Actuopaleontological characterization and molluscan biodiversity of a protected tidal flat and shallow subtidal at the northern Red Sea

Abstract

Tidal flats are abundantly developed throughout the Phanerozoic but actualistic studies on subtropical‒tropical tidal flats are rare. This study of a tidal flat and shallow subtidal of a lagoon protected behind a fringing reef at the northern Red Sea is therefore aimed to contribute to the understanding of the distribution and abundance of paleontologically relevant fauna and its lebensspuren. Muddy sands to sands and beachrock are the predominating benthic habitats. Important ecosystem engineers are the oyster Saccostrea cucullata, which forms decimeter- to meter-scale bioherms in the lower intertidal, the ocypodid crab Dotilla sulcata, which reaches highest abundances on the tidal flat, some tens of meters from the low-tide line, and the callianassid decapod Paratrypaea bouvieri, which is very abundant in the subtidal. The sediment surface in the intertidal was densely covered with lebensspuren of gastropods. Quantitative and qualitative samples revealed 130 species of molluscs from 51 families. Most species found belong to the Veneridae, Lucinidae, Turridae, Cardiidae, Trochidae, and Pyramidellidae in the subtidal and to the Cerithiidae in the intertidal, but overall only 32 species were found alive (13 on the tidal flat, 28 in the subtidal). Distinct assemblages of living and dead molluscs characterized the intertidal and subtidal, which suggests that dead shells can be used to assess regional biodiversity. Numerically, the living assemblage on the tidal flat was strongly dominated by the deposit-feeding potamidid gastropod Pirenella conica, and in the subtidal by infaunal bivalves (Glycymerididae, Lucinidae, Tellinidae, and Veneridae). Predatory gastropods (Naticidae, Melongenidae, Fasciolariidae, Muricidae, and Conidae) were accessory components that were mostly present in qualitative samples in the subtidal and lower intertidal. Accordingly, frequencies of drilling predation for the total assemblage are significantly higher in the subtidal (25.7 %) than in the intertidal (1.2 %). We conclude that the preservable fauna and its lebensspuren are characteristically developed along an environmental gradient and suitable to distinguish intertidal from subtidal habitats in a potential fossil record.