Age determination of Olivancillaria vesica auricularia (Lamarck, 1810) (Mollusca, Gastropoda, Olividae) in the Cassino Beach, Rio Grande, Rio Grande do Sul State, Brazil

Abstract

The age population structure of Olivancillaria vesica auricularia at Cassino beach, Rio Grande – RS, Brazil was studied according to the growth lines on the body whorl. The individuals were collected monthly from December 1996 to June 1998 at the washing and surfing zone. A total of 401 animals (220 males and 181 females) were collected. The shell size ranged between 15.2 mm and 57.0 mm, where 0 to 6 growth lines were distinguished. The growth lines are Orange longitudinal bands, with 1.0 to 2.0 mm width, over the external surface of the body whorl and transversally on the fascicle. The growth lines can be on the external lip (border) or approximately 5.0 mm distant from the border (middle). The number of the growth lines had strong correlation with the shell size (r = 0.86), which indicated that the number of growth lines found on the body whorl is proportional to the size of the shell. No significant differences were detected between sexes considering the number of the growth lines. Probably the formation of the growth lines could be related with seasonal environmental conditions. During the autumn and winter, animals with growth lines on the shell border were predominant, and during the spring and summer, animals with growth lines distant to the border were abundant.