The Sidoarjo mud is the first visible phenomenon of a mud volcano that occurs in a human settlement and which is
subsequently channelled into a river. Clay, aluminium and iron were reported to be the dominant contaminants that could
possibly come into contact with and accumulate on the surface of local fish and initiate alteration in scale microstructure.
The aim of this study was to evaluate the extent of water body contamination in the Sidoarjo mud by evaluating the
chromatophore density and microstructure deformation of fish scales that act as biomarkers. Scale samples were
obtained from caged Mozambique tilapia (Oreochromis mossambicus) fish that were placed downstream and upstream
of the Sidoarjo mud spillway pipes. With respect to melanophore density, it was found that the scales of fish exposed in
the downstream section were significantly lower in chromatophores (<50 chr/mm²) than the control scales in fish from
the upstream station (>100 chr/mm²). This study suggested that the density of chromatophores was closely related to
the concentration of total suspended solids (r = 0.69), which was possibly enhanced by iron (r = 0.56). Using scanning
electron microscopy analysis, some deformation, i.e. irregularity of spherule shape and increasing pits in the space
between ridges, were observed.