lON'ES ; PROBABLE CAUSE OF CRATER WOUNDS 



Figure 3. — Crescentic scar on a living porpoise, Stenella 

 roseiventris Wagner, The Oceanic Institute, Makapuu, 

 Hawaii. 



dibular teeth of Isistius are shed as a unit and 

 that the next set of replacement teeth are al- 

 ready erect and immediately functional when 

 the previous set is shed. He wondered "... 

 which aspects of Isistius biology require such a 

 safeguard." 



OBSERVATIONS 



An opportunity to test the idea came in July 

 1969, during cruise 44 of the RV Toirn^end 

 Cromwell of the NMFS, HAFRC (National Ma- 

 rine Fisheries Service, Hawaii Area Fishery 

 Research Center). Nightly midwater trawl 

 hauls were made along long 145° W between 

 lat 14° N and 3° S in the central Pacific. The 

 trawl catches contained occasional specimens of 

 Isistius, some of which were alive but moribund 

 when brought on board. I stated that this spe- 

 cies of shark might be responsible for the crater 

 wounds which we had observed on tunas a few 

 hours earlier. This led John D. Fowler, Jr., re- 

 search assistant aboard the Toirnsend Cromwell, 

 to iiress the mouth of a moribund Isistius against 

 the side of a dead fish, Cubiceps sp. The shark 

 made a biting motion, producing a crescentic 

 wound (Figure 4) that if completed would have 

 been similar in size and shape to crater wounds 

 observed on tunas. That shark could not be in- 

 duced to repeat its performance, but Fowler's 

 experiment led to further attempts to determine 



Figure 4. — A crater "wound" produced on the surface 

 of a nectarine by pushing the teeth of a dead Isistius 

 into the fruit and then rotating the body of the shark 

 around the point of attachment. In the center is a 

 crescentic "wound" produced by a living Isistius when 

 its mouth was pressed against the side of the dead fish. 



whether adaptations in structure for specialized 

 feeding behavior existed in Isistius. 



The basihyoid cartilage or "tongue" of Isistitts 

 was large and thick in contrast to that structure 

 in galeoid sharks. It was also unusually mov- 

 able; with a pencil I was able to push the tongue 

 caudad to a point just anterior to the first ex- 

 terior gill opening (Figure 5). In that position 

 the posterior margin of the tongue was elevated 

 (dorsad) until the tongue was nearly vertical, 

 fitting closely against the roof of the mouth, and 



Figure 5. — A demonstration of the movability of the 

 tongue of Isistius. 



793 



