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Some Studies on the New Zealand Oysters

[Introduction]

No free swimming oyster larvae have so far been found in plankton collections studied from Wellington Harbour. Under laboratory conditions however, the larvae were observed to be released in various stages of development which may or may not be the case in the field. Oysters were collected from Evans Bay and were quickly removed to the laboratory where they were placed in 7 ½ in culture dishes (3–4 oysters per dish) and covered with sea water. The temperature of this sea water varied between 18.5°C. and 20.5°C. and was usually 2–4 degrees warmer than the temperature prevailing in Evans Bay. In every instance this rise in temperature of the sea water was sufficient to induce the oysters to liberate their larvae. All stages of development were liberated from parent oysters held in the laboratory including embryos and trochospheres, early straight-hinged veliger larvae and fully developed larvae possessing a pair of pigment spots and well developed foot.

The measurements of free swimming larvae of O. edulis vary between 0.16mm and 0.20mm (Erdmann, 1934, p. 6, and Korringa, 1941, p. 101) whereas larvae of O, lutaria at a comparable stage of development vary between 0.31mm and 0.33mm. Swimming in these latter larvae is by means of the velum and is restricted to horizontal movement on the bottom of the dish and short vertical movements. In the horizontal movement, the velum is uppermost and the larvae turn anti-clockwise. The movements as seen by the naked eye is very slow, almost imperceptible. The measurements of seven fully developed larvae of O. lutaria are given to show the size variations in height of larvae with similar length: page 22 0.42mm × 0.28mm; 0.42mm × 0.35mm; 0.42mm × 0.36mm; 0.43mm × 0.31mm; 0.44mm × 0.34mm; 0.45mm × 0.37mm; 0.47mm × 0.39mm.

Since the size of the straight-hinged veliger of O. lutaria is greater than that of O. edulis, it could be anticipated that the fully developed larva of O. lutaria would be larger than that of O. edulis. Measurements of the larvae of O. edulis which have just developed the pigment spot vary between 0.26mm and 0.30mm (Cole, 1939, and Korringa, 1941) and larvae ready to settle measure between 0.27mm and 0.31mm (Erdmann 1934, Cole 1939, and Korringa 1941).

Fully developed larvae of O. lutaria (Text-fig. 5, A, B and C) liberated in the laboratory have been observed to swim rapidly in both horizontal and vertical directions, to rest on the surface of the water and to creep on the bottom of the dish. Larvae that are about to settle do not necessarily confine themselves to crawling and in fact they alternate swimming with crawling phases, always being in a crawling phase immediately prior to attachment. This last type of movement is the exploratory phase described by Cole (1938, p. 478) in which the foot serves as a locomotory organ on the substratum immediately prior to attachment and apparently acts as a tactile organ.

Furthermore, Cole (1938, p. 471), notes that in O. edulis the eyespot (pigment spot) develops only a few days before attachment. Thus it would appear, that if in O. lutaria the foot and pigment spots are also developed only a few days before attachment then the larvae of O. lutaria are ready to settle and become attached immediately after being liberated. Fully developed veliger larvae which do not settle have also been liberated. These larvae are easily recognised by their pigment spots but do not exhibit an exploratory phase which is typical of attaching larvae. These larvae are the same size as larvae that settle and become attached.