The population biology and genetics of the deep–sea spider crab,Encephaloides armstrongiWood–Mason 1891 (Decapoda: Majidae)

Abstract

Numerous specimens of the majid spider crab, Encephaloides armstrongi , were sampled from six stations (populations) between 150 and 650 m depth, on the continental slope off the coast of Oman. This extended the known geographic and bathymetric range of E. armstrongi , which is now known to occur along the continental margins of the northern Indian Ocean from the western coast of Burma to the coast of Oman. This band–like distribution is contiguous to the oxygen minimum zone in this region. The biology and genetics of populations of Encephaloides armstrongi separated by depth were studied. The overall sex ratio of the E. armstrongi sampled was male–biased ( p < 0.01; 3.3 males: 1 female; S o = 0.538). However, sex ratio varied both between populations ( p < 0.01) and between size classes of crabs. Size frequency analysis indicated that the male and female crabs consisted of at least two instars, one between 6 and 16 mm carapace length and one between 16 and 29 mm carapace length, which probably represented the terminal (pubertal) moult for most individuals. Accumulation of female crabs in the terminal instar probably caused the variation of sex ratio with size classes. Some male crabs grew to a larger size (up to 38 mm carapace length), possibly as a result of maturity at later instars. Length frequency distribution was significantly different between sexes (one–way ANOVA p < 0.001). Within sexes, length frequency distributions varied between different populations. In both male and female Encephaloides armstrongi the individuals from a single population located at 150 m depth were significantly smaller than individuals at all other stations and were considered to represent a juvenile cohort. For female crabs no other significant differences were detected in length frequency between populations from 300 m to 650 m depth. Significant differences in length frequency were detected between male crabs from populations between 300 and 650 m depth. Horizontal starch gel electrophoresis was used to detect six enzyme systems coding for eight loci for individuals sampled from each population of Encephaloides armstrongi . Genetic identity ( I ) values between populations of E. armstrongi ( I = 0.98−1.00) were within the normal range for conspecific populations. Observed heterozygosity ( H o = 0.080−0.146) was lower than expected heterozygosity ( H e = 0.111−0.160), but in the normal range detected for eukaryotic organisms. F –statistics were used to analyse between population ( F ST ) and within population ( F ) genetic structure. For both male and female E. armstrongi significant genetic differentiation was detected between the population located at 150 m depth and all other populations. Analyses of F IS and F ST , excluding the 150 m population indicated that for female E. armstrongi there was no significant structuring within or between populations. For male E. armstrongi significant heterozygote deficiencies were detected within populations and significant genetic differentiation between populations. The most likely explanations for the observations of the present study are: the population of Encephaloides armstrongi located at 150 m depth represented a juvenile cohort that is genetically distinct from deeper populations; female E. armstrongi formed a single population between 300 m and 650 m depth in the sampling area; male E. armstrongi were from two or more genetically distinct populations which are represented by different numbers of individuals at stations between 300 m and 650 m depth. This caused the observed significant differences in morphology (size distribition) and allele frequencies of male populations. It is likely that E. armstrongi exhibits gender–biased dispersal and that the crabs collected between 300 m and 650 m depth formed spawning aggressions. This also explains the bias in sex ratio of individuals sampled in the present study.

Patterns of morphological and genetic variability in UK populations of the shore crab, Carcinus maenas Linnaeus, 1758 (Crustacea: Decapoda: Brachyura)

“…Hence, the results of this study demonstrate that the expression of inter-population variability in the morphology of C. maenas has both environmental and genetic components that need to be accounted for in future biomonitoring, taxonomic and population-level research. Buffer systems described in Wendel and Weeden (1989) and Creasey et al (1997). Clustering is based on UPGMA.…”

“…The gel and bridging buffers were prepared according to Wendel and Weeden (1989) and Creasey et al (1997). Staining protocols were obtained from Cheliak and Pitel (1984).…”

Genetic diversity in the Gammarus pecos species complex: Implications for conservation and regional biogeography in the Chihuahuan Desert

“…Heterozygote deficiency seems to be a common feature of deepsea invertebrates (Creasey et al 1997). Three main processes are thought to have induced these deficiencies in marine crustaceans: natural selection, assortative mating, and the presence of cryptic species (Creasey et al 1997). Usually, heterozygote deficiency caused by selection occurs only at specific loci, whereas it is found across all loci when it is due to inbreeding (Creasey et al 1997).…”

“…Heterozygote deficiency seems to be a common feature of deepsea invertebrates (Creasey et al 1997). Three main processes are thought to have induced these deficiencies in marine crustaceans: natural selection, assortative mating, and the presence of cryptic species (Creasey et al 1997).…”

Evidence for poecilogony in Pygospio elegans (Polychaeta:Spionidae)

“…Observed heterozygote deficiency may result from (1) mis-scoring of gels though the presence of null alleles, (2) bottlenecks in population size, and founder effects, (3) selection against heterozygotes, or for a specific allele, and (4) the Wahlund effect. Null alleles appear to be rare (Creasey et al 1997). Pygospio elegans exhibits typical opportunistic behaviour, and population densities may fluctuate dramatically in space and time (Morgan 1997), potentially resulting in bottlenecks and founder effects.…”