Arabian muds: A 21st-century natural history on crab plovers, crabs and molluscs

The Sultanate of Oman harbours an incredibly rich intertidal mudflat ecosystem: Barr Al Hikman. The place is different. A number of shorebirds, crabs and molluscs seems to have evolved exceptionally well-developed armature. In this thesis I aim to reveal part of an important evolutionary story. Most shorebirds in the area feed on crabs, that hide in burrows when they sense danger. The exception is the crab plover. This bird is specialized in catching mobile swimming crabs; crabs with powerful claws that ferociously defend themselves when attacked. Crab plovers owe the unique talent of catching crabs to their evenly uniquely shaped, dagger-like bill. Contrary to almost all other intertidal mudflat ecosystems in the world we observed hardly any molluscivorous shorebirds at Barr Al Hikman. At first we could not explain this observation as molluscs are abundant in the area. Yet detailed measurements showed that molluscs have particularly well-developed armature, notably thick shells. This armature restricts predation opportunities for birds. Swimming crabs however, with their powerful claws, appeared more adept at preying upon molluscs. We presume that the thick-shelled molluscs, the powerful claws of the swimming crabs and the dagger-like bill of the crab plover have evolved in so-called evolutionary arms races. Our results are in line with earlier work on coastal areas, showing that evolutionary arms races can escalate within the Indo-West Pacific region. Previous observations on the intertidal mudflat ecosystems in this region did not reveal the extent to which local species’ armature had developed. Our research shows that complex regional processes can affect the morphology and distribution of species. In the light of the conservation of species and global chance it is important to know these processes