The method used by researchers in Stony Brook, New York and Menai Bridge, Wales to differentiate the 3 west-coast bay-mussel species, Mytilus galloprovincialis, M. trossulus, and M. edulis involves morphometric differences in shell shape and feature.  First, the authors use allozyme characters to choose geographical locations that contain only a single species.  They then employ a multivariate analysis¹ of 18 morphometric features to find the weighting of characters that maximise distances among the species. The 18 measurements used are shown in the figure above. After application of a canonical variates analysis, the 3 species separate visually as shown in the depiction on the Right.  Fewer morphometric characters² can be used in the analysis, but the ones omitted must be those that contribute least to the canonical function.  Conversely, if too few characters are used, then the canonical variates may overlap so much that differentiation of the species is impossible.  The study shows that no “silver bullet” morphometric measurement is available to separate the 3 species.  The authors acknowledge, however, that inclusion of other characters³ suggested in past studies might yield even better discrimination among the species.  McDonald et al. 1991 Mar Biol 111: 323.

NOTE¹  known as canonical variates analysis

NOTE²  the authors remark that even the 2 characters that show the greatest differences among species, namely, length of anterior adductor scar and length of hinge plate (numbers 9 and 4 in the diagram) lead to considerable overlap when used singly

NOTE³  such as mantle colour, angle of ligament, and position of point of maximum shell width

Sea mussels Mytilus californianus can be easily distinguished by eye from the bay mussels M. trossulus and M. galloprovincialis when they are adults, but some help is needed when they are juveniles, say, at about 1.5mm in shell length.  Here are 5 chief features of difference, all using M. californianus as the reference:

1. dorsal apex is more posterior
2. posterior adductor scar is larger
3. posterior adductor scar is located closer to the dorsal apex line
4. hinge-teeth demarcations are more prominent
5. anterior-dorsal angle is shallower  Martel et al. 1999 Invert Biol 118: 149.

Mytilus californianus above ( 0.2X) and
M. trossulus below (1.2X). Latter photo
courtesy Dave Cowles, Walla Walla University,
Washington rosario.wallawalla.edu

Research Study 1 above enables juvenile mussels of Mytilus californianus and M. trossullus/galloprovincialis to be differentiated.  A follow-up study by these same researchers addresses the need for an easy and quick method to differentiate these species at the end of the larval life in the plankton and/or at the time of settlement at 250-700µm shell length.  The authors determine that in comparison with bay mussels Mytilus trossulus, sea mussels M. californianus have:

1. a shallower and flatter umbo (i.e., lower prodissoconch I curvature; see photos on Left)
2. a greater prodissoconch I length = length between the ends of the curved lines in the drawings on the upper Right
3. a wider separation between the provincular lateral teeth. These are demarcated by reddish-colored spots on either side of the umbo = distance between arrows (see lower set of drawings on Right)

The researchers confirm the validity of the morphological data by comparing PCR products of genomic DNA of provisionally identified postlarvae.  The authors add that the methods represent an economical and effective alternative to more costly and time-consuming DNA analysis, and they work equally well for differentiating postlarvae of southern Californian M. galloprovincialis from those of M. californianus.  Martel et al. 2000 Mar Biol 137: 811.

NOTE  this is the embryonic or larval shell, as opposed to the adult shell, or dissoconch