Table 1 Reproductive contents of the follicles of 1816 Chilean oysters, O. chilensis, sampled from Foveaux Strait,
New Zealand, from April 1970 to April 1971 Type of reproductive material
Number of Percentage of
Ž . present in the follicles
oysters total sample
Male and female 1305
71.9 Solely male
443 24.4
Solely female 60
3.3 None
8 0.4
Total 1816
100
Ž .
developed and verified for this oyster Jeffs, 1998, 1999 . This technique relies on scoring different features of the gametogenic cycle for each gonad. A visual estimate
was made of the percentage of all the male and female reproductive material over the entire gonad. The presence of ova and spermatozoa in each gonad was each scored on a
scale of 0 to 3, where 0 was the total absence of gametes from a gonad, 1 was a trace in the lumen of one or more follicles of the gonad, 2 was a small quantity in many
follicles, and 3 was abundance in most follicles. The abundance of haemocytes
Ž .
reabsorping reproductive material phagocytosis and the release of ripe gametes from Ž
. the lumen of the follicles spawning were scored in the same manner, where 0 was the
absence of gamete loss, 1 was a trace, 2 was a small quantity in many follicles, and 3 was abundance in most follicles. Thus, an individual score of 3 for gamete loss indicated
a major loss of gametes, i.e., a major spawning.
Only a small percentage of the Foveaux Strait oyster population incubate larvae, Ž
. hence only a few incubators were expected in the monthly samples Cranfield, 1979 .
Therefore, to establish the size range of oysters that brood larvae, about 1000 oysters were taken from each site on 6 December 1970, in the height of the brooding season. All
oysters were opened and the shell height of all brooding oysters was measured.
3. Results
3.1. Animals sampled A total of 1898 oysters was collected in the monthly samples from the four sites.
These oysters ranged in shell height from 19 to 109 mm. In the subsequent 27-year storage period, nine oyster gonad preparations were irreparably damaged. Of the
Ž .
remaining oysters, the gonads of 73 3.9 had been rendered sterile by infection with
Fig. 2. Differences in the contents of gonads in relation to oyster size of the Chilean oyster, O. chilensis, Ž .
sampled from Foveaux Strait, New Zealand, over 1 year from April 1970. A Mean percentage of gonads Ž
. Ž . Ž
. Ž . containing male reproductive material qS.E. . B Mean scores for abundance of ova in gonads qS.E. . C
Ž . Ž .
Mean scores for abundance of spermatozoa in gonads qS.E. . D Mean scores for loss of gametes from Ž
. Ž . Ž
. follicles qS.E. . E Mean scores for abundance of phagocytosis of gametes in follicles qS.E. .
the trematode parasite, Bucephalus longicornutus, and were therefore excluded from further analysis. Although Bonamia sp. was known to be present in Foveaux Strait
Ž .
oysters well before 1970–1971 Hine and Jones, 1994 , the disease was not apparent in a random subsample of 100 gonads examined microscopically.
3.2. Site trends Visual inspection of plots of mean values for all reproductive features showed that
the general trends in reproductive behaviour were very consistent among the four sites. Therefore, the combined data from all sites was used for analyses of results.
3.3. Sexual phases and size Reproductive material was completely absent in very few of the 1816 oysters
Ž .
Ž .
sampled Table 1 . Only one small oyster 27 mm lacked fully developed follicles. Overall, most oysters contained some male reproductive material, while female material
Ž .
was less common Table 1 . Nearly a quarter of the oysters contained solely-male reproductive material and this condition was most common among oysters of 19–50 mm
height. Oysters in this same size range were predominantly male, with the follicles only Ž
occasionally containing small amounts of immature female material i.e., oogonia and . Ž
. oocytes
Fig. 2a and b . The presence of female material increased in oysters 50 mm height, on average to around 40 of the overall follicle area for oysters 75 mm, and
Ž .
ova also became much more common Fig. 2a and b . Despite these trends, spermatozoa maintained a higher overall mean abundance in the follicles of oysters of a larger size
Ž .
Fig. 2c . Solely-female oysters were uncommon, making up - 5 of oysters over 56 mm, which was the smallest size for a wholly female oyster. Mostly, these oysters
contained large amounts of ripe ova with very little or no oogonia or oocytes. Ž
. About 70 1206 of the oysters containing spermatozoa had released gametes of
Ž .
some kind, mainly spermatozoa. By comparison, 63 857 of the oysters containing ova had released gametes, mainly spermatozoa, since hermaphroditic oysters holding
ova in the follicles were able to simultaneously release spermatozoa. The mean amount of loss and phagocytosis of gametes from the follicles remained similar for all sizes of
Ž .
oysters Fig. 2d and e . Ž
. Only 12 0.6 of the 1898 oysters collected in monthly samples were brooding
larvae. The brooding oysters were between 63 and 97 mm and were found in the months between July 1970 and January 1971. The additional samples taken in December 1970
yielded a further 105 brooding oysters that ranged in size from 60 to 100 mm. This strongly suggested that only oysters from 60 mm in size were able to brood larvae.
Therefore, of all the oysters G 60 mm taken in the monthly samples, only 0.8 was
Ž .
brooding, and the largest proportion 5.9 was in the December sample. Assuming that Ž
. this population has a 30-day larval incubation period Cranfield and Allen, 1977 , it was
Fig. 3. Annual pattern of male features of the gonads of the Chilean oyster, O. chilensis, sampled from Ž .
Foveaux Strait, New Zealand, over 1 year from April 1970. A Mean percentage of gonads containing male Ž
. Ž . Ž
. Ž . reproductive material S.E. . B Mean scores for abundance of spermatozoa in gonads S.E. . C
Ž .
Ž .
Percentage of monthly sample containing abundant score of 3 spermatozoa unbroken line , and abundant Ž
. Ž
. spermatozoa together with extensive score of 3 phagocytosis broken line .
estimated that between 7 and 10 of the adult population of oysters was brooding in this year.
3.4. Seasonal trends Mature reproductive material, especially spermatozoa, dominated the oyster gonads
Ž .
Ž throughout the year Figs. 3 and 4 . Spermatozoa were abundant defined as a score of
. Ž
3 in over 90 of all oysters in monthly samples from July to November austral .
Ž mid-winter–spring , but this declined rapidly from December to March austral sum-
. Ž .
mer–early autumn Fig. 3c .
Ž .
Ova were least common in September to October austral spring and most common Ž
. Ž .
in March to June austral autumn–early winter Fig. 4 . However, ova were abundant
Ž .
defined as a score of 3 in about half of the oysters greater than 50 mm in monthly
Fig. 4. Annual pattern of female features of the gonads of the Chilean oyster, O. chilensis, sampled from Ž .
Foveaux Strait, New Zealand, over 1 year from April 1970. A Mean scores for abundance of ova in gonads Ž
. Ž . Ž
. S.E. . B Percentage of monthly sample of oysters 50 mm containing abundant score of 3 ova
Ž .
Ž .
Ž .
unbroken line , and abundant ova together with extensive score of 3 phagocytosis broken line .
Ž .
samples for most of the year Fig. 4b . Fewer oysters contained abundant ova from the commencement of brooding from July onward.
The release and phagocytosis of gametes from the follicles occurred year round and Ž
. both were lowest during June to October austral mid winter–mid spring and highest in
Ž . Ž
. March austral early autumn
Fig. 5 . Ž
. Extensive phagocytosis in mature male oysters defined as a phagocytosis score of 3
was uncommon throughout much of the year, except during January when it reached 29 of the oysters containing abundant spermatozoa. This amounted to 13 of all the
Ž .
oysters in the monthly sample Fig. 3c . Extensive phagocytosis in mature female Ž
. oysters defined as a phagocytosis score of 3 was low throughout much of the year,
except from January to March when it reached 23 of oysters 50 mm with abundant Ž
. ova. This amounted to 13 of all oysters 50 mm in the monthly sample Fig. 4b .
Fig. 5. Annual pattern of loss and phagocytosis of gametes from follicles of the Chilean oyster, O. chilensis, Ž .
sampled from Foveaux Strait, New Zealand, over 1 year from April 1970. A Mean scores for loss of gametes Ž
. Ž . Ž
. from follicles S.E. . B Mean scores for abundance of phagocytosis of gametes in follicles S.E. .
The follicles of the 12 brooding oysters were characterised by a large loss of gametes, as well as extensive phagocytosis of the remaining reproductive material. Phagocytosis
Ž .
was extensive defined as a score of 3 in the gonads of 7 of these 12 brooding oysters and was associated with ripe gametes remaining in the follicles. Ripe spermatozoa
remained in the gonads of only 2 of the 12 brooding oysters, while ripe ova remained in eight oysters despite the commencement of brooding.
4. Discussion
