Egg-Laying Behavior, Embryonic Development and Hatched Larva of the Pygmy Cuttlefish, Idiosepius pygmaeus paradoxus ORTMANN

Seven species or subspecies of the Idiosepiid cuttlefish measuring less than 15mm in mantle length have been reported in the world. The pygmy cuttlefish Idiosepius pygmaeus paradoxus ORTMANN, is widely distributed in the littoral waters of Japan. During the periods from April to June 1969 and May and July 1970, 194 live adults of I. pygmaeus paradoxus were collected from the Ariake Sound, Kyushu, Western Japan, and some of them were kept in a plastic aquarium or glass jars for the study of the egg-laying. During 1 to 7 days after captivity, female parents of the cuttlefish laid eggs on the walls or bottom of the containers. Each female laid eggs once or twice with 3 days intervals. The total number of eggs laid by one female ranged from 25 to 64. The female parents died in 1 or 2 days after their final egg-laying. The process of egg-laying behaviors observed in the glass jar is as follows: A) The female parent adheres to the bottom of the glass jar with adhering organ of the back and holds the body upside-down. B) An egg is ejected at a time from the funnel and received in her arms. C) The parent sets herself by grasping the bottom with dorsal, dorsolateral and ventrolateral arms and stickes the egg to the bottom with tentacles. D) After sticking the egg, the parent slides backward by hard flapping of fins and repeats egg-laying. One process of egg-laying takes about 30 seconds. The egg of I. pygmaeus paradoxus is wrapped up in the egg-capsule. The egg-capule is almost elliptical in shape being constructed with 8-10 layers of translucent membranes and measuring 1.4-1.6mm in length and 1.2-1.4mm in width in the early developmental stage. The egg is elliptical in shape measuring 0.87-0.91mm in length and 0.67-0.72mm in width in the early stage. Both the egg and the egg-capsule increase in size during the coures of embryonic development. The egg is the smallest among the eggs of the Sepioid cuttlefishes. The course of the embryonic development is shown in Plates I-IV. It is almost the same as that of the other cuttlefishes. It takes 15-17 days for incubation at the water temperature of 18.5°-22.6℃. The newly hatched larva is 1.16-1.22mm in dorsal mantle length and 2.30 -2.44mm in total length. The arm formula of the larva is 2>1>3>4, but the tentacle is not observed yet. The chromatophore-distribution in the head and arms of the larva is characteristic.(1) ヒメイカ(コウイカ目,ヒメイカ科)の成熟個体を,夜間に水中集魚灯下で採集し,実験室内の小型水槽で飼育し産卵させ,その産卵行動,卵発生,ふ化稚仔を観察した｡ (2) 産卵は飼育開始後,1～7日後にみられ,1尾の雌は1～2回産卵し,2回産卵した場合は,その間隔は3日であり,1尾の産卵総数は25～64個であった｡雌親イカは,いずれも,最後の産卵ののち1～2日で死亡した｡ (3) 雌親イカは,その外套背面にある付着器官で水槽底に付着し,体を背腹さかさにしたまま,漏斗より卵を1個づつおし出し,それを触腕で底面につぎつぎに産みつけてゆく.1個の卵を産みつけるのに要する時間はやく30秒であった｡ (4) ヒメイカの卵は,コウイカ科のものににており,1個づつ卵嚢につつまれている｡卵嚢は,ほぼ楕円体で,産卵直後の大きさは,長径1.4～1.6mm,短径1.2～1.4mmであり,卵発生が進むにつれて大きくなる｡ 卵は長楕円体であり,発生初期における大きさは,長径0.87～0.91mm,短径0.67～0.72mmであるが,卵発生が進むにつれて大きくなり,ふ化前には発生初期の大きさの1.5～1.8倍に達する.この卵および卵嚢は,コウイカ目のイカ類の卵のなかでは最小のものである｡ (5) 卵発生の経過は,Pls.I～IVに図示し,説明しているように,既報のイカ類の卵のそれと大差ない｡ふ化までに要する時間は,水温18.5°～22.6℃で15～17日である｡ (6) ふ化稚仔の大きさは,外套背長1.16～1.22mm,全長2.30～2.44mmであり,卵黄をほとんど吸収した状態でふ化する.脚式は2＞1＞3＞4であり,触腕は観察されなかった.頭部および腕部の色素胞は,特徴的な分布を示す