First record of planktonic egg masses of the diamond squid, Thysanoteuthis rhombus Troschel, in the Sea of Japan

抄録

This paper provides the first evidence that Thysanoteuthis rhombus spawns in the Sea of Japan. Five planktonic egg masses were collected in the southern Sea of Japan during 29 October to 24 November 2004 and transported to onshore laboratories for observation. Part of each egg mass was reared in indoor, aerated tanks supplied with filtered running seawater. The egg masses (ca. 60–120 cm in total length and 13–15 cm in diameter) were cylindrical with rounded ends and consisted of a resilient, transparent gelatinous core with a pair of egg rows forming spiral loops around the core. The embryos had dome-shaped mantles covered with many chromatophores and slowly rotated inside the spherical egg capsules. Hatching was observed in all the egg masses 3–10 days after collection. Morphological characteristics of the egg masses, eggs and embryos, especially the large number of chromatophores present in the early embryonic stages, agreed with the descriptions in previous studies. Near the collections sites, surface water temperature and salinity ranged 18–22 °C and 33.3–33.7, respectively, both of which were lower than the optimum conditions for T. rhombus spawning. The early embryonic stages of the egg masses at collection such as cleavage and slow currents at the sea surface suggest the egg masses were spawned near the collection sites. Increased abundance of T. rhombus in the Sea of Japan and increased sampling efforts were proposed as two possible causes for the egg-mass discoveries.