Mass recruitment of the lancelet Branchiostoma japonicum at a sandbank in the western Seto Inland Sea in 2002 and subsequent age structure of the population

抄録

In 2001, many planktonic larvae of the lancelet Branchiostoma japonicum were found in the western Seto Inland Sea. To know their subsequent state, the benthic population at a small sandbank, named Ohzu, located there was surveyed with a grab sampler in total 26 times from November 2001 to November 2005. The mean population abundance at six stations was around 100 ind. m⁻² until July 2002, and abruptly increased to 1041 ind. m⁻² until November 2002 by a mass recruitment. Thereafter, the abundance decreased with fluctuations to around 200 ind. m⁻² in November 2005. The 2002-year class continued to be the strongest year class until at least 2005. Planktonic larvae collected in July 2005 at 4-km intervals along an 80-km line showed strongly patchy aggregations with scales ranging 4 to 12 km. Their patchy aggregation probably resulted from a spawning habit known for lancelets, which spawn within a short time at night and synchronously a few times in summer. The mass recruitment in 2002 and the large interannual fluctuation of the benthic population were most likely caused by the accidental settlement of patchy larval aggregation. Populations at Ohzu and a site (Kaihara) about 6.5 km apart in June 2006 showed a similar age structure having the strongest age class is still regarded as the 2002-year class, suggesting that the two sites were settled by the same larval patch or different patches with different fertilization dates.