Comparison of the activity patterns of nine molluscan grazers on a boulder shore at Amakusa, Japan

The activity patterns of 9 co-occurring species of molluscan grazers were observed on a gently sloping boulder shore at Amakusa, west Kyushu, Japan. Surveys on their activity were carried out four times in summer: twice during neap tides and twice during spring tides. Snails (Lunella coronata, Nerita japonica and Littorina breviculd) moving on the upper surface of the boulders were considered as _eactive_f, and those not moving but staying on the surface as _estationary_f. With limpets (Nipponacmea concinna, N. nigrans, N. schrenckii, N. temmachii and Patelloida pygmaed) and chitons (Acanthochitona defilippi), all individuals on the upper surface of the boulders were considered as _eactive_f, irrespective of whether they were moving or staying. Interspecific variations related to day/night and tidal conditions were recognized. Two species of snail, N. japonica and L. brevicula, were mainly active when _eemerged_f (exposed to air) in the nighttime. Chitons and Nipponacmea limpets were active mainly when _eawash_f (affected by wave action) and emerged in the nighttime. L. coronata and P. pygmaea were active in the nighttime irrespective of the tidal conditions. Relationships between the activity patterns and inorganic contents of faeces were observed; _elimited activity species_f (A. defilippi, Nipponacmea spp. , L. brevicula and N. japonica) , which are active mainly during the _eemerged_f and _eawash_f conditions in the nighttime, produce faeces of low inorganic content, while _ebroad activity species_f (P. pygmaea and L. coronata, including Monodonta labio) , which are active in the nighttime irrespective of the tidal conditions, produce faeces of high inorganic content. Probably, the _ebroad activity species_f have more chance to ingest fine inorganic particles accumulated on the surface of boulders during their feeding excursions when submerged than the _elimited activity species_f.