マサゴハゼの生活史

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タイトル別名
  • The life history of the gobioid fish, Acentrogobius masago (Tomiyama)
  • マサゴハゼ ノ セイカツシ

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Acentrogobius masago (Tomiyama) is a small gobioid fish growing about 25 mm in total length ; it is distributed in Middle and Southern Japan, living a bottom life on sandy-mud bottom in estuaries (Fig. 1). The author collected over 700 individuals of this gobioid fish, including growth stages, from Kyushu and Inland Sea, and examined the external and internal body-features, e. g. number of fin-rays, scale-phases, tongue, alimentary duct, genital organs, number of vertebrae, etc. (Fig. 2). The sex-dimorphism remarkably developed in the form of genital papillae. The mature female fish is generally larger than the male (Fig. 3). The black nuptial coloration appeared only in mature male fish on the second dorsal and anal fins. The ripe ovary contained two groups of eggs : ones, 0.48～0.57 mm in diameter, and the immature ones, less than 0.11 mm. The number of the mature ovarian egg of one individual was enumerated as 264～961 in 14 specimes (Table 1) ; and the mature eggs seemed to be spawn all at a time. The spawning-season seemed to extend from May to September in the vicinity of Fukuoka City, Kyushu. The spawning-behaviour in natural habitat is not observed. The fertilized egg by artificial insemination is spindle in shape, 1.2 mm in long axis, and 0.4 mm in short axis, with a bundle of adhesive filaments at the basal end. In early developmental stages, 3 8 oil-globules appeared in the translucent yellow yolk (Fig. 4). The incubation-period was about 4 days at the water-temperature 21-25℃. The newly hatched larva is 2.0 mm in total length (Fig. 5). The prelarvae, 6～8 mm in total length, appeared from June to September in the vicinity of Fukuoka City, living a swimming-life in the littoral zone. The juveniles, 8～10 mm in total length, were found to live a bottom life in estuaries (Fig. 5). The gut-contents of the post-larvae, carring a swimming-life, consisted chiefly of planktonic copepods, whereas those of the young and adult fishes, living a bottom-life, mainly of organic detritus on the bottom. The examination of the size-frequency of the collected specimens shows that this gobioid fish attains 16～25 mm in total length, and the most individuals become mature in a year. The minimum adult seems to be 17 mm in total length in both sexes, and the life-span over 2 years.

マサゴハゼは中, 南部日本の各地に分布する成魚全長25mm前後の小型のハゼ類の一種であり, 内湾にそそぐ川の川頃氣水域の砂泥底上でアベハゼ,ヒモハゼなどの他のハゼ類と共に底棲生活を送つている(Fig. 1). 筆者はマサゴハゼの体の内, 外部各形質(頭部の形状, 各鰭の条数, 鱗, 舌および舌咽骨, 消化管, 生殖腺, 脊柱骨数など)について調べたがその頭部は特異な形状を示す(Fig. 2). 二次性徴は生殖孔突起の雌雄差に最も著しく現われ, 黒色の婚姻色は雄成熟魚だけに, その第2背鰭および臀鰭上に現われる. 雌成魚は一般に雄成魚より大きい(Fig. 3). 成熟卵巣は成熟卵群(卵径0.48～0.57mm)と未熟卵群(卵径0.11mm以下)との著しく形状の異つた2卵群よりなり, 成熟卵群はその形状からみて同時に産み出されると考えられるが, その数は1尾について264～916個(Table 1)を数えた. 卵はまだ採集されず, 産卵習性も不明. 産卵期は福岡市近郊で5天月然から9月までの長期に亘るようである. 人工授精によつて得た受精卵は紡錘形をなし, 長径1.2mm, 短径0.4mm, 卵膜先端は鈍く尖り, その後端には附着絲叢がある. 卵発生初期の油球数3～8個. 孵化時間は21～25℃の水温で約4日間(Fig. 4). 直後の仔魚は全長2.0mm. 全長6mm以孵化下の後期仔魚はまだ採集されていない. 全長6～8mmの後期仔魚は福岡市近郊では6月から9月の間に成魚の棲む水域の汀線附近で欝泳生活を送つているのが見られた. 全長8～10mmの初期稚魚は成魚と同じ棲息場で游泳生活から底棲生活に移り, そこで全生涯を送る. 游泳期の仔魚の消化管内には橈脚類がみられたが底棲生活を送つていた稚魚, 若魚, 未成魚, 成魚の各発育期の消化管内容はdetritusが主であつた. マサゴハゼは生後満1年で全長16～25mmとなり, その大部分は成体となつて産卵に与り(最小成体は雌, 雄ともに全長17mm), 寿命は満2年を越えることが考えられる. 全標本中の最大魚は雌全長35mm, 雄28mmであつた.

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