Identification of pen shells (Bivalvia: Ostreida: Pinnidae) collected off northern Iloilo, Philippines using their morphological characters

This study describes morphological characteristics of pen shells collected along the coast of northern Iloilo, Philippines. A total of 380 pen shell individuals with intact shells and their adductor muscles were carefully examined and measured. Eleven species were studied, namely: Atrina pectinata (Linnaeus, 1767), Atrina inflata (Dillwyn, 1817), Atrina vexillum (Born, 1778), Atrina seminuda (Lamarck, 1819), Atrina rigida ([Lightfoot], 1786), Pinna bicolor Gmelin, 1791, Pinna atropurpurea Sowerby, 1825, Pinna deltodes Menke, 1843, Pinna muricata Linnaeus, 1758, Pinna incurva Gmelin, 1791, and Pinna nobilis Linnaeus, 1758. The species were identified based on the nine characteristics of the valve for the genus Pinna and eight for the genus Atrina. These characteristics were then correlated with their adductor muscles’ morphology. Analysis of variance revealed that the three most dominant species investigated under genus Atrina, namely Atrina pectinata, Atrina inflata, and Atrina vexillum, were found to be significantly different in the eight characters of the valve (P < 0.05). Moreover, three representative Pinna species, Pinna bicolor, Pinna atropurpurea, and Pinna deltodes, were significantly (P < 0.05) different based on shell width, dorsal posterior shell margin, sulcus width, and dorsal posterior margin to dorsal nacreous margin. The observed high correlation (P < 0.05) between adductor muscle properties and different shell length characteristics for five dominant species can be used as a predictor of growth and suggests that the increase in the size of adductor muscle correlates to the increase in the size of the mentioned shell characteristics. No previous study of this kind was conducted in the Philippines. This work provides relevant information for related biological research on other pen shell species and for the management of pen shell resources in northern Iloilo and, possibly, other regions in the world.