Development and verification of a new maintenance and management method considering the geoenvironmental dynamics for a habitat of the Manila clam Ruditapes philippinarum

Abstract

The purpose of this study is to suggest a new maintenance and management method for artificial tidal flats as a habitat for the Manila clam Ruditapes philippinarum by performing laboratory and field experiments of the geoenvironment and geoenvironmental dynamics to prevent the naticid snail Glossaulax didyma from preying on the Manila clam.In laboratory experiments, no Manila clam was predated by G. didyma in tanks with gravel only laid to a thickness of 100 mm. On the other hand, in tanks with sand only laid to a thickness of 100 mm, the predation rates ranged from 45 to 75% at the end of the experiments (after about one month). The results of field experiments revealed that replacement of a 100 mm-thick surface layer in a tidal flat with gravel having a diameter of 10–20 mm, the same size as the Manila clam, was effective in protecting the Manila clam from predation by G. didyma. The survival rates of the Manila clam were 88.0% in the gravel plots, whereas those in the sand plots were 1.0%, about two months after the start of the experiment. The growth rates of the Manila clam, in terms of the mean shell lengths, were significantly different between the sand and gravel plots in the field experiments (Student’s t-test, P = 0.0001). Despite a typhoon passing close to the site of the field experiment, neither sand accumulation nor apparent flow-out of gravel was observed, indicating the geomorphological/geoenvironmental stability of the placement of the gravel.This method was shown to be effective in keeping the topography and providing a geomorphologically stable habitat. Artificial tidal flats constructed so far have been composed of a single material, namely, sand, and are simple in structure. However, it has been reported that tidal flats and revetments composed of heterogenetic materials and having complicated structures are effective for reducing the predation pressure from predators and for biodiversity. Our proposed method, in which the gravel-covered areas were arranged on the original sandy tidal flats, is effective not only for preventing feeding damage but also for creating various habitats for diverse benthic organisms.