Before shell formation, the appearance and development of the embryo are almost identical for Asian clams and the two species and hybrids of dreissenids. The identification of preshell larvae can be quite troublesome; however, Asian clams can be readily separated from dreissenids by knowing the location of the preshell larvae at the time of collection.
Embryological development in Asian clams occurs inside the female in the brood pouch (i.e., gill chamber). This differs significantly for dreissenids because the embryos and preshell larvae can readily be collected only in the water column.
Embryological development in Asian clams occurs inside the female in the brood pouch (i.e., gill chamber).
Whereas larvae of dreissenidae are completely pelagic (i.e., free-floating in the water column), other freshwater bivalves (e.g., Corbiculidae, Unionidae, Sphaeriidae) brood their larvae through early developmental stages in marsupial sacs on the gill. Shelled juveniles (arrows) of a fingernail clam, Musculium sp. (Sphaeriidae) can be seen in this scanning electron micrograph.
This differs significantly for dreissenids because the embryos and preshell larvae can readily be collected in the water column and not internally within the female.
Separation of preshell dreissenids (quagga, zebra, and their hybrids) is nearly impossible utilizing standard light microscopy characters. The size of the sperm in the two dreissenid species and their hybrids differs. The sperm size in the quagga mussel is slightly larger, about 9-12 µm, while in the zebra mussel the sperm size ranges from only 7 to 9 µm. This is probably not a very reliable character. Shape is also different, but this can only be seen with high power magnification.