Fossil coelacanths, and their only living representative Latimeria cha
lumnae, possess a small, supplementary epicaudal fin of unknown functi
on at the tip of the large tail. Movements of this fin were analyzed o
n film sequences taken with submersibles in the fish's natural habitat
off Grande Comore, Indian Ocean. Epicaudal fin beats were not coordin
ated with strokes of paired and unpaired lobed fins, and the epicaudal
fin did not move continuously. When in action, successive fin beats w
ere performed more-or-less rhythmically but varied considerably in dur
ation. Epicaudal fin beats occurred mainly in three different situatio
ns: during curve swimming, with slow forward motion of the body; durin
g narrow turns on a point without significant forward motion of the bo
dy; and during station holding without forward, backward, or sideward
motion of the body. In the latter case, the fin regularly stood still
at maximum deflexion for several seconds. In turns and curves, epicaud
al fin beats appeared to have a locomotory function, whereas during ho
vering, with standstills of the fin at maximum deflexion, beats presum
ably have a sensory function. The lateral-line canal extends to the ve
ry tip of the epicaudal lobe. If the lobe is held laterally at an angl
e to a local current parallel to the main body axis or to a current re
sulting from backward motions, water displacement in front of the lobe
could be detected by the lateral line on the lobe.