Bioacoustics and reproductive ecology of the damselfish Dascyllus albisella
Mann, David A.
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While many fishes are known to produce sounds during courtship and aggression, the information contained in the sounds and their role in reproduction is not well understood. This thesis is an intensive investigation of the sounds produced by the damselfish Dascyllus albisella, the effect of the environment on their acoustic signals, and how the sounds relate to reproduction. D. albisella males produce pulsed sounds during the signal jump, visiting by females, mating, aggression to heterospecifics and conspecifics, and nest preparation. Females make only aggressive sounds. The pulse period of aggressive sounds was shorter than courtship sounds. There was no difference between visiting and mating sounds, except in pulse duration. Two types of aggressive sounds were produced, pops and chirps. Pops were more commonly made towards heterospecifics than conspecifics. There were no differences in courtship sounds made by males from Johnston Atoll and Hawaii, except in pulse duration, which are likely due to differences in the recording environment. The pulsed sounds produced during the signal jump of D. albisella were analyzed to determine what information they contain about the signal jump and how they change with propagation. There was no relationship between signal jump speed or distance with the number of pulses or pulse period of the sound. There was no consistent 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 1-2m to 5-10 dB at 11-12 m. It is unlikely that D. albisella can detect sounds at or beyond 11-12m from the sound source, based on noise masking data from other fishes. Pulse period is least affected by propagation when compared to peak frequency, pulse duration, inter-pulse interval, and coefficient of variation of pulse amplitudes within a call. These results suggest that the sound produced during the signal jump acts over short distances and that the pulse period provides the most reliable basis for signal detection. A passive acoustic detection system was developed to continuously record sound production activity of individual males in the field. The rate of sound production could be used to determine the timing of spawning. The daily rate of sound production increased until the day of spawning, after which it decreased by over half. Additionally, the amount of sound production at night was highest just before spawning. The passive acoustic detector also revealed that D. albisella had regular peaks of calling at dawn, similar to the dawn chorus in birds. Patterns of male reproductive success varied for individual males over successive reproductive cycles and was not correlated to male size. The variation in reproductive success suggests that females choose males based on characters that vary from cycle to cycle. Data from the passive acoustic detector showed rates of courtship were positively correlated with reproductive success for three males. The continuous time-series of sound production were analyzed to determine appropriate sampling strategies to measure male sound production over shorter time periods using SCUBA. However, short samples of sound production (10 minutes or 60 minutes per male per day) were poor estimators of peak calling rates and daily calling rates. The rich variation in male courtship rates may contain information about male condition that has been previously ignored. Two reproductive synchrony measures were developed and used in randomization tests to test for synchronization of reproduction within five sites in the Johnston Atoll lagoon. Groups of isolated fish spawned in synchrony, but not in synchrony with other groups, even as close as 20-30 m. There was no apparent selective pressure for synchronous spawning when brood size, brood loss, and brood failure were considered. It is possible, though currently untestable, that there is a benefit of synchronous spawning for larval survival. It is unlikely that reproduction is synchronized in response to an environmental cue, because the scale of synchronization is small. Synchronization might develop through the courtship sound, because it regularly increases and decreases with spawning and the range of detectability is on the order of the range of synchronization. But, it is also possible that males are responding to chemical cues released by females. Spawning synchrony was also analyzed for 10 damsel fish species. D. albisella was among the most synchronized species, along with Abudefduf troschelii. Using a phylogenetic analysis of Chrornis, Amphiprion, and Dascyllus there are three viable hypotheses concerning the evolution of reproductive synchrony in D. albisella 1) it is an evolutionary relict that is no longer selected for and possibly maladaptive, 2) it evolved as part of the harernic lifestyle of the common ancestor of the Dascyllus genus, or 3) it evolved as the result of selection pressure for synchronization during the larval stage.
Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the Massachusetts Institute of Technology and the Woods Hole Oceanographic Institution September 1995
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