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Some Studies on the New Zealand Oysters

Maturity and Sexual Phases

page 10

Maturity and Sexual Phases

Sexual cycles of many marine organisms are more often than not governed by the complexity of their reproductive organs and the environmental conditions during the spawning months. However, the anatomical simplicity of the reproductive organs of oysters and other bivalves in general reflects their simple sexual reactions.

Sperm balls are generally considered to be characteristic of hermaphroditic and larviparous species of oyster. Sperm balls are characteristic of O. lutaria so that it could be expected also to be hermaphroditic and larviparous; this proved to be the case. The argument that sperm balls indicate hermaphroditism and the larviparous condition in oysters that possess them is taken further to indicate that such oysters are also protandric, and have a rhythmical alternation of female and male phases throughout the remainder of their lives. As regards the last two characteristics, it must be mentioned here that although O. lutaria appears to be protandric it is not proven and although sex change does occur, the lack of tagged individuals has made it impossible at this stage to say whether the sex change is a rhythmical alternation.

The young were never observed to become sexually mature in the summer during which they attached themselves. Maturity was only reached during the second summer when oysters measured approximately 20mm in length. In general, this agrees with the observations of Sparck (1925) on O. edulis when he noted that the young of this species may in exceptional circumstances produce eggs in the second summer. Individuals of O. lutaria were in their third summer at least and more commonly in their fourth summer before egg development commenced.

Although no experiments were carried out with O. lutaria as regards temperature and time requirements for maturation, it is noted that an oyster collected in August at a water temperature of 10.0°C, liberated larvae in the laboratory. The fact that the surface water temperatures in the previous few weeks was below 10.0°C. (average 9.4°C.) clearly indicates that O. lutaria is capable of maturation and spawning at temperatures as low as 10.0°C. Whether this was an exceptional case of spawning is unknown and it can only be said here that during the course of this study no other oysters were collected which spawned at similar temperatures.

Sexual phases: During the course of this study several young oysters were collected. These young oysters ranged in size from 4mm to 25mm, the latter being about one year old. The gonads gave no appearance of development until the oyster was at least a year old, or 20mm in length. Smears made of these gonads produced on all occasions spermatids and mature sperm balls. Whether or not the primary gonad is bisexual is not known since the limited material was insufficient to enable the author to clarify this point. Protandry is considered to dominate the sexuality of larviparous and oviparous species of oysters (Stafford, 1913; Coe, 1931, 1932) and it would appear that further investigation on O. lutaria will prove it to be protandric.

The sequence of sexual phases is hard to study in the oyster for two reasons: (1) there is a need for tagged oysters so that an all year round check can be made on the gonads that were originally known to be either male or female, and (2) the sequence is further complicated by the fact that different regions of the same gonad may have different sexual phases at the same time. Thus, the tubules further away from the main area of proliferation, i.e., beneath the hinge may be in an advanced state of spermatogenesis while the area of the gonad nearer the pericardium may contain ripe eggs.

In the present study sections and smears were made of just over 100 gonads to show the types of sexual phase exhibited by O. lutaria. The phases were as page 11 follows: hermaphrodite individuals with ripe sperm and eggs; females but with some early proliferation of spermatogonia; males, with no trace of oogenesis; males with oogonia present; males with oocytes lining the wall of the follicles. From this list it can be seen that no females were observed which did not show some phase of spermatogenesis. Orton (1927, p. 976) regards all oysters in which ripe ova are present as essentially functional females, irrespective of the amount of spermatogenesis occurring in the secondary and primary follicles.

The number of sexual phases that O. lutaria may pass through in a year, or even in a life-time, is unknown. In O. edulis however, the indifferent phase is followed by spermatogenesis. Thus, the lack of eggs in young O. lutaria suggests that in the New Zealand mud-oyster also spermatogenesis succeeds an indifferent phase. The exact age of O. lutaria when oogonia begin to develop is unknown. Nonetheless, the gonads of oysters in their third summer exhibit a characteristic male phase with ripe sperm morulae in the lumen of the follicles and young oogonia and oocytes lining the walls of the follicles. The development of this first observed female phase proceeds concurrently with the development and further maturation of the sperm morulae.

In the transitional phase between male and female phases the developing oocytes enlarge, thus leaving the ripe sperm morulae isolated in the lumen of the follicles. Spermatogonia that would appear to belong to a subsequent male phase lie between the developing ova round the wall of the follicle. The fact that ripe sperm balls and mature eggs occur together in the same follicle does not mean that self-fertilisation will necessarily result, for the individual spermatozoa of the sperm ball are tightly held together until they are liberated by contact with the sea water. Only if the eggs and sperm are liberated simultaneously from the gonad will there be opportunity for self-fertilisation. If self-fertilisation does occur, then the ripe sperm and eggs have been developed in an hermaphrodite gonad.

Hermaphroditism: Coe (1932, p. 136) notes that hermaphroditism is not rare in the genus Ostrea as at least ten of the more than sixty described species are known to be monoecious and viviparous. There has been a suggestion that hermaphrodite oysters "are derived from individuals which spawned incompletely, at the end of the previous season leaving behind in the gonad a fair number of unripe eggs which were retained throughout the winter'' (Cole, 1942). The true factors governing the formation of hermaphrodite follicles are still unknown. Cole's suggestion when considered relative to the presence of hermaphrodite follicles at the beginning of a season seems reasonable, but it is hard to see how it can account for hermaphrodite follicles being present all through the breeding season.

Korringa (1941, p. 46) notes that "as the eggs develop, the oysters remain functioning as males for a considerable time and often sperm production continues until about ten days before the discharge of the eggs. When we consider that a few days after the shedding of the eggs the gonad again contains sperm morulae, we may conclude that the purely female phase can be very short, about three weeks". In O. lutaria the follicles that possessed developing or mature eggs always possessed in addition developing spermatozoa. Thus the gonads of three year old and older oysters of O. lutaria always appeared to be hermaphroditic and in a variety of intersexual forms.

Gonads after ovulation: Serial sections were prepared of the gonads of oysters that had liberated larvae in the laboratory. Sections were also made of gonads while the parent oyster was still incubating veliger larvae. In all these sections it could be seen that the secondary follicles of the gonad had collapsed, having become very irregular in outline and considerably lessened in size. The page 12 genital canal connecting the lumen of the secondary follicles was also noticed to be considerably narrower than that found in sexually mature oysters. Phagocytes were present throughout the primary and secondary follicles and in the surrounding connective tissue. A few degenerated eggs were noticed scattered in the follicles.

Orton (1927, p. 974) shows that it is normal for unspent eggs to be voided from the gonad after the initial main act of spawning. He also considered that if the residual ova were fertilised upon spawning, they would then give rise to a successful but spurious second spawning. Thus, two stages of developing larvae would be likely to be found within the inhalent chamber. Such a condition was not observed in O. lutaria.

Orton (1927, p. 974) suggests that in O. edulis degeneration with absorption of eggs usually and possibly always occurs, while Coe (1932, p. 133) states that in O. lurida the few remaining ripe ova may eventually undergo degenerative changes and phagocytosis. This latter course of development occurs in O. lutaria where phagocytes containing the nuclear material of the unspent ova occur in the lumen of the follicles and similar phagocytes without the nuclear contents occur in the surrounding connective tissue. Phagocytes were present in the follicles of oysters that had spawned some ten to fourteen days earlier. This persistence of phagocytes in the follicles of O. lutaria is in direct contrast to that recorded for other oysters.

Coe (1932) and Orton (1927) state in O. edulis, O. lurida and other incubatory oysters, spermatogenesis begins immediately after the eggs have been liberated from the follicles and that the follicles have abundant mature sperm balls at the time when the larvae are fully developed and ready to be liberated. Spermatogenesis in these oysters continues until approximately eight weeks after oxidation.

In the twelve post-spawned gonads that were sectioned, there was complete absence of spermatogenesis and oogenesis. The twelve gonads were representative of early summer and late summer spawners. Since there were no adequate facilities for keeping the spawned oysters alive indefinitely, the interval between ovulation and the commencement of spermatogenesis is unknown. The length of time elapsing between the first and second female phase is also unknown.

Resting period and wintering condition: After the ripe sperm balls and eggs have been discharged the appearance of the body alters considerably, for the tissues become flabby, watery and translucent. This translucent condition which enables the brownish digestive diverticular tubules to be observed, remains until the next sex phase is entered upon. Oysters that appeared to be in a resting phase were observed throughout the year but with more frequency during the months of March, April, May, June and July. During these months, the resting condition may last longer than in the summer months. Sections of the gonads during these months showed that oogenesis and spermatogenesis still occurred but not to the point where ripe eggs and sperm were present. Cole (1942, p. 343) notes that the "stage at which an individual passes the winter is conditioned by the phase it reaches at the close of the previous season".

The duration of the spawning season for O. lutaria is eight months at least and possibly all the year round when favourable conditions are present. Thus there is never a long period of over-wintering but rather a resting period with the spermatocytes and oocytes ready to mature when the conditions become more favourable. Since mature eggs and sperm were never found in the sections made of over-wintering gonads, in O. lutaria it may be assumed that phagocytes have broken down and absorbed any ripe products left from the previous season. The functional sexual phase in the following season will be that of the dominant sex cells in the resting follicles.