Population dynamics ofSesarma rectum(Crustacea: Brachyura: Grapsidae) in the Ariquindá River mangrove, north-east of Brazil

Abstract

This study was carried out in order to provide basic information on the population ecology of the crab Sesarma rectum in the Ariquindá River mangrove, Tamandaré, State of Pernambuco, Brazil. The population was analysed with regard to the following aspects, in particular: the size-class frequency distribution of carapace width (CW), mean body size (CW) of males and females, morphological sexual maturity, sex-ratio, reproductive period, and recruitment. Samples were collected monthly from April 2008 through to March 2009; the crabs were collected manually, with a capture effort by one person for 30 minutes, during low tide. The specimens obtained were measured for CW, length of the propodus of males, and abdomen width of females; and the sex and ovigerous condition were noted. Altogether, we obtained 511 specimens (132 juvenile and 137 adult males, and 171 juvenile and 71 adult females, of which 32 were ovigerous). The median CW of males (16.15 mm) was significantly larger than that of females (13.82 mm) (P , 0.05). The size at morphological sexual maturity was 15.73 mm in males and 16.71 mm in females. The sex-ratio for the total of specimens analysed was 1.11:1 (male:female) (P . 0.05). The sex-ratio by size-class showed an anomalous pattern, with a greater abundance of males in the larger sizeclasses. The reproductive period was continuous and the highest frequency of ovigerous females was recorded in the spring and summer. The major pulse of recruitment occurred during autumn and winter, which is related to greater reproductive activity during the warmer months of the year.

Population structure and reproductive biology of the crab Sesarma rectum Randall, 1840 (Brachyura, Sesarmidae) in an impacted tropical mangrove in northeast Brazil

“…Castiglioni et al (2011) also found ovigerous females of S. rectum throughout the year in the Arinquindá River, Pernambuco, indicating a continuous reproductive period with two peaks of intensity, one during spring and other in summer. The monthly frequency of ovigerous females found by Castiglioni et al (2011) was positively correlated with air and burrow temperatures.…”

“…The sex ratio found in the present study (1 : 0.87) did not differ significantly from the Fisherian proportion of 1 : 1, although there was a trend for males to be slightly more numerous than females. Similarly, Silva et al (2007) found no deviations from the expected ratio for a population of S. rectum in Paraty, Rio de Janeiro, and neither did Silva & Chacur (2002) for S. rectum in Ubatuba, nor Castiglioni et al (2011) for S. rectum in Arinquindá, Pernambuco. However, in these populations there was also a slight trend toward a greater abundance of males.…”

“…However, it is important to keep in mind that this could also be due to collection bias. Leme (2002) and Castiglioni et al (2011) also found significantly larger males than females for a population of Sesarma rectum in a subtropical mangrove forest in southeastern Brazil and in a tropical mangrove in northeastern Brazil, respectively. However, it should be noted that in some populations of crabs, females may be larger than males, as found by Bedê et al (2008) for Uca (Minuca) victoriana Von Hagen, 1987 in Rio de Janeiro;and by Costa (2000) and Bezerra & Matthews-Cascon (2006) for Uca (Minuca) thayeri in Ubatuba, São Paulo and Uca (Leptuca) leptodactyla (Smith, 1898) in the Pacoti River, Ceará, respectively, although the differences were not significant.…”

“…Studies of sesarmid crabs in Brazil have shown that the animals in different populations reproduce continuously throughout the year (Leme, 2002(Leme, , 2005Nicolau & Oshiro, 2007;Castiglioni et al, 2011). However, even tropical species of crustaceans may show a bimodal distribution, as found by Bezerra & Matthews-Cascon (2007) for Uca (Minuca) thayeri Rathbun, 1900 in a tropical mangrove of the Pacoti River in Ceará.…”

“…Studies of the population biology and reproductive biology of sesarmid crabs are numerous, including S. rectum from subtropical mangroves in southeastern Brazil (Mantelatto et al, 1999;Silva & Chacur, 2002;Silva et al, 2007), but relatively few studies have been carried out in tropical mangroves in Brazil (Castiglioni et al, 2011). Comparisons among populations will help to evaluate the differences between them, as well as to understand the environmental and biological constraints from which those differences originate.…”