Seasonal Fluctuation in a Marine Red Alga, Gracilaria vermiculophylla (Gracilariales, Rhodophyta), from Nokonoshima Island, Southern Japan

Seasonal fluctuation of Gracilaria vermiculophylla population was investigated at Nokonoshima Island, Fukuoka, southern Japan from November 2008 to April 2010. The wet weight and the length of randomly collected five plants were monthly measured, and the size class distribution of the excised thalli (categorized into nine size classes based on the thallus length) was followed. The proportions of reproductive types (spermatangial, cystocarpic or tetrasporangial) were also followed for one year (from May 2009 to April 2010) on additional collection to make a total of 100 plants. As the results, the mean wet weight and thallus length were highest in spring (April to June) and lowest in autumn (September to November). The size class distribution fluctuated greatly throughout the year and the smallest size class (Class I; <5.0cm) was conspicuous from October to December both in 2008 and 2009, while the largest size class (Class IX; 40.0–44.9 cm) was only found on May 2009, reaching 41.2 cm in length. The proportions of the three types of plants fluctuated throughout the year with the tetrasporangial plants constantly present and mostly dominant. From these results, it was known that new thalli started to grow from October, and that they could mature rapidly even in short size, suggesting that the renewal of the population is made yearly on sporic basis (tetraspores and carpospores).