Differentiating Parasitism and Other Interactions in Fossilized Colonial Organisms

Colonial species occur in a wide range of aquatic invertebrates, some having excellent fossil records, notably corals, bryozoans and graptolite hemichordates. In contrast to unitary animals, colonial animals grow by adding repetitive modules known as zooids. The ability of colonies to endure partial mortality and the typically plastic growth of benthic colonial species facilitates the formation of macrosymbiotic associations, some of which may be parasitic. However, as with unitary fossils, it is notoriously difficult to identify whether the symbioses are parasitisms (+/-) or mutualisms (+/+). Intergrowths between host colonies of stromatoporoid sponges, corals or bryozoans, and skeletal or soft-bodied symbionts are particularly common in Ordovician-Devonian shallow-water deposits. Soft-bodied symbionts in such intergrowths are represented by moulds in the host skeletons, a process of preservation termed bioclaustration. As yet, however, there is a lack of convincing data showing that any of these symbionts were parasites. By comparison with modern analogues, some fossil galls provide more convincing examples of parasitism, and the destructive effects of borings into the skeletons of benthic colonies also argue in favour of parasitism. Pelagic graptoloid hemichordates from the Early Palaeozoic occasionally contain cysts or tubes that have been attributed to parasites on the grounds that they would have adversely affected the hydrodynamics of the floating colonies. Future studies should test for parasitism by comparing the sizes of colonies hosting symbionts with those lacking symbionts.