Immersion panel survey of the depth range of Spirobranchus sp. 1 on the Echizen coast of the Sea of Japan, central Japan

Abstract

Fossils of sessile organisms can be used to reconstruct relative sea-level changes in the past. The upper limit of fossil colonies of the serpulid worm Spirobranchus sp. 1 (sensu Simon et al., 2019) is an excellent indicator of palaeo-shorelines. This method was defined on the Pacific coast where the tidal range is large and Spirobranchus colonies are well developed; however, the method is difficult to apply to the coast of the Sea of Japan because the tidal range is smaller and colonies are poorly developed in this area. Therefore, we investigated the vertical zonation of intertidal living sessile organisms over almost four years using immersion panels, mainly to assess the accuracy of Spirobranchus sp. 1 as a shoreline indicator on the Sea of Japan coast. The vertical zonation was investigated by measuring the number and size of calcareous shells and tubes attached to the panels. Spirobranchus sp. 1 occurs only in a 35-cm vertical zone near mean sea level and is useful as a palaeo-shoreline indicator. In addition, we obtained information on the vertical zonation of other sessile organisms in the intertidal zone and information on changes in the status of organism attachment over time. This information is useful for recognizing palaeo-shorelines and interpreting age data on the Sea of Japan coast.