LIFE HISTORY OF NEOISOROPHUSELLA LANEI RECONSIDERED

Two hundred and fifty specimens of the Late Mississippian (Chesterian) edrioasteroid, Neoisorophusella lanei, from a single obrution horizon in the Kinkaid Formation of Southern Illinois, were examined to investigate their life history. The sample consisted of two slabs each bearing greater than one hundred and twenty individuals upon which specimen diameters were measured. Edrioasteroids were attached in clusters to isolated islands of hard substrate, primarily shells of the bivalve Promytilus. One cluster consists of many individuals attached to a large member of the same species; the first time that this behavior has been reported. Size distribution of the general population is a normal curve skewed towards large individuals as found in previous studies (Cole, 1975; Kammer et al., 1987), with a complex underlying pattern. The bimodal distribution reported by previous studies was not found. The edrioasteroids occur in size-delimited clusters on suitable hard substrate, such that the typical size range of individuals on any given bivalve shell is more similar to itself than the range of individuals in other clusters. Population density was measured for the general population, and for each cluster. Juveniles attach to hard substrate with space between individuals. The clusters remain stable in population and size distribution until the thecae enlarge making contact. Edrioasteroids rarely overgrow conspecific neighbors, leading to intense crowding and mortality of some members of the cluster, providing substrate for the expansion of large individuals and the late recruitment of juveniles within established clusters. The population density per cluster decreases markedly with increased average individual size. Clusters of large individuals often have juveniles located in interstices or along the edge of the shell, suggesting continuous reproduction and recruitment. The combination of size limited clustering and mortality patterns also suggest that in the post-larval stage, the primary control of the density and distribution of this population is competition for the available hard substrate.