Does mollusc shell debris determine patterns of macrofaunal recolonisation on a tidal flat? Experimental evidence from reciprocal transplantations

• Effect of shell debris on macrofauna was assessed using reciprocal transplantations.
• Structural complexity of the shell matrix had no effect on assemblage structure.
• Assemblages in transplanted cores became similar to controls in destination area.
• Transplanted cores differed from controls at the area where they originated.
• Species-specific responses determine differences between shelled and bare habitats.

Mollusc shells introduce complexity and heterogeneity into soft-sediment habitats and might regulate the structure and functioning of benthic assemblages. Two areas of an unvegetated tidal flat located at the mouth of Paranaguá Bay (southern Brazil) largely differ in substrata characteristics and macrofaunal assemblage structure. Sediments of the inner part of the studied site are structurally more complex than those of the peripheral portion, due to high densities of empty shells of the bivalve Anomalocardia brasiliana. The effects of this biogenic matrix on macrofaunal recolonisation during a three-month period were assessed through a reciprocal transplantation experiment of defaunated sediment between shelled and bare habitats. Hypotheses were tested to distinguish between three possible causes of variation: (i) habitat complexity, (ii) intrinsic factors of each area (regardless of the presence/absence of shells), and (iii) their interaction. Habitat structural heterogeneity provided by empty shells does not explain the large differences between macrofauna in shelled and bare habitats. Assemblages that colonised transplanted sediments became similar to those in controls at the area to which they were moved, but differed from those in controls at the area where they originated. The lack of a consistent response of assemblages to the presence of shells can be explained not only by the extremely fast recovery following small-scale experimental disturbances, but also by the dispersal mechanisms related to reproductive traits of the dominant taxa. Changes in assemblages were greatly influenced by the tanaid Monokalliapseudes schubarti, the numerically-dominant species that showed consistent differences among treatments. The reduced dispersal capacity due to the lack of planktonic larval stages associated with high fecundity, continuous reproduction and fast growth were determinant features for the rapid colonisation of defaunated sediments, regardless of habitat complexity. Despite the well-recognised impact of mollusc shell production on soft-sediment habitats, our study showed that its effects are scale-dependent, both spatially and temporally, and might be regulated by the structure of local assemblages and species-specific responses related to dispersal mechanisms.