An accidental island
On the remote Indonesian island of Buru, a population of largetooth sawfish (Pristis pristis) lives in complete isolation. Unlike most of their relatives who migrate between freshwater rivers and coastal marine habitats, these sawfish are confined to the island's freshwater lakes. This population, unknown to science until 1991, represents a unique evolutionary snapshot. Geological changes, likely a fall in sea level, are thought to have trapped a marine population inland, cutting them off from the ocean and forcing a permanent adaptation to freshwater life. This isolation has made them genetically distinct from their marine counterparts, a separate branch on the family tree.
The sawfish itself is an extraordinary animal. Despite its shark-like appearance, it is a type of ray. Its most famous feature, the long, flat rostrum lined with teeth, can be up to a third of its total body length. This "saw" is a weapon and a sophisticated sensory tool. It is covered with thousands of pores called ampullae of Lorenzini, which function as electroreceptors. These organs allow the sawfish to detect the faint electrical fields produced by prey like fish and invertebrates hiding in murky water or buried in sediment. Once located, the rostrum is used to stun or impale prey with powerful side-to-side swipes.