Surface traces and bioturbate textures from bubbler crabs: an indicator of subtropical to tropical tidal flat environments

Abstract

The surface traces and bioturbate textures genereated by the bubbler crabsScopimera sp.,Dotilla blanfordi, Ilyoplax pusilla, etc., occurring in the tropical supra- to shallow-subtidal environments of the east coast of India have been studied. During foraging these organisms make various patterns on the substrate by their feeding pellets. These patterns are primarily dependant on grain-size characteristics, moisture conditions, and emergence level of the substrate. The crabs also produce typical ichnofabrics caused by back-filling of the burrows which do not exceed 10 cm in length. The internal fabric of the burrow-fills shows both concave-upward and concave-downward laminae resulting from upward or downward movement of the organism within the sediment. In sections perpendicular to burrow orientation the process of back-filling generates concentric laminae. Such a fabric occurs not only in coastal intertidal sediments, but also in sediments of estuarine point bars located 50 kilometres inland where these tiny crabs also occur. Such subsurface bioturbation features may represent a better tool for the identification of tidal environment than surface traces since the latter have a low preservation potential because of the erosional effects of wave and current action. The bioturbate texture, however, occurs over a wide range of the intertidal zone and, hence, in fossil record this bioturbate texture can be useful to identify tidal environments. These crabs can withstand a definite range of salinity and, therefore, this texture can also provide information about salinity conditions of palaeo-tidal environments.