Mass spawning of crabs: ecological implications in subtropical Australia

We review links between autotrophic production, crab herbivory and estuarine food chains in southeastern Australia, a region where mangrove and saltmarsh co-exist towards mangrove poleward limits. Studies in estuaries and embayments across the region have demonstrated a consistent and important role of crab larvae released in synchronised spawning during the spring tide in supplementing the diet of zooplanktivorous fishes. These numerically dominant fishes have been linked through isotopic studies to commercially important predator species. The role of macrophytic vegetation (mangrove and saltmarsh) in the diet of crabs remains controversial, though isotopic analysis has demonstrated the contribution of the C4 grass Sporobolus virginicus to crabs feeding in saltmarsh. Other dietary sources include microphytic algae, and potentially stranded zooplankton. The link between sediment organic matter, crab and zooplanktivore isotopic composition remained tightly coupled in spite of variation in anthropogenic N between estuaries and the isotopic signature of mangroves. Long-term threats to these trophic linkages include coastal squeeze associated with sea-level rise, and the fragmentation and disconnection of habitats.