Rémy Rochette

RAD genotyping reveals fine-scale genetic structuring and provides powerful population assignment in a widely distributed marine species, the American lobster (Homarus americanus)

Deciphering genetic structure and inferring connectivity in marine species have been challenging due to weak genetic differentiation and limited resolution offered by traditional genotypic methods. The main goal of this study was to assess how a population genomics framework could help delineate the genetic structure of the American lobster (Homarus americanus) throughout much of the species’ range and increase the assignment success of individuals to their location of origin. We genotyped 10 156 filtered SNPs using RAD sequencing to delineate genetic structure and perform population assignment for 586 American lobsters collected in 17 locations distributed across a large portion of the species’ natural distribution range. Our results revealed the existence of a hierarchical genetic structure, first separating lobsters from the northern and southern part of the range (FCT = 0.0011; P‐value = 0.0002) and then revealing a total of 11 genetically distinguishable populations (mean FST = 0.00185; CI: 0.0007–0.0021, P‐value < 0.0002), providing strong evidence for weak, albeit fine‐scale population structuring within each region. A resampling procedure showed that assignment success was highest with a subset of 3000 SNPs having the highest FST. Applying Andersons (Molecular Ecology Resources, 2010, 10, 701) method to avoid ‘high‐grading bias’, 94.2% and 80.8% of individuals were correctly assigned to their region and location of origin, respectively. Lastly, we showed that assignment success was positively associated with sample size. These results demonstrate that using a large number of SNPs improves fine‐scale population structure delineation and population assignment success in a context of weak genetic structure. We discuss the implications of these findings for the conservation and management of highly connected marine species, particularly regarding the geographic scale of demographic independence.

Mechanism of a plastic phenotypic response: predator-induced shell thickening in the intertidal gastropod Littorina obtusata.

Phenotypic plasticity has been the object of considerable interest over the past several decades, but in few cases are mechanisms underlying plastic responses well understood. For example, it is unclear whether predator‐induced changes in gastropod shell morphology represent an active physiological response or a by‐product of reduced feeding. We address this question by manipulating feeding and growth of intertidal snails, Littorina obtusata, using two approaches: (i) exposure to predation cues from green crabs Carcinus maenas and (ii) reduced food availability, and quantifying growth in shell length, shell mass, and body mass, as well as production of faecal material and shell micro‐structural characteristics (mineralogy and organic fraction) after 96 days. We demonstrate that L. obtusata actively increases calcification rate in response to predation threat, and that this response entails energetic and developmental costs. That this induced response is not strictly tied to the animals behaviour should enhance its evolutionary potential.

Chemically-mediated predator-recognition learning in a marine gastropod

AbstractWe performed laboratory experiments to examine whether short-term exposure to predation threat increases the intensity of the escape response of juvenile whelks, Buccinum undatum L., when confronted with the predatory asteroid Leptasterias polaris (Muller & Troschel). In the first experiment, we exposed whelks for 4 days to brief encounters with several stimuli associated with predation threat, including contact with and odors of L. polaris and potential alarm signals from conspecifics. The responsiveness of predator-conditioned whelks increased significantly during the first three days of conditioning. On the fourth day, predator-conditioned whelks displayed stronger responses to contact with L. polaris than whelks held under control conditions. The second experiment involved exposing three groups of whelks for 30 minutes every hour, over a 6-hour period, to (i) the odors of L. polaris (predator group), (ii) the odors of L. polaris feeding on whelks and of alarmed conspecifics (predator-whelk gro...

Seascape genomics provides evidence for thermal adaptation and current-mediated population structure in American lobster (Homarus americanus).

Investigating how environmental features shape the genetic structure of populations is crucial for understanding how they are potentially adapted to their habitats, as well as for sound management. In this study, we assessed the relative importance of spatial distribution, ocean currents and sea surface temperature (SST) on patterns of putatively neutral and adaptive genetic variation among American lobster from 19 locations using population differentiation (PD) approaches combined with environmental association (EA) analyses. First, PD approaches (using bayescan, arlequin and outflank) found 28 outlier SNPs putatively under divergent selection and 9770 neutral SNPs in common. Redundancy analysis revealed that spatial distribution, ocean current‐mediated larval connectivity and SST explained 31.7% of the neutral genetic differentiation, with ocean currents driving the majority of this relationship (21.0%). After removing the influence of spatial distribution, no SST were significant for putatively neutral genetic variation whereas minimum annual SST still had a significant impact and explained 8.1% of the putatively adaptive genetic variation. Second, EA analyses (using Pearson correlation tests, bayescenv and lfmm) jointly identified seven SNPs as candidates for thermal adaptation. Covariation at these SNPs was assessed with a spatial multivariate analysis that highlighted a significant temperature association, after accounting for the influence of spatial distribution. Among the 505 candidate SNPs detected by at least one of the three approaches, we discovered three polymorphisms located in genes previously shown to play a role in thermal adaptation. Our results have implications for the management of the American lobster and provide a foundation on which to predict how this species will cope with climate change.

Does vulnerability influence trade-offs made by whelks between predation risk and feeding opportunities?

In the northern Gulf of St Lawrence, eastern Canada, the whelk Buccinum undatum occasionally aggregates near the feeding asteroid Leptasterias polaris to obtain food. This is surprising considering that whelks are frequently preyed upon by L. polaris and have developed a violent escape response to this predator. Almost all whelks in aggregations near feeding asteroids are large. This study suggests that small whelks do not approach feeding L. polaris because they are more vulnerable to this predator. Predation experiments demonstrated that the vulnerability of whelks to L. polaris decreases with increasing whelk size. Sampling in the field showed that 97% of whelks measuring more than 5 cm in shell length are found in the sediment zone, where L. polaris extracts large endobenthic bivalves, but whelks less than 5 cm are equally abundant in the rocky (49.5%) and sediment (50.5%) zones. Field experiments showed that whelks within 50 cm of large bivalves being ingested by L. polaris were larger than those within 50 cm of bivalves placed by themselves on the bottom. Finally, laboratory measurements showed that the influence of predator threat on the time whelks spent feeding decreased with whelk size, but the influence of feeding motivation on the time whelks spent feeding increased with whelk size. In the situation where motivation to feed was high and size-related diVerences in vulnerability were large, the impact of the predator decreased significantly with increasing whelk size. Whelks show threat-sensitive decision making adjusted to potential feeding benefits and vulnerability to predation.

Differential snail predation by an exotic crab and the geography of shell-claw covariance in the Northwest Atlantic.

Abstract Here we investigate if predation by the European green crab (Carcinus maenas) differs between two congeneric snails in the northwest Atlantic (Littorina littorea and L. obtusata), and ask if differential predation can help explain the geography of claw and shell forms among geographically separated populations. First, correlations between crusher-claw size and shell mass—tested across a wide size range of animals—were highly significant among populations of C. maenas and L. obtusata, whereas only a small number of significant correlations were found between C. maenas and L. littorea, and these were limited to the smaller size classes of snails and crabs. Moreover, among populations, L. obtusata shells were more frequently scarred than those of L. littorea, and L. obtusata were attacked and killed more frequently than L. littorea during field- and laboratory-predation experiments. Combined, results suggest L. obtusata is currently under greater selection by C. maenas than L. littorea for more crab-resistant shell forms. One possible explanation for these patterns is that L. littorea may have interacted with green crabs for centuries (in Europe) prior to their reintroduction to green crabs in America, thus predator-resistance may had already evolved.

Aggregation of whelks, Buccinum undatum, near feeding predators: the role of reproductive requirements

In the Mingan Islands, northern Gulf of St Lawrence (eastern Canada), the whelk Buccinum undatum displays a strong escape response to its predator, the asteroid Leptasterias polaris, nevertheless large sexually mature individuals occasionally approach feeding L. polaris to obtain food. In this study, we investigated the hypothesis that reproductive requirements increase the tendency of sexually mature whelks to approach feeding asteroids. Prior to egg laying, females (which invest more energy than males into the production of reproductive structures) represented 72% of the adult whelks that approached feeding L. polaris, but only 36% of the adults randomly collected from the study area. Furthermore, females that were attracted to feeding asteroids had smaller reproductive organs (after accounting for body size) than females randomly collected from the study area. Similarly, prior to egg laying, females fed longer and ingested more food than males when tested in the presence of L. polaris in the laboratory. After egg laying, however, females and males displayed a similar tendency to feed in the presence of a predator, both in the field and in the laboratory. Predator-impact indices, computed by contrasting the feeding activity of whelks in the absence and presence of a predator, indicated that females (but not males) responded more boldly to predators prior to than after egg laying, despite a general decrease in feeding activity at that period. Taken together, our observations indicate that the tendency of adult whelks to approach feeding predators is influenced by potential reproductive gains. Because such gains are presumably more directly linked to a given feeding opportunity in sexually active individuals, whelks may be selected to display increased levels of boldness towards predators with the onset of sexual maturity. Thus, potential reproductive benefits may partly explain the size-dependent tendency of whelks to approach feeding asteroids. Copyright 2001 The Association for the Study of Animal Behaviour.

A field test of threat sensitivity in a marine gastropod

In the Mingan Islands, the whelk Buccinum undatum displays defensive manoeuvres to both contact and water-borne chemical cues from the predatory asteroid Leptasterias polarisIn spite of this, whelks occasionally aggregate in great numbers near L. polaris while it is ingesting a prey; they then attempt to steal food from their predator and also wait for leftovers. In this study, the response of whelks in different types of encounters with L. polaris was examined to test the hypothesis that whelks are sensitive to the magnitude of the threat their predator represents. In a field experiment, whelks consistently fled both non-feeding and feeding L. polaris (asteroids used were consuming small prey items that were unlikely to provide food for whelks). When current flow was stable, whelks fled more directly down current and more frequently displayed violent defensive behaviours, in response to non-feeding L. polariswhich presented a higher risk, than in response to feeding asteroids (lower risk; 47% versus 2%). Consequently, whelks tested with non-feeding asteroids more rapidly distanced themselves from the predators than did whelks tested with feeding asteroids. In a field survey, there were more active whelks in the vicinity of cruising (higher risk) than stationary (lower risk) L. polaris (53% versus 14%). Among those whelks that were active, defensive behaviour patterns such as shell rocking and leaping escape movements were frequently shown by whelks near cruising predators (69%), but never by whelks near stationary predators (0%). The discriminative capabilities apparent in these results are likely to be adaptive, because they enable whelks to limit the cost of escape responses while still keeping predation risk low, and also because they facilitate a close association with L. polaris from which the whelks receive feeding benefits.Copyright 1997 The Association for the Study of Animal Behaviour1997The Association for the Study of Animal Behaviour

A flexible response to a major predator provides the whelk Buccinum undatum L. with nutritional gains

We examined the association of the common whelk Buccinum undatum with the predatory asteroid Leptasterias polaris in the Mingan Islands, northern Gulf of St. Lawrence. Quantitative sampling, using SCUBA diving, of surfaces of the sea floor in the vicinity (within 150 cm) of feeding and non feeding L. polaris and surfaces devoid of asteroids showed that both immature and mature whelks generally avoid L. polaris. Surprisingly, densities of mature whelks up to 12 times the average were occasionally encountered near asteroids when they were consuming a large prey, usually the surf clam Spisula polynyma. Comparisons of the stomach-content mass of whelks approaching L. polaris feeding on S. polynyma with that of whelks which were departing from the same type of feeding bout demonstrated that whelks obtained significant food gains by approaching L. polaris. Previous studies failed to identify the food resources that could account for the high biomass of whelks in the Mingan Islands. The association with L. polaris potentially explains a substantial part of the whelks diet. Whelks of the Mingan Islands have evolved a flexible behavioural response to L. polaris which permits them to make adaptive and risk-sensitive decisions when choosing between avoiding a major predator and taking advantage of food resources it makes available.

The geography and bathymetry of American lobster benthic recruitment as measured by diver-based suction sampling and passive collectors

Abstract We present results of an international collaboration to survey American lobster Homarus americanus Milne-Edwards, 1837 nurseries in Atlantic Canada and the northeast United States from 2007 to 2009 under a standardized protocol involving two sampling methods, diver-based suction sampling and passive collectors. We surveyed young-of-year and older juveniles at 191 sampling sites over 39 sampling areas considerably expanding the known depth range and geographic limits of benthic recruitment. Young-of-year densities were strongly correlated in space with the abundance of older juveniles, signifying consistently strong settlement in the Gulf of Maine, lower Bay of Fundy, southwestern Nova Scotia and southern Gulf of St. Lawrence, and relatively weak settlement in southern New England, eastern coastal Nova Scotia and Newfoundland, a pattern consistent with commercial lobster harvests. Passive collectors elucidated bathymetric patterns of young-of-year recruitment in oceanographically contrasting regions. Although we observed young-of-year lobsters as deep as 80 m, they were most abundant above the thermocline in summer-stratified regions, such as the western Gulf of Maine and southern New England, and depth-wise differences were less extreme in thermally mixed waters of the eastern Gulf of Maine–Fundy region, a finding consistent with previous observations that postlarvae concentrate above the thermocline. Between the two samplers, we detected no sampling bias for young-of-year lobsters, although collectors may slightly under-represent older juveniles entering from the surrounding sea bed. Finally, we found that interactions between juvenile lobsters and suspected predators or competitors in collectors, such as crabs and fishes, are weak and unlikely to bias collector results.