The Biology of the Eleotrid Fishes, Eviota abax and Eviota zonura

Eviota abax (JORDAN et SNYDER) and Eviota zonura JORDAN et SEALE are small eleotrid gobies. In Japan both the fishes are distributed in the south-western districts. The distribution-limit of E. abax is more nothern than that of E. zonura. Both the fishes lived in tide pools on rocky or coral-reef shores and in coastal shallow waters near the pools (P1. I, Figs. 1, 2). In some instances of the collections made by the authors in Kyushu and the Ryukyus, both the fishes were collected from the same tide pool. The fishes fed on small Copepods, Isopods, Curastaceans, Polychaetas and fish larvae. The male E. abax grows to 44mm in total length and the female to 40mm (P1. I, Figs. 3, 4). The male E. zonura grows to 40mm and the female to 33mm (P1. I, Figs. 5, 6). Some individuals of both the fishes become the adults, which are over 26mm in total length, in a year, but the other individuals become the adults in two years. The sex-dimorphisms appeared in the first dorsal fin and urogenital papilla of both the fishes. The first spine of the first dorsal fin of the male fish is filamentous, but the spine of the female is not filamentous. The papilla of the female E. abax is similar in shape to the papilla of the female E. zonura, but the papilla of the male E. abax is different in shape from the papilla of the male of the allied species, E. zonura (Fig. 1). The ripe ovarian eggs of E. abax ranged from 0.5mm to 0.6mm in diameter, and they amounted from 329 to 790 in the cases of five females, ranging from 28mm to 32mm in total length. The ripe ovarian eggs of E. zonura were similar in size to the eggs of E. abax, and they amounted from 336 to 1,026 in the cases of five females, ranging 26mm to 31mm. The ripe eggs of both the fishes seemed to be spawned some times during a spawning-period. The spawning-period of both the fishes extended from May to August in Kyushu. A pair of the mature fish of E. abax had been collected from a tidal pit on the rocky shore of Tsuyazaki in Northern Kyushu on the 13th of July, 1960. The fish was kept in a glass jar, which was provided with sand bottom, for 16 days. The fish deposited an eggcluster on the inner side of the shell, Spondylus sP., which was laid on the sand bottom as an artificial nest, on the 14th of July. The egg-cluster contained about 250 eggs. The egg was adhesive one, and it was spindle in shape. It was 2.4mm in long axis and 1.0mm in short axis, and it had a yellow yolk (Fig. 2). The translucent granules appeared in the embryo of the egg at the early developmental stages and disappeared at the later stage (Fig. 2, K). Many agrippa eggs, in which the embryo-heads faced to the bundle of the adhesive filaments, were observed in the egg-cluster. The discharged unfertilized egg of E. zonura was similar in shape to the egg of E. abax, and it was smaller than the latter. It was 1.7mm in long axis and 0.65mm in short axis, and it had a light blue yolk (Fig. 3, A). The newly hatched larva of E. abax was 4.4mm in total length, and it had 24 myomeres. It consumed its yolk in 36 hours and swam in a glass jar (Fig. 2, I). The post larvae of both the fishes had not been collected from tide pools, but the juveniles of the fishes, over 8mm in total length, appeared in tide pools where the adult fishes lived. In Kyushu the juveniles usually appeared in tide pools after August, about three months after the commencement of the spawningperiod of the fishes. These facts suggest that the larvae, which hatch out in the coastal zone, leave the coastal zone for off-shore waters and spend a pelagic life there about three months, and they come again to the coastal zone when they become the juvenules (Fig. 3, B). The authors aim to use E. abax and E. zonura together with other tide-pool gobies for living fishing baits, and for this purpose they are studying on the ecology of the gobies.イソハゼおよびミドリハゼはともに日本では西南域に産する小形のハゼであり,前者は後者より北方域まで分布する.両者ともに岩礁およびさんご礁海岸の潮だまりおよびその附近の沿岸浅所にすみ,等脚類・橈脚類・甲殻類・多毛類・稚魚などの小動物をたべており,両者がおなじ潮だまりから同時にとれた例もある. イソハゼ・ミドリハゼはともに全長5cm以下の小形魚であるが,前者は後者よりややおおきくなり,またともに雄が雌よりややおおきくなる.最小成体はともに全長26mmであり,生後1年で成体となる個体と2年で成体となる個体との両方があり,そのいずれにも産卵ののちもいきのこるものがある.九州西南部における産卵期は両者ともに5月から8月のあいだであり,雌は一産卵期に数回にわたって産卵するとおもわれるが,卵径0.5～0.6mmの成熟卵巣卵数は全長28～32mmのイソハゼで329～790,全長26～31mmミドリハゼで336～1,026をかぞえた. 第2次性徴はイソハゼ・ミドリハゼともに第1背びれおよび泌尿生殖突起にあらわれる.すなわち,両者ともに雄の第1背びれの第1棘は糸状にのびているが,雌の棘は糸状をなしていない.生殖孔突起の形は雌・雄でことなり,また雌ではイソハゼとミドリハゼの突起は同形であるが,雄の突起は両者ではかたちがことなる. 天然における産卵習性は両者ともにわかっていないが,イソハゼについては,1960年7月に,福岡県津屋崎町の岩礁海岸の潮だまりから採集した成熟魚の雌雄一対を水槽で飼育し,それに産卵巣としてウミギクの貝殻を与えて産卵させることができた. イソハゼの卵は長径2.3～2.5m,短径1.0～1.1mm,の紡錘形をした沈性付着卵であり,卵黄は黄色であった.ミドリハゼの受精卵はまだとれていないが,海水のなかに放出された未受精の熟卵はイソハゼの卵とおなじく紡錘形をした沈性付着卵であり,そのおきさは長径1.7mm,短径0.65mmで,イソハゼの卵よりややちいさく,卵黄は淡青色であった. イソハゼのふ化直後の仔魚は全長4.4mmで,体節数は24をかぞえた.イソハゼ・ミドリハゼはともに後期仔魚はまだ採集されていないが,これまでの採集結果からみると,両者ともに海岸域でうみつけられた卵からふ化した仔魚は潮・海流にながされていちど外海にでてゆき,そこで浮遊生活をおくりながら後期仔魚期をすごし,いその生活にてきした形態をそなえた全長10mm前後の稚魚になったのちにふたたび海岸域にきてそこにすみつくものとかんがえられる.九州各地では,イソハゼとミドリハゼの稚魚はともに例年8月いごになると親魚のすんでいる潮だまりから採集できるが,後期仔魚は潮だまり内ににまあらわれない. 筆者はイソハゼ・ミドリハゼを含めた潮だまりのなかにすむハゼ類を採集して有用魚のつりのいきえさとして利用することをめざしており,このために,いそにすむハゼ類の生態について実験・観察をすすめている