What struck Scott Bennett most were the razor clams.

The long saltwater clams, resembling old-fashioned razors, normally burrow into sand to avoid predators. But when Bennett, an ecologist, visited South Australia’s Great Southern Reef last month, he saw thousands of them rotting on the sea floor.

“100% of them were dead and wasting away on the bottom,” Bennett told CNN.

Since March, a harmful algal bloom, fueled by a marine heat wave, has been choking South Australia’s coastline, turning once-colorful ecosystems filled with thriving marine life into underwater graveyards.

The bloom has killed about 15,000 animals from over 450 species, according to observations on the citizen science site iNaturalist. They include longfinned worm eels, surf crabs, warty prowfish, leafy seadragons, hairy mussels and common bottlenose dolphins.