Is the occurrence of a sigmoidal ventral border in Cyprideis torosa (Jones) valves linked to salinity? A morphometrical analysis approach

Abstract. The living euryhaline species Cyprideis torosa (Jones) undergoes morphometric variations in size, noding and sieve-pore shape linked to environmental parameters. In particular it is known that salinity values around 8–9‰ represent the osmoregulation threshold and the turning point between smaller and greater valves and prevailingly noded as opposed to un-noded valves. Here, a character never studied before is analysed: the relationship between salinity and the C. torosa valve outlines, especially the morphology of the ventral border. Geometric–morphometric analysis was carried out on different populations of C. torosa collected in oligohaline (Massaciuccoli marsh, Tuscany, and Lake Trasimeno, Umbria) and hyperhaline waterbodies (Trapani–Paceco salt pans, Sicily). The results indicate that the species displays two morphotypes: a dominant one, characterized by a straight ventral border; and a morphotype with a sigmoidal profile ventral border. This latter morphotype seems to be related to higher salinity: the oligohaline waters are characterized only by C. torosa individuals with straight ventral outline, while the hyperhaline environments are marked by the occurrence of 17.6% of sigmoidal individuals. Furthermore, the sigmoidal ventral border is more emphasized in males than in females.