Donald R. Nelson

Prey Capture by the Pacific Angel Shark, Squatina californica: Visually Mediated Strikes and Ambush-Site Characteristics

Eleven of 23 sharks tested attacked a fish model from beneath a transparent partition, which was designed to block near-field mechanical cues. Head-on approaches by the model elicited a higher frequency of attacks and shorter attack latencies than caudal approaches (frequency, P = 0.04; latency, P = 0.03). Night attacks may have been facilitated by turbulence-generated bioluminescence. Resting sharks and vacated depressions were usually adjacent to reefs, either facing or aligned parallel to margins of nearby reefs, and oriented toward upslope directions. Four sharks that were identified based on natural pigment patterns returned to the near proximities (< 3 m) of previously occupied ambush sites. Based on our experiments and observations, we suggest that (1) prey movement sensed visually is the most important cue for eliciting daytime attacks, (2) the sharks probably possess an anterodorsally directed field of vision, and (3) ambush sites are selected based on substrata characteristics likely to be associated with prey availability.

Acoustic attraction of Pacific reef sharks: effect of pulse intermittency and variability.

Abstract 1. 1. Responses to low-frequency, pulsed sounds were observed in 5 species of reef sharks at Eniwetok Atoll, Marshall Islands. During 45 sound playback periods 253 sharks were sighted, while 44 were seen during 45 corresponding control periods. 2. 2. Of three artificial attractive sounds of identical frequency (25–100 Hz), significantly more sharks were seen during Sound 2 (10 pulses/sec, intermittent) and Sound 3 (15−7·5 pulses/sec, intermittent) than during Sound 1 (10 pulses/ sec, continuous). Response intensities of attracted individuals were highest for Sound 3, somewhat less for Sound 2 and least for Sound 1. 3. 3. Results indicate that pulse intermittency clearly enhances attractive value and is probably more important in this regard than pulse-rate variability.

Diel Activity Rhythms in the Nocturnal, Bottom-Dwelling Sharks, Heterodontus francisci and Cephaloscyllium ventriosum

The horn shark, Heterodontus francisci, and the swell shark, Cephaloscyllium ventriosum, exhibited distinct nocturnal activity patterns when observed underwater in the natural environment. Swimming activity began shortly after dusk and continued until dawn, after which very little activity was seen. Laboratory records for the horn shark under fixed and shifting LD 12:12 (12 hr light-12 hr dark) regimes indicated an exogenous rhythm, with an activity onset directly controlled by the onset of darkness. Horn sharks showed a high level of aperiodic activity in constant darkness and a low level of aperiodic activity in constant light. Activity in a swell shark appeared initiated by the onset of darkness during fixed LD 12:12 regimes, but slightly preceded darkness during a shifting (1 hr later/day) LD regime. This swell shark exhibited an endogenous (i.e., circadian) rhythm of shortened period (advancing phase drift) in constant darkness, and lengthened period (delaying phase drift) in constant light.