Habitat dependence in the functional traits of Austrohelice crassa, a key bioturbating species

Bioturbating macrofauna can have major effects on their physical, biological and bio-geochemical surroundings, altering ecosystem functioning. Burrow builders increase the amount of sediment water interface over which oxidative exchange occurs, whilst organisms that 'bulldoze' through sediments homogenise and oxygenate the surface layers. Here we demonstrate that the bioturbating crab Austrohelice crassa performs the role of burrow builder in muddy and bulldozer in sandy sediments. Sediment reworking rates and expansion of the sediment water interface through A. crassa bioturbation were estimated from 4 parameters: burrow and crab density, burrow morphology, burrow permanency, and burrow maintenance, measured across a sedimentary gradient. Burrow decay rates (k, d(-1)), in combination with differences in burrow and crab densities, were responsible for changes in sediment reworking rates among habitats. Mean burrow permanency (1/k) varied from 25.7 d in mud to 10.8 d in muddy sand to 1.4 d in sand. In mud, where adult burrow density was greatest (207 m(-2)) and structures were stable, A. crassa burrows extended the sediment water interface by 147%, compared with 50% in muddy sand and 22% in short-lived sand burrows. In sand, 82% of the sediment available to A. crassa was mixed per lunar month (rising to 130% when including burrow maintenance values), whereas in mud and muddy sand, mean rates of sediment reworking dropped to 7.4%. These differences in the key bioturbational role of A. crassa between sediment types indicate that the impact of this species on ecosystem functioning will be highly habitat-dependent.