The population biology and genetics of the deep–sea spider crab,Encephaloides armstrongiWood–Mason 1891 (Decapoda: Majidae)
Authors: Creasey Simon , Rogers Alex D. , Tyler Paul , Young Craig , Gage John 
Published: October 2002

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 ofE. 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 ofEncephaloides armstrongiseparated by depth were studied. The overall sex ratio of theE. armstrongisampled was male–biased (p< 0.01; 3.3 males: 1 female;So= 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 ANOVAp< 0.001). Within sexes, length frequency distributions varied between different populations. In both male and femaleEncephaloides armstrongithe 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 ofEncephaloides armstrongi. Genetic identity (I) values between populations ofE. armstrongi(I= 0.98−1.00) were within the normal range for conspecific populations. Observed heterozygosity (Ho= 0.080−0.146) was lower than expected heterozygosity (He= 0.111−0.160), but in the normal range detected for eukaryotic organisms.

F–statistics were used to analyse between population (FST) and within population (F) genetic structure. For both male and femaleE. armstrongisignificant genetic differentiation was detected between the population located at 150 m depth and all other populations. Analyses ofFISandFST, excluding the 150 m population indicated that for femaleE. armstrongithere was no significant structuring within or between populations. For maleE. armstrongisignificant 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 ofEncephaloides armstrongilocated at 150 m depth represented a juvenile cohort that is genetically distinct from deeper populations; femaleE. armstrongiformed a single population between 300 m and 650 m depth in the sampling area; maleE. armstrongiwere 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 thatE. armstrongiexhibits 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.