Form and Internal Structure of Recent Algal Nodules (Rhodolites) from Bermuda

The growth forms and internal structure of living coralline algal nodules (here termed rhodolites) from a shallow Bermuda bay contain a surprisingly sensitive record of the frequency of their movement by wave action. The growing rhodolites of Whalebone Bay are pebble to cobble-sized spheroids found at depths of 1-2 m. In section, a nucleus (either algal or foreign) is surrounded by a laminated shell or coralline algae with two different growth forms: laminar and columnar. In almost all the larger rhodolites the nucleus is invariably succeeded first by laminar and then columnar structure. Growing spheroidal and ellipsoidal rhodolites with smooth surfaces (laminar structure) are concentrated in sandy channels; those flattened ones with bumpy surfaces (columnar structure) are half buried in skeletal sand or trapped in rock-floor depressions. Evidently, the spheroidal form and laminar structure indicate frequent movement, while the flattened form and columnar structure indicate immobility. The frequency of movement recorded by the laminar structure was determined by observation of marked rhodolites. Specimens of 80 mm were moved 3 m in the sandy channels by waves generated by winds of 15 knots. Winds of 15 knots or more occur monthly throughout the year and much more frequently during the fall and winter. The relationships between form and internal structure and frequency of movement observed in these shallow-water rhodolites may also prove to be useful for interpreting the growth conditions of other Recent rhodolites from deeper water, 10-100 m, as well as those of fossil rhodolites.