Use of seagrass Zostera novazelandica (Setchell, 1933) as habitat and food by the crab Macrophthalmus hirtipes (Heller, 1862) (Brachyura: Ocypodidae) on rocky intertidal platforms in southern New Zealand

Abstract

Along rocky reefs at Kaikoura, New Zealand, the crab Macrophthalmus hirtipes (Heller, 1862) constructs its burrows exclusively in patches of the intertidal seagrass Zostera novazelandica (Setchell, 1933). The occupation of seagrass patches is widespread, with 63% of patches having crab burrows. The number of externally visible burrows is highly correlated with the number of crabs inhabiting seagrass patches. Plaster casts of burrows revealed that burrows situated at the edge of seagrass patches differed from those away from the edge in having on average more entrances and passageways and, therefore, greater total burrow length. There was an average burrow density of 9.4 (± 1.04 S.E.) per m 2 of seagrass, but burrows were several times as numerous on the edges of patches, especially those that bordered tide pools. The diet of M. hirtipes was found to consist mostly of living seagrass (50.4 ± 3.40% of foregut contents) and sediment (39.2 ± 3.11% of foregut contents), with small amounts of isopods, amphipods and conspecifics also consumed. There were no intersexual or site-specific differences in diet. Ontogenetic differences in diet were found, with smaller crabs consuming more sediment and larger crabs consuming more seagrass. Crabs contributed significantly to erosion of seagrass patches. There was an average of 22.5% (± 5.27) loss of patch area where crabs were present compared to 1.8% (± 0.37) where crabs were absent from patches. Laboratory tests showed that burrow construction took around 250% longer in seagrass patches that had denser blade coverage. In the field, burrows persisted for up to 3 months, even if vacated by crabs. Their abundance at patch edges, combined with the loss of sediment and reduced binding of seagrass roots, accelerated erosion of seagrass patches.