Population Demographics

Population demographics sometimes demonstrate a lack of large individuals within the population, (Litulo, 2005; Branco et al., 2002; Turra et al., 2002.) This could be due to a high mortality rate of certain sized crabs, (Zar, 1999) or due to a lack of large available shells, (Branco et al., 2002.) Sex ratios have commonly been found to be female biased, which is actually frequently observed among other species as well, (Wada et al., 1995; Litulo, 2005.) There are a multitude of possible reasons as to why females are more abundant than males; differences in longevity and growth times, differential migration, mortality, and differential production of gametes, (Litulo, 2005.) The abundance of females, according to Turra & Leite (2000), may be because of a higher mortality rate of males resulting from habitat partitioning, differential feeding restrictions or spatial dispersion between sexes. It is also likely that males undergo longer distance migrations in the search to find the rarer large shells, (Litulo, 2005.)

The number of available shells may limit populations. Dominant males are able to obtain and defend their shells better than their subordinates, and therefore are able to acquire the highest quality shells available. Having the ability to obtain the best and largest shells allows them to to grow even larger, ever increasing their dominance among other individuals, (Bertness, 1981)