perimental evidence, the whole question of the 

 factors influencing the behavior of oyster larvae 

 at the time of setting needs to be examined. 



TO THE ANGLE OF SURFACE 



French oyster growers take advantage of the 

 preference of oyster larvae for the under surfaces 

 of submerged objects and use special spat collec- 

 tors made of tiers of tiles set one upon the other 

 with their concave surfaces underneath. New 

 spat is always found in larger numbers on the 

 lower surfaces. According to Cole and Kjiight- 

 Jones (1939) the larvae of 0. edulis reared in large 

 tanks in Conway, Wales, set more mtensely on 

 the under surfaces of test shells. A study of the 

 effect of the angle of a flat surface on the attach- 

 ment of larvae was made by Hopkins (1935) in 

 his work on 0. birida of the Pacific Coast. He 

 used glass plates, each 2,400 scj. in., placed at 

 different angles over the oyster grounds. Tlie 

 under hox-izontal surface was designated as 0° and 

 the upper horizontal as 1S0°. Other plates were 

 set at 45° intervals between the two extremes. 

 The average number of larvae attached to each 

 surface were: 



0° (under horizontal) 1, 195 



45° 181 



90° 11 



135° 3 



180° (upper horizontal) I 



Similar observations were made by Schaefer 

 (1937) with the larvae of C. gigas. He set 150 

 glass plates in positions varjang by 45°; some of 

 the plates were parallel to the du'ection of the 

 tidal current while others were transverse to it. 

 The plates were left for 5 days before the spat 

 were counted. There is a functional relationship 

 between the intensity of setting and the angle of 

 the surface on which the larvae set, the number 

 of larvae being greatest on the under horizontal 

 surface (0° angle) and lowest as the angle ap- 

 proaches 180°. The curve shown in figru-e 354 

 is drawn between the points taken as the average 

 numbers of larvae attaching during 5 days on a 

 glass surface held at different angles. The curve 

 is hyperbolic. Schaefer attributes the setting be- 

 havior of C. gigas to the upward position of the 

 foot of the swimming larvae and possibly to nega- 

 tive geotaxis. No experimental evidence is given 

 to substantiate either point. 



The behavior of oyster larvae does not differ 

 from that of many other fouling organisms which 



372 



2 



CO 



li- 

 o 



cr 

 iij 

 m 



2 



45 90 135 



ANGLE OF SURFACE 



180 



Figure 354. — Effect of angle of the surface of plate glass 

 on the number of spat of C. virginica which attach to 

 it within 5 days, (llass area 2,400 sq. in.; 0° is under 

 horizontal surface. (Figure 1 from Schaefer, 1937). 



were found by Pomerat and Reiner (1942) to 

 attach in greatest abundance to the imder sm'- 

 faces of plates held in a horizontal position. 



Contradictory residts were obtamed, however, 

 in experiments with cement covered boards held 

 at several angles either suspended in water or 

 placed near the bottom. These tests made by 

 Butler (1955) on oyster bottoms near Pensacola, 

 Fla., showed a preponderence of spat on the upper 

 surfaces of the boards. Setting on upper sur- 

 faces (135° and 180°) comprised 78 percent of 

 the total number set, and only 18 percent were 

 counted on the lower surfaces. The remaining 4 

 percent were found on vertical boards. Butler 

 was not able to confirm the results of Pomerat's 

 and Riener's tests made earlier at Pensacola in 

 which frosted glass plates suspended in water 

 attracted the greater percentage of oysters (and 

 barnacles) to the lower surfaces. Bonnot (1937, 

 1940) built a spat collector from plywood strips 

 dipped in a mixture of cement and sand. He 

 fastened five or six strips of plywood one above the 

 other so tliat the horizontal surfaces were sepa- 

 rated by three-fom-ths of an inch. Setting of 0. 

 Iiin'da averaged 98 larva per square inch on the 

 upper surfaces and 67 per square inch on the 

 lower ones. The turbulent cm-rent caused by 

 narrow spaces between the strips possibly was 



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