イボニシのマガキ稚貝に對する加害機構について

Abstract

The present paper includes the results of my work on the injuring mechanisms of Purpura clavigera of 32mm in mean height to young oysters of 15mm in mean height in a summer month from August 7th to September 7th, 1947, at Mangokuura, Miyagi Prefecture. An oyster drill can kill 40% of 60 young oysters in a summer month and an oyster about in a day. Drills injure oysters to death by boring a hole always on the right valve of oyster in 3 manners of attack; thus the penetrated hole, the non-penetrated hole on the valve and the edge of valve is encroached semicircularly. Oysters were all killed by these 3 manners of attack. Drills seem not to select the peculiar part of oyster because every part of.right valve is attacked (Fig. 1). But the holes bored are by far the most abundant at the peripheral part of valve. The definite relationship between the situation of hole and the size of oyster and drill is unrecognizable, but there is a tendency that the larger the individuals are, the more frequently oysters are bored at their periphery and that the smaller the individuals are, the much more oysters are killed by the drills. The situation of hole on the valve of oyster seems to be determined by the size of oyster drill, the place and the direction of attack and the length of proboscis of drill. A single oyster has always a single hole on the valve and no more than 2 holes. The penetrated hole is inverted conical in shape with the outer hole being always larger than the inner hole. A greater part of holes are nearly circular or elliptical in outline. The major axes of outer and inner holes are in line with or parallel with each other or nearly so. The outer and inner holes are measured 1, 3mm and 1mm respectively on an average in their major axes. The minor axis of inner hole is about 3 times as wide as the width of radula. The larger drill can usually bore the larger hole than smaller drill can do. Drills move the proboscis back and forth from the gently inclining slope towards the steep slope of inner surface of hole in sliding right and left by width of radula as they bore a hole on the valve. The inclining angle of inner surface of hole ranges about from 45° to 50° when the inner hole is situated nearly in the central part of the outer hole. We can judge the position and direction of attack, the poise of drill from the features characteristic of the hole because the drill seems usually not to shift the position, the poise of body and the movement of proboscis during the time of boring hole on the oyster. A central tooth has a large central cusp at the middle, a small lateral cusp on both sides of it and 3 to 8 marginal denticles on both sides of the lateral cusp (Table 2), the base is laterally oblong in shape. A lateral tooth is sickle-shaped with broad base; the cusp tapering towards pointed tip with entire margin. The width of radula and the size of drill can be estimated from the minor axis of inner hole. Kinoshita was of the opinion that the size of hole will be determined by the distance between 2 lateral cusps of central tooth of radula and the boring speed of drill. The minor axis of inner hole ranges from 7.5 to 8 times as wide as the distance between the lateral cusps, while it is about 3 times as wide as the entire width of radula. Thus it seems adequate to consider that the entire radula will concern in boring hole. The mechanical action of radula may be recognizable.