Morphotypes of the Foraminiferan Homotrema Rubrum (Lamarck): Distribution and Relative Abundance on Reefs in Bermuda

Five morphotypes of the foraminiferan Homotrema rubrum were identified from a study of shape variation on 4,002 tests collected from Bermuda reefs. They are, in order of decreasing relative abundance, hemispherical (37.3%), globose (18.5%), knobby (17.1%), encrusting (12.3%), and columnar (3.4%). Test sphericity, measured as the rate of change in height with diameter (=slope), ranged from more spherical globose tests (0.73) to flattened (0.01) encrusting forms. Mean shape ratios (height/mean diameter) differed for each morphotype and were also useful in distinguishing morphs. Mean test size ranged from large globose (25 mm³), to med sized hemispherical (8 mm³), and small columnar, knobby, and encrusting tests (mean = 4 mm³). Thin sections revealed similarities among some morphotypes, with globose, hemispherical, and columnar H. rubrum having test bases that resembled the small, compact chambers of the encrusting test matrix. This, combined with smallest average size and diameter, suggests that juvenile H. rubrum are encrusting. Large globose tests are likely reproductive adults and hemispherical or columnar tests intermediates in ontogeny. Tests may continue to grow in diameter and remain encrusting, but if chambers are added at a greater rate to test height than diameter either a hemispherical or columnar shape will develop, depending on the size of the test base at the time. Knobby H. rubrum were distinct, having fragile tests with loosely organized chambers and often an abundance of sponge spicules projecting from test wall areolae. Spicules may directly regulate development of a knobby form. Morphological comparisons over large spatial scales (e.g., north, south, west; nearshore, midshore, offshore), of varying degrees of exposure, showed similar frequencies of the five morphotypes in all environments. However, morphological variation in response to environmental conditions was evident at smaller scales (e.g., in and out of reef cavities). Knobby and globose tests dominated restricted microenvironments while exposed reef substrates were characterized by hemispherical and encrusting morphs. However because all five morphotypes were found in all habitats surveyed, it is likely that variation in H. rubrum morphology is regulated by both ontogeny and environmental conditions.