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Reef fish larvae may respond to changes in the concentration of water chemistry. These chemical cues can come from multiple sources in the larvae’s environment. The larvae may also respond to changes in water temperature, because a change in temperature affects the dispersal and activity of the chemical stimuli. The ability to sense these chemical cues depends on the presence and sensitivity of sense organs. The larvae also have to be able to determine the source of the cues to orient themselves and swim accordingly (Kingsford et al. 2002). Chemical stimuli may be of biotic or abiotic origin, both of which are very useful in reef detection. Metabolites from reef organisms, amino acids, fatty acids, and alcohols are all stimuli of biotic origin. These cues are useful because they signify to the larvae that there is a concentration of life in the area meaning food and mates will be available. Useful abiotic cues are salinity, temperature, and calcium carbonate from reefs (Kingsford et al. 2002). Different salinities and temperatures are found on both vertical and horizontal planes and can be excellent cues. For example, warm reef plumes may provide directional cues, within a range of a kilometer or two. Chemical cues are utilized by reef fish larvae for settlement at small spatial scales, and used in navigation at larger spatial scales (Kingsford et al. 2002). Though it is difficult to test how the larvae sense these chemical cues, experiments have been performed on the olfactory organs of reef fish larvae. Details of this will be discussed below. Results confirm that this is one way the larvae detect chemical cues.
Many species of reef fish (specifically species of apogonids, pomacentrids, blennies, and gobies) are able to use odor cues to locate a settlement habitat (Wright et al. 2005). In a physiological study done by K.J. Wright et al., 2005, the olfactory organs and abilities of pomacentrids (damsel fish) were examined using the non-invasive EOG technique (electroolfactogram) which “measures olfactory transduction by recording the change in the negative electrical potential at the surface of the nasal epithelium,” (Wright et al. 2005). This group of reef fish is one of the dominant fish families on coral reefs, both in biomass and number of species. The ability of pre- and post-settlement individuals to sense the amino acid L-alanine using olfaction was tested. Amino acids are useful chemical cues for locating a reef because there is a concentrated source of amino acids in the reef environment from the high density of organisms living there (Wright et al. 2005). The pomacentrids in this experiment were able to process olfactory information. The olfactory responses to the amino acid L-alanine of pre- and post-settlement individuals were the same. This indicates that an acute olfaction sense is needed at the early pre-settlement life stage. This experiment concluded that pomacentrids use odor cues during the larval phase. This physiological study supports previous behavioral studies testing larval use of olfaction in detecting reef habitats (Wright et al. 2005).
Sound travels well underwater, independent of currents, and so, is a plausible cue for fish larvae locating a reef. Fish and invertebrates living on coral reefs create noises that can be heard several kilometers away. These noises tend to occur and be loudest at night when most reef organisms are active (Leis et al. 2003, Simpson et al. 2005). Physiological studies have shown that the otoliths of larval fish – a structure in the inner ear canal that aids in hearing- are present at the early developmental stages (Leis et al. 2003). Most teleosts are able to hear quite well within a range of 100 to 2000 Hz, (Leis et al. 2003). In fact, hearing in an adult fish is as good as the hearing abilities of adult mammals and birds. Both physical and biological sound sources can be used by larvae. Breaking waves, which can vary daily and seasonally, are physical sound sources, whereas noises made by organisms are biological sounds (Kingsford et al. 2002). In order for these sound cues to be helpful, the larvae have to be able to detect the sound, and locate the source.
Simpson et al., 2004, conducted a behavioral experiment testing larval ability to sense and localize sound. Twenty-four patch reefs were built. Speakers broadcasting pre-recorded reef noise (mostly snapping shrimp and fish calls) were submersed in 12 of these patch reefs, while the other 12 had no noise broadcast. Reef fish larvae preferred the noisy reefs to the silent ones, with more taxa represented on the noisy patch reefs (see Figure 1(A) and (B)). Simpson et al. tried this procedure again, but using high frequencies greater then 570 Hz (primarily shrimp) in some patch reefs, and low frequency noises less than 570 Hz (primarily fish) in other patch reefs. These treatments were compared to the silent patch reefs. This time, almost four times as many larvae arrived on the noisy reefs. As before, the silent reef received less settlement. Interestingly, some species preferred settling on the reefs broadcasting lower frequency sounds, and some preferred the reefs broadcasting higher frequency sounds (see Figure 4(C) and (D)). For instance, pomacentrids were attracted to the higher frequency reefs. This suggests that some species are more sensitive to either high or low frequencies, possibly the organisms making these sounds. The results of this study confirm that sound cues are used by settling reef fish larvae (Simpson et al. 2004).
Figure 1 (Simpson et al. 2005): Comparison of catches from patch reefs with different sound treatments. (A and B) Reefs broadcasting reef noise (black) or silent reefs (white). (C and D) Reefs with high-frequency (black) or low-frequency (grey) reef noise or silent reefs (white). Statistical results are for (A) Chi-squared analyses, (B) Wilcoxon’s matched pairs test, (C) pairwise Chi-squared analyses with Bonferroni corrections, and (D) pairwise Wilcoxon’s matched pairs test with Bonferroni corrections (ms, P< 0.1;*, P< 0.05;**, P< 0.01). All apogonids and pomacentrids were excluded from the analyses in (B) and (D).
To support behavioral studies like those described above, a physiological study was done to determine the capabilities of the auditory organ of reef fish larvae (Wright et al. 2005). This was done by measuring the auditory brain stem responses of the larvae on an audiogram, by using sub-dermal electrodes on larvae that were placed in a fish holder. Sounds were broadcast at different intensities and frequencies. The results of the study indicated that reef fish larvae were capable of using auditory organs to process sounds of frequencies between 50 and 2000 Hz. The results for both pre- and post-settlement larvae were similar, suggesting pre-settlement larvae use their auditory organs for some purpose, possibly sensing and locating reef sounds (Wright et al. 2005).
Most reef fish larvae possess some form of eyes, present at a very early developmental stage. Vision is a well developed sense in reef fish (Lecchini et al. 2005). Because objects underwater are most likely visible within about 50 meters, visual cues are probably only used over small spatial scales (Kingsford et al. 2002). In a study done by David Lecchini et al., 2005, the larvae were tested on whether or not they used short range visual cues. A large aquarium like the one pictured in Figure 2 was used and tests were run to validate the absence of external effects on the aquarium. The goal of the experiment was to test if larvae could visually process conspecifics and/or coral habitat, and use these as cues in movement toward a suitable habitat. First, the larvae were placed in the central compartment, (A), and were able to move freely to either compartment (E) or (D) (see Figure 2). Tanks 1 and 2 were separated from the main tank in order to keep out any cues other than visual ones. In this way, visual cues were responsible for the movement of the larvae. Heterospecifics were placed in tank 2 and conspecifics in tank 1. After a period of 2 minutes the compartment in which the larvae were present was recorded. The distribution of larvae in this test was compared to a test run where no cues were present, visual or otherwise. In a second experiment, Lecchini et al. used coral habitat from where the larvae will generally settle. This time coral habitat was placed in tank 1. Results of the first test indicated that in the presence of visual cues from conspecifics, 10 out of 18 species studied used these cues to move significantly toward the conspecifics. Results of the second test indicated that in the presence of visual cues from coral habitat, none of the species tested showed significant movement toward the coral. These results imply, at least in the species studied, visual cues come from conspecifics and not coral colonies (Lecchini et al. 2005). It is suggested that those species that did not show significant movement toward conspecifics, could still possibly be using the visual cues of conspecifics in their movement (Kingsford et al. 2002). By staying out of the compartment nearest the conspecifics they may be staying out of danger. Some larvae may use visual cues of territorial conspecifics as a warning to avoid the area. More studies are needed to determine if this is the case. In the above described experiment, Lecchini et al., 2005, confirmed the use of visual cues by reef fish larvae.
Figure 2 (Lecchini et al. 2005): The aquarium system used to evaluate the sensory modalities of coral reef fish larvae at settlement. The special experimental system consists of an aquarium with five compartments (A-E), with A, B and C interconnected via funnels and D and E isolated from central compartments via plastic panels affixed with removable opaque barriers. Additional tanks (labeled no.1 and 2) are isolated from the five-compartment aquarium and mounted upon separate platforms to prevent transfer of vibratory signals. Larvae are introduced into the central compartment of the aquarium (A) and can remain in it or move toward the adjacent compartments (B and C) due to funnels (anti-return system).
The mechanisms of reef larval dispersal are important factors in understanding coral reef population dynamics. Current studies done on reef fish with a bipartite life cycle indicate that these fish in the larval stage are far from passive drifters. They are capable of sensing the environment around them, processing the stimuli, and using their excellent swimming abilities to find a settlement. The replenishment of reef fish populations and the structure of the coral reef community depend upon new recruits. Considering how these new recruits are brought into the reef community is essential to helping understand the ecology of coral reefs.
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