Population dynamics of Marginopora kudakajimensis Gudmundsson (Foraminifera : Soritidae) in the Ryukyu Islands, the subtropical northwest Pacific

The population dynamics of Marginopora kudakajimensis Gudmundsson, a dinofl agellate endosymbiont-bearing soritid foraminifer, was studied in the Ryuky u Islands, the subtropical northwest Pacific. Macroalgal samples were colle cted monthly between November 1995 and November 1996 at a 1-m-deep lagoonal site colonized by this species. Monthly variations in the size-frequency d istributions and population density indicate that this foraminiferal popula tion replaces itself in one year. Asexual reproduction occurs twice a year, in late spring and winter; in late spring, some of the adult individuals r eproduce by multiple fission simultaneously. As a result, two cohorts (a la te spring and a winter cohort) are found in the population during a year. L ife span of the former cohort is up to one year, while that of the latter c ohort is up to six months. Megalospheric specimens comprise up to 99% of th e population with a few microspheric individuals throughout the year. The l ife tables and survivorship curves revealed that size specific mortality ra tes were very low during the first several size classes and increased there after, indicating low juvenile mortality with high mortality later in life. The carbonate production rate by this Marginopora population is approximat ely 5 kg CaCO3 m(-2) yr(-1), which is extremely higher than those reported for other larger foraminiferal species. (C) 2000 Elsevier Science B.V. All rights reserved.