PELAGIC DURATION, DISPERSAL, AND THE DISTRIBUTION OF 

 INDO-PACIFIC CORAL-REEF FISHES 



Edward B. Brothers 



3 Sunset West 



Ithaca, New York 14850 



Ronald E. Thresher 



Division of Fisheries Research 



C.S.I.R.O. Marine Laboratories 



Cronulla, N.S.W. , 2230 Australia 



ABSTRACT 



The breadth of distribution of most Indo-Pacific coral-reef fishes does 

 not clearly correlate with the duration of their pelagic developmental stages. 

 Rather, there is a threshold effect at approximately 45 days, above which 

 species tend to be broadly distributed, but below which pelagic duration and 

 distribution appear unrelated. Long larval durations also characterize species 

 that have colonized geographically isolated areas, such as the Hawaiian Islands 

 and the Eastern Pacific. Comparison of larval durations with estimates of time 

 required to cross the "East Pacific Barrier" further suggests colonization of 

 the New World to be an infrequent event, most likely associated with unusually 

 strong development of the Equatorial Countercurrent. 



INTRODUCTION 



The pelagic larval stage characteristic of many invertebrates and fishes 

 is widely assumed to serve as a dispersal mechanism (Scheltema, 1971, 1978; 

 Crisp, 1976; Barlow, 1981; Smith, 1982). A logical corollary of this assumption, 

 also widely accepted, is that long larval duration correlates with broad 

 dispersal and consequent broad distribution (Shuto, 1974; Strathmann, 1974; 

 Zinsmeister and Emerson, 1979; Reaka, 1980; Ayal and Safriel, 1982; Jablonski 

 and Lutz, 1983). We tested this hypothesis by comparing the distributions 

 of 115 species of Indo-Pacific coral-reef fishes, in 22 families, with the 

 duration of their pelagic developmental stages, determined by examination of 

 otolith microstructure. The data suggest a threshold effect, with long pelagic 

 durations correlating with broad distributions only above a species mean value 

 of approximately 45 days. Only 20% of the species we examined have pelagic 

 durations this long, a value in surprisingly close agreement with independent 

 estimates of the frequency of "true long-distance" pelagic larvae of marine 

 invertebrates (Thorson, 1961). We also examine the question of whether long 

 pelagic durations are typical of those species that have colonized geographically 

 isolated areas, such as the Hawaiian Islands, and, particularly, the Eastern 

 Pacific. 



METHODS 



Duration of the pelagic larval stages was determined either by aging of 

 individuals newly recruited to the reef (Brothers, et al . , 1983) or by examination 



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