POPULATION-DYNAMICS OF THE PLANKTIC FORAMINIFER GLOBIGERINOIDES-SACCULIFER (BRADY) FROM THE CENTRAL RED-SEA

In the central Red Sea the planktic foraminifer Globigerinoides saccul ifer (Brady) lives in the upper 80 m of the water column and depth pre ferences for different size classes could clearly be established. Repr oduction takes place at full moon, at about 80 m water depth, probably within the chlorophyll maximum. Juveniles ascend in the water column and reach the surface after less than approximately 2 weeks, before th ey are 100 mum in diameter. Pre-adult stages of ca. 200 mum steadily d escend within 9-10 days to the reproductive depth. We identify four mo rphotypes within G. sacculifer. On the basis of shape of the last cham ber, two basic morphotypes are defined; normalform and sac-like. The r elative size of the last chamber determines two further, secondary, mo rphotypes; kummerform and kummersac, respectively. In terms of earlier classifications, the kummersac morphotype forms an intergrade between kummerform and sac-like. Analysis of size, depth distribution and tim e of morphotype formation do not demonstrate a distinct relation betwe en the kummersac and one of the primary morphotypes. Kummerform and ku mmersac formation reach a peak shortly before full moon, whereas the h ighest frequencies of sac-like chambers are found around new moon. Sin ce formation of these morphotypes leads gametogenesis by 24-48 h, it c ould indicate that the reproduction of sac-like morphotypes is isolate d from the rest. Although additional evidence is required to decide on the taxonomic status of the normalform and the sac-like morphotype, i t is tempting to speculate that reproductive isolation over time can p lay an important role in the process of speciation. The mean and media n sizes of the living population and of the flux assemblage differ mai nly because of the growth component in the standing stock and differen tial sedimentation speed of dead and gametogenic specimens. Difference s between the flux assemblage and the thanatocoenosis cannot be explai ned by simple dissolution phenomena.