Sediment samples were collected by SCUBA diving using a box core (= sampling unit) with an effective sampling area of 100 cm2. The box core was pushed 10 cm into the bottom collecting approximately 1000 cm3 of sediments. Generally, three sample units were collected per site. Sediments were sieved through a 500-μm sieve and preserved in 70% ethanol. All mollusk shells were sorted and counted, in the case of the bivalves, each disarticulated valve was counted as an individual. This dataset includes the complete shells and the large skeletal fragments retaining the apex or more than half of the hinge line.