PhD Dissertation Title: Flow, Feeding, and Form: Consequences of Coloniality
Colonial animals can grow in a wide variety of shapes, but some forms have evolved several times in different taxa suggesting convergence. Previous hypotheses to explain the evolution of colony form in bryozoans have generally focused on spatial limitation or risk of dislodgement, and only more recently has the selective role of food acquisition in shaping colony form been explored. I measured the effects of a range of free-stream velocities on ingestion rate in several species of bryozoans of different growth forms in a flow tank. To compliment the flow tank measurements, I measured the effect of velocity on abundance, growth, and survival of bryozoans in the field. Regardless of water velocity, one species of bryozoan, Membranipora membranacea, generally had a higher ingestion rate, grew faster, and survived longer than other species tested. I investigated some potential reasons why Membranipora captures food so effectively, including how water flow interacts with the shape and size of the colony as well as the spacing of feeding zooids within the colony to affect feeding success. The size and shape of the colony as well as the spacing of zooids within the colony all affected feeding rate; however, the higher feeding success of Membranipora is most likely explained by the advantage of closely spaced feeding zooids. The feeding advantage of densely packing zooids may help explain the increase in zooid integration over evolutionary time in the bryozoan phylum. The effect of flow on feeding success not only may have influenced the evolution of colony form, but having high feeding success in a wide range of flow velocities may allow certain species, such as Membranipora, to become successful invaders in new habitats.