Relation of form to life habit in free-living cupuladriid bryozoans

Abstract

Since the late Mesozoic, several bryozoan groups have occupied unstable soft-sediment habitats by adopting a free-living and motile mode of life. Today, the free-living bryozoans often dominate epibenthic faunal communities in these expansive habitats, yet their biology and ecology remain poorly understood. This study examines their unique mode of life by exploring the relationship between form and function in the free-living Cupuladriidae of tropical America. Cupuladriid species occupy distinct niches in water depth and sediment type, and these variables correlate with a variety of morphological traits amongst species, thereby helping formulate hypotheses on functional morphology. These hypotheses were tested with disturbance, burial and mobility experiments. Species deal both passively and actively with disturbance and burial. Colony size and shape passively influences colony stability on the sea floor, susceptibility to burial and the ability to emerge once buried. Movable mandibles (setae) assist in emergence following burial, actively increase colony stability, and deter the settlement and disruption of encrusting growth of other organisms. Mandibles also enable colonies to move over the sea floor, but such behaviour appears to be of minimal benefit to avoid predation or to influence dispersal. Disturbance on the sea floor is the most important factor in driving the convergent evolution of the mobile free-living form. Colony shape and size and the employment of mandibles to improve stability, return to the surface after burial, and remove epibiota are central to the post-Mesozoic success of free-living bryozoans.