The three dreissenid species present in North America may be difficult to distinguish at first glance. Their life cycles, life history, and general morphology are similar. If one looks at three specific areas, however, it is possible to tell adult zebra mussels from adult quagga mussels and both from adult false dark mussels, Mytilopsis leucophaeata.
These three areas span the range from a broad ecological characteristic (habitat), to an external morphological character (ventral shell edge and margin), and finally to an internal morphological character (apophysis on myophore plate). If all three are considered, it is relatively easy to distinguish among these species.
Habitat
Both zebra and quagga mussels are found in areas of relatively low salinity (see the Ecology section for more detail). In general, zebra and quagga mussels cannot tolerate salinity levels above 3 ppt. Mytilopsis lives predominantly in brackish water areas where the salinity ranges from 3 to 15 ppt (Claudi and Mackie 1994). Thus if the mussel in question has come from a brackish area and has the physical features described below, it is most likely Mytilopsis.
External Morphology
External Shell Characteristics of the Three Dreissenid Species.
If one examines the ventral shell margin and ventral shell edge of the mussels, differences are visible. Zebra mussels have a concave or flattened ventral shell edge, and "keels" are formed by their acutely angled shell margins. Both features provide additional stability for the attached mussel. If one places representatives from all three species on a flat dish, only the zebra mussel will be able to stay upright. Both quagga mussels and Mytilopsis have a convex ventral edge and a rounded ventral margin.
Modified from Claudi and Mackie (1994) with permission.
Internal Morphology
All three dreissenid mussels have a shelf or myophore plate immediately posterior to the umbone on the internal portion of the shell. Only Mytilopsis has an apophysis or projection on the lateral margin of its myophore plate. This protrusion, used for muscle attachment, is absent in both zebra and quagga mussels.
Veliger Identification
Thus far, the information presented has been concerned with adult bivalves. Positive identification of larval or veliger forms is difficult, especially among closely related species such as the zebra and quagga mussels. Methods of collecting veligers will be discussed later in the section on detection and monitoring. For now, assume that a quantity of water has been collected from a freshwater source containing live bivalve veligers. Several types of bivalves can be eliminated as possibilities immediately.
Thus there are only three remaining possibilities: Corbicula fluminea (Asian Clam), Dreissena polymorpha (zebra mussel), or Dreissena bugensis (guagga mussel) -- all of which have free-swimming veliger larvae. Distinguishing between veligers from the two species of Dreissena is almost impossible until the ventral shell edge and margin develop. Differentiating between Corbicula and Dreissena veligers is possible. It is somewhat simplified if both animals have developed to the straight hinge form and it is easier if umbonal forms are involved.
The key feature for identifying Corbicula larvae is the slightly flattened hinge during the early development stages (Marsden 1992). Corbicula larvae may also have noticeable striations on the shell valves. Dreissena larvae have slightly rounded hinges and smooth shell valves.
As indicated previously, the identification of free-swimming bivalve larvae is extremely difficult. The key feature for identifying Corbicula sp. larvae is the flattened hinge during the early development stages (Marsden 1992). Corbicula larvae may also have noticeable striations on the shell valves. The straight-hinged larvae of Dreissena lack those striations.
The section on detection and monitoring provides a more in-depth discussion of larval collection and detection. Given the brief overview provided by this section, it should be possible to identify adult bivalves at least to the superfamily stage. Using this information makes it possible to develop appropriate risk assessment, monitoring, and control programs for a particular area.