Jm. Elliott et al., MORPHOTYPES OF THE FORAMINIFERAN HOMOTREMA-RUBRUM (LAMARCK) - DISTRIBUTION AND RELATIVE ABUNDANCE ON REEFS IN BERMUDA, Bulletin of marine science, 58(1), 1996, pp. 261-276
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, hemispher
ical (37.3%), globose (18.5%), knobby (17.1%), encrusting (12.3%), and
columnar (3.4%). Test sphericity, measured as the rate of change in h
eight with diameter (=slope), ranged from more spherical globose tests
(0.73) to flattened (0.01) encrusting forms. Mean shape ratios (heigh
t/mean diameter) differed for each morphotype and were also useful in
distinguishing morphs. Mean test size ranged from large globose (25 mm
(3)), to med sized hemispherical (8 mm(3)), and small columnar, knobby
, and encrusting tests (mean = 4 mm(3)). Thin sections revealed simila
rities among some morphotypes, with globose, hemispherical, and column
ar H. rubrum having test bases that resembled the small, compact chamb
ers of the encrusting test matrix. This, combined with smallest averag
e size and diameter, suggests that juvenile H. rubrum are encrusting.
Large globose tests are likely reproductive adults and hemispherical o
r columnar tests intermediates in ontogeny. Tests may continue to grow
in diameter and remain encrusting, but if chambers are added at a gre
ater rate to test height than diameter either a hemispherical or colum
nar shape will develop, depending on the size of the test base at the
time. Knobby H. rubrum were distinct, having fragile tests with loosel
y organized chambers and often an abundance of sponge spicules project
ing 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 morphotyp
es in all environments. However, morphological variation in response t
o 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 morphoty
pes were found in all habitats surveyed, it is likely that variation i
n H. rubrum morphology is regulated by both ontogeny and environmental
conditions.