200 Years. 200 Stories. Story 191: “The Coelacast’s Upside-down Fin ”

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the cast of the coelacanth
The Academy's "coelacast." The inverted pectoral fin is the one closest to the head.

The Coelacast’s Upside-down Fin

Although the Academy’s Ichthyology Collection does not have a true specimen of the coelacanth, Latimeria chalumnae, it can boast the next best thing: a 64-inch plasterware cast of the same. It was prepared in 1972 from a preserved specimen at the Muséum national d'Histoire naturelle in Paris at the request of Dr. Arthur DuBois of the University of Pennsylvania. Dr. Dubois proposed to study the hydrodynamic features of several oceanic fishes (swordfish, marlin, and tuna) and intended to compare them to those of a model coelacanth. In 1991, Dr. DuBois, then at Yale University, donated the cast to the Academy as a gift to Dr. Keith Thomson, author of Living Fossil, The Story of the Coelacanth and the institution’s president at the time.

Dr. Thomson recently remarked that the coelacast has a minor flaw: the pectoral fins are mounted upside down! In the true coelacanth, the dorsal rays of the pectoral fin are longer and appear as such when the fin is fully adducted (pulled close to the body). However, the Academy cast depicts the ventral rays as longest in the adducted pectoral fin. Ours is not the only coelacanth cast with transposed pectoral fins! Identical casts with the same flaw have been on display at the Carnegie Museum in Pittsburgh, the Field Museum of Natural History in Chicago, and the British Museum of Natural History in London. The French modelers are probably not to blame for the world’s aberrant coelacanth casts. Coelacanths have the extraordinary ability to rotate their paired fins and do so in a sculling motion while swimming. When extended away from the body, the pectoral fin is rotated such that the longest rays appear ventrally.

Read more fantastic, funny, and downright frightening stories about fishes!

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