The function of astrorhizae in stromatoporoids: quantitative tests

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Abstract
Using the diameters of the channels at branch points, we quantitatively test three alternative models of the function of astrorhizae in stromatoporoids. The distribution of diameters at branch points is significantly different from the distribution that would be predicted from models of either a diffusive function or a bulk-flow system in which resistance to flow was constant at all levels of the branching hierarchy. The distribution of channel diameters is virtually identical to that predicted by a model (Murray's law) that simultaneously minimizes resistance to flow and some volume-related cost function. Astrorhizae thus carried a bulk flow of fluid and can be inferred to have been lined with cellular elements; the exchange sites associated with the fluid-transport system were distributed throughout the soft tissues of the stromatoporoid animal. The most parsimonious hypothesis of function, that the fluid-transport system was associated with suspension feeding, implies strong similarities between the structure of the stromatoporoid animal and living sponges.

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