Pleistocene sedimentary rocks exposed in the intertidal and shallow subtidal zones of the northeastern Gulf of California coastline are being significantly weathered and eroded by a diverse suite of biologic agents. Macroscopic bioerosion of carbonate substrates in this region is universal, although the distribution patterns of particular taxa of borers are patchy.
In the vicinity of Puerto Penasco (Sonora, Mexico), where the tidal range achieves a maximum of 9 m (30 ft), the dominant macroboring organisms include mytilid bivalves (Lithophaga), sipunculid worms (Phascolosoma and Themiste), and clinoid sponges (Cliona). Abundances are locally high (e.g., up to 120 sipunculids per 1,000 m3 of rock). Other prominent but slightly less abundant borers include bryozoans, regular echinoids, and polychaete annelids (eunicids, spionids, and possibly sabellids). Nestlers, which are organisms that occupy and sometimes modify or enlarge preexisting borings, are common. They include bivalves (mainly arcids and petricolids) and crustaceans (various crabs and shrimps).
Data on the distribution of borers with respect to intertidal microfacies are not sufficient to permit much generalization at this point in the investigation. However, it is clear that substrate character is an important factor. Poorly cemented beachrock (sandstone composed of bioclasts and volcanic rocks fragments) is bored intensely by bivalves and sipunculids. Limestone coquina is colonized by dense populations of boring bivalves and sponges. Loose shell material commonly contains borings of sponges and polychaetes.
To determine bioerosion rates and colonization sequences of boring taxa, experiments with marble slabs staked out at numerous sites are in progress.