| Summary: | A-D. Stereoscopic micrographs of the ligament area of mussel valves. Note that scale bars differ between A-C and D. Strip of the prismatic layer under the ligament nympha is indicated by arrows. A, B. E-morphotypes: the space under the ligament nympha is totally (A) or partially (B) covered by the nacre. C, D. T-morphotypes: a strip of uncovered prismatic layer under the ligament nympha is dark and wide (C; typical of most examined populations) or pale and narrow, recognizable by a scar separating it from the nacreous layer (D; typical of the Gulf of Maine populations). E. External and internal features of the shell valves of M . trossulus (right) and M . edulis (left) genotypes from the Kola Bay (from sample Sev.17 in S1 Table ). In most cases T-morphotypes (marked by *) and E-morphotypes could be distinguished by an unaided eye. (TIFF)
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