Evolution of erect helical colony form in the Bryozoa: Phylogenetic, functional, and ecological factors

Abstract

Erect helical colony forms have evolved at least six separate times within the Bryozoa, but only among those in which branches are composed of a single layer of feeding zooids. The four best known genera with helical colony forms evolved independently, and each occupied different benthic marine environments, achieved different growth habits, and utilized different aspects of an array of functional potentials resulting from the radially symmetrical colonies. Examination of the distribution of these four genera (Archimedes, Bugula, Crisidmonea, and Retiflustra) within a theoretical morphospace of hypothetical helical colony form reveals that each occupies its own characteristic region of morphospace, broadly overlapping in some dimensions but separated in others. The genera differ markedly in the branching densities within their filtration-sheet whorls, reflecting their phylogenetic legacies rather than constructional or functional constraints associated with helical growth. In contrast, all tend toward helices in which the radiating whorls of the unilaminate branches are held at an average of 50-60° to the central axis of the colony, and this may reflect a common functional optimum associated with the cilia-driven, auto-generated currents within the helix. The region of morphospace characterized by high values of surface area - i.e. helical geometries with branches orientated at very low angles to the central axis, and with very closely spaced whorls along the axis - is entirely empty of bryozoans, and these geometries apparently represent functionally unrealistic colony forms. © 2003 The Linnean Society of London.