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Some Studies on the New Zealand Oysters

Metamorphosis

Metamorphosis.

Knowledge of the metamorphosis of the larva of the American oyster, V. virginica, is considerable, owing mainly to the work of Stafford (1913). Cole (1937, 1938) describes the less well-known metamorphosis of incubatory oysters. The following is a brief account of the anatomical reorganisation of the spat of O. lutaria when it takes on the fixed life of the adult oyster.

The shell: The dissoconch is the post larval shell; it is formed during the 24 hours following attachment. It appears around the distal margin of the prodissoconch, but is very thin and indefinite at first. The dissoconch shell growth can be divided into two phases, (1) the silphologic (spat) phase, and (2) the adult shell phase. The silphologic shell phase has further been divided into five stages by Jackson (1888) most of which have been identified in O. lutaria. The lower left valve as it grows, becomes closely attached to the attachment area by becoming flattened. Lateral wings appear on the valves, usually on the anterior side. Growth continues until the lower valve reaches the margin of its attachment when it page 24 proceeds to grow upwards so that the general form of the adult shell with concave lower valve and flattened upper valve is soon apparent. Another feature of the dissoconch silphologic stages is the subnacreous layer which is formed unevenly at first until it covers the entire inside lining of both valves.

The foot: The foot persists for the first 24 hours following attachment but then rapidly degenerates and is carried forward by the rotation of the mouth until it reaches a mid-ventral position. The statocysts are thought to persist in the adult oyster of other species but no trace was ever found of them in O. lutaria.

The velum: The velum persists as an identifiable organ for about 12 hours following attachment. It is contained within the valves and the movement of the cilia is quite clear. The velum finally collapses and shrinks in size, becoming converted eventually into the outer palps of the adult oyster (Text-fig. 5, D). Cole (1938) describes how the velum of O. edulis shrinks after attachment and is carried forward and upward by the rotation of the body so that about 24 hours after attachment, the velar remnants "consist of a little of the typical epithelium of the thickened edge and some of the muscle fibres of the interior ... subsequently the much thickened upper lip spreads out laterally and gives rise to short rounded lobes which project, one on each side, at the upper corners of the mouth ". These are the rudiments of the outer palps of the adult oyster. The outer palps are initially large but gradually shorten as the inner palps are formed. The beginning of the formation of the inner palps was never observed in O. lutaria and as far as the writer is aware, has not been observed in other species of oysters. However, they are present in spat measuring 5.5mm in length.

The mantle: The mantle edge is visible in a fully-developed larva and in the attached spat. Associated with the mantle lobes are prominent radiating vessels (Text-fig. 5, D) and in spat measuring 5mm they appear to act as pulsating vessels or accessory hearts. Tentacles were not observed in recently settled spat. In older spat measuring 5mm the mantle lobes are well defined being united anteriorly beneath the hinge and posteriorly with the gills. The mantle margin has three folds as in the adult but the similarity ends there. The outer fold has no tentacles and is unpigmented, and the middle fold is more well-developed, being tentacular. There are two types of tentacles arranged in no particular order. The majority are small and regular, as tall as they are broad and the remainder are twice as tall as they are broad and are scattered amongst the smaller more regular tentacles. Both types of tentacles are brown apically. The inner fold which is the pallial curtain is present as a simple flap, smaller than the outer fold and similarly without tentacles.

The gills: At the time of attachment, the larva has between eight and ten gill filaments (Text-fig. 5, C). This is in contrast to the six filaments recorded for O. edulis by Erdmann (1934) and seven filaments recorded by Cole (1937, p. 413). Cole further notes that eyed-spat have eight gill filaments; spat measuring 0.35mm have ten gill filaments and that the 11th, 12th and 13th gill filaments appear on the left side before the spat is 90 hours old. Such spat measure approximately 0.6mm in diameter. At this stage there are also seven gill filaments on the right side. Spat of O. lutaria measuring 0.6mm (Text-fig. 5, D) in diameter have been settled for only 72 hours in contrast to the 90 hours of O. edulis described by Cole (1937). Such spat have 11–12 gill filaments on the left and about five gill filaments on the right (not figured). Thus the larvae of O. lutaria possess more gill filaments is in keeping with the much larger size of the settling larvae of O. lutaria. Yonge (1926) figures a spat of O. edulis which had probably been settled for five or six days. 1.2mm in length, with 20 gill page 25 filaments on the left and 13 gill filaments. Spat of O. lutaria of a similar stage were never found. However, spat measuring 5mm in diameter had 90 gill filaments on the left. The filaments were simple and similar in structure, there being no transitional or principal filaments or even any indication of plication. The filaments are spaced their own distance apart and are ciliated laterally and apically and are connected apically to adjacent filaments by a thin membrane. In these spat the filaments had split longitudinally so as to form the respective inner and outer lamellae. This splitting did not include the head of the filament. Two rows of interfilamentar junctions are present on the left inner demibranch but none could be seen on the corresponding right demibranch. The interfilamentar junctions transform the linear filamentous spaces into a series of fenestrae.

Stafford (1913, p. 68) stated that weight and pressure acting unequally upon the two sides of the gill soon effects a marked difference in the gill growth rate. The left gill in O. lutaria grows much faster than the right gill. The right and left gills of a recently settled spat correspond to the inner demibranchs of the adult. The measurements of a spat with the beginnings of the right outer demibranch are unknown but the left outer demibranch is seen to appear from the mantle and the gill axes of the left inner demibranch in a spat measuring 5mm × 5.5mm. The appearance of the left outer demibranch in other incubatory species of oysters is unknown as far as the author is aware. Stafford (1913, p. 68) also mentions that in C. virginica the right outer demibranch is formed when the spat is 2.5mm tall and the left outer demibranch is formed when the spat is 3mm tall. A spat 3mm tall has 50 gill filaments in the left inner gill. This is considerably less than the 90 filaments that O. lutaria has when the outer left demibranch appears.

Stafford furthermore notes that a spat of C. virginica 1.5mm in height has 23 gill filaments which is very similar to the 1.2mm spat of O. edulis which has 20 gill filaments. Therefore it is quite reasonable to estimate that a spat of O. edulis measuring about 3mm in height will show incipient left outer demibranch. In contrast, the spat of O. lutaria takes longer to develop all the demibranchs even though it settles at a much larger size and possesses more gill filaments at the time of attachment.

Adductor muscles: The fully developed larva possesses an anterior and posterior adductor muscle more or less symmetrically placed with regard to the hinge and of similar size. However, after attachment the anterior adductor muscle is moved upwards and outwards towards the edge of the prodissoconch where it finally disappears (Text-fig. 5, D). The posterior adductor muscle moves downwards and towards the centre of the viscera. In a spat measuring about 0.5mm in height, the posterior adductor muscle is situated between the edge of the prodissoconch and the dissoconch, until it finally moves entirely onto the dissoconch (Text-fig. 5, D). As the muscle moves centrally, it enlarges but does not as yet show any differentiation into quick and catch areas. The muscle is also becoming characteristically lunate as it moves ventrally on to the dissoconch.

Pigment spots: The pigment spots are lost during the 20 hours immediately following attachment.

Spat axes: In the larva, the mouth and the anus develop ventrally but when the larva becomes attached the anus moves dorsally. The mouth also rotates dorsally to rest against the ventral side of the anterior adductor muscle, and the posterior adductor muscle develops on the ventral side of the intestine. The palps and gills have also rotated dorsally from a former ventral position. The hinge of the prodissoconch is dorsal but after metamorphosis a reorientation of the body takes place so that the hinge becomes anterodorsal.