Distribution and bathymetric zonation of shell-boring endoliths in Recent reef and shelf environments: Cozumel, Yucatan (Mexico)

Pelecypod shells from bottom sediments of the shelf and upper slope off Cozumel (Mexico), at depths ranging from 1 to 47 m, were studied with regard to the taxonomy, frequency and distribution of microborings produced by endolithic organisms. Scanning electron microscopy of plastic casts of the borings reveals the existence of 29 microboring types, distinguished on the base of shape, size, mode of branching, overall boring pattern and sporangia. The endoliths include cyanobacteria, chlorophycean algae, rhodophycean algae, fungi, sponges and several microborers of uncertain taxonomic affinities.

Although most of the borings exhibit a wide bathymetrical range, a subdivision of the photic zone is indicated by the abundance and vertical distribution of some species. The upper part of the photic zone (1–30 m) is characterized by higher species diversity, by the green algae Phaeophila engleri and Entocladia testarum, and by the abundance of the cyanobacteria Hyella gigas. Fungi occur at all depths but are more common in the lower photic zone (30–47 m). The bathymetrical position of the boundary might be influenced by the vertical sampling distance. Generally, there is a good agreement with other bathymetrical patterns of recent endoliths, especially with those from the Puerto Rican carbonate shelf.

There is no distinct difference in the frequency or taxonomic composition of shell-boring endoliths from reefs and interreef areas (platforms covered by bioclastic sands). Boring intensity, measured by point-counting of SEM photographs, is generally high; in 58% of the samples 60 to 99% of the outer shell surface is bored, predominantly by fungi and by the green alga Phaeophila engleri. Mean boring intensities show no relationship with water depths.