Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: 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 that line the river from its source to south of Jacksonville begin to transition into salt marshes east of the city. Mayport is home to approximately 20 shrimping vessels that use the mouth of the St. Johns to access the Atlantic Ocean.
What are the type of mollusk that is left in mounds many feet high?