Materials and Methods

Transcription

1

2 158 ~ 40 (1971) this purpose the life history in the early stage of the gill trematodes must be made clear, especially in the form and ecology of the miarcidium as well as of the egg. This paper has been intended to clarify the life history of the yellow-tail's gill parasites in the earlier stages, especially eggs. Materials and Methods In the present study the yellow-tail's gill trematodes and their eggs are used. The gill parasites used in this study were found stuck fast to the yellow-tails which had been reared in the floating net cages of our Institute located at Shirahama during the period between May, 1962 and Angust, After left alone in a glassware full of sea water for a short time, the parasites laid their eggs in the water, the number of which was counted, their diameters being respectively measured by means of a micrometer. A hatching experiment was made on the eggs by our putting them into a glassware full of boiled and filtered sea water, a constant temperature favorable to the eggs being kept by means of a thermostat. In measuring the miracidium, we first put it on a slide covered with a piece of cover glass and then measured it by means of a micrometer. Results and discussion 1. The number of eggs spawned by a single gill trematode Our researches on the 18 specimens of gill trematodes have shown that one of them lays at a time as many as 111 ~ 627 eggs, 360 eggs per each being laid on the average. During the period between Dec. 30, 1962 and March 4, 1963, when the sea water was at a temperature between 10 and 15 C, our reseach was made on the 18 specimens of gill trematodes and found that only a single one of them had laid out at a time as many as 111 ~ 627 eggs, 350 eggs being laid by each specimen on the average. 2. The shape of the egg The egg is oval in shape, somewhat elongated to a long axis, its long diameter being O. 144 ~ mm. ( mm. on the average) and short diameter being ~O. 077 mm. (0.073 mm. on the average), respectively (plate. 1, a).

3 '. " Outside, each egg is covered with a colorless and transparent egg shell, but inside, it is light yellow and transparent with some fine granular sculptures. Each egg is rather narrow at its extremity, but rather round in its basal part. At its basal part, the egg has a transparent, cohesive thread which is very thin and long with a light yellow color. When these cohesive thread with an egg at their respective further ends are put together, they get twisted about, forming themselves into a bunch of thread with a number of eggs in different spots, the sight of which would remind you of those melons growing in clusters on the minor stems branched out from the main stem. These bunches of thread are so cohesive that they are often found stuck fast to the fish cultivating nets, floating casks or yellow-tails themselves (plate 1, b). 3. Hatching The first experiment was made on how the eggs would be hatched at A. M. on March 3,1963. We used two glasswares for our experiment. One of them contained the boiled and filtered sea water of 300 cc., and the eggs of 3 specimens of gill trematodes were put into it. The other glassware was filled with the sea water only boiled of 300 CC., and the eggs of 1 specimen of the parasites were put into it. The sea water in both glasswares was always kept at 25 C in a thermostat. At A. M. on March 9, 1963 nearly all the lids of the eggs in the two glasswares were taken off and the miracidiums were seen swimming from place to place at a fairly rapid speed, which showed us that it took the eggs 5 days and 22 hours to get hatched in the sea water with the temperature of 25 C. Another experiment was made to learn how days the eggs took to get hatched at high temperature. Some eggs which had been laid at 9.00 P. M. on July 27, 1963 were put into the glassware filled with the boiled and filtered sea water with the temperature of 29 C. Nearly all the eggs were hatched at 3.00 P. M. on July 31 st. In the sea water with the temperature of 29 C it took the eggs only 3 days and 6 hours to get hatched. The oocytes turned more and more opaque about the day before the eggs being hatched, and some wrinkles which were likely to be the outline of the body of the miracidium began to appear, till at last a cap-like ring showed itself on the one-sixth portion of the long axis from the egg's external extremity (plate 1, c). Such a ring

4 160 ~ 4 % (1971) appeared at least a few hours before the egg being hatched. About this time the outline of the body and something like the rudimental sucking organs of the miracidium were clearly visible. The miracidium in its egg shell was seen moving vigorously with its body twisted, not only towards the short axis of the egg, but also towards the long axis, especially immediately before the egg being hatched, till at last it turned round and round all over the egg shell (plate 1, d). When the lid of the egg opened like a hatch, the miracidum stretched out of its anterior half, expanding and contracting itself for a while (plate 1, e), and when it had stealed out its posterior half, it got into the sea water and was seen swimming there at full speed. 4. Miracidium The miracidium is a depressed worm, nearly transparent and colorless, and elliptical in shape. It is 0.53 mm. in total length and something like a rudimental organ of the posterior sucker of is visible (plate 1, f). When it swims in the sea water, it quickly waves its lateral membranes, both right and left (plate 1, h). The worm can swim as fast as an ordinary ciliate or even faster. The miracidium has by this time been endowed with adhesive power and is often seen stuck fast to the slide or to the glassware. On the day following the egg being hatched, strange to say, no miracidium could be seen in the water within the glassware. 5. Adhesive aspect of the egg to the net plate As an egg-attachment tool, we used a frame of wires 5 em. square with a "Kureharon" net of 8 knots attached to it. Two different places were chosen where the attachment tools were to be hung: One was the middile part of the fish cultivating net and the other, 1 meter down from the outside of the same net. In these two places 5 attachment tools fastened to one rope were hung 1 meter below the surface of the sea water. The results obtained are shown in Table 1. The number of the eggs found stuck fast to the net is not constant, varying with the time of the year. No conspicuous differnce can be seen in number as to whether the eggs belong to one-year-old yellow-tails or to the younger ones. Those fish cultivating nets with many of the eggs stuck fast are probable hot beds for the parasites. Judging from the results of our experiment made on the eggs being hatched, the best way of getting rid of the gill trematodes as far as possible is, we

5 ~EE!tlI!Ji.ffi!ff~iEA : /\7-HiM',z!.U Heteraxine heterocerca O)y~ 2::miGff.:!l. 161 Table 1. Adhesive aspect of the egg to the net plate. Date Aug Aug I Situation of Depth of net plate (m) *net plate center center outside Sept I center center Oct outside * Hanging situation of the net plate. Center is the middle part of the fish cultivating net. Outside is the situation separating from net to outward 1 m. believe, to replace the fish cultivating net by a newer one as often as posible. If this is to be done every 7 ~ 10 days, much fewer eggs will be found stuck to the net and there will be far less probabability of the miracidiums being hatched. The miracidium just hatched has naturally a strong adhesive power and on the day following the eggs being hatched no miracidium was found swimming in the sea water within the glassware. These facts have confirmed us in believing the following: the miracidiums on being hatched out steal spontaneously into the gills of yellow-tails within a few hours. If they were to remain swimming in the sea water for a longer time without hurrying into the gills of yellow tails, it would mean death to them. Summary 1. A single gill trematode of the yellow-tail produces as many as 111~627 eggs (350 on the average) at a time in the sea water which has a temperature of 10~15 C. 2. The egg is of an ovoid in shape, elongated to a long axis. The longer diameter is O. 150 mm. long, while the short one is mm. long on the average. Each egg has a transparent, cohesive thread at its basal part and when put together, these threads of the respective eggs form themselves into a bunch of thicker thread. 3. So far as the eggs to be found stuck to the fish cultivating net are concerned, there is no conspicuous difference in number, whether the eggs belong to oneyear-old yellow-tails or the younger ones.

6 162 ~ 4~' (1971) Much more eggs are found stuck to the fish cultivating net in its middle part, while fewer eggs, on its outside. In the sea 1 ~ 5 meters deep the eggs are found distributed, regardless of the depth of the sea. 4. It takes the eggs 5 days and 22 hours to get hatched in the sea water with a temperature of 25 C, while, in the sea water with a temperature of 29 C, it takes the eggs 3 days and 6 hours. 5. The miracidium is a depressed worm which is nearly transparent and colorless, and elliptical in shape. It is 0.53 mm. in total length, and something like the rudimental organs of posterior suckers of 5+5 can be visible. References GOTO, S.: ]ourn. ColI. Sci. Tokyo Imp. Univ., 8, (1895). YAMAGUTI, S.: Syslema Hel minthum, 4, (1963). AKAZAKI M.; HARADA T.; U)!EDA S. & H. KUMAI: This Bull. (2) (1965). AKAZAKI M.: lap. ]. Ecol. 15 (41, (1965). MATSU~ATO T.: Fish Pathology. 2 (2), (1968).

7 Plate 1 1. Egg and miracidium of the yellow- tai I' s gill trematode, Heteraxine helerocerca. A. B. D... egg, C... hatching, E. F... miracidium.