Abstract
Reef fish sustain populations on isolated reefs and show genetic diversity between nearby reefs even though larvae of many species are swept away from the natal site during pelagic dispersal. Retention or recruitment to natal reefs requires orientation capabilities that enable larvae to find their way. Although olfactory and acoustically based orientation has been implicated in homing when larvae are in the reef’s vicinity, it is still unclear how they cope with greater distances. Here we show evidence for a sun compass mechanism that can bring the larvae to the vicinity of their natal reef. In a circular arena, pre-settlement larvae and early settlers (<24 hours) of the cardinal fish, Ostorhinchus doederleini, showed a strong SSE directional swimming response, which most likely has evolved to compensate for the locally prevailing large scale NNW current drift. When fish were clock-shifted 6 hours, they changed their orientation by ca. 180° as predicted by the tropical sun curve at One Tree Island, i.e. they used a time-compensated sun compass. Furthermore, the fish oriented most consistently at times of the day when the sun azimuth is easy to determine. Microsatellite markers showed that the larvae that had just arrived at One Tree Island genetically belonged to either the local reef population or to Fitzroy Reef located 12 kilometers to the SSE. The use of a sun compass adds a missing long-distance link to the hierarchy of other sensory abilities that can direct larvae to the region of origin, including their natal reef. Predominant local recruitment, in turn, can contribute to genetic isolation and potential speciation.
Highlights
The persistence of reef fish populations on isolated oceanic islands demonstrates that a significant number of their larvae return to their natal reef after their pelagic dispersal phase [1,2,3,4]
Far, no plausible mechanism has been demonstrated that could allow pelagic larvae to locate the natal reef from distances beyond a few kilometers after their initial period of passive dispersal during which they are not capable of sustained directional swimming
For description of the circular diagram, see legend to Figure 2. doi:10.1371/journal.pone.0066039.g003 (1) some larvae remain close to the reef, and (2) others drift further away on prevailing currents and may or may not return to the natal reef
Summary
The persistence of reef fish populations on isolated oceanic islands demonstrates that a significant number of their larvae return to their natal reef after their pelagic dispersal phase [1,2,3,4] Such ‘‘self-recruitment’’ has been shown using transgenerational otolith tagging [5,6] and genetic markers e.g. Swimming capabilities of larval reef fishes are surprisingly strong, late in the pre-settlement period [9,10]; and sensory guidance based on acoustic [11,12,13] and olfactory [14,15] mechanisms have been suggested Both odor halos [14] and acoustic cues [16] become fainter with distance and are unlikely to provide useful directional information beyond a few kilometers. The aim of the present paper is to investigate whether reef fish larvae have a compass mechanism that could be helpful for long-distance homing
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