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有明海産コイチの初期生活史に関する研究


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Title: 有明海産コイチの初期生活史に関する研究
Other Titles: Studies on the Early Life History of Nibea albiflora (Richardson) in Ariake Sound
Authors: 田北, 徹
Authors (alternative): Takita, Toru
Issue Date: Dec-1974
Publisher: 長崎大学水産学部
Citation: 長崎大学水産学部研究報告, v.38, pp.1-55; 1974
Abstract: 有明海に分布するコイチについて,成魚の形態,分布と産卵生態に関する知見をもとに初期生活史を研究し,初期成育期の形態,生態およびそれら相互の関係を明らかにした。成魚の形態と分布:有明海産コイチの形態は従来の本種に関する記載にほぼ合致するが,本種の特徴の1つとされている眼隔の頭長に対する割合については,東シナ海産の本種との間に有意の差が認められた。有明海のコイチは4~5月に湾内を北上し,春から秋まで湾奥部の"前の海",西側湾入部の"諫早湾"と湾中央部東側のいずれも最低潮時の水深が2~5mの浅海域を中心に分布する。それらは10~12月に次第に浅海域を離れ,冬季は湾口部附近の比較的深い水域に移動して越冬する。このように本種は全生活史を湾内で送る。産卵:本種は5~8月に,この時期の主な分布域である湾奥部浅海を中心に産卵し,5,6月がその盛期であるが.この期間中の産卵域における水温は18~29℃(盛期は19~23℃),海水濃度については塩素量17.5‰以下のかなり低濃度の海水が分布するが,受精に不適当なほど低くはない。産卵は午後3~7時の間に行なわれ,卵巣卵の受精能力はこの間に最大になる変化を示す。海中には産卵時刻に発生初期の卵が現われ,時刻の経過につれてほぼ一斉に発生が進行する。卵と卵内発生:卵はほぼ透明,球形で,囲卵腔は狭く,直径0.17mm前後の油球を1個持っている。卵径は親魚によってかなり変化するが,平均卵径は0.76mm,卵径範囲は0.69~0.83mmである。海水濃度による卵径の変異はごく小さい。卵の比重は媒質の比重によって変化し,産卵域の海水濃度(塩素量15~18‰)では24℃において1.018~1.020である。コイチ卵の受精率と媒質の塩素量との関係については,13.55‰以上では受精率が比較的高く,5,48‰以下では全く受精しない。卵は水温19℃では37時間30分,29℃では15時間40分でふ化する。仔稚魚の形態:ふ化直後の仔魚は全長1.5mm,筋肉節数8+19=27。その後3日間に口が開き,目に黒色素が現われ,胸鰭が形成され,尾部が伸長する。色素胞については 黄色胞は体全体に分布し,黒色胞は成長にともない体の背面から側面と腹面へ移動する。尾部中央に両色素胞による明瞭な1横帯が形成される。後期仔魚と稚魚については,体長2.8mmで歯が形成され,約4mmで垂直鰭の原基が形成され,約6mmで各鰭条が完成して稚魚期にはいる。体長7~8mmから体側に黒色胞が現われはじめ,体長約11mmで鼻孔が完成し,15mm前後に鱗が形成されはじめる。臀鰭の直後にある1つの大きな黒色胞叢は後期仔魚からこの大きさの稚魚までの特徴の1つである。体長70~80mmで各形質の変化はほぼ終り,体表に成魚の斑紋があらわれて若魚期にはいる。鰾は外見的にはふ化後3日で形成され,体長25mmからその背側面に樹枝状突起が発達しはじめ,その形態は約80mmで成魚に等しくなる。鰾に付随し,発音に関係する腹腔壁筋肉は鰾完成時から形成されはじめ,体長120mm以上でなきはじめる。消化管は前期仔魚では食道から直腸までほぼまっ直ぐに連絡しているが,卵黄吸収時に1回転してそこに胃が形成され,体長4~7mmで腸管が折れ曲がり,体長9mmで成魚と同じ形態に達する。各体部の比較によれば,頭胴長―体長,尾部の高さ―尾部長,頭部の高さ―頭長,頭部の高さと体高との関係,上顎長―頭長および上顎の体軸に対する角度は仔魚の相対値が成魚と最もかけ離れ,ある体長まで変化したのち一定に達し,仔魚の特徴的な体型からの変化を表わす。尾鰭長―全長は体長12~30mmに極大に達し,稚魚期初期の特徴的な形態を示す。腹腔壁筋肉は体長約80mmで形成され,その重さの体重に対する割合は雄は120mm,雌は140mmで大きく変化し,急速な増大を示す。体長16~150mmにおける体長(Lcm)と体重(Wg)との関係は W=0.0165L3.010で表わされる。仔稚魚と若魚の月別体長組成および産卵期の成魚の体長組成によれば,仔稚魚は5~8月に現われ,その年の終りまでに最大は体長180mm,最小は50mmとなり,生後満1年の体長モードは約170mm,満2年で230mm前後に達する。卵から若魚期までの分布:表層と底層における卵分布量の比較から,比重20.98~22.63の海中において卵は発生初期には底層に多く,産卵後5時間以後は表層に多く分布するという垂直分布の変化が認められ,コイチは底層で産卵し,卵は環境水の比重が卵の比重より大きい場合は徐々に表層に浮上すると考えられた。有明海のほぼ全域に及ぶ11地点の表層と水面下4mの層で行なった稚魚網採集の結果によれば,コイチ卵は主な産卵域である湾奥部浅海を中心に分布し,湾中央部東側の浅海にも若干分布する。湾奥部と中央部における底層曳稚魚網採集によれば,体長15mm以下の仔稚魚は湾奥部沿岸の最低潮時の水深5m以浅の水域に多く分布し,大型個体ほど岸寄りの浅い水域に分布する傾向がある。また仔稚魚の多くは湾奥部沿岸の限られた水域に集中して分布し,その傾向も大型個体に顕著に認められる。稚魚と若魚の分布については,諫早湾奥部に注ぐ本明川の河口域,前の海最奥部に当たる筑後川河口沖の干潟域,それに続く低潮線附近の浅海域と水深5m前後の湾奥部沖合域において採集を行なった。上記の各水域別の出現期と体長組成の月別,水域別変化によれば,体長6~20mmの稚魚は河口域,干潟域から浅海域,21~80mmは干潟域から浅海域に分布し,81mm以上の若魚は成魚が分布する湾奥部の水深2~5mの沖合域に分布し,稚魚と若魚は成長にともなって沖へ移動する。各成育期の生息域の環境条件については,産卵域の環境水温は18~29℃,塩素量は15~18‰,の若魚と成魚が9~11月に分布する沖合域の水温は15~27℃,塩素量は16~18‰である。これに対し,河口域で稚魚が多く分布する環境の水温は22~33℃,塩素量は1~17‰で,コイチ稚魚は分布環境に対する適応が極めて広い特徴がある。食性:湾奥部で採集された仔稚魚,若魚と成魚の胃内容によれば,仔魚と体長40mm以下の小型稚魚は橈脚類を,大型稚魚はアミ類,ヨコエビ類,アキアミ,若魚はその他のエビ類とカニ類を主に捕食し,成魚はエビ類とカニ類のほかにイワシ類,ヨコエビ類,ワレカラ類も多く捕食しており,コイチの食性は成長にともなってほぼこの順序で変化するとみられる。コイチの餌生物に対する撰択性は許容範囲が広いが,対象とする餌生物の変化は基本的には仔魚期と稚魚期初期の浮遊性から稚魚期後期の遊泳性と付着性へ,稚魚期後期の食性から若魚,成魚期のほふく性も含めた広い範囲の食性へという生態的な要素および小型から大型へという大きさの要素に規定される。初期生活史の主要な餌である橈脚類,アミ類,アキアミ,シバエビの分布とコイチの分布は時期的,地域的によく対応しており,餌条件については,有明海湾奥部はコイチの初期成育に好適な条件をそなえている。初期生活史における形態と生態との開係:相対成長,各形質の発達,分布,食性の対比により,コイチは仔魚期と稚魚期に形態的な対応のもとに独特な環境で特徴的な生活をし,若魚期に形態的,生態的に成魚と同じ段階に達し,この間に,卵黄期,体長3~6mm,6~12mm,12~27mm,25~80mm,60~160mmの6期の各々特徴ある成育期を経て成長することが明らかになった。 / Early life history of Nibea albiflora (Richardson) was studied morphologically and ecologically on the basis of morphology and distribution of the adult and the reproduction in Ariake Sound which is situated in the west part of Kyushu. 1) Morphology and distribution of the adult Though morphological characters of N. albiflora distributed in Ariake Sound are almost in agreement with the discription of the fish reported, significant difference is recognized between ones from the sound and the ones from East China Sea where the fish is abundantly distributed in the ratio of interorbital length to head length. The adults of N. albiflora move toward shallower waters of the northern innermost region or the eastern region of the sound in April and May, and stay in the coastal area of 2~5m in depth of low tidal level till autumn. They leave the area in October~December, and are distributed in quite deep area of more than 30m in depth near the mouth of the sound in cold season. Morphological characters and distribution of the fish suggest that the fish of the species in this sound belongs to a population which spends most of the life in the sound and does not leave away from the sound. 2) Spawning Spawning of N. albiflora is mainly carried out in May and June in the shallow coastal area of 2~5m in depth of low tidal level, above all in the northern innermost area where the fish is abundantly distributed. Water-temperature of the spawning area in the season is 18~29℃, and the usual concentration of the sea water in the area in the season is more than 13.55‰ in chlorinity, in which concentration fertilization of egg of N. albiflora is found to be successfully performed. Spawning of the fish is carried out from three to seven o'clock in the afternoon, and the time the ovarian eggs attain to the maximum capacity of fertilization coincides with the spawning time. 3) Embryonic development and early morphological growth Egg of N. albiflora is almost transparent, colorless and spherical in shape and has a narrow perivitelline space and one oil globule which is about 0.17mm in diameter. Mean and range of diameter of artificially fertilized eggs from five parent fishes are 0.76mm and 0.69~0.83mm respectively. Though effect of sea water concentration on egg diameter is not great, there is a tendency that the higher is the concentration where the egg is fertilized, the smaller is the diameter of the egg. Specific gravity of the egg changes according to that of the circumstance, being 18.3~19.6 in usual concentration of the spawning area (15~18‰ in chlorinity) at 24℃ of water-temperature. Embryonic development shown in Table 9 and Figure 19 is almost the same as common one observed on general pelagic fish egg. Hatching took place 37hr. 30min. at 19℃ and 15hr. 40min. at 29℃ after insemination. Newly hatched larva from artificially fertilized egg is 1.5mm in total length and has 27 myomeres (8+19=27). By three days after hatching, opening of mouth, appearance of black pigments on the eyes, formation of pectoral fins and elongation of tail took place. As for chromatophores on the body, melanophores, being in the dorsal part on the newly hatched larva, generally shift to the body sides and the ventral margin, and assemble thickly on the sides and the ventral margin of middle part of the tail forming a characteristic band, while xanthophores are scattered throughout the most part of the body. Morphological growth in post larval and prejuvenile stages (A. J. Mansueti and J. D. Hardy, Jr., 1967) observed on individuals collected with larval nets in the innermost region of the sound is as follows. Larva of 2.8mm in body length has teeth, and one of 4.0mm is forming rays of vertical fins. Individual of 6~7mm has complete rays on each fin and is regarded to have attained to the prejuvenile stage. In prejuvenile of 7~8mm, the area of body surface covered with melanophores begins to spread on the sides of the trunk, and in one of 15mm, scales begin to be formed. Individual of 70~80mm in which most of the morphological changes have been accopmlished and the typical striped pattern of the adult appears on the sides of the body is regarded to have already attained to the juvenile stage. Air-bladder is formed by three days after hatching, and its external appearance forming two series of arborescent appendixes on the dorsal sides grows to be the same as that in the adult by the stage of 80mm in body length. Muscles in body cavity which are connected to the airbladder and reportedly related to sound production are formed and begin to grow in the stage of 80mm. The fish begins to croak in juvenile stage of more than 120mm. As for morphology of digestive organ, stomach is formed in the stage of yolk absorption (2~3mm in body length), intestine is convoluted in the stage of 4~7mm, and the external appearance of the adult is provided by the stage of 9mm. Larva and prejuvenile of N. albiflora are quite resemble to those of Argyrosomus argentatus which are abundantly distributed in the same area as the former and caught simultaneously. Post larva and prejuvenile of N. albiflora may be discriminated from those of A. argentatus by the differences that, in the former, a clucter of melanophores is located behind the anal fin base more anteriorly, that melanophores of the dody sides first appear on the position anterior to the dorsal fin, while those of A. argentatus appear beneath the fin, and that the clumps of melanophores on the body sides are obscurer. In prejuvenile of more than 20mm and juvenile, there are distinct differences that those of N. albiflora have larger eyes and higher body proportion. According to relative growth examined about larvae, prejuveniles and juveniles, relative values of head and trunk lenght to body length, depth of tail to tail length, depth of head to head length, relation between head depth and body depth, upper jaw length to head length, and angle of upper jaw to body axis are far apart in larval stage from those of the adult, and get nearer to the adult's constants by a certain stage of growth, showing morphological change from typical character of the larva. Ratio of caudal fin length to total length is maximum in the stage of about 10mm in body length, showing typical character of young prejuvenile Growth rate of the sound producing muscles shown as the ratio of the muscle weight in body weight increases in the stage of 120mm (male) and 140mm (female) in body length. Relationship between body weight (Wg) and body length (Lcm) in young period of 16~150mm in body length conforms to the following regression. W=0.0165L3.010 Larvae of N. albiflora appear in the sound from May to August and attain to 50~180mm in body length by the end of the first year. Modes of the body length at age one and age two are about 170mm and 230mm respectively. 4) Ecology in egg and young period As to the vertical distribution of eggs of N. albiflora, it is found that, in the water of higher specific gravity than that of the egg, eggs are more in the bottom layer during some period after the spawning, and after a while the number of eggs increases in the upper layer. This suggests that the spawning is carried out in the bottom layer, and in the water of higher specific gravity than that of the egg, eggs ascend gradually. The eggs are mainly distributed in shallow waters of the innermost region, and rare in the middle area and the area near the mouth of the sound where the water is comparatively oceanic and deep. Larvae and small prejuveniles are mainly distributed being densely assembled in some restricted coastal areas of the innermost region of the sound, and such a tendency is found that the larger the size of individual is, the shallower waters and the nearer to the coast it is distributed. Prejuveniles and juveniles are seen in shallow water areas near the coast of the innermost region that are the mouth area of river, intertidal zone off the mouth of river, and zones just lower than low tidal level from June to September, and in comparatively deep area of the region where the depth at low tidal level is 2~5m from September to December. As for the body length composition in each area, most of individuals in the mouth area of river are 6~20mm, ones in the intertidal zone and ones in the shallow waters near low tidal level, are 6~60mm and 6~80mm respectively, and ones in the comparatively deep area are 111~150mm. These facts about occurrence and body length composition suggest that individuals once recruited to shallow coastal area in larval stage move toward the deeper area with growth, and body length range of the group distributed in certain area varies dependently on depth of the area or distance from the shore. As for the environmental condition in young stage, prejuveniles are distributed in very wide ranges of water-temperature and sea water concentration, being 22~33℃ of water-temperature and 1~17‰ in chlorinity, while the adult lives in waters of 22~29℃ and 16~18‰ in chlorinity in warm season. Stomach contents from individuals in each growth stage show that Copepoda, Mysidacea, Gammaridae, Acetes japonicus, other Natantia, and small Brachyura compose main food of the fish in young period at least in one short period of growth, and changes of their composition with growth in prejuvenile and juvenile stages suggest that these main food animals are preyed alternately in general in the order of Copepoda-Mysidacea, Gammaridae, and Acetes japonicusother Natantia and small Brachyura. Although the fish has comparatively broad selectivity for food animals, alternation of prey composition with growth of the predator seems to be affected by relationship in ecology and size of prey and predator. Comparison of the occurrence of Copepoda, Mysidacea, Acetes japonicus, and Metapenaeus joyneri in Ariake Sound to the distribution of young of N. albiflora shows that occurrence of main food animals and the young corresponds each other in time and space in head region of the sound, and the region forms a good nursery area for young of N. albiflora. 5) Conclusion From an overall review of relative growth, development of morphological characters, changes of distribution and feeding habits which are mentioned above, it is found that N. albiflora spends a typical life in special circumstance and has the morphological characters adapted to the life in larval and prejuvenile stages, and the morphological and ecological characters in juvenile stage attain to almost the same as that in the adult, and six developmental divisions in young stage which have typical characters of morphology and ecology respectively are recognized, namely, yolk sac stage, 3~6mm, 6~12mm, 12~27mm, 25~80mm, 60~160mm in body length.
URI: http://hdl.handle.net/10069/30797
ISSN: 05471427
Type: Departmental Bulletin Paper
Text Version: publisher
Appears in Collections:第38号

Citable URI : http://hdl.handle.net/10069/30797

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