Variation in the depth and morphology of burrows of the mud crab Helice crassa among different types of intertidal sediment in New Zealand

Crabs are among the larger and more active burrowers in intertidal sediments in New Zealand, as in many other parts of the world. Abundances of mud crabs Helice crassa and their burrows were compared among sites differing in the nature of their sediments. Differences in burrow architecture among sites were quantified using casts of burrows made in situ. The effects of bioturbation on sediment geochemistry were also determined with respect to redox potential and the concentrations of 2 chemical phases, acid volatile sulphides (AVS) and iron pyrites (FeS 2 ), that influence the bioavailability of heavy metals. The objective of this study was to identify effects of differences in the extent and nature of burrowing activity of the crab H. crassa among different sediment types on sediment geochemistry, particularly with respect to the bioavailability of heavy metals. Burrows were more abundant at muddy sites (average 22 to 59 burrows 20 cm diameter core-') than at sandy sites (average 12 burrows core -1 ). Although not necessarily deeper at muddy sites, burrows were more complex in structure. Average volumes of casts were larger at muddy than sandy sites by a factor of up to 14.8 and at the most densely-burrowed site, burrows represented 14 % of the volume of the surrounding sediment. The architecture of the burrows of H. crassa is discussed in the context of published models of burrow structure and function for other crustacea. Patterns of differences in chemical variables among sites were not clear cut. Redox profiles generally showed decreases with depth in the sediment and, among the muddy sites, potentials were highest and most variable at the site with greatest burrowing activity. Concentrations of AVS and FeS 2 were lower in sandy sediments than in muddy ones. Among muddy, sites, the concentration of AVS was lowest at the site with the greatest amount of burrowing, consistent with introduction of oxygen to the sediment and the consequent oxidation of AVS. Concentrations of FeS 2 showed a pattern that suggested that burrowing introduces FeS 2 from deeper layers, where concentrations are higher, into shallower, bioturbated layers of the sediment, offsetting effects of oxidation due to burrowing activity. Concentrations of FeS 2 were highest at the site with most burrowing activity.