Who Eats Bryozoans? — and Why It Matters for Interpreting Evolution

Our fundamental understanding of the taxonomy and diversification of bryozoans is based on the evolution of the zooid skeleton. Within the largest clade, Cheilostomata, the evolutionary succession from anascans through ascophorans is interpreted as a progressive trend increasing the extent of calcified, protective armament. Yet no previous comprehensive analysis of living bryozoans has considered their importance as prey for other animals. A survey of colony predators uncovered 399 species in eight phyla. Multivariate analyses revealed predator trophic groups based on dissimilar feeding mechanisms, body sizes, types of locomotion and dietary breadths. Most larger-sized predators take bryozoans as minor dietary components or as incidental by-catch in their pursuit of other invertebrates living on the colonies. Mobile epibionts such as nematodes, polychaetes, and small arthropods showed greater fidelity in their bryozoan diets. Structural defenses including zooid calcification, ovicells, spines and avicularia are known to deter some small predators. These defenses and the material strength of zooid skeletons offer trivial resistance against the mechanical forces generated by larger durophagous predators such as echinoids, decapods or fishes, among the key driving forces of the Marine Mesozoic Revolution. As an alternative, the small epibiont predators (or parasites) causing sublethal damage to colonies are likely determinants of this trend. One may deduce that the ubiquity of skeletal defenses is in some way proportional to the fitness costs of sublethal predation by smaller animals, yet it is only part of the story, since other life history stages are unseen in the fossil record. Changing risks and suites of predators through different bryozoan life stages can have great influence on the evolutionary expression of resource allocation, morphological and developmental lability, and traits closely associated with avoidance, defense and tolerance.