DEPTH TRENDS IN ENCRUSTING COMMUNITIES ON MOLLUSK SHELLS FROM 15M TO 275M

Organisms that encrust the preservable remains of larger organisms are an important part of the fossil record that is often ignored. Encrusting communities develop on a finite habitat and reflect the local paleoenvironment. Community analysis of a modern encrusting fauna aids in interpretation of paleocommunities attached to fossils. A controlled long-term study of biont communities was undertaken on mollusk shells deployed in 5 depositional zones in the Bahamas to depths of 275m. Shells were deployed in mesh bags and tethered to poles to allow retrieval after 2 and 7 years via submersible. Upon retrieval, laboratory analyses were conducted in the field, including photographs and a full assessment the encrusting fauna. Each encruster was identified and its percent cover was estimated. During subsequent lab analyses, Mytilus edulis shells were re-examined under a dissecting microscope in order to focus on bionts with preservable hard parts. In addition to aerial coverage of each species, individual foraminifera were counted, as their small size results in fractional aerial coverage values.

Clear trends in biont community structure correlate with changes in depth. Shallow shells (15m) exhibit the highest percent area coverage but a lower species richness value than shells at 70m. Shells at sites below the photic zone (~100m) show decreases in percent area covered, but species richness values remain high. Typical shallow encrusting communities include serpulids, foraminifera, coralline algae, and bryozoans as surface area dominants. Species richness is highest among foraminifera and serpulids at 15m and foraminifera and bryozoans at 70m. Below the photic zone surface area and species richness values remain highest for foraminifera and serpulids, although total aerial coverage drops dramatically.

High aerial coverage and lower species richness values on the 15m shells reflect dominance by photosynthetic bionts and/or bionts with photosymbionts. The lower aerial coverage and higher species richness values on the 70m shells reflects their location deeper in the photic zone. Results of this study indicate that biont encrustation of fossils may be useful in interpreting ancient light levels, energy, nutrient availability, and even time since death of marine shelly fossils.