Please answer the following question: Given the following context:  Using an unofficial nickname of "The River City", Jacksonville has a culture centered on the St. Johns. An annual footrace named the Gate River Run accepts 18,000 participants who travel a course along and over the river twice. The largest kingfishing tournament in the U.S. is held on a St. Johns tributary, where sport fishers concentrate on king mackerel (Scomberomorus cavalla), cobia (Rachycentron canadum), dolphin (Coryphaena hippurus) and Wahoo (Acanthocybium solandri). The home stadium for the Jacksonville Jaguars faces the river, as does most of the commercial center of downtown. Seven bridges span the St. Johns at Jacksonville; all of them allow tall ships to pass, although some restrict passing times when train or automobile traffic is heavy.Tides cause seawater to enter the mouth of the St. Johns River and can affect the river's level into the middle basin. As a result, much of the river in Jacksonville is part seawater, making it an estuarine ecosystem. The animals and plants in these systems can tolerate both fresh and salt water, and the fluctuations in saline content and temperatures associated with tidal surges and heavy rainfall discharge. Marine animals such as dolphins and sharks can be spotted at times in the St. Johns at Jacksonville as can manatees. Fish such as mullet (Mullidae), flounder (Paralichthys lethostigma), shad (Alosa sapidissima), and blue crabs (Callinectes sapidus) migrate from the ocean to freshwater springs upriver to spawn.Although freshwater invertebrates inhabiting and comprising algae and periphyton make the foundation of food webs in the middle and lower basin, zooplankton and phytoplankton take that role in the estuarine habitat. Mollusks gather at the St. Johns estuary in large numbers, feeding on the bottom of the river and ocean floors. The abundance and importance of oysters (Crassostrea virginica) is apparent in the many middens left by the Timucua in mounds many feet high. Oysters and other mollusks serve as the primary food source of shorebirds. The large trees...  answer the following question:  What are the type of mollusk that is left in mounds many feet high?
Answer:
oysters