Population dynamics of the planktic foraminifer Globigerinoides sacculifer (Brady) from the central Red Sea

Abstract

In the central Red Sea the planktic foraminifer Globigerinoides sacculifer (Brady) lives in the upper 80 m of the water column and depth preferences for different size classes could clearly be established. Reproduction 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 they are 100 μm in diameter. Pre-adult stages of ca. 200 μm steadily descend within 9–10 days to the reproductive depth. We identify four morphotypes within G. sacculifer. On the basis of shape of the last chamber, two basic morphotypes are defined; normalform and sac-like. The relative size of the last chamber determines two further, secondary, morphotypes; 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 time of morphotype formation do not demonstrate a distinct relation between the kummersac and one of the primary morphotypes. Kummerform and kummersac formation reach a peak shortly before full moon, whereas the highest frequencies of sac-like chambers are found around new moon. Since formation of these morphotypes leads gametogenesis by 24–48 h, it could indicate that the reproduction of sac-like morphotypes is isolated from the rest. Although additional evidence is required to decide on the taxonomic status of the normalform and the sac-like morphotype, it is tempting to speculate that reproductive isolation over time can play an important role in the process of speciation. The mean and median sizes of the living population and of the flux assemblage differ mainly because of the growth component in the standing stock and differential sedimentation speed of dead and gametogenic specimens. Differences between the flux assemblage and the thanatocoenosis cannot be explained by simple dissolution phenomena.