Fish with wonky ears less successful on reefs


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Reef fish larvae require symmetrical ears to ensure successful navigating from open waters back to the reef, according to the results of a new study.

Monica Gagliano, Martial Depczynski, Stephen Simpson and James Moore studied the damselfish Pomacentrus amboinensis, a common and abundant species on the Great Barrier Reef.

Following hatching, P. amboinensis larvae undergo a 15- to 23-day pelagic phase in the open ocean before returning to the reef, where they settle directly into adult coral reef habitats.

The authors captured fish larvae using four light traps that were equipped with underwater speakers continuously playing recorded sounds of the reef (made up of a chorus of pops made by nocturnal fishes together with a higher frequency but lower-intensity background crackle produced by snapping shrimps as well as other nocturnal feeding, movement and calling sounds).

Another two light traps similarly equipped with dummy speakers were used to control for any innate attraction of the fish larvae to the floating speakers.

They then analysed the otoliths or ear-bones for symmetry (i.e. whether the left and right otoliths are of similar size and shape).

According to the authors ishes depend on a comparison of left versus right saccular input to the brain to resolve the azimuth of the sound direction...and morphological differences between the right and left paired saccular otoliths can pose problems for sound localization due to incongruity in the movement of the right and left otolith within the saccule...Hence, even a small degree of asymmetry in the shape of the otolith pair can cause significant differences in the acoustic functionality of young fishes, thereby potentially jeopardizing the successful recruitment of larvae to suitable benthic habitats.

The authors found that traps broadcasting high-frequency sound attracted between 9 and 18% more symmetrical recruits than silent traps and that that ...fishes caught in traps broadcasting sound were not only more asymmetrical, but they also reached the reef after a longer pelagic phase compared with conspecifics attracted to the sound.

They interpret this as individuals with higher levels of asymmetry of the otolith pair as encountering more difficulties in detecting and orienting towards the reef habitat, and encountering more detours and delays on reaching the reef to locate suitable settlement sites.

They concluded that larvae with asymmetrical otoliths not only encountered greater difficulties in detecting suitable settlement habitats, but may also suffer significantly higher rates of mortality.

For more information, see the paper: Gagliano, M, M Depczynski, SD Simpson and JAY Moore (2008) Dispersal without errors: symmetrical ears tune into the right frequency for survival. Proceedings of the Royal Society B: Biological Sciences 275, pp. 527"534.