摘要
牡蛎的敌害很多,在养殖过程中死亡率很高,假若从产卵开始,经过孵化、生长,直到成长为商品牡蛎时,就万中难得其一。根据Prytherch(1930)统计,即使从固着的牡蛎苗开始计算,能成长到收获的成贝也仅占0.1%—2.0%。因此,防治敌害对贝类养殖工作说来是十分重要的。根据过去生产实践的经验证明,肉食性腹足类,特别是骨螺类,是牡蛎稚贝的敌害。
In China,Purpura gradata Jonas,a gastropod natural enemy of oysters,is important causative agent of oyster mortality during the culture.period.So far,the general habit of this snail has not been investigated.The present paper gives an account of the breeding and feeding habit of this animal.The studies were carried out in the oyster culture beds at Pao-an,Kwangtung Province in 1957,and the results obtained are summarized as follows:P.gradata is commonly found along the coast from Amoy to the Kwangtung Province,and is very abundant in oyster culture beds.The range of vertical distribution of this snail is.mainly from intertide to 1.5m.below low tide mark;however,it was not found in the deeper culture beds of Ostrea rivularis in Pao-an.In laboratory experiments,it was observed that sea water with a salinity below 4‰was not favorable for the growth of P.gradata,and when placed in sea water with a salinity lower than 1.33‰it died out within 7 days.Thus,during the rainy season it did not occur either in the fattening bed of O.rivularis,where the salinity of the sea water had become much diluted.The breeding season lasted from the middle of April to about the beginning of October and the water temperatures ranged from 22.9 to 29.3℃.The number of capsules deposited by a single adult snail at one laying was recorded(table 3)during the normal spawning season.For 15 snails observed,the number of capsules laid per snail ranged from 49-69;with an average of 65.It was also observed that the female after depositing one group of capsules may move away and later deposit further batches.For 1,000 capsules collected from different stations the number of embryos per capsule may vary from 41-250 with an average of 113.4(table 2).If the average number of capsules laid per female is assumed to be 65,from each of which on the average 113.4 embryos emerge,then each female snail gives rise to 7,371 young per year.In the laboratory,678 capsules,laid by different individuals,were divided into 4.sections,and were incubated experimentally in tanks.At water temperature ranging from 21.6-34.4℃(table 4),the average duration of the incubation period observed was from 17.5-25.5 days and the maximum was 13 days.From a comparison of samples it can be seen that the sex ratio of P.gradata differs for different localities from which it was taken.Table 5 gives the proportion of the two sexes of P.gradata obtained from five stations.It shows that in samples from the test-shelf of How-Hai the number of males to females is nearly equal but the proportion of females reaches a peak of over 87%in samples from Pai-Shih-Chow.The proportion slightly increases to between 60-70%in samples from How-Hai,Ku-Po-Chiao and Swatow.The difference in the sex ratio may be due to different breeding and feeding conditions in each locality.In 4,324 snails the percentage of males to females is 34.2%to 65.8%.The size-distribution curves of the two sexes(fig.5)show that the shell of females was slightly higher than that of the males.By keeping snails enclosed with three kinds of food,i.e.1-year-old spat,Balanus,and Glaucomya,some idea of the potential damage may be obtained.For these experiments.3 lots of snails,varying in size from 8.5 to 31.2 mm.were used.Table 7 shows that the rate of destruction of each kind of food by the snails correlates with the size of the snail.The smaller snails tend to catch the barnacles probably because they are easy to prey on.The results of another experiment(table 8)show that the snail has an ability to destroy a 2-year-old oyster.It also shows that when spats are present oysters are not seriously threatened by the snails.Under natural conditions,the rate of destruction of spats by P.gradata is over 50%,hence in the oyster culture bed this pest must be got rid of.
作者
楼子康
LOU TZE-KONG(Institute of Oceanology,Academia Sinica)
出处
《海洋与湖沼》
1963年第1期56-70,共15页
Oceanologia Et Limnologia Sinica
