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 Anderson's (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.

Two laboratory experiments investigated mate guarding and sperm allocation patterns of adult males with virgin females of the snow crab, Chionoecetes opilio, in relation to sex ratio. Although females outnumbered males in treatments, operational sex ratios were male-biased because females mature asynchronously and have a limited period of sexual attractiveness after their maturity molt. Males guarded females significantly longer as the sex ratio increased: the mean time per female was 2.9 d in a 2 males:20 females treatment compared to 5.6 d in a 6 males:20 females treatment. Female injury and mortality scaled positively to sex ratio. Males that guarded for the greatest number of days were significantly larger, and at experiment's end had significantly smaller vasa deferentia, suggesting greater sperm expense, than males that guarded for fewer days. In both experiments, the spermathecal load (SL)--that is, the quantity of ejaculate stored in a female's spermatheca--was independent of molt date, except in the most female-biased treatment, where it was negatively related. The SL increased as the sex ratio increased, mainly because females accumulated more ejaculates. However, similarly sized males had smaller vasa deferentia and passed smaller ejaculates, such that, at a given sex ratio, the mean SL was 55% less in one experiment than in the other. Some females extruded clutches with few or no fertilized eggs, and their median SL (3-4 mg) was one order of magnitude smaller than that of females with well-fertilized clutches (31-50 mg), indicating sperm limitation. Males economized sperm: all females irrespective of sex ratio were inseminated, but to a varying extent submaximally; each ejaculate represented less than 2.5% of male sperm reserves; and no male was fully exhausted of sperm. Sperm economy is predicted by sperm competition theory for species like snow crab in which polyandry exists, mechanisms of last-male sperm precedence are effective, and the probability that one male fertilizes a female's lifetime production of eggs is small.

Growth and maturation of postlarval male snow crab (Chisnc~ecetes spklio) in an exploited Gulf of Saint Lawrence population were elucidated using size distributions in beam trawl samples taken bimonthly from April 1991 to May 1992, moult and maturity indices, and measures of growth per moult. Males develop in three stages: immature, without spermatophores; adolescent, with spermatophores but undifferentiated chelae; and adult, with spermatophores and differentiated chelae. Males change from immature to adolescent after a puberty moult, and from adolescent to adult after a terminal moult producing a final carapace width (CW) >40 mm. The slope of the regression relating post-to pre-moult CW decreases after puberty. Carapace moult increments are the same at pre-terminal and terminal moults, for a constant premoult CW. Males recruit to the legal size of 95-mm CW at instar XII, 43.7 years or more after settlement. The slope of the regression of gonad weight on CW is less for adult than for adolescent males owing to the formers' participation in reproduction and to greater depletion of sperm stores in large than in small adult males. Gonad weight increases with time since moult, but for large adults, did not reach levels recorded in prefishery times or in an unfished population.IRCsumC : La eroissance et la maturation des stades postlarvaires du male du crabe des neiges (Chioaoecetes spilio) dans une population exploitke du golfe du Saint-Laurent ont Ct C 6lucidCes a 19aide de distributions des tailles dans des Cchantillons bimestriels prClevCs h l'aide d'un chalut B perche d'avril 1991 a mai 1992, d9indices de mue et de maturitC et de mesures dvaccroissement de la taille B la mue. Le male se dCveloppe en trois Ctapes: immature, sans spermatophores; adolescent, avec spermatophores mais h pinces non diffkrencikes; adulte, avec spermatophsses et pinces diffCsenciCes. Le male immature devient adolescent aprks une mue de pubertC et l'adolescent devient adulte aprks une mue terminale rtsultant en une largeur de earapace (LC) finale >40 mm. La pente de la rkgression entre les LC post-et prCmue dCcro"i aprks la pubertC. L'accroissement de la carapace 2 la mue est pareil aux mues prC-terminale et terminale, pour une mCme LC prCmue. Ees mQles recrutent a B a taille l6gale de 95 mm de LC au stade XHI, =8,7 ans ou plus aprks lVCtablissement. La pente de la rCgression du poids des gonades sur la LC est plus faible chez les mlles adultes que chez les m21es adolescents, paree que les premiers participent B la reproduction et que les rkserves de sperme sont plus entamCes chez les gros que chez les petits males aduItes. Le poids des gonades croft en fonction du temps CeoulC depuis la mue, mais n9atteignait pas chez les gros mlles adultes les niveaux enregistr6.s avant l'essor de la pCche ou dans une population vierge.

A total of 1 691 mature female Chionoecetes opilio was collected by beam trawl in Baie Sainte-Marguerite on seven occasions from April 1991 to May 1992. Quantitative analyses of ovaries, brood, eggs, and spermathecal contents were performed on a subsample of 318 females. Females moulted to maturity in March and April. Development of ovaries and brood were phased and lasted 24–27 mo. Eggs hatched mainly from April to June. Spermathecae contained zero to three ejaculates, and the mean blotted weight of freshly deposited ejaculates was significantly greater in primiparous (0.513 g) than in multiparous (0.249 g) females. The weight of stored ejaculates decreased over time independently of spawning events. Fecundity was correlated positively with carapace width and negatively with mean egg diameter. Eggs of primiparous females were on average 1.4–2.7% larger, but 16.4–22.7% fewer per brood, than those of multiparous females. Some large multiparous females had small broods, due possibly to lesser fertility. Females probably hatch only two broods in a lifetime and produce a total of 81 630 to 83 143 larvae at 57.4 mm carapace width. Primiparous females apparently contribute >40% of larvae produced by the population.

Marine crustaceans are known as a group with a high level of morphological and ecological diversity but are difficult to identify by traditional approaches and usually require the help of highly trained taxonomists. A faster identification method, DNA barcoding, was found to be an effective tool for species identification in many metazoan groups including some crustaceans. Here we expand the DNA barcode database with a case study involving 80 malacostracan species from the Estuary and Gulf of St Lawrence. DNA sequences for 460 specimens grouped into clusters corresponding to known morphological species in 95% of cases. Genetic distances between species were on average 25 times higher than within species. Intraspecific divergence was high (3.78-13.6%) in specimens belonging to four morphological species, suggesting the occurrence of cryptic species. Moreover, we detected the presence of an invasive amphipod species in the St Lawrence Estuary. This study reconfirms the usefulness of DNA barcoding for the identification of marine crustaceans.