Large-sized benthic foraminifera at the northern limit of subtropical zone: southern Kochi, Shikoku Island, Japan

Abstract

Large benthic foraminifera (LBF) are relatively large (>0.5 mm), symbiotic with microalgae, and can be found in tropical to subtropical, clear, shallow waters. LBF are commonly studied in lower latitudes but are less studied in higher latitudes close to their current limit of geographic distribution. As climate change progresses, poleward extension is expected to proceed apace, thus it is important to characterize benthic foraminiferal fauna in these higher latitude zones in order to track the progress of their habitat extensions. The present study examined large-sized (0.5 to 2 mm size fraction) benthic foraminifera, both symbiotic and non-symbiotic, in one such region: Kochi Prefecture in Shikoku Island, Japan. Living foraminifera attached to algae as well as in sediment, and dead foraminiferal tests in sediment were collected from three stations in the study area. A total of 64 taxa were identified, of which 17 were found living in algae, 8 living in sediment, and 60 were dead in sediment. Amphistegina lobifera was the dominant species in the region, making up almost 50 % of all specimens identified, followed by other symbiotic foraminifera. Compared to foraminiferal living communities and dead assemblages in lower latitudes (the Ryukyu Island Arc), amphisteginids and textulariids were more common in Kochi. Calcarinids which are dominant in the Ryukyu Island Arc were not found in this study.