ENCRUSTING FORAMINIFERA AT MAYAGUANA, BAHAMAS: DISTRIBUTION AND RELATIONSHIP TO FOOD AVAILABILITY

Benthic foraminifera that are cemented to hard substrates are known as encrusting or attached foraminifera. Their fixed nature makes them more likely than free foraminifera to remain in their original habitat after death; this, combined with their sensitivity to environmental variables such as light and water energy, makes encrusting foraminifera potentially valuable for paleoecology. 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 has been recorded at San Salvador and at Cat Island, Bahamas. In addition, density and size of individuals was shown to decrease from onshore to offshore at Cat Island, suggesting that conditions for optimal growth and reproduction diminish with distance from shore.

The objectives of this actualistic study were (1) to see if the same pattern of distribution occurs at Mayaguana, located some 215 km southeast of San Salvador, and (2) to take water and sediment samples to see if food availability could be involved in the distribution noted. We sampled 7 sites and collected 6 cobbles per site. Syringes were used to take 20-ml water samples under each cobble, and 100-ml samples of sediment were taken under the cobbles after collection.

The distribution of the encrusting foraminifera at Mayaguana closely resembles that of the assemblages at San Salvador and Cat Island, except that Planorbulina was more abundant nearshore than expected. As seen at Cat Island, both density and size of encrusting foraminifera decreased with increasing distance from shore. This pattern seems to be influenced by the amount and type of food items available. Less food appears to have resulted in fewer and smaller foraminifera. In addition, foraminiferal taxa appear to have dietary preferences: H. rubrum is found with abundant crustaceans, whereas Planorbulina is associated with nematodes. The abundance of large tests of Gypsina plana at the platform margin in spite of the low availability of potential food items in the water and sediment samples is likely explained by the presence of photosymbionts in this species.