HABITAT PARTITIONING AND TIERING IN TROPICAL INTERTIDAL CALLIANASSID MOUNDS: AN EXAMPLE FROM THE BAHAMAS

Mounded topography formed by callianassid shrimp is ubiquitous to modern tropical, shallow subtidal to intertidal, sandy carbonate substrates of the wider Caribbean region and beyond. The various species of callianassids are real substrate-surface engineers, a powerful instrument of bioturbation, and the dominant animals of the deep-tier endobenthos. In the Bahamas, extensive tidal flats with distinctive mound and funnel topography created by the callianassid Glypturus acanthochirus are common along the margins of lagoons or “creeks.” With time these callianassid mounds commonly coalesce to form composite mound surfaces that become stabilized with development of a thin microbial mat. This sets the stage for shallow-tier burrowers to colonize and partition these surfaces.

In Pigeon Creek on San Salvador Island, at least three species of burrowers commonly inhabit the stabilized mound surfaces: the upogebiid shrimp Upogebia vasquezi and the fiddler crabs Uca major and Uca spp. U. vasquezi burrows are distinctive, unusual, and complex. A remarkably thick and cohesive, externally pelleted lining encapsulates two U-shaped interlocking but unconnected burrows that penetrate 10-15 cm into the substrate. Each burrow system contains a male and female pair. The burrows of U. major are unlined, have diameters of 2-5 cm, and extend obliquely into the mound substrate, following a gently meandering, somewhat irregular course, with lengths of up to 50 cm. These burrows end with a bulbous turnaround. Uca spp. burrows are similar in form to those of U. major but are much shorter in extent and smaller in diameter, typically about 1 cm. Partitioning of callianassid mound surfaces by U. vasquezi, U. major, and Uca spp. burrows is in a ratio of about 5:1:4.5/m2.

Callianassid-mounded substrates with trace fossils formed by Glypturus acanthochirus and Upogebia vasquezi have been recognized on San Salvador Island in a lagoonal facies of the late Pleistocene Grotto Beach Formation, with the top of the facies representing sea-level stillstand and initial regression at the end of the Last Interglacial. These trace fossils may prove useful as both paleoenvironmental and sea-level position indicators in the Bahamas and other geologically similar areas.