Biology, population structure, and field-growth rates of Porcellionides pruinosus (Brandt, 1833) (Isopoda, Oniscidea) at Garat Naam (Kasserine, Tunisia)

Biology, population structure, and field growth rates of Porcellionides pruinosus were studied at Garat Naam (Kasserine, Tunisia). Recruitment occurred from April/May to November. The proportion of ovigerous females in the population was positively correlated with day length, and the proportion of juveniles was positively correlated with day length and temperature. Female size was positively correlated with fecundity (number of eggs per brood) and fertility (number of embryos hatched per brood). We estimated losses during embryonic development at +/-2.5%. Sex ratio was always in favour of females. We analysed size frequency distributions for recognizable cohorts. Field growth data were used to calibrate a growth model. Minimum average length of newly born cohorts ranged from 3.3 +/- 0.148 mm to 4.14 +/- 0.159 mm total length (TL). Life span was estimated at 11 to 14 months. Cohorts born in summer and autumn survived longer (13 to 14 months) than cohorts born in spring (11 to 12 months). Females took approximately 1 month to reach sexual differentiation (3.96 +/- 0.09 mm TL), while males took slightly less (3.24 +/- 0.14 mm TL). Females from cohorts born in spring took less time (+/-2.5 months corresponding to 3.58 +/- 0.21 mm TL) than females from cohorts born in late summer and autumn, to reach sexual maturity (+/- 3.5 months corresponding to 4.99 +/- 0.23 mm TL). P. pruinosus behaved as a semi-annual species (two or three broods per year), with iteroparous females, and a bivoltine life cycle. A comparison with other studies exhibits the occurrence of large intra-specific variation in the species' reproductive pattern.