Porcelain crabs are ideal for studies of responses to environmental stress because of their species richness and their distribution patterns. The largest genus of porcelain crabs, Petrolisthes, contains over 100 species. Petrolisthes are distributed across both latitudinal and intertidal zone gradients throughout the Pacific Ocean, and sympatric species have clearly delineated vertical zonation patterns. The two distribution gradients result in a great range in exposure to abiotic stresses and microhabitat conditions among species. Thermal habitat characteristics differ dramatically from subtidal species to high-intertidal species, and mean temperatures and thermal habitats differ between temperate and tropical species. In the eastern Pacific there are four species assemblages in different geographic regions (north and south temperate, tropical, and northern Gulf of California endemics). Studies on Petrolisthes whole-animal thermal tolerance limits (Stillman and Somero, 2000; Stillman, 2002), and thermal tolerance limits of cardiac function (Stillman and Somero, 1996; Stillman, 2003), indicate that there has been strong selection to adapt thermal tolerance limits to microhabitat characteristics. Interspecific differences exist in upper thermal limits, lower thermal limits, the effects of acclimation on thermal limits, and the proximity of physiological thermal limits to maximal and minimal habitat temperatures. Petrolisthes species most vulnerable to global warming are those that currently live in the hottest thermal habitats, and that are the most heat tolerant (Stillman 2003). This vulnerability is due to the fact that: i) warm adapted crabs live in a thermal habitat that is at the edge of their upper thermal limits, and ii) warm adapted crabs have the least capacity to acclimate their upper thermal limits. These results are counterintuitive to common thinking about the impacts of climate change on species distributions in that they predict that climate change will most greatly impact heat tolerant species.
What mechanisms set thermal limits and plasticity of thermal tolerance?
The mechanistic bases that set organismal thermal tolerance limits remain poorly resolved and are an area of intense study. Because of the large variation in thermal physiology among species of Petrolisthes, these crabs are an attractive set of organisms for analyses of the physiological and molecular bases that set heat tolerance limits, cold tolerance limits, thermal tolerance range (i.e., eurythermality), and plasticity of thermal limits. Our current research focus is to understand physiological and molecular bases of thermal acclimation of Petrolisthes, with experiments designed to address the following questions: i) What changes in gene expression accompany physiological changes during thermal acclimation?; ii) How does acclimation to constant and fluctuating thermal regimes differ?; and iii) How does thermal acclimation to constant and fluctuating temperatures alter physiological responses to thermal stress? These studies are currently conducted in one species, Petrolisthes cinctipes, which is found in the mid to high intertidal zone from central California to northern British Columbia. Future work will expand these studies to include other species of Petrolisthes that are adapted to different thermal habitats (e.g., temperate subtidal zone, and the Northern Gulf of California). The main tools used in these studies are controlled temperature aquaria for thermal acclimations, an apparatus for determining the warm and cold thermal performance limits of crab cardiac function, and a cDNA microarray for P. cinctipes that has been developed (with sequencing support through a grant from the JGI Community Sequencing Program) using crabs exposed to a wide range of acclimation conditions and stresses.
To assess changes in gene expression during thermal acclimation of cardiac thermal performance limits, transcriptome profiles of crabs are developed during prolonged acclimation to warm and cold temperatures and during acute swaps from warm to cold temperatures, and vice versa.
To assess changes in gene expression during exposure to thermal regimes that fluctuate in a manner that reflects intertidal zone habitat temperatures during the hottest and coldest seasons, transcriptome profiles of crabs are developed during acclimation to cyclical temperatures that reflect summertime and wintertime low tide temperature variation. For example, in one cycling acclimation group, temperatures increase from 8°C to 26°C in a 6 hour period and then return rapidly to 8°C, simulating a common intertidal thermal profile during a summertime low tide period. Transcriptome profiles of tissues removed at regular intervals during the thermal cycles are being used to examine the changes in gene expression that accompany these rapid changes in thermal phenotype.
To assess how thermal acclimation affects physiological responses to thermal stress, crabs acclimated to constant and fluctuating temperatures (above sections) are given a sub-lethal thermal stress (to within 1°C of cardiac warm or cold thermal limits) during a simulated extreme summertime or wintertime low tide period. Following this stress, crabs are placed into a recovery tank and transcriptome profiles are generated from specimens removed during recovery periods of 0.5 to 30 hours. The results of these experiments are essential for interpreting the variation in transcriptome profiles in field-acclimatized crabs subjected to the same sub-lethal thermal stresses, as described below.
What are ecological consequences of thermal phenotype in response to climate change?
How much latitudinal and seasonal variation exists in physiological responses to thermal stress among populations of a species? Ecological range shifts that correlate with increasing sea surface temperatures have been noted in Petrolisthes, suggesting that these crabs may be experiencing thermal stress levels great enough to have population-level consequences (Barry et al 1995 Science). I am currently investigating the ecological significance of thermal stress variation within a species' range by studying crabs from the southern, central, and northern parts of their range (Monterey, CA; Charleston, OR; Bamfield, BC) during winter and summer seasons. The thermal physiology of field acclimatized P. cinctipes is being analyzed in three ways: i) measurement of cardiac thermal performance limits of crabs immediately transported to the laboratory upon collection, ii) transcriptome profiles of crab tissues dissected into RNA preservative immediately upon collection are being generated from samples collected twice per year, once during winter, and once in summer. Ideally, the frequency of sampling will be increased to at least four times per year to as high as during each tidal cycle (bi-monthly), and iii) transcriptome profiles of crabs during recovery following exposure to sub-lethal levels of warm and cold thermal stress, as described above. Differences in thermal physiology among conspecifics distributed across a latitudinal gradient are hypothesized to be due to either local adaptation or to acclimatization. Adaptation would only be possible if genetic differentiation exists among individuals from different populations across the latitudinal gradient. Studies of mitochondrial DNA sequence and microsatellite polymorphism in P. cinctipes have not indicated that there is any phylogeographic structure with respect to latitude in crabs from central California through the Pacific Northwest (R. Toonen, pers. comm.). Thus the hypothesis that there are seasonal and latitudinal acclimatization responses to thermal habitat differences is relevant to animals living in the natural environment.
Funded by the National Science Foundation