HOCHBERG, R; Smithsonian Marine Station: Evolutionary morphology of the head in Thaumastodermatidae (Gastrotricha). Fluid mechanics and evolutionary constraints drive historical transformation of the feeding apparatus

Gastrotrichs live in an environment characterized by extremely low Reynolds numbers. The morphology of the gastrotrich feeding apparatus is therefore adapted to overcome viscosity and enhance prey capture. An investigation of mouth geometry, pharynx structure, and cephalic muscle topology was undertaken using electron microscopy, epifluorescence and confocal scanning laser microscopy to gain insight into the evolutionary morphology of the head and feeding system of thaumastodermatid gastrotrichs. Results suggest that species of Thaumastodermatidae evolved a novel head morphology to exploit the interplay of hydrodynamics, evolutionary constraints, and prey specialization. The thaumastodermatid head is characterized by a distinctly broadened morphology that bears a wide, dorsoventrally-compressed mouth and a weakly developed myoepithelial pharynx, characterized by a low myofilament density and few sarcomeres. The mouth is covered by a dorsolateral expansion of the head called the oral hood that contains extensions of pharynx and body wall. It is hypothesized that an evolutionary loss of radial myofilaments in the pharynx and fluid mechanic principles governing the functional morphology of feeding have driven the evolution of head morphology. A weak pump hypothesis is proposed that predicts a low density of pharyngeal muscle translates into an ineffective muscular pump in a viscous medium. Selection to maximize the surface area of the head to either enclose a small volume of water or create a funnel opening would theoretically increase pumping capacity for food uptake and transport. Based on these observations, it is proposed that moderate evolutionary constraints in pharyngeal organization may channel evolutionary changes in the morphology of the head leading to specializations of the feeding apparatus.