Life history of Ammonia "beccarii" forma 1 on hard substrate in the Ohashi River, southwestern Japan

Abstract

We investigated the life history of Ammonia forma 1 (A. beccarii forma 1 sensu Matoba) inhabiting a bridge pier and surface sediments in the Ohashi River (tidal river between two brackish-water lagoons) in southwestern Japan based on seasonal changes in the standing crop and size distribution. This species commonly occupies shallow infaunal habitats but sometimes is found in attached macrobenthos on hard substrates. Attached macrobenthos at 1 m and 2 m water depth on a concrete bridge pier and surface sediments on the river floor at two sites were studied. Seasonal changes in both the standing crop and size distribution of A. forma 1 were generally different between the bridge pier and surface sediments. There was no statistical correlation between the standing crop of bridge pier A. forma 1 and the abundance of attached macrobenthos on the bridge pier. This finding suggests that bridge pier foraminifera do not simply depend on the abundance of attached macrobenthos on a hard substrate. The presence of small bridge pier individuals indicated that major reproduction occurred in spring and fall during the study period. A. forma 1 from the bridge pier can likely breed in the macrobenthos colonies on hard substrates (i.e., in-situ) and hence recover, even when it has been diminished by physical disturbance (e.g., freshet). Thus, these characteristics of A. forma 1 on the bridge pier likely enable the population to persist in an ephemeral habitat within macrobenthos colonies and an unstable brackish-water environment.