Effects of intraspecific competition and scavenging on growth of the periwinkle Littorina littorea

Abstract

The effects of density of conspecifics, size of experimental cages, and scavenging of ani- mal tissue on the growth rate of Littorina littorea (L.) were tested during the summers of 1991, 1992, and 1994. In 1991, the effects of intraspecific competition were examined by holding individually marked periwinkles in cages of 5 different sizes and at 4 densities. Cages were 25, 50, 100, 200, and 400 cm 2 in area, and were of different sizes to test for caging artifacts. Experimental densities were 50, 100, 200, and 400 m -2 and encompassed normal field densities (x = 299 m -2 ; SD = 205). Growth was measured as final shell height and the amount of shell added at the lip of the aperture. The experiment showed evidence for intraspecific competition and no effect of cage size. Periwinkles, which were between 12 and 16 mm in shell height, added an average of 6.8 mm per 6 wk at the lip when held at a density of 50 m -2 and 1.6 mm per 6 wk when held at 400 m -2 . The results were con- firmed with a 1992 experiment, which used 100 cm 2 cages and densities of 100, 200, and 400 m -2 . Field observations suggested that L. littorea may also scavenge animal tissue, and this was tested in a 1994 experiment in which periwinkles were provided mussel (Mytilus edulis) tissue. Growth rates were nearly doubled when snails were given mussel tissue. While L. littorea is one of the most stud- ied gastropods, this study is the first experimental test of intraspecific competition, and the first demonstration that scavenging can enhance growth rates of periwinkles.