FERRARO: DAILY TIME OF SPAWNING 



tion with the biotic or abiotic environment 

 (Nikolsky 1963; Aschoff 1964; Schwassman 1969). 

 The annual spawning cycle of fishes which may be 

 timed to coincide with the annual production cy- 

 cle, or a period of low predation, etc. (Nikolsky 

 1963; Gushing 1969; Hoar 1969), may be the 

 coarse adjustment, and diel spawning time the 

 fine tuning adjustment to temporally changing 

 environmental conditions. 



Spawning periodicity may be important in syn- 

 chronizing reproduction between the sexes (As- 

 choff 1964; Marshall 1967). The precise daily tim- 

 ing of reproduction may be particularly important 

 in species which engage in mass spawnings. An 

 extreme example are the lancelets (e.g., Bran- 

 chiostoma lanceolatum) which, according to Bre- 

 der and Rosen ( 1966), release eggs and sperm into 

 the water at sunset for chance fertilization. Tem- 

 porally synchronizing spawning in pairing species 

 presumably is more efficient and may optimize the 

 number of receptive encounters. 



Marshall (1967) suggested diel spawning 

 periodicity might serve to increase reproductive 

 isolation between related and morphologically 

 similar species. Reproductive isolation may be im- 

 portant in a species-rich habitat such as a coral 

 reef, but many temperate water marine fishes ap- 

 parently spawn at or about the same time, i.e., in 

 the evening and at night (Tables 4, 5). 



Diel spawning periodicity could be an adaptive 

 behavior of fishes to avoid high incident solar 

 radiation during a very sensitive period of em- 

 bryonic development. Bell and Hoar (1950) and 

 Eisler (1958, 1961) demonstrated the lethal and 

 deleterious effects of light on salmonid embryos, 

 especially during early embryogenesis; and Mari- 

 naro and Bernard (1966) demonstrated lethal ef- 

 fects of light, particularly ultraviolet light, on 

 some marine planktonic fish embryos. Perlmutter 

 (1961) and Breder (1962) believed that some 

 characteristics offish eggs (e.g., transparency, oil 

 droplets, melanophores) were physiological 

 adaptations to minimize deleterious effects of 

 lighten fish embryos, and Perlmutter (1961) listed 

 several spawming behaviors of fishes which he 

 thought were adaptions to avoid or minimize ex- 

 posure of embryos to light. If light is especially 

 harmful to recently fertilized fish eggs, nocturnal 

 spawning could be an adaptive behavior to avoid 

 light during early embryonic development. How- 

 ever, an explanation is then necessary for why 

 light is not a factor for diurnal spawners such as 

 Atlantic mackerel and scup. 



Nikolsky (1963) suggested that some fishes 

 spawn at times of day when spawning adults or 

 their eggs will be least susceptible to predation. 

 The "exhausted" condition of some fishes that have 

 recently spawned (Brawn 1961; Breder and Rosen 

 1966) and evidence of increased vulnerability of 

 some spawning fishes to trawling (Mohr, in Blax- 

 ter 1965) tend to support the idea that spawning 

 time may be an adaption to minimize losses due to 

 predation on the parents. Visual predators, pre- 

 dators with diurnal feeding patterns, or predators 

 which undergo diurnal vertical migrations (e.g., 

 ctenophores; Hirota 1974) could subject 

 planktonic fish embryos to different levels of pre- 

 dation over a diurnal cycle. Synchronizing daily 

 spawning time to coincide with a period of low fish 

 embryo predation minimizes fish embryo mortal- 

 ity due to predation. If a fish embryo predator had 

 a diurnal predation cycle of 12 h high and 12 h low 

 predation. Figure 3 shows that a fish spawning at 

 the beginning of a low predation period ensures 

 50% or more of the embryo incubation time of its 

 progeny will be spent at the low predation level. 

 Qualitatively the results would be the same if 

 changes in predation were gradual and periods of 

 high and low predation were of different dura- 

 tions. Figure 3 also shows that if predation cycles 

 cause diel spawning periodicity, selection for diel 

 spawning periodicity is potentially greater when 

 embryo incubation times are short. 



PREDAT I ON 



100 



X 50 



24 48 72 96 120 144 168 



INCUBATION TIME (h) 



FIGURE 3.— Relationship between percentage of time spent at 

 low predation when spawning occurs at the beginning of a period 

 of low predation and total incubation time of embryos, assuming 

 12 h alternating periods of low (L) and high (H) predation. 



461 



