It’s called a gamma larvae.

A mature river clam can excrete about 3 million glochidia larvae. However, the larvae are relatively small and basically have no ability to survive alone. However, the hooks on the side edges of the larvae's shells are very sharp. If you want to survive, you must rely on parasitism! When the glochidia larvae mature, they are expelled from the body through the drainage holes of the clam and fall to the bottom of the water or suspend in the current.

Gamma larvae are the larvae of bivalve molluscs (such as river mussels). Because the fertilized eggs are adhered to the mucus secreted by the mother's gill valves, they will not be discharged out of the body with the water flow, and will gradually develop into small larvae in the body.