their tendencies toward more fusiform bodies and 

 deeply incised caudal fins, diurnal planktivores 

 have acquired added speed that is adaptive in 

 quickening their return to reef shelter when 

 threatened. Expanding this suggestion, these fea- 

 tures are more developed in planktivores that 

 swim farther from the reef because threats from 

 predators probably increase in more exposed loca- 

 tions. Although morphology that permits faster 

 swimming would also enhance holding station in a 

 current, we believe the major selection pressures 

 shaping these features in planktivores have come 

 from predators. 



Despite the obvious adaptiveness of fusiform 

 bodies and of deeply incised caudal fins in many 

 planktivores, the morphologies of certain other 

 highly successful diurnal planktivores have taken 

 the opposite course. For example, among the fishes 

 we studied, Dascyllus reticulatus (Figure 7) and 

 Amblyglyphidodon curacao are among the deepest 

 bodied of pomacentrids, and yet they range farther 

 into the water column than the species ofChromis 

 or Pomacentrus. Similarly, the many planktivor- 

 ous chaetodontids in Hawaii (e.g., species of 

 Chaetodon and Hemitaurichthys), all deep-bodied 

 forms with truncate caudal fins, are highly suc- 

 cessful planktivores that range widely in the 

 water column (Hobson 1974). 



We suggest that whereas fusiform bodies in- 

 crease the chance of eluding predators, deep bodies 

 increase the chance of discouraging predators. The 

 basis of this second suggestion is the fact that 

 piscivores live with the danger of choking on 

 spiny-rayed prey lodged in their pharynx or 

 esophagus. Over the years we have seen many 

 predators in this predicament — often fatally. Pis- 

 civores generally swallow their prey head-first, 

 frequently after manipulation to ensure proper 

 orientation. Reasons for not swallowing a spiny- 

 rayed fish tail-first are obvious. Assuming, then, 

 that a prey fish is swallowed head-first, the danger 

 of it becoming lodged in the pharynx or esophagus 

 increases with its depth or width. Thus, predators 

 equipped to take prey from among the variety of 

 planktivores in the water column (where those at 

 a given level tend to be about the same length) 

 would find greater risk ingesting deeper bodied 

 forms, especially those with prominent fin spines. 

 Of course, this advantage of a deep body and prom- 

 inent spines in thwarting predators extends 

 beyond planktivores; the entire family Chaeto- 

 dontidae, for example, would benefit (Hobson and 

 Chave 1972). 



152 



FISHERY BULLETIN: VOL. 76, NO. 1 



ACKNOWLEDGMENTS 



We thank Stephen V. Smith, Director, and 

 Laboratory Mangers Philip and Janet Lamberson, 

 of the Mid Pacific Marine Laboratory at Enewetak 

 Atoll, for making facilities available to us. The 

 laboratory is supported by the Division of Biomed- 

 ical and Environmental Research of the U.S. 

 Energy Research and Development Administra- 

 tion and is operated as an extension of the Hawaii 

 Institute of Marine Biology, University of Hawaii. 

 For constructive criticism of the manuscript we 

 thank Carl L. Hubbs and Richard H. Rosenblatt, 

 Scripps Institution of Oceanogrphy; William M. 

 Hamner, Australian Institute of Marine Science; 

 Robert E. Johannes, Hawaii Institute of Marine 

 Biology; and William Lenarz, Tiburon Labora- 

 tory. John E. Randall, Bernice P. Bishop Museum, 

 Honolulu, identified Mirolabrichthys pascalus; 

 Kenneth Raymond, Southwest Fisheries Center 

 La Jolla Laboratory, National Marine Fisheries 

 Service, NOAA, drew Figure 1 and the fishes in 

 Figure 6; and Alice Jellett, Tiburon Laboratory, 

 typed the manuscript. 



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