Studies on the Shipworms II : Monthly Settlement of Shipworm Larvae

Abstract

Monthly settlement of shipworm larvae for a year from May, 1974 through April, 1975, and the existing species were investigated at three test sites in Japan: Onagawa (38° 26'N, 141° 30'E), Miyagi Pref., Tatoku Island (34° 18'N, 139° 49'E), Mie Pref., Noto (37°8'N, 137°E), Ishikawa Pref. The following species were found: Teredo navalis LINNAEUS and Limnoria lignorum (RATHKE) at Onagawa, T. navalis, Lyrodus pedicellatus (QUATREFAGES), Bankia carinata (GRAY) and L. lignorum at Tatok Island, T. navalis, L. pedicellatus, Teredo furcifera VON MARTENS and L. lignorum at Noto. Monthly settlement of shipworm larvae was determined by test panel method (pine wood, 5 × 20 × 2 cm) in which the number of borer apertures on wood surfaces was counted under stereo-microscope. Very light larval settlement was observed for only two months (September and October, 1974) at Onagawa. Shipworm attack on wood at Tatoku Island occurred for six months from June through November, 1974. The extent of larval settlement was very light, fewer than 10 borer apertures per 100 cm^2. Occurrence of larval settlement at Noto began in June when water temperature rose up to around 20℃, and continued for nine months with a peak in September, 1974. The more larval settlement was recorded on the lower surfaces than on the upper surfaces.