FISHERY BULLETIN: VOL. 70. NO. 3 



Observations on Labroides phthirophagus. — 

 In describing evening cover-seeking by La- 

 broides phthirophagus above, data are presented 

 on just one individual. At least five individuals 

 of this species are involved in the nine obser- 

 vations of the initial morning appearance (Fig- 

 ure 11). 



Other fishes seeking cleaning from this spe- 

 cies often hover in typical soliciting fashion at 

 the cleaning stations (see Losey, 1971) for some 

 time before the cleaners have made their initial 

 morning appearance. 



SUMMARY 



The data presented above are summarized in 

 Figures 12 and 13. 



DISCUSSION 



Most fishes on the reefs at Kona behave dif- 

 ferently in daylight than they do at night, and 

 during twilight their actions express a transition 

 between these two modes of behavior. In es- 

 sence, we are concerned with predominantly di- 

 urnal or nocturnal fishes changing from active 

 to relatively inactive states, or the reverse, and 

 the influence that certain characteristics of twi- 

 light itself have on these actions. These con- 

 siderations underlie the following discussion. 



FEEDING LOCATIONS VERSUS 

 SHELTER LOCATIONS 



The daily activities of tropical reef fishes show 

 the overriding influence of two primary con- 

 cerns: to eat and to avoid being eaten. Reef 

 fishes must contend with predators at all times, 

 but during those periods of day when they are 

 active their actions seem dominated by their own 

 feeding. On the other hand, during those pe- 

 riods of the day when they are relatively in- 

 active, resting, their major consideration seems 

 to be security. Thus, for example, the small 

 parrotfish Scariis taeniurus grazes in exposed 

 positions during daylight, but rests in coral crev- 

 ices at night, often encased in a mucous envelope. 

 Winn and Bardach (1959) discussed the func- 

 tion of the mucous envelope as a defense against 

 nocturnal predators. 



100 



10 



01 



0001 



00001 



0.00001 



COVER^SEEKING 

 OF DIURNAL FISHES 



^®x. 





10 



10 20 30 40 



TIME RELATIVE TO SUNSET (mm) 



50 



Figure 12. — Summary of events during the evening 

 transition period. For explanation of symbols, see leg- 

 ends for Figures 2, 4, and 5. In addition, numbered 

 points represent mean values for events. 1) When the 

 last Thalassoma duperrey took cover on five evenings. 

 2) When a certain Labroides phthirophagus took cover 

 on five evenings. 3) When the last Chromis leucurus 

 took cover on three evenings. 4) Abrupt vacating of 

 water column by diurnal fishes; the beginning of the 

 interim, or "quiet," period: 12 evenings. 5) When 

 Apogon snyderi first left cover on four evenings. 6) 

 When Apogon menesemus first left cover on five evenings. 

 7) When Holocentrus sammara first left cover on four 

 evenings. 8) When Myripristis spp. abruptly appeared 

 in large numbers above the reef; the end of the interim, 

 or "quiet," period: eight evenings. 



Tropical reef fishes exhibit either one of two 

 general means of reducing predation during 

 their inactive periods: they school or they seek 

 cover (Hobson, 1968). Thus, many fishes that 

 forage at night, including various clupeids, po- 

 madasyds, mullids, lutjanids, and carangids, 

 characteristically spend the day in large inactive 

 schools that hover in exposed positions on or near 

 the reef. The advantage of such schools in re- 

 ducing predation was discussed earlier (Hobson, 

 1968). Significantly, no diurnal fish is known 



730 



