Branching patterns, generating rules, and astogenetic trajectories in arborescent bryozoans

The terminal growth of bryozoans allows a reconstruction of their growth history and generating rules from the developed pattern. Three species of arborescent bryozoans share a bias in growth rate that favors reverser branches (those whose direction of growth is opposite that of their parent branch); this bias produces a common hummocky appearance to the top margin of the colony. Other differences in the growth rules, however, result in markedly different colony forms. For two species, the cue for splitting of branches seems to be the length of the branch; for the other, the cue seems to be the time since the last splitting. Simulations based on the distinction between reversing and nonreversing branches reproduce the hummocky pattern, but offer little insight into the underlying mechanism. Simulations based on occlusion, the line-of-sight blocking of the branches by each other, give similar patterns with fewer assumptions. These simulations also suggest that the pattern is due to a limited availability of nutrients to occluded growing tips, and that nutrients remain relatively localized within colonies. The actual determinants of colony form are likely to combine both nutrition and geometry.