The Pectinid Bivalve Pedum spondy­loideum (Gmelin 1791): Amount of Surface and Volume Occupied in Host Corals From the Red Sea

Abstract

Abstract. In the northern Red Sea, the nestling and facultatively boring pectinid bivalve Pedum spondyloideum lives embedded in various scleractinian host corals of seven families, including the previously unrecorded hosts Astreopora, Leptastrea and Hydnophora. Pedum density varied according to host species, locality and depth. The bivalve‐occupied coral surface (OCS) was measured on samples and from close‐up photos taken in the field. Based on the 9 × 6 cm photo frames, Pedum density ranged from 1.9 – 18.6 · (100 cm2)– 1. In 14 Montipora, apparently the favourite host genus, mean density was 6.5 · (100 cm2)– 1 and mean OCS 3.4 %, but maximum OCS amounted to 12.5 %. In 19 × 13 cm frames, Pedum density ranged from 0.4 – 10.7 · (100 cm2)– 1. The maximum density occur­red again in Montipora, followed by Porites [6.7 · (100 cm2)– 1] and Cyphastrea [6.0 · (100 cm2)– 1]. The OCS ranged from 0.2 – 7.0 %. The latter was found in Goniastrea, and 6.6 % OCS in Montipora. The mean density of Pedum in 13 Montipora was 3.6 · (100 cm2)– 1 and mean OCS 2.2 %.Heavy infestation does not appear to impact the corals, except in growth form. Shell size and dwelling volume are highly correlated. From the opening length on the coral surface, the expected volume can be calculated using the regression y = 0.2127 x2.7447. A high Pedum density indicates a southerly reef site or near‐shore locality with ample suspended nutrients in the water. Successions of Pedum generations in the same host demonstrate that corals usually outlive their inhabitants by many years. The embedded traces are less distinct than in the coral‐associated mytilid Lithophaga. Compared with the latter, Pedum dwellings have a less regular shape and a much wider opening. Coral overgrowth partly fills and camouflages dwellings. Although no fossil record is known, Pedum traces have a fossilisation potential.