Growth and form in modular animals: ideas on the size and arrangement of zooids

Abstract

Modular (colonial) invertebrates are mostly aquatic, sessile, active or passive suspension feeders. This paper proposes and discusses some generalizations concerning form that apparently are related to the sessile colonial mode of life. In contrast to the size of related unitary forms, the modules are small, maximizing feeding surface relative to metabolic mass and favouring production of a high energy surplus. Increasing colonial integration in ascidians and hydroids is associated with decreasing module size but in Bryozoa, with the lophophore as index, with some increase in size. The smallest lophophores are found in species with apparently primitive, near-linear branching. Among bryozoans with compact encrusting colonies, however, species with larger lophophores can outcompete abutting neighbours with smaller lophophores. Lophophore size may then be a compromise between energetic advantage and competitive disadvantage. Whereas internal filterers tend to have modules grouped to produce larger exhalant openings, favouring stronger discharge flow, in Bryozoa it appears advantageous to attain the maximum coverage of expanded lophophores. In Cheilostomata, lophophores are generally close packed, except at excurrent chimneys, and zooid size and shape are then directly linked to the dimensions of the lophophore. Bryozoa Cyclostomata, however, have evolved away from close-packed lophophores and quincuncial zooids towards fasciculated arrangements, possibly providing structural excurrent channels in a group that lacks the colonial coordination to maintain non-skeletal chimneys. Variations in colony form are related to mode of growth, the disposition of modules to maximize filtration, and interactions with environmental factors. Increasing surface area leads to increased drag imposed by water movements. This may place constraints on growth and form, or may be exploited to augment filtration. Passive filterers often produce erect, branching, planar colonies oriented normal to directional currents. Bilaterally symmetrical, dish-shaped colonies with downstream zooids may occur in unidirectional flow. Erect bryozoan colonies more commonly are irregularly tufted or regularly branched in three dimensions, being then adapted to flows that vary in direction or velocity, or both.