Comparison of associations between the pectinid bivalve Pedum spondyloideum (Bivalvia, Pteriomorphia, Pectinidae) and scleractinian hermatypic corals in the Red Sea and the West Pacific Ocean

Abstract

The pectinid bivalve Pedum spondyloideum (Gmelin, 1791) is an obligate associate of living scleractinian corals that occurs in the Indo-Pacific. It is byssally attached and it lives embedded in the coral skeleton; it is usually completely surrounded by live tissue on the coral surface, but not inside the dwelling. Associations between Pedum and host corals are relatively well known from the Red Sea but very little is known about these associations in the "Coral Triangle" harbouring the most biologically diverse coral reefs in the world. In consequence, the main goals of this field study were to enhance the understanding of the nature of associations between Pedum and its scleractinian host corals in different areas of the "Coral Triangle" in Indonesia located in the West Pacific (i.e. where one can expect to find the most varied associations) and to compare associations between the Red Sea and the West Pacific Ocean. The different areas selected during this study were the Derawan area, located on the northeast coast of Borneo and those of Komodo and Alor, located in the Sonde islands. Observations were carried out in the field by SCUBA-diving. Most of the sites were fringing reefs, with a developed reef crest and a fairly steep reef slope after which a flat gentle slope of sandy habitat dominates. Dives consisted of a slow ascent along the reef in a zigzag path to the shallowest points. Corals were identified in the field or in the laboratory after collection of samples. During this study 10 new associations between Pedum and its scleractinian host corals (Montipora aequituberculata, M. crassituberculta. M. efflorescens. M. hispida, M. monasteriata, M. spongodes, M. turgescens, M. undata, Pontes annae and turbinaria stellulata) have been identified, which increases by more than 20% the number of coral species known to be infested. Field studies in the Red Sea and the "coral triangle" in Indonesia have shown that Pedum can be associated with about fifty coral species. Pedum is associated more frequently and in higher densities with encrusting to semimassive and even branching Montipora species in the Red Sea, whereas it is mainly associated with massive Porites species in Indonesia (between 65 and 85%, depending on location). Moreover, although some coral species are common to the different studied sites, Pedum do not infest the same species, indicating that the asssociations differ regionally. The majority of the infestations take place at low depth (0.5 to 20 m) in the zone corresponding to the higher coral density. As yet, it is unclear what type of association exists between Pedum spondyloideum and its host coral. It is certain, however, that the distribution of P. spondyloideum on different coral species is not random and that there is a distinct habitat preference for certain corals over others.