MAGNUSON and HEITZ: GILL RAKER APPARATUS 



Several authors have pointed out that fish 

 tend to select the largest food organisms avail- 

 able to them (Hayashi, 1956, as cited in Yasuda, 

 1960b; Ivlev, 1961; Brooks, 1968). The large 

 mouth of larval scombrids facilitates capture 

 of large copepods at first feeding and contributes 

 to their rapid early growth I'ates (Shirota, 1970) . 

 The responsiveness of at least one scombrid to 

 food is influenced by the size of the food or- 

 ganism — A', pelamis ate whole shrimp and squid 

 at the beginning of a feeding, but as they be- 

 came sated, they would only eat cut-up pieces 

 of smaller size (Nakamura, 1962). Feeding be- 

 havior of Atlantic mackerel, Scomber- scombnis 

 Linnaeus, (Sette, 19.50) and northern anchovy, 

 EngrauUs mordax Girard, (Leong and O'Connell, 

 1969) changes with the size of food. When small 

 food is present, they open the mouth wide and 

 flare the opercles in a filter feeding mode, but 

 with larger food they make individual biting 

 attacks. S. japonicus eats food smaller than 

 would be predicted by gill raker gap (Hiyama 

 and Yasuda, 1957). The spiny process we ob- 

 served on the rakers of S. japonicus probably 

 form an even finer sieve than is formed by the 

 rakers themselves. Regardless of the mode of 

 selection (anatomical, behavioral, or perceptu- 

 al), the selective capabilities of scombrids and 

 coryphaenids would appear to be correlated with 

 the anatomy of the gill raker apparatus. 



An individual scombrid is able to prey on or- 

 ganisms differing greatly in size. It is capable 

 of engulfing and retaining crustaceans, small 

 fishes, and squid by means of a well-developed 

 gill raker apparatus. It is also capable of pur- 

 suing, capturing, and ingesting fast-moving 

 fishes and squids, provided they are not too large 

 to be swallowed whole. The gill raker gap and 

 maximum distensibility of the mouth and esoph- 

 agus then would be expected to set limits on the 

 range of food sizes eaten by scombrids. Within 

 this size i-ange a diverse faunal assemblage exists 

 in the sea that includes numerous species of 

 ci'ustaceans, fishes, and molluscs. The diversity 

 of species in the size range consumed by an in- 

 dividual scombrid has, to a great extent, masked 

 the selectivity that does occur. The present pa- 

 per provides some evidence for selection of or- 



ganisms above a minimum size determined by 

 the magnitude of gill raker gaps. 



ACKNOWLEDGMENTS 



We thank Reginald Gooding for assistance 

 with collection of data, Marian Yong and Betty 

 Ann Keala for computer processing the data, 

 and William H. Neill and Dr. James F. Kitchell, 

 Laboratory of Limnology, Madison; Witold L. 

 Klawe, Inter-American Tropical Tuna Commis- 

 sion, and Dr. Maurice Blackburn, Scripps Insti- 

 tution of Oceanography, who critically reviewed 

 the manuscript. This project was supported 

 entirely by the Bureau of Commercial Fisheries 

 Biological Laboratory, Honolulu. 



LITERATURE CITED 



Alverson, F. G. 



196.3. The food of yellowfin and skipjack tunas in 

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1953. Observations on the food and feeding of the 

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 Blackburn, M. 



1968. Micronekton of the eastern tropical Pacific 

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 HrvAMA, Y., and F. Yasuda. 



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IVERSEN, E. S., AND H. O. YOSHIDA. 



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369 



