Morphogenetic gradients in Paleozoic bryozoan colonies

Abstract

Subcolony boundaries within Paleozoic bryozoan colonies, not otherwise easily observable, can be located precisely by computer mapping of variables derived from Fourier series. The monticule-centered subcolonies were maintained by morphogenetic gradients that controlled zooecial size and form, zooidal degeneration (producing brown bodies), and the spacing of monticules with respect to each other. These gradients were probably determined by the release of a budding inhibitor from polymorphic zooids within the monticule. In colonies with active monticular budding the morphogenetic gradients represent ontogenetic series, as the zooids that arose in the monticule gradually migrated outward along modified spiral pathways. Polymorphism, including reproductive functions, is probably reflected in the spatial order of zooids away from a monticule. In subcolonies with monticular budding, polymorphs represent different stages in the life history of a single zooid, with gradual transitions in zooecial form between castes.

The structural similarity between monticules and oghurd dunes supports the idea that they also functioned as excurrent water outlets, possibly with reproductive polymorphs clustered near the outflow. The variability of monticular size and spacing in some colonies indicates that hydrodynamic efficiency was compromised by the need for developmental plasticity. The development of active budding within monticules is considered an ecophenotypic response to the external environment, suggesting that all monticules within the Stenolaemata were potential budding sites. Monticules were successful adaptations in bryozoans because they performed a multiplicity of functions. While regulating the morphogenesis of the colony, they simultaneously served the metabolic functions of waste removal, feeding and reproduction.