An investigation of the distribution and abundance of Otala punctata (Müller, 1774) in the Baћrija area

Abstract

In Malta, a second population of Otala punctata has established itself in the Baћrija valley system. This helicid, native to the western Mediterranean, is alien to the Maltese Islands, and therefore, the present study was carried out in order to investigate the spatial distribution and abundance of Otala punctata in Baћrija. Spatial distribution was determined by physically surveying the area of study and noting down if the helicid’s presence was observed, or not. The abundance of the helicid was investigated along a line transect along the Baћrija Valley watercourse and through random plot sampling. The latter methodology involved quadrat sampling while recording the number of occurring Otala punctata and Eobania vermiculata (a native mollusc having similar traits to the alien helicid), recording the micro habitat and measuring the maximum diameter of the occurring Otala punctata specimen. Otala punctata was found to have colonised three areas along the Baћrija valley system. The total spatial distribution was found to cover an area of approximately 0.6 km², while the maximum abundance found along the transect line was that of 1.98 individuals per m². By means of the plot sampling method it was determined that although the alien helicid had a higher abundance in the area close to its possible origin, it did not seem to have affected the population density of the indigenous Eobania vermiculata. The alien helicid was recorded to be most abundant in low (virescent) vegetation. Shell measurements indicated that there is a higher abundance of juveniles at the extremities of the area of distribution. This could indicate that the snails are dispersing outwards, possibly to escape competition for resources. Additionally, although it was observed that the spatial distribution may be slowed down or even temporally halted due to some physical barriers, this study saw that to the transport of agricultural produce from Baћrija to markets elsewhere, has a very high potential to disperse this alien species to new areas in the Maltese Islands.