Phillip S. Lobel

The role of herbivorous fishes and urchins in coral reef communities

SynopsisHerbivorous fishes and invertebrates are conspicious elements of coral reef communities where they predominate both in numbers and biomass. Herbivores and the coral reef algae on which they feed represent a co-evolved system of defense and counter-defense. Algal species have developed toxic, structural, spatial and temporal defense or escape mechanisms, while the herbivores employ strategies that involve anatomical, physiological and behavioral adaptations. Current research demonstrates that many reef fishes are highly selective in the algae they consume. Food selection in these fishes may be correlated with their morphological and digestive capabilities to rupture algal cell walls. Sea urchins select more in accordance with relative abundance, although certain algal species are clearly avoided.The determinants of community structure on coral reefs have yet to be established but evidence indicates a strong influence by herbivores. Reef herbivores may reduce the abundance of certain competitively superior algae, thus allowing corals and cementing coralline algae to survive. We discuss how the foraging activities of tropical marine herbivores affect the distribution and abundance of algae and how these activities contribute to the development of coral reef structure and the fish assemblages which are intimately associated with reef structure.

Aggregation behavior of the grey reef shark, Carcharhinus amblyrhynchos, at Johnston Atoll, Central Pacific Ocean

Free-ranging female grey reef sharks, Carcharhinus amblyrhynchos, were observed forming daily aggregations offshore Sand Island, Johnston Atoll between March and late May from 1992 to 1995. Daily water temperatures were recorded at the aggregation area from 1993 to 1995. The annual aggregation cycle did not coincide with maximum or minimum annual water temperatures. During the 1994 aggregation, temperatures were recorded at least every hour at 7 sites in the Atoll. The sharks aggregated most frequently and in highest numbers at the largest and shallowest site, which also contained the fewest underwater structures. The water temperature at this site was 1–2 °C warmer than at neighboring sites and at a reef channel between the lagoon and the open ocean. The pattern of movement of sharks to and from the aggregation area was correlated with daily fluctuations of water temperature (r=0.38; p < 0.001), tide (r=0.30; p < 0.001) and light level (r=0.21; p < 0.001) after adjustment for autocorrelated errors. The daily maximum number of aggregating sharks coincided with the daily maximum water temperature. However, the number of aggregating sharks did not necessarily increase during days of higher water temperature. During the 1994 aggregation period, four sharks were fed ultrasonic telemetry tags, and telemetry stations were deployed at three sites within the aggregation area. Individual sharks were tracked returning to the aggregation area for durations of one to five days. The end of detection of tagged sharks is thought to be due to the regurgitation of the indigestible tags, rather than the sharks permanent departure from the aggregating area. Sharks aggregated only during the day; none of the tagged sharks were recorded and no sharks were ever seen at the aggregation area during night time.

Propagation of damselfish (Pomacentridae) courtship sounds

Many damselfishes perform a courtship behavior known as the signal jump, in which they rise in the water column and then rapidly swim downward while producing a pulsed sound. Pulsed sounds produced during the signal jump of the damselfish Dascyllus albisella were analyzed to determine whether they were correlated with the signal jump distance or speed, and how they changed with propagation. No statistical relationship was found between signal jump speed or distance with the number of pulses, pulse period, or change in the peak frequency of pulses in a call. If echoes were present in the sound, the change in echo delay would likely have been too small for damselfish to detect. Sounds attenuated with distance such that the signal-to-noise ratio decreased from 17–25 dB at 1 to 2 m to 5–10 dB at 11 to 12 m. It is unlikely that D. albisella can detect sounds at or beyond 11–12 m from the sound source, based on noise masking data from other fishes. Pulse period is least affected by propagation when compared to ...

SPAWNING SOUNDS OF THE DAMSELFISH, DASCYLLUS ALBISELLA (POMACENTRIDAE), AND RELATIONSHIP TO MALE SIZE

ABSTRACT Synchronous audio-video recordings were made of free-living Dascyllus albisella on coral reefs at Johnston Atoll, Central Pacifc Ocean. Males produced distinct and consistent sounds during courtship and mating. The courtship sound is a well-known feature of pomacentrid behavior, and is produced during the signal jump. Male D. albisella also produced a mating sound, which has not been previously described for any other pomacentrid. The mating sound is produced as the male quivers during spawning. The courtship sound differed from the mating sound by having a greater number of pulses (6±4 vs. 3±1, mean ±SD) and a longer duration (262±57 vs. 127±45 msec). The courtship sounds of larger males were lower in frequency than those of smaller males (r2-0.64, power regression). The median dominant frequency of small males (20 to 40 g) was 390 Hz (n=12 males), compared to 334 Hz (n=7 males) for large males (40 to 60 g).

Sounds produced by spawning fishes

SynopsisLow frequency sounds are shown to be associated with the spawning of two Caribbean coral reef fishes: the hamlet, Hypoplectrus unicolor (Serranidae) and the striped parrotfish, Scarus iserti (Scaridae). Both fishes produce distinctive sounds while broadcasting gametes in midwater. H. unicolor produces sounds via muscle stimulation of the swimbladder. Fin movements among group spawning S. iserti produce hydrodynamic noise. Although reproductive behaviors of these two species have been previously studied in detail, the association of sounds with mating is new. The mating sounds cannot be easily detected by human hearing underwater but are recordable using a hydrophone. The sounds are distinct and recognizable enough to allow counting and acoustic mapping of mating events in these species.

THE CONTINUING ENIGMA OF CIGUATERA

Research on ciguatera fish poisoning has expanded significantly over the last decade. In large part, this increase in effort is due to the identification of several benthic dinoflagellates as the toxin producers, a discovery soon followed by a series of field and laboratory studies on their distribution, abundance, growth characteristics, and toxin production. Equally important have been advances in the analytical techniques and equipment needed to chemically characterize the toxins. Much of that work benefited significantly from the rapid progress in chemical research on the numerous other toxins produced by marine dinoflagellates.Despite this surge in activity (summarized in the proceedings of four recent conferences or workshops: Ragelis, 1984; Salvat, 1985; Anderson et al., 1985; this issue), the general state of knowledge on ciguatera remains relatively poor, both in terms of toxin chemistry and the physiological ecology of the causative dinoflagellates. Some important generalizations are gaining acc...

Acoustic behavior of the damselfish Dascyllus albisella: behavioral and geographic variation

Behavioral and geographic variation in animal communication has been well-studied in insects, frogs, birds, and mammals, but little is known about variation in fishes. We used underwater audio-video recordings of the behavior and associated sounds produced by the domino damselfish, Dascyllus albisella, at Johnston Atoll and Hawaii, which are separated by 1000 km, to study behavioral and geographic variation in communication sounds. Males produced pulsed sounds during the courtship behavior known as the signal jump, visiting by females (during pseudospawning), mating, aggression to heterospecifics and conspecifics, and nest preparation. Females made only aggressive sounds. The following features of the sounds were measured: number of pulses, pulse rate, pulse duration, inter-pulse interval, dominant frequency, and frequency envelope. The only difference between visiting and mating sounds was a small difference in pulse duration. Two types of aggressive sounds were produced, pops and chirps. Pops contained only one or two pulses and were more commonly made towards heterospecifics than conspecifics. Aggressive chirps had between 3–11 pulses and were made most often towards conspecifics. The pulse rate of aggressive chirps was faster than signal jump sounds. The only difference in signal jump sounds made by males from Johnston Atoll and Hawaii, was a small difference in pulse duration, which was likely due to differences in the depths of the recording environment and not in the sounds produced.

PASSIVE ACOUSTIC DETECTION OF SOUNDS PRODUCED BY THE DAMSELFISH, DASCYLLUS ALBISELLA (POMACENTRIDAE)

ABSTRACT We developed and field-tested a passive acoustic detector that collects data on sound production by sonic fish. The detector was deployed to measure the timing of sound production by males of the damselfish Dascyllus albisella (Pomacentridae), at Johnston Atoll, Central Pacific Ocean. Sound production rates were higher during the reproductive season (April) than during the non-reproductive season (October). The highest rates of sound production occurred on the day before and day of egg-laying. Sound production rates decreased during brood care, and increased again after hatching. The correlation of sound-production rate with the spawning cycle provided a reliable acoustic signal that was monitored by the detector. This new technology provides a capability for obtaining detailed measurements of reproductive activity over long time periods. Multiple detectors can be used simultaneously to monitor reproduction over large spatial scales.

Possible species specific courtship sounds by two sympatric cichlid fishes in Lake Malawi, Africa

Sounds were produced by the males of two species of cichlid fishes while courting females. Each courtship sound consisted of a series of distinct pulses occurring in rapid succession. Courtship sounds produced by Tramitichromis cf. intermedius and Copadichromis conophorus were significantly different in pulse rate and individual pulse durations. For C. conophorus calls (n=127) the mean ± sd number of pulses per call was 10 ± 3 and call duration was 181 ± 59 ms. There was a significant positive linear relationship between call duration and the number of pulses (r2=0.912, p < 0.001). The dominant frequency of the pulses in calls was 471 ± 50 (range 372–594) Hz (n=40 calls). T. cf. intermedius also produced a pulsed courtship call; data (mean ± sd) from two male T. cf. intermedius: 9 ± 2 pulses per call and duration 199 ± 44 ms (n=20 calls). The linear regression between call duration and number of pulses was positive (r2=0.463, p=0.001). Pulse rate within calls of T. cf. intermedius compared to C. conophorus were significantly different (p=0.018). Individual pulse durations were also significantly different (p=0.043) between species. However, interpulse intervals were not significantly different (p=0.177). These cichlids produced courtship sounds that were distinct by individual pulse durations and by pulse repition rate in a call.

Ocean current variability and the spawning season of Hawaiian reef fishes

SynopsisThe spawning patterns of four Hawaiian reef fishes with similar reproductive habits, but different biogeographic distributions were studied from September 1980 to October 1981. Two species are Hawaiian endemics [Centropyge potteri (Pomacanthidae) and Chaetodon multicinctus (Chaetodontidae)] and the other two have pan-tropical Pacific distributions [Ctenochaetus strigosus and Zebrasoma flavescens (Acanthuridae)]. All showed increased spawning activity from January to July, the prevalent pattern among a majority of coastal marine fishes in Hawaii. Environmental correlatives to the period of peak reproduction include cycles of (a) daylight length and temperature which probably function as proximate cues and (b) seasonal variations in ocean current patterns which may ultimately affect survival of larvae and dispersal. Peak reproduction takes place during months when (1) mesoscale eddies most likely occur, and (2) the probability is greatest of drifting objects remaining near the islands.