Journal of the Faculty of Fisheries and Animal Husbandry, Hiroshima University Volume 11 Issue 1
1972-07-31 発行


An Ecological Study on the So-called Mogai (Anadara subcrenata (LISCHKE)) cultured in Kasaoka Bay
Ting, Yun-yuan
Kasahara, Shogoro
Nakamura, Nakaroku
The so-called "Mogai", Anadara subcrenata (LISCHKE) (a kind of ark shell) is one of the most important bivalves in the Seto Inland Sea, but in recent years the area of its culturing grounds is gradually decreasing due to reclamation works and water pollution through the release of industrial wastes. This study was carried out mainly to clarify the environmental conditions of Mogai culturing grounds in Kasaoka Bay, Okayama Prefecture; the growth of Mogai as well as the maturation of gonad; the artificial inducing of spawning and the effect of chlorinity upon the fertilization and the early development. The results obtained are as follows:

(1) Several items on the quality of water and bottom mud are shown in Fig. 2 and Table 2 & 3, signifying the fitness of the ground as to the culturing of this species.
(2) The growth of shell length in one year period from May 1970 on was from 0.8 cm to 3.4 cm and from 2.7 cm to 4.8 cm in the one year old and two years old classes respectively.
(3) The relations between shell length and total weight, shell height and also shell width which were calculated on the data obtained in 2,300 of cultured individuals are shown in Fig. 5, 6 and 7 respectively.
(4) The shell width of Mogai derived from Kyushu was smaller than that from Osaka at the same size, particularly in their young stage.
(5) The seasonal variation in fatness can be divided roughly into two periods, namely one from August to December when its evaluation is considerably low, and another from January to July. The difference of the evaluation of fatness between both sexes was not recognized.
(6) Microscopic photographs of gonads of both sexes in each stages of development are shown in Fig. 12.
(7) The spawning season seemed to happen from June to December. In the examined district, it was especially flourishing from July to September.
(8) The biological minimum of this clam was about 1.5 cm in shell length and 1.0 g in weight in both sexes. Sex ratio observed in spawning season was 1:1.
(9) The discharge of sexual products of this species was effectively induced by treatments of raising the water temperature, immersion in sperm suspended sea water (only to egg discharge) or suitable concentration of ammoniated sea water, but treatment by NaOH and KOH was not effective.
In regard to the influence of chlorinity upon the discharge of sexual elements, no particular effect was observed in the range of Cl 14.08‰ to 24.40‰. However, discharge reactions were obstructed in chlorinity of 8.16‰ and 26.0‰.
(10) Mogai of 3-5 cm in shell length discharged about five to ten millions eggs, the average diameter being 51.38±3.78µ.
(11) On the treatment of unfertilized eggs in water of different grades of chlorinity, the eggs were safely kept the longest period (13 hours at 22℃) in chlorinity of about 16.6‰.
(12) It was recognized that about 15‰ of chlorinity suits best the development of fertilized eggs of this species as can be judged by the survival rate up to the D-shaped larvae.