Probable existencia de dos ecofenotipos de Megapitaria squalida (Sowerby, 1835) en Bahía Concepción Baja California Sur, México

Abstract

From April 1988 to August 1989, bivalve mollusc samples were taken from five localities in Concepcion Bay, Gulf of California. These samples included the following species: Argopecten circularis (Sowerby, 1835), Megapitaria squalida, (Sowerby, 1835), Laevicardium elatum (Sowerby, 1833), Dosinia ponderosa (Gray, 1838) and Glicimeris gigantea (Reeve, 1843); growth rates and gonad ripeness of these species are being investigated. After examining shells of M. squalida from the Santispak area, two different shell forms were discovered. The first type, here after called variety A, is characterised by a thicker and darker shell, the hinge width is also greater, the interior side of the hinge is bent, and the posterior adductor muscle scar is deeply sculptured. Variety A was collected from all five of the studied areas in Concepcion Bay, and also in Bay of La Paz. The second type of shell, variety B, is whiter and lighter, with a thinner hinge, its interior side slightly bent, and its teeth smaller than in variety A; the posterior adductor muscle scar is not sculptured.To quantify the differences between these two varieties with regards to shell size, the longitudinal, dorsoventral and transversal axes were measured along with shell weight. The aboye measures were used to calculate the following Index: I = 1000*W/ (L*11*-1), where, W = shell weight, L= length, H= height, T= thickness. When comparing the shell weight to the calculated indices, two groups are distinguishable, although they overlap slightly at weights below 40 g. Frequency curves for Index values of the two clam varieties are depicted and differences between means were highly significant (99 Confidence Level).