Parity and disparity between two Chama oysters: the reproductive biology of the IndoPacific C.pacifica Broderip, invasive to the Mediterranean Sea; and C.savignyi Lamy, indigenous to the Red Sea

Members of the oyster family Chamidae are common in Indo-Pacific subtidal habitats, adhering to the sides of rocks, where they are usually well hidden by a fine growth of algae that extends equally over the rocks and the oyster shells (Purchon 1968). Chamidae species are known for their efficient filter-feeding abilities and thus may compete with other benthic filter-feeders for both space and food (Raj 2008). In Eilat (Northern Gulf of Aqaba, Red Sea), Chama savignyi Lamy, 1921, is quite abundant in reefal habitats, growing among coral colonies (Fig. 1A), from shallow subtidal rocky formations down to 20 m. Chama savignyi outnumbers all other oyster species there. However, its congener, Chama pacifica Broderip, 1835, is seldom found in Eilat (see Results). The opening of the Suez Canal in 1869 connected the Eastern Mediterranean Sea and the Gulf of Suez (Northern Red Sea), linking two biogeographic provinces, the Atlanto-Mediterranean and the Indo-Pacific (Spanier & Galil 1991). This triggered an invasion of Indo-Pacific (Erythrean) biota into the Mediterranean Sea (termed the