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Simon Creasey1,2, Alex Rogers1, Paul Tyler2, Craig Young3 and John Gage4
1. Marine Biological Association of the U.K., The Laboratory, Citadel Hill, Plymouth, Devon, PL1 2PB, U.K.
2. Department of Oceanography, Southampton University, Southampton Oceanography Centre, Empress Dock, Southampton, SO14 3ZH, U.K.
3. Harbor Branch Oceanographic Institution, 5600 U.S. 1 North, Fort Pierce, FL 34946.
4. The Scottish Association for Marine Science, P.O. Box 3, Oban, Argyll, PA34 4AD, U.K.
Samples of the majid spider crab, Encephaloides armstrongi, were collected from six stations along the continental slope of the western Arabian Sea at depths of 150 - 650 m. The biology and genetics of horizontally and vertically separated populations were studied.
The overall sex ratio of crabs was male-biased (p<0.01), although variation in sex ratio was observed in individual populations and between size classes of crab. Size frequency data indicated that male crabs comprised at least three instars. Female crabs comprised at least two instars, the second of which probably represented a terminal moult. Accumulation of female crabs in the terminal instar probably caused the variation of sex ratio with size classes.
Length frequency distribution differed significantly between the sexes (p<0.001). Within sexes, length frequency distribution varied between populations. In both sexes of Encephaloides armstrongi individuals from the 150m population were significantly smaller than those from the remaining populations. It is concluded that the 150m population represents a juvenile cohort. Additional length frequency differences were observed between populations in male crabs, but none were detected in female crabs.
Eight enzyme loci were detected using starch gel electrophoresis from each population of Encephaloides armstrongi. Genetic identity (I) values were in the normal range observed (I= 0.98 - 1.00) for conspecific populations. Observed heterozygosity (Ho= 0.080 - 0.146) was lower than expected (He= 0.111 - 0.160), but consistent to the range detected in deep-sea decapods and crustacea in general.
Genetic structure of populations was examined using F-statistics. In both sexes of Encephaloides armstrongi significant genetic differentiation was detected between the 150m population and all other populations. When the 150 m population was excluded, the levels of within population and between population genetic structure were not significant in female crabs, but remained significant in male crabs as a result of significant heterozygote deficiency.
It is hypothesised that: (i) The 150m population represents a genetically distinct cohort of Encephaloides armstrongi, (ii) female E. armstrongi form a single panmictic population between 300 and 650 m, (iii) male crabs are from two or more genetically distinct populations at depths of 300-650m. This caused the observed differences in morphology and allele frequency between male populations. E. armstrongi may exhibit male gender-biased dispersal, with the 300-650m populations representing spawning aggregations.
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