whether mimic body temperatures differ from the external environment and among species and microhabitats. For all three species, microhabitat temperatures were considerably higher than the body temperatures, with differences as great as 11.1°C on horizontal rocky substrata. Across microhabitats, daily maximal temperatures of the limpet Cellana tramoserica were on average 2.1 and 3.1°C higher than body temperatures of the whelk Dicathais orbita and the barnacle Tesseropora rosea respectively. Among-microhabitat variation in each species\u27 temperature was equally as variable as differences among species within microhabitats. Daily maximal body temperatures of barnacles placed on southerly facing vertical rock surfaces were on average 2.4°C cooler than those on horizontal rock. Likewise, daily maximal body temperatures of whelks were on average 3.1°C cooler within shallow rock pools than on horizontal rock. Our results provide new evidence that unique thermal properties and microhabitat preferences may be important determinants of species\u27 capacity to cope with climate change

Association between heamolymph osmolality and head orientation (differences in orientation from downwards) in <i>Cellana tramoserica</i>.

<p>(a) Patch 1 Time 1 n = 46 (b) Patch 2 Time 1 n = 64 (c) Patch 3 Time 2 n = 89 (d) Patch 4 Time 2 n = 85 Downwards is defined as between 135° and 225°, where 0° was straight up [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0150200#pone.0150200.ref032" target="_blank">32</a>].</p

Comparison of mean (+ s.e) body and anterior temperature of limpets orientated in different directions and different habitats (n (combined across sampling times) = 10).

<p>(a) body temperature (b) anterior temperature. Treatments were control limpets (C) facing downwards, individuals rotated 360° (P) and individuals rotated 180° to face upwards (T) in the shade (grey bars) and sunlight (white bars).</p