Mechanisms of Dispersal and Settlement of Coral Reef Fish Larvae

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MECHANISMS OF DISPERSAL AND SETTLEMENT OF CORAL REEF FISH LARVAE
Understanding larval ecology and the mechanisms used in dispersal and habitat selection helps to better understand the population dynamics of coral reef communities (Browne & Zimmer 2001). Until recently, the ways in which planktonic larvae find a suitable settlement site on coral reefs was unknown. It was assumed that these larvae dispersed in a passive manner, drifting with the currents. This assumption meant that the larvae did not actively participate in the dispersal process. More recent studies indicate that this is not the case. A number of reef organisms are active participants in dispersal and selection of a proper habitat among the coral reef community; this paper will focus on reef fish larvae dispersal and settlement. These larvae possess exceptional swimming abilities and make use of chemical, sound and visual sensory cues. Today the larval stages of most reef fish and some reef invertebrates are known to actively disperse and select a suitable reef environment in which to settle.
Most reef organisms must survive a planktonic larval dispersal phase. During this phase they are required to either actively or passively find their way back to a reef after being driven out by the currents. This is not an easy task, as most larvae are food for planktivores (Levinton 2001). Reef fish larvae are known to actively disperse using sophisticated mechanisms. Reef fish are excellent swimmers and combine this ability with sensory mechanisms to locate a suitable reef habitat. This stage of life is an important one, and selection of and dispersal to the ÒrightÓ reef habitat are key to surviving in the coral reef environment.
REEF FISH LIFE CYCLE & LARVAL DISPERSAL
The life cycle of most reef fish is a bipartite one with a larval dispersal phase followed by a reef adult phase. Reproduction is sexual and fertilization occurs externally. Most reef fish are iteroparous with high fecundity (Sale 1991). Many species display broadcast spawning where they form aggregations and gametes are broadcast into the water column. Some species, however, prefer to spawn in pairs rather than in large masses (Sale 1991). Reef fish follow spatial and temporal spawning patterns. The larval biology hypothesis proposes that these spatial and temporal reproductive patterns accommodate the larval needs, instead of the adult needs, in order to maximize recruitment (Robertson et al. 1990, Sale 1991). The majority of parents take little or no care of their young, as they are broadcast spawners. Yet, there exists some variation, suggesting adaptation (Sale 1991). For example, the yellow head jaw fish found in the Caribbean is a mouth brooder. To insure his offspring will hatch, the male will brood the eggs in his mouth (Mustard 2005). This could facilitate imprinting of the reef environment, for use when the larvae must locate a settlement (Atema et al. 2002). Those that do reproduce by broadcast spawning, however, produce immense amounts of eggs, while only a small percentage of these will survive to adulthood. These eggs and larvae, while in the plankton, will be food for planktivores (Byatt et al. 2001, Levinton 2001). Depending on the species, the larvae can be anywhere from 1mm Ð 20mm long (Levinton 2001). The larval phase of most reef fish species lasts about three to six weeks (Sale 1991). During this phase they are forced to find their way back to the reef, as they are advected Ð horizontally move with the currents - from their home reef at the beginning of their larval stage. Once they find their way to a suitable reef they will live in the benthic habitat in their juvenile phase (Sale 1991, Mustard 2005). Larvae sometimes return to their natal reef, but how they accomplish this is not clear (Kingsford et al. 2002). It was once thought that reef fish larvae were passive drifters, their final destination determined only by the currents. However, passively drifting cannot explain the recruitment success demonstrated by so many species of reef fish. Physiological and behavioral studies have shown that reef fish are capable of active dispersal, using superb swimming abilities and sensory mechanisms to guide them to a suitable reef settlement.
Swimming Abilities of Reef Fish Larvae
The incredible swimming abilities of reef fish larvae surprised researchers, causing scientists to question the passive dispersal hypothesis. Reef fish larvae are strong, effective swimmers, capable of swimming long distances for long periods of time (Arvedlund 2005, Leis et al. 2002). Some think they should be considered ÒmicronektonÓ instead of plankton because of their incredible mobility (Leis et al. 2003). In one experiment (Arvedlund 2005), scientists tested the endurance of 51 species, and speeds of 50 species of reef fish larvae in a laboratory swimming-chamber. The larvae swam at mean speeds of 20.6 centimeters per second, and some as fast as 65 centimeters per second. Incredibly, these speeds can exceed those of local ambient currents. If a human were capable of comparable swimming speeds, he/she could swim 100 meters in 3.6 seconds (Arvedlund 2005). The results of the endurance tests demonstrated that the species tested swam an average of 40.7 kilometers, with a maximum of 140 kilometers, before becoming exhausted. The mean time to exhaustion was 83.7 hours, the longest time being 288.5 hours. If a human could match these results, he/she would swim almost 4000 kilometers before exhaustion (Arvedlund 2005). This study indicated that reef fish larvae are very effective swimmers.
It is possible for the young fish to save energy by locating a current that would carry them to a proper reef habitat (partial navigation). This, however, requires not only swimming but also sensing of reefs and currents (Kingsford et al. 2002). Being able to swim for long periods of time, and quickly is only useful if the larvae can orient themselves toward a settlement site (Kingsford et al. 2002). These effective swimming abilities combined with sensory abilities maximize the possibility of the larvae finding a reef.
Sensory Abilities of Reef Fish Larvae
Reef fish larvae are known to use sensory cues in their environment to guide them to a home reef. Experiments have tested and confirmed that larvae can sense a reef by chemical cues, sound cues, and visual cues. Depending on the species, the larvae may use a combination of these cues, or only one. Some may even use a hierarchy of cues to sense a reef (Kingsford et al. 2002). Reef fish larvae have been known to frequently inspect a reef, making use of sensory organs, and swim away if the particular reef is not what they are looking for (Leis et al. 2002). Detecting the stimuli allows the fish to navigate and locate a settlement.

Chemical Cues
Reef fish larvae may respond to changes in the concentration of water chemistry. These chemical cues can come from multiple sources in the larvaeÕs 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 Òmeasures olfactory transduction by recording the change in the negative electrical potential at the surface of the nasal epithelium,Ó (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 Cues
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 Ð 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) WilcoxonÕs matched pairs test, (C) pairwise Chi-squared analyses with Bonferroni corrections, and (D) pairwise WilcoxonÕs 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).
Visual Cues
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).
CONCLUSION
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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