Observations on the morphology, ecology and biology of Xylophaga dorsalis (Turton) (Mollusca: Xylophagainae) in the Trondheimsfjord (Western Norway)

IRG/WP 466

L N Santhakumaran

The paper deals with the various aspects of the systematics, ecology and biology of the little known pholad wood-borer, Xyophaga dorsalis (Turton) of the Trondheimsfjord (Western Norway). Diagnostic characters of the species have been given, based on examination of fresh living specimens and accomodating the several morphological variations. A key to the identification of Norwegian species of Xylophaga is also included. Seasonal intensity of the species, based on incidence on a number of pine wood panels, exposed for overlapping periods for 1 year has been analysed. In shallow waters, larval settlement is restricted to summer and ear1y autumn, whereas at greater depths, the same extends even during the winter and spring. Inhibitive action of fouling organisms on the settlement of Xylophaga dorsalis has been indicated. Vertical distribution of the species at every 3 m from intertidal level to 30 m, and also at depths of 40 m, 70 m, and 100 m has been described. Intensity of attack increases with depth with maximum concentration near the mud-line. The borer prefers the silted upper surface of the panel for settlement than its lower surface and sides. Growth-rate of the borer has been studied based on burrow length measurements as well as on length and height of the shell valves. Boring activity is extremely rapid in the Trondheimsfjord, maximum length of burrow being 58.50 mm attained in less than 8 months. Data on growth-rate at different levels show that the length and height of the shell valves increase gradually as the depth increases, and larger individuals are always metwith in deeper levels. Burrow length measurements also indicated the same trend except close to the mud-line where the size declined possibly due to overcrowding. Distribution of specimens of various length classes at different depths also followed the same pattern with individuals belonging to larger size groups being predominant at deeper levels. Length and height of the shell valves, length of burrow and distribution of specimens of various size groups at 40 m, 70 m, and 100 m depths also demonstrated a similar trend. Preliminary observation on salinity tolerance indicates that the borer is very sensitive to decrease in salinity, and below 18.77% they are unable to survive for more than 24 hours. Incidence of Xylophaga dorsalis on various timber species illustrated that the borer is capable of attacking all timber species, although in a few the larvae do not succeed beyond making tiny pits. The results are briefly compared at appropriate places with those of earlier workers.


Conference: 81-05-11/15 Sarajevo, Yugoslavia

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