See Shell Orientation and Measurements for diagrams.
It is not known whether such hybridization occurs in the field. However, hybrids have been produced under laboratory conditions. Large overlap in a certain percentage of individuals in terms of size and shape in field-collected individuals suggests that hybridization may be possible in nature. Again, the separation of the two species of dreissenid veligers is difficult, at best, and only tentative conclusions can be reached.
Zebra Mussel Identification