Relationship between seasonal seagrass growth and egg case dynamics of the oval squid (Sepioteuthis lessoniana Férussac in Lesson, 1832) in the coastal waters of the Ryukyu Archipelago

Abstract

The oval squid (Sepioteuthis lessoniana Férussac in Lesson, 1832 often lays eggs on seaweed in coastal waters. However, in the coastal waters of the Ryukyu Archipelago, Japan, it specifically lays its eggs on seagrass, a behavior also observed in mainland Japan. Spawning dynamics in relation to seagrass growth have not been previously documented, and it remains unclear why oval squids specifically select seagrass as a spawning substrate. Therefore, we conducted a field survey to investigate the seasonal growth of seagrass communities (total area of 2,400 m2) and determine the abundance of egg cases deposited by oval squid on seagrass. Seasonal quadrat monitoring was conducted in the coastal waters of central Okinawa Island, Ryukyu Archipelago, Japan, from 2018 to 2020. Seagrass (Thalassia hemprichii (Ehrenberg) Ascherson, 1871 and Syringodium isoetifolium (Ascherson) Dandy, 1939) increased in length and coverage area from late June and peaked in late August; thereafter, the seagrass gradually withered. Furthermore, oval squid eggs, each containing 1–3 ova, were deposited on seagrass from mid-July 2018 and late June 2020, and the largest numbers were deposited in mid-August 2018 and late July 2020. The number of eggs laid on the seagrass gradually decreased until late September 2018, when they were no longer observed. However, in 2020, the number of eggs spawned decreased, followed by an increase between August and September, and eggs were no longer found in mid-October. The function of the seagrass community as a specific spawning substrate and nursery ground for hatched juvenile oval squid is also emphasized.