Tree Lagoon Research Articles

The role of habitat in determining the distribution of a sponge-red alga symbiosis on a coral reef

At One Tree Reef, populations of the symbiotic association between the sponge, Haliclona cymiformis (Esper, 1794), and the red macroalga, Ceratodictyon spongiosum Zanardini (1878), occur predominantly on the rubble banks inside the northeastern side of the reef crest surrounding One Tree Lagoon, immediately behind the most exposed section of the reef crest. There is only one population in the centre of One Tree Lagoon, where the substratum is sand rather than coral rubble. When clumps of the association were transplanted from the edge to the sandy centre of the lagoon, to areas where it did not occur naturally, some clumps survived for at least 349 days and more than tripled their original biomass, while control clumps left at the rubble bank did not grow noticeably larger. Fusion experiments between individuals collected from different sites showed some histocompatibility, suggesting that all the existing populations of Haliclona/ Ceratodictyon may have originated, via fragmentation, from the same parent population. These experiments suggest that the lack of Haliclona/ Ceratodictyon in the centre of the lagoon may be due to an inability of fragments or propagules of the association to colonize these sites, because of a lack of solid substrata for attachment, rather than to differences in the physical environment.

Population dynamics of an association between a coral reef sponge and a red macroalga

Sponges are often as abundant as corals on tropical coral reefs and many species are symbiotic with algae. These associations may contribute significantly to reef primary productivity. This paper describes the first study ever made of the population biology of any of the known associations between sponges and macroalgae. Populations of the symbiotic association between the sponge, Haliclona cymiformis (Esper), and the red macroalga, Ceratodictyon spongiosum Zanardini, were studied in One Tree Lagoon, southern Great Barrier Reef. The association primarily occurs in areas where the substratum consists of dead coral rubble (rubble banks), at the edge of the lagoon, where it can reach a biomass of up to 270 g wet weight m−2. Fragmentation is the primary mode of reproduction of the Haliclona/Ceratodictyon association at One Tree Reef. Although algal sporangia were frequently encountered during the summer, sperm cysts were infrequently found in the sponge, and oocytes and larvae were never observed. The size-frequency distribution of the clumps at the rubble bank sites was strongly skewed toward small individuals (longest branch length generally ≤80 mm). Haliclona/Ceratodictyon was also abundant at one site in the centre of the lagoon; at this site, which was not affected by strong currents or wave energy, the size-frequency distribution of the association was normal. Populations of the association at the rubble bank sites are very mobile, with intact clumps moved more than 30 cm per day under calm conditions and up to 1 m per day during rough weather. Survivorship of fragments or mobile clumps of the association is very high. At one site, the biomass of the association was not affected by two cyclones and at least one severe storm. At another site, up to 50% of the biomass of the population was lost following each of these storms, but the recovery of the biomass was largely complete within 9 months. The Haliclona/Ceratodictyon association has a rapid growth rate, of 8 mg (g wet weight)−1 day−1, which exceeds that of many other coral reef sponges, but it is within the range of growth rates reported for other species of marine macroalgae.

Relationships between recruitment and postrecruitment processes in lagoonal populations of two coral reef fishes

Pomacentrus amboinensis Bleeker and Dascyllus aruanus (L.) exhibit similar large scale patterns in recruitment to One Tree lagoon, southern Great Barrier Reef. They recruit in greatest numbers near the windward perimeter compared with the centre of the lagoon, and at the perimeter, recruit preferentially to deeper sites (4–8 m depth). This pattern was observed in recruitment to standard coral heads of Pocillopora damicornis (L.), a preferred substratum transplanted to a range of locations and depths. As substratum was held constant, spatial variation in recruitment was not determined by the availability of suitable habitat. Patterns in growth and mortality of the two fish species were monitored for juveniles transplanted to another set of standard coral heads, in order to determine whether post-recruitment processes would reinforce or disrupt patterns established at the time of recruitment. The two species exhibited contrasting patterns. Pomacentrus amboinensis exhibited faster growth and better survivorship at deeper sites, regardless of the location in the lagoon, reinforcing the initial pattern. Dascyllus aruanus exhibited faster growth at perimeter sites and significantly better survivorship at central sites, patterns that would tend to disrupt the initial pattern. Within locations, individuals exhibiting slower growth rates ( Pomacentrus amboinensis) or smaller initial size ( Dascyllus aruanus) had reduced life expectancy. The results suggest that location selection by settlers of Pomacentrus amboinensis results in improved growth and/or survival. This was not the case for Dascyllus aruanus, which is probably more influenced by the presence of conspecifics. However, even for Pomacentrus amboinensis, large-scale patterns probably reflect constraints on habitat selection. Recruitment in Pomacentrus amboinensis was consistently lower to sites immediately down current of other reefs, regardless of their position. I suggest that the absence of recruits of both species to central sites indicates that this region is a recruitment-shadow. Juveniles settle in preferred habitat at the first opportunity after entering the lagoon, leaving much of the preferred habitat in the centre of the lagoon unoccupied. Patterns are initially recruitment-determined, but either reinforcement or disruption mean that the numerical abundance of adults cannot be predicted on the basis of recruitment data alone.

Variation in the genetic composition of coral (Pocillopora damicornis and Acropora palifera) populations from different reef habitats

The genetic structure of populations of the corals Pocillopora damicornis and Acropora palifera was examined in three habitats at One Tree Island during March and April 1993, using electrophoretically detectable variation at six allozyme loci. There were significant genetic differences among populations of P. damicornis within each of the reef crest, lagoon and microatoll habitats. The level of differentiation among populations was similar in each of the habitats. Differences between populations of P. damicornis from lagoon and microatolls were no greater than that within habitats, but genetic differentiation of these from crest populations was much higher. There was no difference in the genetic composition of A. palifera populations within or between the lagoon and microatolls, the only habitats where this species was found. Both coral species had observed:expected (G O:GE) genotypic diversity rations >0.80, indicating predominantly sexual reproduction. These data, the high genotype diversity and general conformance of genotype frequencies to those expected under conditions of Hardy-Weinberg, suggested panmixis at each site. The high degree of sexual reproduction in the P. damicornis populations is unusual for a species where asexual reproduction has been the dominant mode of reproduction reported to date. Gene flow in both species was considerable between the lagoon and the closed microatolls. The genetic differences between populations of P. damicornis in these habitats and the reef crest may reflect the relative isolation of all populations within the closed One Tree Lagoon from those outside. However, local currents appear to offer effective means of dispersal between the habitats, suggesting that the genetic differences result from natural selection in the different environments within One Tree Lagoon and the reef crest.

Experimental evaluation of the effects of habitat structure and competitive interactions on the juveniles of two coral reef fishes

Dascyllus aruanus (L.) and Pomacentrus amboinensis (Bleeker) are small and abundant planktivorous fishes that colonize small patch reefs in One Tree Lagoon (Great Barrier Reef, Australia). The former prefers a live Pocillopora damicornis (L.) coral substratum, and the latter is more common on Porites sp. reefs, whether live or dead. This paper describes an experiment which tests for effects of Pocillopora and Porites substrata on the survival and growth of juveniles of these two fish species. It also examines the outcomes of intra- and interspecific competition on these two coral types. Coral substratum was found to have much greater effects than competition on juvenile survival over a 1-yr period, with greater numbers of both fish species surviving on Pocillopora reefs. There were no indications of competitive dominance of one fish species over another, or a reversal of dominance in relation to substratum type. Slight positive effects of density on survival were obtained in P. amboinensis, and slight negative effects were apparent in D. aruanus. Effects of coral substratum on growth were less clear, although there is some suggestion that both species grow better on Pocillopora reefs. There were substantial effects of intraspecific competition on growth in P. amboinensis, but not in D. aruanus, and no effects of interspecific competition on growth in either species. Overall, the experiment suggests that the factors controlling growth and mortality are complex, and highlights the need for further multifactorial studies focused at the level of the species.

Some interactions between residents and recruits in two coral reef fishes

Two experiments were conducted to evaluate potential effects of resident Dascyllus aruanus (L.) and Pomacentrus amboinensis Bleeker on the recruitment of other common pomacentrid fishes in One Tree Lagoon, on the Great Barrier Reef. Dascyllus aruanus had a marked positive influence on the recruitment of conspecifics to Pocillopora damicornis (L.) reefs, its preferred coral substratum. Recruitment of Pomacentrus amboinensis was unaffected by the presence of Dascyllus aruanus, but a third species, Pomacentrus sp., recruited in lower numbers when Dascyllus aruanus was present. Pomacentrus amboinensis residents had no effect on the recruitment of conspecifics or any confamilial species. The consequences of gregarious recruitment in Dascyllus aruanus were investigated in a third experiment. Proportional survivorship of juveniles declined as the densities of juveniles increased, although the effect was not a substantial one. Preliminary data suggest that survival may be enhanced in the presence of older conspecifics. The mean standard length attained by survivors was not affected by initial recruit density or by older residents. There appear to be differences among the species in the timing of the major recruitment peaks, D. aruanus tending to recruit early, Pomacentrus amboinensis midway and Pomacentrus sp. toward the end of summer. A further experiment showed that early recruits of Dascyllus aruanus did not affect the survival or growth of subsequent recruits of Pomacentrus amboinensis. However, these late recruits did show reduced growth in the presence of conspecifics that had recruited earlier. The discussion emphasizes the species-specific nature of the identified interactions, and cautions against treating guild members as a homogeneous group when examining patterns of recruitment.

Estimates of adult and juvenile mortality for labrid fishes at One Tree Reef, Great Barrier Reef

Few estimates of natural mortality have been reported for coral reef fishes, yet this information is essential for predicting the effects of recruitment fluctuations on adult populations. In this study, mortality of adult (10 species) and juvenile (11 species) labrid fishes resident on ten isolated patch reefs in One Tree Lagoon, southern Great Barrier Reef, was estimated by visual censuses covering 30 mo, from November 1981 to April 1984. Numbers of adults of all species declined linearly over this period. Mortality rates differed significantly among species, ranging from 4.9 to 69.5% per annum. Average life-span for fishes that had survived the first year of life varied from 1.6 to 11.5 yr, depending on species. Average juvenile mortality varied among species from 25 to 91.8% per annum and was highest for abundant schooling species. Different cohorts of the same species also experienced significantly different rates of mortality. No clear relationship emerged between the number of new recruits and the number of fishes surviving to one year of age. The influence of recruitment fluctuations on adult populations of labrid fishes may be difficult to predict.

Significance of pelagic bacteria as a trophic resource in a coral reef lagoon, One Tree Island, Great Barrier Reef

Bacterial numbers and frequency of dividing cells (FDC) were estimated in surface waters at seven stations along a transeet across One Tree lagoon and reefs at 2 h intervals over one tidal cycle in May 1984 and once a day on 8 d in December 1984/January 1985. Cell numbers ranged between 1.24 and 13.10x105 ml-1, with an overall mean value of 4.81x105. There was a progressive depletion in standing stock from windward to leeward stations across the lagoon, also reflected in the FDC, which generally showed similar dynamic patterns to cell numbers throughout the study. Predator-free seawater (filtered through 2 μm pore-size filters) and mucus-enriched incubations were also used to establish the relationship between the growth rate (μ) and FDC for coral-reef populations. Average growth rates predicted from FDC values ranged from 0.062 to 0.174 h-1, which is equivalent to doubling times of 5.74 to 16.00 h, with an overall mean value of 11.5 h. These fast doubling times suggest that bacteria respond rapidly to pulses of enrichment as they float over the lagoon and reefs, characteristically achieving about one doubling per tidal cycle. This probably ensures that the bulk of labile organic matter and dissolved nutrients is conserved within the immediate coral-reef environment. Estimates of bacterial production also suggest that they may contribute up to 40% of total picoplankton production and about 25% of total microplankton production. Pelagic bacteria are therefore potentially a major food resource for benthic filter-feeders, especially for those mainly dependent on the smaller (pico) components of the plankton.

Food availability affects growth in a coral reef fish.

Pomacentrus amboinensis is common on small patch reefs within One Tree Lagoon (Great Barrier Reef), where it preferentially settles onto deep reefs. A preliminary experiment, in which juveniles were transplanted to identical reef structures at two sites, within two depth strata, indicated that juvenile growth and survivorship were better in deeper water. The hypothesis that this difference was due to food availability was tested by a supplemental feeding experiment, carried out at another two randomly chosen sites, within the same two depth strata. Fish were fed each day over a one month period, during which no mortality was observed. The growth rates of juveniles were markedly higher on all food-supplemented reefs, when compared to controls. Growth differed between depth strata, but there was no interaction between the food x depth factors, which would have suggested a greater effect of food supplementation in either habitat. Thus, although the difference between depths cannot be attributed to food availability, the results have a more general significance. Food appears to be a limiting resource (in terms of growth) in both the marginal shallow habitat, and the more suitable deeper habitat.

Feeding by larvae of Hypoatherina tropicalis (Pisces: Atherinidae) and its relation to prey availability in One Tree Lagoon, Great Barrier Reef, Australia

Feeding of and food availability for larvae of Hypoatherina tropicalis were investigated in One Tree Lagoon, Great Barrier Reef, Australia, during November 1981 and January 1982. These surface-dwelling larvae and their microzooplankton prey were sampled as near to simultaneously as possible on 12 occasions during the daytime. Larvae of all sizes (5–17 mm SL) fed successfully over the observed range of mean prey densities (12–235 per liter), and the overall feeding incidence was 98.9%. Larger larvae consumed greater numbers and more categories of prey than did smaller larvae. Larvae selected copepods of all sizes, and nauplii, gastropods, bivalves, and foraminiferans that were greater than 75 ¢s mm in width. Tintinnids (mostly 37–74 µm in width) were generally avoided by larvae, but were occasionally important in the diets when they constituted more than 60% of the total available prey, regardless of the density of the selected prey categories. Larvae less than 14 mm SL ingested meroplankton (gastropods, bivalves, foraminiferans, and polychaetes) in direct relation to the densities available, and without regard to the densities of copepods available. However, the largest larvae (14–17 mm SL) ingested meroplankton in inverse relation to the density of copepods available, indicating that larvae consumed more meroplankton when the concentration of copepods was low. Such flexibility and opportunism in feeding behavior may increase the larvae's chances of obtaining adequate nutrition during periods of suboptimal feeding conditions.

Length of larval life in twelve families of fishes at ?One Tree Lagoon?, Great Barrier Reef, Australia

The length of larval life in some coral reef fishes was estimated from the number of growth increments in the otoliths of newly settled fishes. We examined 210 individuals comprising 38 species and 5 unidentified taxa, and belonging to 12 families. During 2 successive austral summers (1976–1977 and 1978–1979), specimens were collected from the lagoon at One Tree Reef, Great Barrier Reef, Australia. By assuming that growth increments in otoliths are laid down on a regular daily cycle commencing near the time of hatching, we calculated typical ages ranging from 3 to 6 wk with a minimum of just over 2 wk and a maximum of 12 wk. The otoliths also contain distinctive microstructural features which can serve as approximate temporal markers for the change from the postlarval to juvenile life stage.

Spatial and temporal patterns of recuruitment of juvenile coral reef fishes to coral habitats within ?One Tree Lagoon?, great barrier reef

Recruitment of juvenile fishes to coral grids, each comprising 4 colonies of 3 species of coral (Acropora formosa, Seriatopora hystrix and Pocillopora damicornis), was examined at 4 widely separated sites within the lagoon of “One Tree Reef” over two successive summers and intervening months (November 1976–January 1978). Recruitment was highly seasonal, with most recruitment occurring during summer. For many species, numbers settling differed greatly from year to year with total numbers (over all sites) differing as much as an order of magnitude between summers. Many fish species demonstrated marked preference among the three coral species as settling sites. The distribution of each of the 20 commonest species across the 4 lagoon sites differed significantly from a random pattern. Differences of an order of magnitude were common in the numbers of a given species recruited to different sites. Each site was preferred by at least one species. In each of 5 cases examined, the pattern of settlement of the species across the 4 sites changed significantly from one summer to the next. The distribution of recruits of a number of species corresponded to the distribution of adults, but for other species there was no correspondence. It is concluded that, at the spatial scale examined, patterns of recruitment of some taxa are consistently more variable than those of other taxa. The implications of spatial variability of recruitment for the distribution of adult fish is discussed and the importance of being able to discriminate such “natural” variability from other kinds of change is stressed.

Seasonal changes in resident zooplankton sampled by emergence traps in one tree lagoon, great barrier reef

Small resident Zooplankton species which rise from the substratum after sunset were sampled during the winter and summer seasons of 1977 and 1978. Modified emergence traps were used to obtain quantitative overnight samples from five habitats on and around two lagoonal patch reefs: live coral, and coralline rock, both on top of the reefs, sand at undercut and at vertical reef bases, and open sand 1–3 m away from the reef. In summer, at each habitat, many more animals and more species rise off the substratum than in winter. Estimated mean densities of animals rising overnight ranged from 664.9 · m −2 in winter (at undercut reef base) to 5034.7 · m −2 in summer (rock on top of reef). In winter the abundance of animals rising from open sand is significantly lower than that from the top of the reef. A total of 261 taxa was recognized. Samples from all habitats are dominated by copepods, mainly small calanoid and harpacticoid species. The most abundant of these include Stephos sp., Pseudodiaptomus cornutus Nicholls, Peltidium spp., Tisbe sp., and an unidentified thalestrid. In summer, additional copepod and peracarid species become numerous at night over the reef and over open sand. Other major taxonomic groups collected were ostracods, polychaetes, mysids, cumaceans, tanaids, gammarids, and the zoeae of some carid species. Most species appear to have a limited vertical range. Fauna rising from the top of patch reefs is rich compared with that from open sand, and significantly richer in summer than in winter. Faunal analyses indicate the localized distribution of a “coral” fauna and a “sand” fauna. Spatial distribution of these characteristic faunas is more discrete in winter than in summer. The reef base habitats appear to be a transitional faunal zone which contains elements of both “coral” and “sand” faunas. Seasonal fluctuations in abundance, composition and spatial distribution are similar to seasonal trends found in light-trap sampling of resident Zooplankton in the same seasons, and in similar locations, on Heron Reef. Sampling techniques, limitations, and results are compared with those of single-season emergence trap studies in other coral reef regions.

Seasonal changes in resident zooplankton sampled by emergence traps in One Tree Lagoon, Great Barrier Reef : McWilliam, P.S., P.F. Sale and D.T. Anderson, 1981 J. exp. mar. Biol. Ecol., 52(2/3):185–203

Seasonal changes in resident zooplankton sampled by emergence traps in One Tree Lagoon, Great Barrier Reef : McWilliam, P.S., P.F. Sale and D.T. Anderson, 1981 J. exp. mar. Biol. Ecol., 52(2/3):185–203