Settlement Patterns of Dascyllus flavicaudus and D. aruanus on Pocillopora spp. Moira Decima and Holly Kindsvater Abstract Settlement patterns of two Dascyllus species provide insight into the relationship between recruits and adult distribution of reef fish larvae because of the close association between damselfish and settlement substrate. In Moorea, experimental coral heads were monitored for sixteen days for gregarious settlement by D. flavicaudus and D. aruanus. Three replicates of treatments stocked with three conspecifics each were surveyed and compared with four accumulation treatments (left alone) and eight control replicates (blanks). Except for the accumulation treatment, corals were cleared periodically. Corals were analyzed qualitatively for possible cues affecting settlement. A positive effect of conspecifics was observed for D. aruanus. The same effect was not seen for D. flavicaudus, though this species seemed to prefer corals deemed more structurally complex. A current net was placed upstream of the experimental coral site to attempt to connect levels of larval input with observed settlement patterns, though very few Pomacentrids were caught. Introduction While there have been many studies documenting patterns of settlement of fish on coral reefs, understanding the mechanisms underlying these patterns has proved elusive. Temporal variation, spatial variation, and variation in species life history influence larval recruitment and thus adult population distribution (Jones and McCormick, 2002). Although these areas are well-studied (Schmitt and Holbrook, 1999; Dufour, 1994; and Doherty, 2002), empirical generality is scarce. It is known that settlers are many and vulnerable, while adults are fewer and resilient. Additionally, settlers have highly developed sensory aparati (Raimondi, pers. comm. 2002). Consequently larvae should show strong settlement behavior that locates them in sites that afford protection from predators and environment, minimize negative effects of competition, and allow mating opportunities (and fertilization success). Reef fish life history is a strong determinant of coral reef community structure. Prior to settlement, many coral reef fish have a pelagic phase that occurs in the plankton for a period between 18 to 94 days (Lo-Yat, pers. comm., 2002), depending on species. After this pelagic period, larval fish recruit to reef habitats at night (Victor, Sale old). Once colonization occurs, settlers are thought to swim until they reach a suitable habitat for settlement. Evidence show that larval behavior plays a crucial role in the selection of this suitable habitat; often rejecting the nearest reefs, in pursuit of better-suited ones (Leis and McCormick, Sale 2000). Mature reef fish spawn frequently and have a high reproductive output. Some species of damselfish in the genus Dascyllus preferentially settle on specific corals or anemones (Schmitt and Holbrook). They are associated with the settlement patch until near-adulthood. Dascyllus flavicaudus, the yellowtail damsel, and Dascyullus aruanus, the humbug damsel, recruit to branching coral species that are common (Schmitt and Holbrook). However the distribution of settlement is not directly related to the distribution of available habitat. In Moorea, French Polynesia, some coral heads are saturated with recruits and others in apparent good health are empty.
Upon encountering favorable habitat larvae may settle as a group and are not substantially displaced after settlement. If the resulting species composition from one coral to the next is very different, it may be because the groups are successful by chance in a sort of lottery model the first group to settle wins. Alternatively, intraspecific or interspecific interactions may lead to a pattern of group settlement. For instance, a visual or chemical cue given by conspecifics may draw individual larvae to a coral patch, by indicating good settlement habitat (Sweatman, 1983). Sensory cues would be advantageous because larvae have little information about the quality of one substrate patch versus another, and presence of other species may indicate good habitat. If drawn to a cue, larvae would still appear to settle gregariously but would not necessarily be associated in the plankton. Another way to look at patterns of distribution in the reef community is in the context of recruitment limitation versus saturation. In a recruitment-limited system the population is never at carrying capacity, while in a saturated system, the adult population may be regulated by post-settlement events. The behavior of group settlers may follow either one of these two models depending on the mechanisms that stimulate group settlement. If individuals recruit gregariously forming groups upon settlement the system could be saturated. This pattern would occur if fish detect settlement cues from characteristics of the coral habitat or other species present. However, if larval groups associate prior to settlement and stay grouped even though there is more open space on the reef, the system is recruitment limited. To approach these questions, we used two different and complementary methods: crest current nets and experimental coral settlement plots with Dascyllus flavicaudus and D. aruanus. We specifically tested the following hypotheses: There is uneven settlement by D. flavicaudus and D. aruanus on coral heads because if they come across the crest in groups and settle or they respond to chemical or visual cues, or if distribution is random. In addition we looked for a correlation between settlement and coral quality and complexity; and settlement and resident predators. Specifically we tested if there was preferential recruitment to coral heads with established conspecifics. We did this to see if visual or chemical cues promote group settlement. Finally we used a current net designed to sample larvae near the crest in order to observe the degree to which input rate affects community structure, mainly to see if the system is recruitment limited. Although few damselfish were found in the crest current net, we were able to monitor general recruitment trends on the reef with the current net, as well as explore the possibility of using this as an accurate sampling method for quantitatively assessing the abundance of larvae that cross the barrier reef. Materials and Methods Site and species West Opunohu (S17 29 288, W 149 52 253 ), a crest patch reef located to the west of the pass at Opunohu Bay on Moorea was chosen as the site for this study because of a sandy flat between the reef edge and the boat channel, approximately 300m from the crest. This open flat was chosen for two reasons. First, it is an area where we could
isolate treatment coral heads from the reef. It is also a place with little current and its location right before the channel makes it the last possibility for larvae to settle, before being swept out the Opunohu Pass. Thus, we expected to have a good chance of larvae settling here. The species chosen for this study were Dascyllus flavicaudus and Dascyllus aruanus because of their fidelity to settlement substrate as juveniles. Dascyllus aruanus are protogynous hermaphrodites that live in groups mainly on Pocillopora spp. They belong to the group of smaller Dascyllus, and as such never abandon their coral head, laying their eggs at the base of the latter (Bernardi, pers comm. 2002). They respond to conspecific chemical cues for settlement; once they ve settled they inhibit other species from settling on the same colonies (Doherty, Sale 2000). Dascyllus flavicaudus belong to the groups of larger damselfishes, it is not yet known if they are hermaphrodites or gonadal. This species settles on branching coral heads as well, and may also inhibit the settlement of other fishes. (Victor and Wellington, 2000) Both species are diurnal planktivores that lay benthic eggs. These hatch approximately 3 days later and enter the plankton, where they remain for 22-24 days (Schmitt and Holbrook, 1998). These species are observed to preferentially recruit to corals in the genus Pocillopora as well as Montipora, Acropora, and other branching coral species. Experimental design We placed fourteen Pocillopora spp. corals at West Opunohu, and surveyed each daily for recruitment. Corals heads were set in a row parallel to the crest, approximately 300m inshore from it. The corals were anchored with cable ties attached to rebar stakes. Coral heads were placed approximately 5 m away, a distance we considered to be enough to prevent migration. We assumed that an observed increase in fish numbers was due to recruitment, and a decrease was due to post settlement mortality Fourteen of the coral heads were cleared periodically of recruits to isolate patterns of recruitment by larvae. Three of the coral heads were stocked with young Dascyllus aruanus and three with young D. flavicaudus in order to test for differential recruitment between corals with stocked conspecifics and corals without these. Other species of fish seen infrequently on the corals were also noted. In conjunction with Zacher and Engel we set up four coral heads, which were surveyed and not cleared, to observe community accumulation. All corals, mostly Pocillopora verrucosa, were evaluated (using Scion image analysis) for open space, health and depth. Naturally occurring predators (such as crabs, crouchers, and arceye hawkfish) on the corals were noted. These factors were combined into a qualitative assessment of coral complexity and desirability. A new way of larval sampling: the current net A current net was placed close to the reef crest upstream from our experimental heads to monitor a connection between levels of larvae coming over the crest and recruitment seen on the coral heads. This net was checked daily and the larval catch
identified. The net was designed to sample the water column for larval fish newly arrived over the crest. The opening frame was 0.25 m 2 and the pyramid tail was approximately four meters long. The mesh was a little more than 50% open each opening was slightly over 1 mm square. The codend was loosely anchored and buoyed so that the net floated and did not rub on the reef. The net was anchored about 0.5 m below the surface to a low coral head approximately 25 m inshore from the reef crest, and about 200m upstream from the coral head array. 1,2). Results Recruitment was observed to follow a temporal pattern for both species (Figure Least Squares Means 1.0 LNUMBERS 0.5 0.0-0.5 Figure 1. Average daily recruitment for D. flavicaudus across treatments -1.0 1 2 3 4 5 7 8 9 10 11 12 DAYS
Least Squares Means 1.0 LNUMBERS 0.5 0.0-0.5 Figure 2. Average daily recruitment for D. aruanus across treatments -1.0 1 2 3 4 5 7 8 9 10 11 12 DAYS As the figure shows, there were pulses of recruitment throughout experiment. These pulses differed between species. Larval association before settlement Over the course of our experiment, the empty coral heads did not experience large scale differences in recruitment of either Dascyllus species. We looked at recruitment at each replicate vs. days, taking groups numbers into account. There appeared to be a slight group effect for D. flavicaudus (Figures 3-4).
15 10 DAY 5 0 0 4 8 12 16 REP NF 5 4 3 2 1 0 Figure 3. Recruitment of D. flavicaudus. Replicates were plotted versus days, the size of the circles in the figure proportional to the group size that settled to the given coral head. It appears to be randomly dispersed 15 10. DAY 5 0 0 4 8 12 16 REP NA 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 Figure 4.. Recruitment of D. aruanus. Replicates are plotted against days, the size of the circle proportional to the size of the group. Here there may be an attractor because the data is not as randomly dispersed as for D. flavicaudus. Effect of conspecifics on recruitment During periods of high recruitment, comparison of settlement of D. aruanus on corals stocked with conspecifics show significantly higher recruitment than those with D. flavicaudus or the blank controls (Figure 5, Table 1). There was no effect for the recruitment patterns of D. flavicaudus. (Figure 4 and 9, Table 2).
Least Squares Means 1.0 0.8 LNUMBERS 0.6 0.4 0.2 Figure 5. Effect of treatment on recruitment levels for D. aruanus. 0.0 accum aruanus TTT control flavi Table 1. Analysis of Variance for Dascyllus aruanus Source Sum-of-Squares df Mean-Square F-ratio P TTT$ 0.346038 3 0.115346 5.423349 0.001428 DAYS 0.450808 10 0.045081 2.119612 0.026018 TTT$*DAYS 1.067520 30 0.035584 1.673090 0.023662 Error 3.254071 153 0.021268
Least Squares Means 1.0 0.8 LNUMBERS 0.6 0.4 0.2 Figure 6. Effect of treatment for recruitment numbers of D. flavicaudus 0.0 accum aruanus TTT control flavi Table 2. Analysis of Variance for D. flavicaudus Source Sum-of-Squares df Mean-Square F-ratio P TTT$ 0.065578 3 0.021859 0.715911 0.543915 DAYS 1.110705 10 0.111070 3.637626 0.000227 TTT$*DAYS 1.114644 30 0.037155 1.216843 0.220332 Error 4.671667 153 0.030534 We analyzed the effect of coral quality on recruitment. We ran a GML model independently for D.aruanus and D.flavicaudus. There was no significant correlation between D.aruanus recruitment and coral open space. There was a significant effect on the recruitment of D. flavicaudus (df=1, p=0.037995) fig 7 20 Total flavi recruits 15 10 5 Fig 7. Total number of D. flavicaudus recruits vs. open space. Max and minum percentages are noted. 0 36 44 52 60 68 Open Space (%)
We analyzed the input of new recruits to the different treatments (accumulation, flavicaudus, aruanus and control) (Figure 8). The treatments of D. aruanus and accumulation were statistically different from the control and D. flavicaudus (Table 3), Least Squares Means 1.0 0.8 RAT2 0.6 0.4 Figure 8. The number of recruits vs days is plotted for every different treatment. The recruits on the control represent the assumed larval input. 0.2 0.0 accum aruanus TTM control flavis We plotted the number of recruits vs days, for every different treatment. The recruits on the control represent the assumed larval input. Table 3. Using model MSE of 0.094 with 154 df. Matrix of pairwise mean differences: 1 2 3 4 1 0.000000 2 0.099452 0.000000 3-0.245117-0.344569 0.000000 4-0.381324-0.480776-0.136207 0.000000 Tukey HSD Multiple Comparisons. Matrix of pairwise comparison probabilities: 1 2 3 4 1 1.000000 2 0.537587 1.000000 3 0.000370 0.000006 1.000000 4 0.000006 0.000001 0.237217 1.000000
Figure 9. Summary of recruitment on different treatments over the sampling period. Accumulation D.aruanus
9 8 7 Sum of recruits 6 5 4 3 2 1 0 0 5 10 15 DAY Sum of recruits 7 6 5 4 3 2 1 Control D. flavicaudus 0 0 5 10 15 DAY Sum of recruits 10 9 8 7 6 5 4 3 2 1 0 0 5 10 15 D. flavicaudus D.aruanus Current net sampling
The current net did not prove to be effective at sampling the larval abundance of either Dascyllus aruanus or Dascyllus flavicaudus. However, it did capture a wide variety of over larval species, including Apogonids, Labrids, Scarids, Microdesmids, Acanthurids, Anguilliformes, Chaetodons and even a few Dascyllus. (Table 4). 30 20 V 10 0 Acanthurus t Anguilliform Apogonidae Chaetodon tr Chromis viri Dascyllus fl Dascyllus tr Elecatinus Gobiidae (+ Grammistinae SPECIES Labridae (+ Microdesmida Other leptoc Scorpaenidae Stegastes fa Syngnathidad UNIDENTIFIED
Table 4. Discussion After monitoring the experimental array of coral heads for sixteen days, we observed spatial and temporal patterns of settlement for both species. Both species experienced pulses of recruitment around Day 7 of sampling, which occurred four days after the full moon, in a period of little moon (Zacher and Engel, pers. comm..) (Figures 1,2, and 9). Though we did not clear gregarious settlement of groups on the empty corals treatments, there are patterns of recruitment that can be distinguished for each species. D. flavicaudus appears to randomly populate corals with little regard to the presence of conspecifics. D. aruanus seems to recruit more consistently to the same corals, signifying that there may be unknown settlement preferences in this species that attracts settlers (Figures 3,4, and 9). There was a significant positive effect of treatment for D. aruanas (p=0.0014), as well as temporal and a combined effect (Table 1, Figure 5). This can be interpreted as a link between high levels of recruitment and the preference of D. aruanus for patches with conspecifics present. D. flavicaudus did not exhibit the same preference, instead settling without regard to conspecific cues during periods of high recruitment (Table 2, Figure 6). In addition, we found a positive correlation between coral complexity and settlement of D. flavicaudus (p=0.0379). These results suggest that while D.aruanas depend on cues of their conspecifics for settling, D.flavicaudus actually prefer coral heads with more branching complexity and available open space. These characteristics offer more protection from predators. So, while D.aruanas relies on the survival cues of
it s conspecifics for information about the site where they will settle, D. flavicaudus may actually be able to asses this at the time of settling. Finally, some inferences about the relationship between input to a reefal system and the resulting community can be drawn from a comparison of the ratio of newcomers (recruits) of D. aruanus to the sum of recruits (both species totals) (Figure 7). Amount of input of D. aruanus is statistically similar for both the D. aruanus treatment and for the accumulation treatment. The control treatments are statistically similar to the D. flavicaudus treatment, and both groups are different from each other. This suggests that even though on average more D. flavicaudus came into our system, D. aruanus seemed to outcompete them as they were more represented in the accumulation treatments, even though their input numbers were much lower. This has implications for the community structure observed in the wild, especially when combined with other observations of the researchers. D. aruanus was observed to recruit in greater numbers first, thus colonizing the accumulation treatment first. It s interesting to note that this may be depictive of a Lottery model functioning at a group level. Once a few D. aruanus came in and settled on the accumulation treatment they inhibited D.flavicaudus, since they were the first there they won the coral head. As Figure 7 shows, in this respect D. flavicaudus appears to be inhibited by the presence of D. aruanus settlers on a potential habitat patch, a finding that is substantiated by the work of Jones (1987b) but not by Schmitt and Holbrook (1999). However, Schmitt and Holbrook analyzed the D. aruanas population over a period of 72 days, monitoring population growth size on manipulated coral heads. They did not have their corals supplied naturally, and therefore were not as affected by the temporal pulses of larvae. Therefore it s possible that if D. flavicaudus would have come in first, we may have observed an effect similar to our results with D. aruanas, as the findings of Schmitt and Holbrook predict. The current net A single current net did not catch a substantial number of larvae that are directly correlated with our experiment. However, the demonstration of the use of this net design proved that it is a feasible way of collecting larvae, and if a considerable number of replicates were to be made, a good estimate of the larvae coming over the reef crest would be possible. Unlike the light trapping methods and diving methods, it s an unbiased method for catching larvae at the colonization stage. The fact that we got a good variety of larvae, although numbers were low indicates the validity of this method for sampling larvae. The quantitative aspect may be remedied by increasing the number of replicates. This pilot study has also led to a number of possible designs that would allow for more efficient sampling. A better design would have an inverted net in front of the actual net. Mesh size would be substantially larger. This inverted net would prevent catching big pieces of algae in the net. This would make clearing the codend easier and reduce drag (increasing the net s life expectancy).
This inverted net would need to have a PVC skeleton; because its structural robustness is different from the net. This net should be bolted to coral with cable ties, like the current net, but should be unattached to the latter to avoid excessive interaction at times of strong current (Figure 10)
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