POPULATION STRUCTURE AND DyNAMICS Of Porcellio lAeVis ( LATREILLE , 1804 ) IN NORTHERN TUNISIA

– The population structure and dynamics of Porcellio laevis Latreille, 1804, a terrestrial isopod, were studied in its natural population at Wadi Joumin in Bizerte (Northern Tunisia), over 18 months from February 2013 to July 2014, by monthly sampling. The ovigerous females were only present from March to August. This species exhibits seasonal reproduction followed by sexual rest. Fecundity and fertility were both estimated to better understand the different steps of reproduction from sexual rest to manca release. Results showed a highly significant difference between oocyte incount during sexual rest and the first step of reproduction. Then, a non-significant difference between both steps of (eggs-embryos) and (embryos-mancas) was obtained. The sex-ratio was female biased and a high significant difference was statistically observed. Analysis of the size frequency polymodal distribution suggested the presence of four cohorts on the first sampling date and 7 new ones during the next 17 months. Lifespan was estimated at 10-12 months. At this study site, P. laevis appeared as a semiannual species, with iteroparous females producing at least two broods, and exhibited a bivoltine life cycle. Porcellio lAeVis SEASONAL REPRODUCTION fECUNDITy SEX-RATIO GROWTHCOHORTS LIfE SPAN 210 C. GHEMARI, M. f. BOUSLAMA, A. AyARI, K. NASRI-AMMAR Vie milieu, 2016, 66 (2) study site and sampling: The sampling station was located at Wadi Joumin, in Bizerte, Northern Tunisia, at the entrance of the natural reserve Ichkeul; situated at 37°0’37”N and 09°41’21”E (fig. 1) far away from any source of pollution and where P. laevis was the only abundant species at this locality. In the study area, the relative air humidity showed a variation from 53 % to 85 % during the entire sampling. Monthly mean temperature was measured in situ using thermo-hygrometer and varied from 10 °C in february to 28 °C in July. Specimens of P. laevis were found beneath stones and decaying leaves. They were collected by hand in the early morning hours (2 h each time), once a month during 18 months from february 2013 to July 2014. We notice that animals were collected from the same area (90 m2) during the sampling period, with the respective substratum and kept in polystyrene perforated boxes to provide ventilation. laboratory procedures and data analysis: In the laboratory, specimens were preserved for measurements. Body length (BL) was measured using a stereo-microscope (Leica MS5). The sex determination was based on the presence or absence of oostegites or a brood pouch in females and of a genital apophyse and developed endopodite at the pleopod I and II in males. In adult males, the endopodite of pleopod I was more differentiated, and the pereiopod VII showed a brush of setose at the carpus and the merus. In juvenile males, the endopodites of pleopod I and pleopod II are not much differentiated. The females are also divided into two groups: – Non reproductive females, grouping juveniles and adults without marsupium, also called (non-ovigerous females) – Reproductive females showing a brood pouch full of eggs or embryos (ovigerous females) or an empty marsupium. fig. 1. – Sampling locality (●) at Wadi Joumin (Northern of Tunisia, Bizerte). POPULATION STRUCTURE Of Porcellio lAeVis 211 Vie milieu, 2016, 66 (2) In immature specimens, the body length does not exceed 5.6 mm. Dissection of females in sexual rest collected on february 2013 (N = 15) and during the breeding period on March 2013 (N = 15) allowed to determine microscopically the number and size of oocytes. In order to estimate the fecundity (number of eggs produced by brood) and fertility (number of embryos for brood) parameters, the eggs and embryos found in the brood pouch of 15 ovigerous females for each of the two stages were removed and counted. In the laboratory, 15 gravid females collected in the field in April 2013 were sized and were individually placed, under quasi-natural conditions of temperature and photoperiod, in translucent plastic boxes (20 × 35 × 10 cm) containing a loamy soil about 3 cm height. They were fed on disks of carrot, and sporadically, on some fresh linden leaves. females were daily observed in order to record the time and the number of the mancas released. field growth rates were estimated by tracking recognizable modal distributions in the population using size-frequency of distributions (0.3 mm length classes) in successive sample dates, following a methodology previously described (Pardal et al. 2000, Marques et al. 2003). Modal distributions are assumed to result from pulses in recruitment, conventionally referred to as cohorts. Size-frequency analysis was carried out using the probability paper method (Harding 1949), as performed by Cassie (1954, 1963). statistical analysis: The observed and expected values of the sex ratio (the ratio of males to females) were compared using a χ2 test. The relationship between fecundity or fertility and the total body length of ovigerous females was described using simple linear regression. A one-way analysis of variance (ANOVA) was also performed to test the significance of differences between number of eggs, embryos or mancas inside or outside the marsupium using XLSTAT. Computations used the ANAMOD software (Nogueira 1992), and the reliability of the distribution separation method was tested with both χ2 and G tests (p ≤ 0.05).