Reproductive biology of the isopod Excirolana braziliensis at the southern edge of its geographical range

Abstract

A full analysis of the reproductive biology of the isopod Excirolana braziliensis Richardson 1912 was conducted in a sandy beach of Uruguay, located at the southernmost edge of its distributional range in the Atlantic Ocean. Reproductive and recruitment periods of E. braziliensis were concentrated in austral summer. Females with oostegites appeared in November, whereas total biomass, individual sizes and fecundity of ovigerous females peaked between December and January. These concurrent traits were responsible for the significant peak of juveniles in January. The size at maturity was 9.88 mm. Four embryonic developmental stages were described and identified: mean length linearly increased from stages I to III, whereas dry weight exponentially decreased from stages I to IV. The high reproductive output (0.41-0.58), reported for the first time in this isopod, exceeds the rates documented for other isopods. Reproduction of E. braziliensis at the southern edge of its range is semelparous: females produce one brood during the reproductive season, exhaust their energy reserves during incubation, and probably die at the end of the reproductive season. A macroscale comparison suggests that E. braziliensis at the southern edge of its range counteracts its narrow reproductive period by a short incubation period with larger individual mature female and embryo sizes, higher fecundity and a higher percentage of ovigerous females than in subtropical and tropical populations. These extreme reproductive indicators could be attributed to the internal retention of embryos that assures offspring survival, coupled with a high adaptation capability to environmental variations across its range.

Production of Excirolana armata (Dana, 1853) (Isopoda, Cirolanidae) on an exposed sandy beach in southeastern Brazil

“…Eggs/embryos incubated in the marsupium of females collected in the first year (December 2003to November 2004 were removed, counted, and categorized into four developmental stages according to Jones (1970) and Martínez and Defeo (2006) (see also Petracco et al 2010). In order to obtain the ash-free dry weight, the individuals were pooled into 0.25 mm length classes and dried at 70°C for 48 h to measure the dry weight (DW).…”

“…Since the incubation period estimated for E. armata is longer than 1 month (ca 2 months; Petracco et al 2010), only the ovigerous females bearing embryos in the developmental stages three and four described by Martínez and Defeo (2006) were considered to estimate the abundance of eggs/embryos in each month. Annual gonad production (P g ) is given by the equation:…”

Population biology of Excirolana armata (Dana, 1853) (Isopoda, Cirolanidae) on an exposed sandy beach in Southeastern Brazil

“…Thus, E. armata on Una beach must be semelparous, as observed for E. braziliensis in Chile (Zuñiga et al. 1985) and in Uruguay (Martínez & Defeo 2006).…”

“…1). Martínez & Defeo (2006) and Zuñiga et al. (1985) estimated that the incubation period of E. braziliensis in Uruguay ( c. 33° S) and Chile ( c. 23° S) was 1 and 3 months, respectively.…”

“…Recent studies have shown that life history traits in sandy beach populations can be highly plastic over latitudinal gradients, leading to an adjustment of the phenotype-environmental relationship (Defeo & McLachlan 2005). In this context, marked differences in structure, population dynamics, and in the reproductive biology of E. braziliensis and Excirolana hirsuticauda were observed according to the latitudinal variation (Cardoso & Defeo 2003Contreras & Jaramillo 2003;Martínez & Defeo 2006). Because E. armata is one of the most abundant species on Una beach (Petracco 2008), Southeastern Brazil, the aim of this study is to analyse the population biology of E. armata in this beach and to test the hypothesis that the life history traits of this isopod undergo a latitudinal variation throughout its distribution range.…”