Experimental evidence for the effects of the thalassinidean sandprawn Callianassa kraussi on macrobenthic communities

Bioturbation by thalassinidean sandprawns is known to structure soft-bottom communities, and field observations have suggested that the sandprawn Callianassa kraussi is a significant force influencing macrofaunal communities. To investigate causal relationships, a field experiment was undertaken in Durban Bay, South Africa, in which experimental cages were used to exclude or include C. kraussi and the abundance of macrofauna in these treatments documented. Cage effects were assessed by comparing macrofauna in inclusion cages with that of unmanipulated areas containing high densities of C. kraussi equivalent to those in inclusion cages. Measurements were made in 3 months, in March, June and September 2005. Total abundance and species richness of macrofauna were significantly greater in exclusion cages than in inclusion treatments during all sampling seasons, while diversity differed between these treatments in June and September only. Ordinations indicated that macrofaunal assemblages in exclusion cages differed statistically from inclusion and control treatments in all three sampling seasons. In general, the surface-grazing gastropod Nassarius kraussianus and suspension and deposit-feeding species such as the polychaetes Prionospio sexoculata and Desdemona ornata, cumaceans, and the bivalves Dosinia hepatica and Eumarcia paupercula were significantly more abundant in prawn-exclusion plots, implying that they are negatively affected by bioturbation by C. kraussi, whereas burrowing infauna were not affected.