Approximately 14 mm in length.

The salinity where I found it usually ranges between 18 and 23, but sometimes the salinity varies a lot beyond that range in both directions. The Harlem River is an estuarine environment, part of the Hudson River Estuary.

This intact shell in perfect condition was nestled inside the almost-closed valves of an empty, half-grown shell of Crassostrea virginica (image 6). The oyster shell had both valves still attached, but one valve of the oyster shell was damaged, so that end was slightly more open. The oyster shell had Bay Barnacles and Conopeum bryozoan growing on the outside and inside of both valves.

There is a cluster of byssus threads on the inside of the oyster -- possibly from this Quagga Mussel, or maybe from another different mussel species.

The Quagga Mussel is not supposed to be able to tolerate salinity, at least in the US, but this looks as if the Quagga mussel grew to this size within the partially broken valves of a dead oyster, and within the estuary of the Hudson River.

The fact that it was inside an oyster shell seems to suggest that the Quagga Mussel managed to live and reach this size in a semi-saline environment.