Function of the heterocercal tail in white sturgeon: Flow visualization during steady swimming and vertical maneuvering

Basal ray-finned fishes possess a heterocercal tail in which the dorsal lob e containing the extension of the vertebral column is longer than the ventr al lobe, Clarifying the function of the heterocercal tail has proved elusiv e because of the difficulty of measuring the direction of force produced re lative to body position in the aquatic medium, We measured the direction of force produced by the heterocercal tail of the white sturgeon (Acipenser t ransmontanus) by visualizing flow in the wake of the tail using digital par ticle image velocimetry (DPIV) while simultaneously recording body position and motion using high-speed video, To quantify tail function, we measured the vertical body velocity, the body angle and the path angle of the body f rom video recordings and the vortex ring axis angle and vortex jet angle fr om DPIV recordings of the wake downstream from the tail, These variables we re measured for sturgeon exhibiting three swimming behaviors at 1.2 L s(-1) , where L is total body length: rising through the water column, holding ve rtical position, and sinking through the water column, For vertical body ve locity, body angle and path angle values, all behaviors were significantly different from one another, For vortex ring axis angle and vortex jet angle , rising and holding behavior were not significantly different from each ot her, but both were significantly different from sinking behavior, During st eady horizontal swimming, the sturgeon tail generates a lift force relative to the path of motion but no rotational moment because the reaction force passes through the center of mass. For a rising sturgeon, the tail does not produce a lift force but causes the tail to rotate ventrally in relation t o the head since the reaction force passes ventral to the center of mass. W hile sinking, the direction of the fluid jet produced by the tail relative to the path of motion causes a lift force to be created and causes the tail to rotate dorsally in relation to the head since the reaction force passes dorsal to the center of mass, These data provide evidence that sturgeon ca n actively control the direction of force produced by their tail while mane uvering through the water column because the relationship between vortex je t angle and body angle is not constant.