Larval Behavior 



Another factor that can affect larval mortality and dispersal is the 

 behavior of fish larvae at sea. It is generally conceded (although with little 

 direct evidence) that fish larvae disperse passively in offshore currents (Sale, 

 1970; Leis and Miller, 1976) and, therefore, that their final destinations are 

 subject to hydrographic conditions. Alternatively, it has been argued that 

 larval drift is not passive (Sale, 1980). In Hawaii, larvae from species with 

 pelagic eggs (labrids, chaetodontids, acanthurids) are more common offshore 

 than species with demersal eggs (pomacentrids and gobies), which are more common 

 inshore (Miller, 1974; Leis and Miller, 1976; Watson and Leis, 1974). This is 

 explained by invoking active larval mechanisms. However, it may also be 

 explained merely by initial passive drift of pelagic eggs. Nevertheless, once 

 fish larvae begin to swim, vertical movements could place them in currents that 

 would hold them inshore or advect them away. Unfortunately very little is 

 known about larval behaviors. Several investigations have established that 

 many pelagic invertebrates undertake marked diel vertical migrations, ascending 

 toward the surface at night and to deeper depths during the daytime (see Segal, 

 1970, for overview). Recent investigations also have revealed that reef 

 meroplankton emerge high into the water column at night, especially during the 

 dark phase of the moon (Porter, et aU_, 1977; Porter and Porter, 1977; Alldredge 

 and King, 1977, 1980; Hobson and Chess, 1979; Robichaux, et^aK, 1981; Ohlhorst, 

 1982, 1985). These vertical movements take place over a few meters. Similar 

 vertical distributions seem to occur in coral reef fish larvae, at least nearshore 

 (Hobson and McFarland, preliminary unpubl. data). It is known, however, that 

 reverse tidal currents can occur over shallow depths, as they do at St. Croix 

 (Lee, et al . , 1977), and it is possible, even likely, that fish and invertebrate 

 larvae use these currents, in combination with vertical migratory behavior, in 

 thei r dispersal . 



Certainly broad parallels occur between benthic marine invertebrates and 

 fishes. In tropical regions, 80-85% of the invertebrates produce long lived, 

 planktotrophic larvae that will hatch, as in fishes, from relatively small eggs 

 (Thorson, 1950). In general, larval pelagic existence is longer in tropical 

 invertebrates than in species from higher latitudes (see Thorson, 1950; 

 Mileikovsky, 1971; and Jablonski and Lutz, 1983, for review). Extended larval 

 periods appear to enhance dispersion and are associated with broad geographic 

 distributions in tropical invertebrates (Scheltema, 1968, 1971), and this 

 pattern seems, at least tentatively, to also hold for coral reef fishes (Brothers 

 and Thresher, 1985). 



In summation, there is no simple explanation in terms of dispersal versus 

 antipredatory mechanisms that resolves what major selective processes have 

 caused virtually all coral reef fishes to produce pelagic propagules. In fact 

 the data cited show such wide variation that it is likely that in some species 

 dispersal was the preeminent selective factor, while in other species antipreda- 

 tion was the most important. In any case, dispersal and antipredation are not 

 mutually exclusive selective factors, and certainly both act to increase 

 survival. Nevertheless, it is difficult not to wonder, as Barlow (1981) has, 

 why more coral reef fishes do not invest in postzygotic activities to reduce 

 the duration of the larval pelagic phase, or even eliminate it. 



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