FACE DOWN IN THE SEDIMENT: IMPLICATIONS FOR THE ONSHORE-OFFSHORE DISTRIBUTION OF ENCRUSTING FORAMINIFERA IN THE BAHAMAS

Benthic foraminifera that are cemented to hard substrates are known as encrusting or attached foraminifera. They are common in modern-day coral reef environments, but they are most abundant in reef rubble at the base of coral heads, not on the exposed surfaces of the coral colonies. The clasts on the seafloor typically have very different upper and lower surfaces: the upper sides bear soft and crustose coralline algae and the undersides, where competition for space is less intense, have abundant encrusting foraminifera as well as polychaetes and bryozoans. Like other foraminifera, the attached forms use their reticulopodia to feed, but because of their inverted orientation they do not reach up into the water column as we usually think of sessile suspension feeders doing. Instead they are reaching down -- feeding on the benthos near the sediment-water interface. Often the clasts are partly buried in the sediment, so that the foraminifers have to feed on the infaunal interstitial biota (meiofauna).

Actualistic studies done on San Salvador, Cat Island, and Mayaguana all show a pattern of onshore-to-offshore distribution with Homotrema rubrum dominating assemblages nearshore, Planorbulina common in diverse mid-shelf assemblages, and large Gypsina plana found at relatively deep sites at platform margins. In addition, density and size of individuals (except for G. plana) was shown to decrease from onshore to offshore at Cat Island and Mayaguana, suggesting that conditions for optimal growth and reproduction diminish with distance from shore.

In this study, we continue to test the hypothesis that the abundance and assemblage characteristics of the potential food items (the meiofauna) play an important role in determining the distribution of encrusting foraminifera. Seafloor sediment samples were taken under cobbles on San Salvador in March 2019 and are analyzed as done previously for Mayaguana samples. Results to date from both islands suggest that less food in the relatively deep water at platform margins is reflected in fewer and smaller foraminifera. (The large tests of G. plana are apparently explained by the presence of photosymbionts.) The apparent dietary preferences of H. rubrum (crustaceans) and Planorbulina (nematodes) are currently being evaluated.