The crab Macrophthalmus japonicus burrows on a tidal flat of the Yellow River Delta in China: Their 3D morphological variability in relation to physicochemical conditions and palaeoichnological perspective

Several environmental parameters, sediment grain size and its total organic carbon (TOC) content, turbidity and salinity of water, have been measured, and burrows of the crab Macrophthalmus japonicus were studied by highresolution CT imaging of sediment cores sampled at 32 stations along transects from supratidal to subtidal settings of the Yellow River Delta in China. The burrows are present only at some stations in muddy and sandysilty sediments. They are concentrated in some patchy areas affected by low hydrodynamic conditions and high food abundance near a muddy tidal creek, exposed to slightly higher energy close to creeks, or far from tidal creeks. However, they are absent in other places with similar organic matter content and/or distance from tidal creeks, irrelevant to salinity, turbidity, and substrate consistency. The distribution and morphology of the burrows cannot be correlated with simple environmental parameters. Possibly, the crabs created their own environment the best suitable for their life. The burrows are unlined and show I-, J-, L-, U- and Y-shape resembling the small version of the trace fossil Psilonichnus, the Y-shaped Polykladichus, the U-shaped Arenicolites, whereas the complex, shallow M. japonicus burrows are similar to Pholeus bifurcatus, and the multi-branched, mostly inclined galleries superficially to Thalassinoides paradoxicus. These analogical trace fossils, although in the sediment record ascribed to occur in different ichnofacies and to have been probably produced by various animals, may occur together in the studied deposits wherein they have been produced by the same animal in the same habitat.