THE POPULATION BIOLOGY AND GENETICS OF THE DEEP-SEA SPIDER CRAB, ENCEPHALOIDES-ARMSTRONGI WOOD-MASON 1891 (DECAPODA, MAJIDAE)

Numerous specimens of the majid spider crab, Encephaloides armstrongi, were sampled from six stations (populations), between 150 and 650 m d epth, on the continental slope off the coast of Oman. This extended th e known geographic and bathymetric range of E. armstrongi, which is no w 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 re gion. The biology and generics of populations of Encephaloides armstro ngi separated by depth were studied. The overall sex ratio of the E. a rmstrongi 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 cara pace length, which probably represented the terminal (pubertal) moult for most individuals. Accumulation of female crabs in the terminal ins tar probably caused the variation of sex ratio with size classes. Some male crabs grew to a larger size (up to 38 mm carapace length), possi bly as a result of maturity at later instars. Length frequency distrib ution was significantly different between sexes (one-way ANOVA p < 0.0 01). Within sexes, length frequency distributions varied between diffe rent populations. In both male and female Encephaloides armstrongi the individuals from a single population located at 150 m depth were sign ificantly smaller than individuals at all other stations and were cons idered to represent a juvenile cohort. For female crabs no other signi ficant differences were detected in length frequency between populatio ns from 300 m to 650 m depth. Significant differences in length freque ncy 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) valu es between populations of E. armstrongi (I = 0.98-1.00) were within th e 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-s tatistics were used to analyse between population (F-ST) and within po pulation (F-IS) genetic structure. For both male and female E. armstro ngi significant genetic differentiation was detected between the popul ation 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 p opulations. For male E. armstrongi significant heterozygote deficienci es were detected within populations and significant genetic differenti ation between populations. The most likely explanations for the observ ations of the present study are: the population of Encephaloides armst rongi located at 150 m depth represented a juvenile cohort that is gen etically distinct from deeper populations; female E. armstrongi formed a single population between 300 m and 650 m depth in the sampling are a; male E. armstrongi were from two or more genetically distinct popul ations which are represented by different numbers of individuals at st ations between 300 m and 650 m depth. This caused the observed signifi cant differences in morphology (size distribution) and allele frequenc ies of male populations. It is likely that E. armstrongi exhibits gend er-biased dispersal and that the crabs collected between 300 m and 650 m depth formed spawning aggregations. This also explains the bias in ses ratio of individuals sampled in the present study.