Distribution of Tylos spp. in the Maltese Islands and population dynamics of Tylos europaeus

Abstract

Two species of the oniscid genus Tylos occur in the Maltese Islands, T. sardous and T. europaeus, which are allopatric and restricted to just one and two sandy beaches, respectively. The dynamics of the largest locallyoccurring T. europaeus population were investigated during the period 2001-2003. Seasonal variation in the sex ratio, length of the 5th segment of the pereion as a proxy for age, and the proportion of adults and juveniles in the population were assessed during each calendar season. The vertical distribution of male, female and juvenile individuals in the sand was determined in the field during summer 2003. Laboratory experiments were made to test sand moisture preferences. The surface activity of the isopods was studied by means of pitfall trap constellations whilst zonation on the beach was studied by sieving sand collected from quadrats placed at regular intervals along a shore-normal transect starting from mean sea-level (MSL), and counting the number of individuals in each sample. For the Tylos europaeus population studied, males outnumbered females in seven of eight seasonal sampling sessions, with a mean male:female ratio of 1.46, although differences between the abundances of adult males and females were only statistically significantly different during the two spring seasons. Juveniles were consistently more abundant than adults, abundances ranging between 220-450 individuals/m3 for juveniles and between 450-3200 individuals/m3 for adults. Males consistently exhibited larger pereion sizes than females: mean pereion length was 1.91mm (± 0.43mm) for males and 1.79mm (± 0.39mm) for females.