Morphological allometry and life history including reproductive swarming of nereidid polychaete Tylorrhynchus osawai in a tidal river in Kyushu, Japan

Tylorrhynchus osawai, nereidid polychaete that is considered to support estuarine biodiversity, undergoes a unique epitokous metamorphosis for reproductive swarming. The reproductive swarming of this species has been observed in Japan for more than 100 years; however, the benthic phase of this species has not been well researched. In this study, we aimed to elucidate the morphological allometry and life history, specifically the reproductive swarming, of this species. To accomplish this, surveys were conducted to collect atokes and epitokes from a tidal river in Kyushu, Japan. A total of 1670 T. osawai specimens were collected and preserved in 80% ethanol. The body widths of these specimens were measured, while several intact specimens underwent additional measurements of body length and the number of chaetigers. The morphological allometry of this species differed between atokous and epitokous forms, indicating that its heteronereis form might be suitable for migration from a tidal river to the ocean. The survey results pertaining to the benthic phase of this species indicated that its lifespan was approximately one year or longer. In addition, the settlement, growth, and maturity of this species exhibited a degree of plasticity, potentially influenced by habitat landscapes and sediment conditions. Based on the surveys conducted on reproductive swarming, climate change could have caused the peak of reproductive swarming to shift slightly later than that of several historical observations. Our findings contribute to elucidating the ecological significance of epitokous metamorphosis in nereidid polychaetes and to conserving T. osawai populations.