Tolotti et al.: Vertical movements of oceanic whitetip sharks ( Carcharhinus longimanus ) 
391 
al., 2015; Tyminski et al., 2015). A 12-h periodicity in 
vertical behavior, however, has not been reported as 
often for pelagic species. Typically, a spectral peak at 
12 h is observed for coastal species, and such peaks 
are strongly linked to tidal cycles (Urmy et al., 2012). 
Similar to the peak observed for shark AOCS3, the 
strongest spectral peak in the periodogram of a bask- 
ing shark in another study had a period of 12.35 h 
(Shepard et al., 2006). The authors associated this peak 
with the tidal cycle because the shark remained on the 
continental shelf where there is clearly a tidal influ- 
ence. In contrast, shark AOCS3 remained in oceanic 
waters (Suppl. Fig. 1) with a potentially weaker tidal 
signal. Therefore, it appears unlikely that the 12-h 
spectral peak observed for this oceanic whitetip shark 
was linked to any tidal cycle. 
Instead, the 12-h peak appears to be related to 
vertical crepuscular movements. The oceanic whitetip 
sharks analyzed in this study occupied considerably 
shallower depths during the hours of dawn and dusk 
than during other periods of the day (Fig. 1). Coinci- 
dentally, at equatorial latitudes, day and night have 
similar lengths, resulting in a crepuscular event ev- 
ery 12 h (World of Earth Science, 2003). Shark AOCS3 
stayed at equatorial latitudes during its entire track- 
ing period, indicating that the 12-h spectral peak of 
vertical movement matches the crepuscular cycle. This 
dawn and dusk pattern of vertical movement was con- 
sistent throughout the time series of all 6 individu- 
als, and it has been identified also in the behavior of 
oceanic whitetip sharks tagged in the Pacific Ocean 
(Musyl et al., 2011). The sharpness of the 12-h peak in- 
dicates that this crepuscular pattern represents a fre- 
quent and pronounced feature in the oceanic whitetip 
shark behavioral repertoire. Similar patterns have 
been reported for other shark species and have been 
associated with foraging behavior and maintaining a 
preferred isolume (Nelson et al., 1997; Vianna et al., 
2013). In short, the variations in luminosity, charac- 
teristic of twilight hours, may represent the cues that 
regulate the behavioral shifts and feeding activity of 
this species. 
