Filamentous sediment-binding blue-green algae, principally Scizothrix, are present throughout the tide flats of the west coast of Andros Island, Bahamas, from storm-tide levels to below low tide, a range in excess of 3 m. However, extensive flat-laminated stromatolitic deposits are restricted to about ½ m in the upper intertidal and supratidal zones. A similar restriction is known from other areas, notably Florida Bay and the Persian Gulf.

Two small gastropods which feed on algae-coated surface-sediment particles are the principal cause of this restriction, but deposit-feeding polychaetes are also responsible. The gastropods, Cerithidea costata and Batillaria minima, in concentrations of 500-2,000/sq m, intermittently are exposed on the margins of shallow subtidal ponds. When flooded they feed so voraciously that about 100 fecal pellets are excreted per individual per hour. On the basis of the size of pellets (0.026 mm³), rate of feeding (almost equivalent to the rate of excretion), number of individuals (1,000/sq m), and the percentage of time spent feeding (50%), I calculate that the topmost millimeter of sediment is reworked over the entire surface in 1 month.

If the gastropods and other deposit feeders were absent from Bahamian environments, the vertical range of stromatolitic deposits could be extended there from ½ m to several meters. Restriction or absence of deposit feeders could be caused by extreme salinity or temperature in other Holocene or Phanerozoic environments. Complete absence of deposit feeders in Precambrian time should have allowed stromatolites to develop to their maximum vertical range.

End_of_Article - Last_Page 848------------