Gaya Crab project

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Baseline Demographic and Morphometric Attributes of Two Shore-dwelling Crab Populations on Pulau Gaya, Malaysian Borneo

FIGURE 1. Details of MA and MB morphology.

Crabs play a central role in island ecosystems as primary consumers of shore detritus, enabling predators and other organisms indirectly to utilize the energy stored in decaying organic matter. It is therefore of central importance to gain a better understanding of similarities and differences among divergent crab populations, as well as periodically to assess the condition of insular crab communities. With these two goals in mind, we investigated the demographic and morphometric attributes of two shore-dwelling crab populations in the area surrounding Padang Point, Pulau Gaya, Sabah, Malaysia. Two morphologically distinct populations, labeled Morphospecies A (MA) and Morphospecies B (MB), were sampled. MA inhabits the ~10 meter wide stretch of sandy beach east of the Padang Point jetty, while MB can be found most readily to the west of the jetty in an area of sand overlain with dark rocks. Individuals of MA exhibit light coloration with dark speckling, whereas individuals of MB display solid coloration, ranging from dark brown to beige. Both species live in sandy burrows.

The MA population was sampled using a 50 meter belt transect. All individuals within three meters of the transect line were collected. The MB population could not be sampled by transect because individuals were not visible on rock surfaces; a quadrat sampling method was employed instead, in which rocks in a designated 5x5 meter square were overturned and all crabs exposed collected. 27 individuals of MA and 60 individual of MB were collected in total. The carapace length and width, the sex, and the claw dominance of each individual collected were recorded.

Statistical analysis was conducted using the R environment (R version 2.11.0 (2010-04-22) Copyright (C) 2010 The R Foundation for Statistical Computing). For each morphospecies, the percentages of male and female right-clawed and left-clawed individuals were calculated. Carapace area was estimated using a rectangular model by multiplying the carapace length and width for each individual. For the MB population, a chi-squared test was performed to determine whether there was a relationship between sex and claw dominance, and a t-test was performed to determine whether there was a similar relationship between sex and body size. Neither test revealed a significant relationship (p = 0.98 and 0.11, respectively). Relationships between morphospecies were also assessed. A t-test for association between body size and morphospecies revealed that the body sizes of MB individuals were significantly greater than the body sizes of MA individuals (P = 2.2 * (10^-16)). Chi-squared tests for association between sex ratio and morphospecies and association between claw dominance and morphospecies both returned significant results (P = 1*(10^-9), P = 0.01).

FIGURE 2. Summary of intra-morphospecific results.

We find several of our results interesting and unexpected. First, the lack of a significant relationship between sex and body size in the MB population contradicts the general rule that crab females are larger than crab males for reasons having to do with reproductive fitness. It is possible that the observed homogenization of body size is the result of selective pressure toward small body size operating on the MB population; however, a quantitative body size comparison of MB with closely-related species will be necessary before a firm explanation of our results can be provided. We conjecture on the basis of our claw dominance measurements that patterns of claw dominance may be selection-independent attributes of crab populations and that, assuming that claw dominance is genetically determined (and depending on details of how it is determined), the difference we observe may be an indirect measurement of the divergence of the two populations. Claw dominance statistics could thus possibly serve as a convenient proxy for more labor-intensive genetic methods.

By far the most striking observation resulting from the study is the extreme difference in sex ratios between the two populations: MB individuals were predominantly female, where as all 27 MA individuals were male. Sex ratios of less than one female in fifty males are not unheard of in island shore communities like the ones we sampled. Nevertheless, it is unclear whether the striking scarcity of females in the MA population represents a normal feature of MA demographics or a community under considerable stress. Given the ecological significance of shore-dwelling crab species, further investigation into this issue is recommended.