Distribution and changes in shell form of Thais SPP. (Gastropoda) near Bamfield, B.C.

Abstract

The distribution and shell form of the three species of Thais present have been investigated along the southeast shore of Barkley Sound (Vancouver Island) along a gradient of decreasing exposure to waves, from Pachena Point (open Pacific) to Grappler Inlet (extreme shelter). The 20 stations comprised rocky promontories and adjacent boulder beaches, the latter giving some local shelter. Thais emarginata (Deshayes), which has the lowest shell weight per unit of shell capacity, was found to range (mainly in the upper littoral) from full exposure to waves to considerable shelter. It corresponds most nearly in range of wave-exposure to the North Atlantic Nucella (= Thais) lapillus (L.) and the New Zealand Lepsiella Iredale, but is probably more tolerant of wave-exposure and less of shelter than these. Its shell was found to be lighter per unit of capacity. T. canaliculata (Duclos) was normally confined to low levels on the shore and to wave-exposed situations; it is somewhat better armoured. T. lamellosa (Gmelin) was found to occupy low levels from moderate to extreme shelter. It is very heavily armoured, no doubt in relation to the greater risk of predation inherent in its habitat. In laboratory tests it was found to be much more resistant than T. emarginata to crabs. Measurements of 477 Thais emarginata, divided between 9 stations, showed that the shell form changed from short and broad on the open coast to tall and narrow on sheltered promontories and on boulder beaches. With increasing shelter the whorl ratio ( R) and apical angle decreased and spiral angle (as defined by D'Arcy Thompson) increased. The ratio of increase of the tubular cavity of the shell per turn of the spiral (proportional to R 2) may reflect the growth rate of the animal.