The coastline of South Australia once had 1500 km of oyster reefs along its coastline. Heidi Alleway (Tall Poppy Award Winner) did a very clever piece of work at the University of Adelaide, piecing together historical evidence that showed that these oyster reefs existed and were completely fished out and removed between the late 1800s and early 1900s (Alleway and Connell, 2015).
These shellfish reefs likely played an important role in removing nutrients and sedimentation from coastal waters, provided habitat for marine life. By keeping the waters clean, this would have created the right conditions for seagrass habitat which again would have provided important habitat for marine life. Australia has a very poor record in this regard, with most of our oyster reefs functionally extinct.