Elizabeth G. Boulding

Claw morphology, prey size selection and foraging efficiency in generalist and specialist shell-breaking crabs

Claw morphology, and claw-closing forces of four species of intertidal crabs from San Juan Island, Washington were compared and related these findings were related to prey size selection, shell breaking times and total handling times on their snail prey, Littorina sitkana Philippi. Two functional groups of crabs emerged: generalists and specialists on hard-shelled prey. The generalist, Hemigrapsus nudus (Dana), has an omnivorous diet and possesses weak claws with small, fine denticles and mechanical advantage (MA) of the claws lever system 0.3. The claws of the generalist, H. nudus, exhibited weaker claw closing forces (5 N) than those of similar sized specialists (>12 N). When crabs of similar weight were offered four size categories of Littorina sitkana, the generalist, Hemigrapsus nudus, exhibited a consistent preference for the smallest size categories, while the three specialists attacked all size classes offered. Hemigrapsus nudus took significantly longer (134 s) than the specialists (30–52 s) to break open a 4 mm L. sitkana. This difference in shell-breaking time between the generalist and the specialists increased with increasing prey size. The rate of successful attacks on increasingly larger L. sitkana decreased with prey size in the generalist (70% on 4 mm, 37% on 6 mm, and 0% on 8 mm snails), but remained high in the specialists (70–100%). Strength limitation of the claws is the best hypothesis to explain the avoidance of large snails by the generalist, H. nudus.

Nonallopatric and parallel origin of local reproductive barriers between two snail ecotypes.

Theory suggests that speciation is possible without physical isolation of populations (hereafter, nonallopatric speciation), but recent nonallopatric models need the support of irrefutable empirical examples. We collected snails (Littorina saxatilis) from three areas on the NW coast of Spain to investigate the population genetic structure of two ecotypes. Earlier studies suggest that these ecotypes may represent incipient species: a large, thick‐shelled ‘RB’ ecotype living among the barnacles in the upper intertidal zone and a small, thin‐shelled ‘SU’ ecotype living among the mussels in the lower intertidal zone only 10–30 m away. The two ecotypes overlap and hybridize in a midshore zone only 1–3 m wide. Three different types of molecular markers [allozymes, mitochondrial DNA (mtDNA) and microsatellites] consistently indicated partial reproductive isolation between the RB and the SU ecotypes at a particular site. However, each ecotype was related more closely to the other ecotype from the same site than to the same ecotype from another site further along the Galician coast (25–77 km away). These findings supported earlier results based solely on allozyme variation and we could now reject the possibility that selection produced these patterns. The patterns of genetic variation supported a nonallopatric model in which the ecotypes are formed independently at each site by parallel evolution and where the reproductive barriers are a byproduct of divergent selection for body size. We argue that neither our laboratory hybridization experiments nor our molecular data are compatible with a model based on allopatric ecotype formation, secondary overlap and introgression.

Quantitative genetics of shell form of an intertidal snail : constraints on short-term response to selection

We investigated the genetic and environmental determinants of shell form in an intertidal snail (Prosobranchia: Littorina sp.) to identify constraints on the short‐term response to selection. Our quantitative genetic parameters were estimated from a half‐sib experimental design using 288 broods of snails. Each brood was divided into two treatments differing in snail population density, and therefore in grazing area per snail. Differences in population density induced marked differences in shell form. Snails in the low density treatment grew faster and had lighter shells with narrower whorls and narrower apertures than their siblings at high density. Despite this environmental plasticity in shell shape we found significant additive genetic variance for components of shell shape. We discuss two mechanisms that may maintain additive genetic variance for shell shape in intertidal snail populations: migration between environments with different selective pressures and migration between environments with different mean growth rates. We also estimated a genetic variance‐covariance matrix for shell form traits and used the matrix to identify constraints on the short‐term response to selection. We predict the rate of response to selection for predator‐resistant morphology such as would occur upon invasion of predatory crabs. The large negative genetic correlation between relative spire height and shell weight would facilitate simultaneous selection for a lower spire and a heavier shell, both of which would increase resistance to predatory crabs.

State-dependent habitat selection by an intertidal snail: the costs of selecting a physically stressful microhabitat

Abstract Animals often modify their behavior in response to changing environmental conditions. As physical conditions become more severe animals in stressful habitats pay higher costs and should expend additional resources to search for less stressful habitats. We tested whether conditions resulting in more severe thermal and desiccation stress increased selection of more protective microhabitats by the intertidal gastropod Littorina sitkana , Philippi. We found that complex microhabitats such as barnacles and macroalgae were better on warm days because they provided cooler temperatures than less complex microhabitats such as crevices and bare rock surfaces. On warm days on the shore, snails foraged for shorter periods before selecting a microhabitat and large snails chose more complex microhabitats such as barnacles and algae more often than on cool days. We then used artificial substrates made entirely of clay in a laboratory experiment to show that this microhabitat selection by the snails was based on a preference for high topographic complexity. In this experiment snails that selected microhabitats early in the observation period chose topographically complex microhabitats significantly more often than snails that selected microhabitats late; and this became even more marked at higher temperatures. These results demonstrated state-dependent microhabitat selection by L. sitkana . The short-term cost of selecting stressful microhabitats on the shore was increased dehydration during low tide. The long-term costs of remaining in less complex areas without barnacles were lower rates of growth and survivorship. We conclude that topographically complex microhabitats allow the persistence of L. sitkana in the high intertidal zone.

Spatial and temporal population genetic structure of four northeastern Pacific littorinid gastropods: the effect of mode of larval development on variation at one mitochondrial and two nuclear DNA markers

We investigated the effect of development mode on the spatial and temporal population genetic structure of four littorinid gastropod species. Snails were collected from the same three sites on the west coast of Vancouver Island, Canada in 1997 and again in 2007. DNA sequences were obtained for one mitochondrial gene, cytochrome b (Cyt b), and for up to two nuclear genes, heat shock cognate 70 (HSC70) and aminopeptidase N intron (APN54). We found that the mean level of genetic diversity and long‐term effective population sizes (Ne) were significantly greater for two species, Littorina scutulata and L. plena, that had a planktotrophic larval stage than for two species, Littorina sitkana and L. subrotundata, that laid benthic egg masses which hatched directly into crawl‐away juveniles. Predictably, two poorly dispersing species, L. sitkana and L. subrotundata, showed significant spatial genetic structure at an 11‐ to 65‐km geographical scale that was not observed in the two planktotrophic species. Conversely, the two planktotrophic species had more temporal genetic structure over a 10‐year interval than did the two direct‐developing species and showed highly significant temporal structure for spatially pooled samples. The greater temporal genetic variation of the two planktotrophic species may have been caused by their high fecundity, high larval dispersal, and low but spatially correlated early survivorship. The sweepstakes‐like reproductive success of the planktotrophic species could allow a few related females to populate hundreds of kilometres of coastline and may explain their substantially larger temporal genetic variance but lower spatial genetic variance relative to the direct‐developing species.

Changes in selection on gastropod shell size and thickness with wave-exposure on northeastern pacific shores.

Abstract Patterns of selection on gastropod shell morphology are generally believed to be different on wave-exposed and wave-sheltered shores. The heavy surf on wave-exposed shores is thought to select for small size whereas the high risk of shell-breaking predation on wave-sheltered shores is thought to select for increased shell size and thickness. We compared the risk of shell-breaking predation to littorinid gastropods of different sizes and shell-thicknesses by tethering them on wave-exposed and wave-sheltered shores of the Northeastern Pacific. Over 2 years we found that the predation rate on the direct-developing gastropod Littorina sitkana was consistently much lower at two moderately wave-exposed sites (less than 0.01% d −1 ) than on the two wave-sheltered sites (8% d −1 and 2% d −1 respectively). At least 30% of the shell-breaking predation resulted in diagnostic “peeled” shell breakage patterns that could be directly attributed to predatory crabs. Observations with SCUBA at high tide suggested that most of the remainder of the shell-breaking predation was from the red rock crab, Cancer productus , and that only a small amount was from pile perch, Rhacochilus vacca . In contrast to our expectations, the smallest size-class of L. sitkana suffered significantly lower rates of predation than the largest size-class at one of the wave-sheltered sites. The effect of shell thickness on predation mortality was as predicted from previous laboratory experiments. The thin-shelled littorinid species, Littorina subrotundata , suffered significantly higher rates of predation than two thicker-shelled species, L. sitkana and L. scutulata s.l ., at three of our four sites. We conclude that the higher rates of shell-breaking predation on wave-sheltered shores of the Northeastern Pacific selects for L. sitkana with thicker but not necessarily larger shells than those on wave-exposed shores.

Genetic and demographic parameters determining population persistence after a discrete change in the environment

Field studies suggest that populations often go extinct following discrete changes in the environment. However, populations may avoid extinction by rapidly adapting to their altered environment. We used a stochastic finite-locus model to estimate the distance the optimal value of a quantitative trait could shift in a single step Δθ c without causing more than 5% of the replicate populations to go extinct. We found that evolution increased the magnitude of Δθc by at least two phenotypic standard deviations and that such evolution could take place within 5–10 generations. Indeed (Δθc) 2 increased approximately linearly with the logarithm of the initial population size and the rate of this increase was much greater when heritability was high or when stabilizing selection was weak. (Δθc) 2 also increased approximately linearly with the logarithm of per capita fecundity. To our surprise there was no ‘demographic rescue’ effect from migration; a population augmented with migrants from a neighbouring population where environmental conditions were unchanged was always more likely to go extinct. The addition of mutation, more loci, density-dependence, or environmental stochasticity had only small effects on the outcome. We were able to compare our results for closed populations with density-independent population growth to those from an analytical model and found good agreement so long as the proportion of the offspring surviving selection in the initial generations was at least 1%.

Variation in temperature tolerance among families of Atlantic salmon (Salmo salar) is associated with hypoxia tolerance, ventricle size and myoglobin level

SUMMARY In fishes, performance failure at high temperature is thought to be due to a limitation on oxygen delivery (the theory of oxygen and capacity limited thermal tolerance, OCLTT), which suggests that thermal tolerance and hypoxia tolerance might be functionally associated. Here we examined variation in temperature and hypoxia tolerance among 41 families of Atlantic salmon (Salmo salar), which allowed us to evaluate the association between these two traits. Both temperature and hypoxia tolerance varied significantly among families and there was a significant positive correlation between critical maximum temperature (CTmax) and hypoxia tolerance, supporting the OCLTT concept. At the organ and cellular levels, we also discovered support for the OCLTT concept as relative ventricle mass (RVM) and cardiac myoglobin (Mb) levels both correlated positively with CTmax (R2=0.21, P<0.001 and R2=0.17, P=0.003, respectively). A large RVM has previously been shown to be associated with high cardiac output, which might facilitate tissue oxygen supply during elevated oxygen demand at high temperatures, while Mb facilitates the oxygen transfer from the blood to tissues, especially during hypoxia. The data presented here demonstrate for the first time that RVM and Mb are correlated with increased upper temperature tolerance in fish. High phenotypic variation between families and greater similarity among full- and half-siblings suggests that there is substantial standing genetic variation in thermal and hypoxia tolerance, which could respond to selection either in aquaculture or in response to anthropogenic stressors such as global climate change.

Conservation genomics of Atlantic salmon: SNPs associated with QTLs for adaptive traits in parr from four trans-Atlantic backcrosses

European Atlantic salmon (Salmo salar) differ in skin pigmentation and shape from the North American lineage of Atlantic salmon but the genetic basis of these differences are poorly understood. We created four large (N=300) backcross families by crossing F1 hybrid male siblings to two females from the European and two from the North American aquacultural strains. We recorded 15 morphological landmarks and two skin pigmentation, three growth and three condition traits on parr. The backcross families were genotyped for at least 129 SNPs (single nucleotide polymorphisms) within expressed sequence tags (ESTs) spaced throughout the Atlantic salmon linkage map. The high polymorphism and low rates of crossover in our hybrid sires provided enough statistical power to detect 79 significant associations between SNP markers and quantitative traits after experiment-wide permutation analysis for all families within traits. Linkage group AS22 contained a quantitative trait loci (QTL) for parr mark number; its homolog AS24 contained a large QTL, which explained 26% of the phenotypic variance in parr mark contrast. We found 25 highly significant QTLs for body shape and fin position on seven different linkage groups, and 16 for growth and condition on six different linkage groups. QTL(s) for pectoral fin position, caudal peduncle position, late parr growth and condition index were associated with an SNP on linkage group AS1, which was linked to the sex-determining locus. Our work adds to the evidence that much of the variation in growth rate, shape and skin pigmentation observed among Atlantic salmon parr from different natal streams is genetic.

Mitochondrial DNA variation in space and time in the northeastern Pacific gastropod, Littorina keenae

The present population structure of a species reflects the influence of population history as well as contemporary processes. To examine the relative importance of these factors in shaping the current population structure of Littorina keenae, we sequenced 762 base pairs of the mitochondrial ND6 and cytochrome b genes in 584 snails from 13 sites along the northeastern Pacific coast. Haplotype network analysis revealed a ‘star‐like’ genealogy indicative of a recent population expansion. Nested clade and mismatch analyses also supported the hypothesis of sudden population expansion following a population bottleneck during the Last Glacial Maximum. Analysis of molecular variance and pairwise ΦST showed no significant spatial population differentiation from Mexico to Oregon – not even across the recognized biogeographic boundary at Point Conception. This is probably due to high contemporary gene flow during the free‐swimming larval stage of this snail. Surprisingly, we found a highly significant temporal population differentiation between a San Pedro sample from 1996 and one from 2005, which gave an estimate of effective population size (Ne) of only 135. Nearly statistically significant changes in the frequency of a particular haplotype in three other populations over 2–3 years further support Hedgecocks ‘sweepstakes’ hypothesis. When by chance nearly all of the progeny from an aggregation of highly fecund sisters that possess a rare haplotype successfully recruit to become the next generation, the rare haplotype can become temporarily common across the entire species’ range. This modification of the sweepstakes hypothesis can explain why the temporal variation that we observed was much greater than the spatial variation.