Comparison of absolute and relative growth patterns among five <Emphasis Type="Italic">Pinna nobilis</Emphasis> populations along the Tunisian coastline: an information theory approach

The variability in absolute and relative growth of Pinna nobilis along the Tunisian coastline was investigated. Five populations of P. nobilis were sampled, three from northern and two from eastern Tunisia. The specimens were aged and ten morphometric characters were measured on each individual. To test if differences existed in absolute and relative growth patterns among the different populations an information theory approach was followed. For absolute growth, von Bertalanffy, Gompertz, the logistic and the power models were fitted in combination with three assumptions regarding inter-population differences in absolute growth patterns: no differences, differences among all five populations or just between northern and eastern populations. The assumption of common absolute growth parameters among all five populations had the greatest support by the data, whereas the assumption of different growth patterns among all five populations had no support. Von Bertalanffy growth model and the power model were both equally supported by the data (while Gompertz had considerably less support and the logistic model had no support), and thus it may not be definitely concluded whether P. nobilis grows asymptotically or not. The P. nobilis populations of the Tunisian coastline had a slow growth and up to an age of ∼ 9 years their shells were smaller than from all other reported populations in the Mediterranean. For relative growth, apart from the classical allometric model Y = aX ^b , relating the size of a part of a body Y to another reference dimension X, more complicated models were used in combination with the three abovementioned assumptions regarding inter-population differences. Those models, of the form logY = f (logX), either assumed breakpoints in the relative growth trajectories or non-linearities. For most morphometric characters, the classical allometric model had no support by the data and more complicated models were necessary. In most cases, different relative growth either among all five populations or between the northern and eastern population groups was supported by the data. Further investigation is needed to relate the morphological differences observed among different populations of P. nobilis to environmental factors.     

Life-history and demographic spatial variation in Mediterranean populations of the opportunistic polychaete <Emphasis Type="Italic">Ophryotrocha labronica</Emphasis> (Polychaeta,Dorvilleidae)

The spatial scale of life-history and demographic variation was investigated in the opportunistic polychaete Ophryotrocha labronica La Greca and Bacci. Individuals were collected along the Italian coasts from three thermally different biogeographical regions of the Mediterranean Sea. For each region, populations from four harbours were considered, and for each harbour, two sites were examined. Life-history and demographic traits were investigated after one generation under a common garden experiment, and their variation at the three spatial scales was assessed. All the traits showed high variability with regard to site. A number of life-history and all demographic traits also varied according to the biogeographical region. Conversely, no differences were found between harbours, suggesting that geographical isolation did not contribute to phenotypic variation. Results confirmed the central role of local conditions for the evolution of life history in species colonizing heterogeneous environments, but they also pointed to the importance of large-scale factors in shaping the phenotypic responses of O. labronica, demonstrating the need for a multi-scale approach for obtaining a good measure of natural variation in widespread opportunistic species.     

Phytoplankton communities in aquaculture system (integration of shrimp and seaweed)

ABSTRACT

Knowledge about phytoplankton communities is important for aquaculture due to their influence on environmental variables. The goal of the study was to evaluate the ecological effect of seaweed Gracilaria birdiae at three different densities: 2.5 (LG2.5); 5.0 (LG5.0) and 7.5?kg m^?3 (LG7.5), on water quality variables and the phytoplankton communities in integrated aquaculture systems (shrimp and seaweed) with a control (without seaweed) (LB) using biofloc technology. Settleable and total suspended solids were significantly correlated with the phytoplankton groups. In addition, orthophosphate was the main variable controlling phytoplankton growth and gross primary production. No significant differences were identified among the treatments in terms of the phytoplankton assemblages (p?>?0.05) but differences were found between the communities (ANOSIM, R global?=?0.873) in relation to the days of the experiment. However, the LG2.5 treatment had higher diversity and lower species dominance, as well as higher productivity and final shrimp weight. The results indicate that seaweed inhibit phytoplankton growth, mainly of toxic species. They suggest that phytoplankton composition in integrated aquaculture systems in the presence of seaweed at a density of 2.5?kg m^?3 is more diversified and provides better water quality and shrimp performance by inhibiting the abundance of harmful species.     

Effect of temperature on nitrogen-limited growth rate and chemical composition of Ulva curvata (Ulvales: Chlorophyta)

Ulva curvata (Kutz.) de Toni growing in shallow estuaries experiences a highly variable supply of dissolved inorganic N, which can limit growth rates. The effects of N supply variability and annual temperature and light variation on growth rates and chemical composition were assessed in plants grown in outdoor tanks supplied with running seawater and either pulsed or continuous N additions for 8 d. Environmental variables were measured every 12 h, growth rates every 2 d, and plant nitrogenous constituents every 4 d. The experiment was repeated 7 times over the course of a year. The slopes of growth rate-tissue N and growth rate-N supply plots increased with temperature and varied at periods at least as short as 2 d. Temperature explained 44% of variation in growth rate, and temperature, light, and N supply together explained 53% of variation in growth rate. N-limitation occurred only above 20°C, the approximate temperature for the maximum growth rate. Growth rate and tissue N were not affected by N supply frequency. Tissue N, ribulose-bisphosphate carboxylase activity, and chlorophyll contents were correlated with each other and with environmental variables but not with growth rate. U. curvata thus does not maintain balanced growth in the strict sense, but rather stores nutrients supplied as pulses for use when they become growth-limiting. Since the relative importance of any single growth-limiting factor is highly dynamic, assessing the importance of multiple limiting factors requires longterm, high-frequency sampling of principal limiting factors and seaweed growth rates.     

Growth-related intraclonal genetic changes in Gracilaria chilensis (Gracilariales: Rhodophyta)

Gracilaria chilensis exhibits noticeable intraclonal variation, some of which is presumed to result from mitotic recombinations or other types of DNA turnover associated with replication activities during cellular division. To test this, genetic variability (determined by multilocus fingerprinting markers using the randomly amplified polymorphic DNA technique, RAPD) and total growth were simultaneously measured over time in clonal replicates of G. chilensis incubated under controlled laboratory conditions. The results suggest that genetic variability increases as growth occurs and biomass accumulates, supporting the hypothesis of growth-related increases in genetic heterogeneity. For species massively propagated by thallus fragmentation in either naturally or farmed populations, growth-dependent genetic changes may constitute a powerful means of generating intra-population variation without thalli becoming reproductively mature, and, as a consequence, bypassing meiosis and/or sexual recombination. Received: 6 May 1998 / Accepted: 30 July 1999     

Population structure and phylogeography of an acorn barnacle with induced defense and its gastropod predator in the Gulf of California

Using sequence variation in the mitochondrial cytochrome oxidase I locus, we estimated the population structure and phylogeography of the intertidal acorn barnacle, Chthamalus anisopoma, and its gastropod predator, Mexacanthina lugubris angelica. Both are endemic to the Gulf of California, being derived from taxa on the Pacific coast of the Baja peninsula, and both exhibit phenotypic plasticity for traits affecting their coevolutionary interactions. Consistent with expectations based on differences in dispersal, C. anisopoma populations generally lack geographic structure, while those of M. l. angelica are more strongly structured. However, the variable degree of differentiation in both species suggests that the extent of reciprocal selection and local adaptation in the species will vary geographically, a result consistent with the concept of a geographic mosaic of coevolution. The pattern of variation in C. anisopoma shows clear evidence of recent spatial expansion, possibly due to increased habitat availability following the last glacial maximum. Phylogeographic analyses suggest that M. l. angelica diversified into three distinct clades after the colonization of the Gulf. Overall, our results illustrate how dispersal potential, geological and climatic events, and recent population growth have impacted the pattern of sequence variation in the two species.     

Food and habitat choice in floating seaweed clumps: the obligate opportunistic nature of the associated macrofauna

The species composition of macrofauna associated with floating seaweed rafts is highly variable and influenced by many factors like spatial and temporal variation, period since detachment and probably also the seaweed species. The presence of seaweed preferences was assessed by a combination of in situ seaweed samplings and multiple-choice aquarium experiments in a controlled environment, using the seaweed-associated grazing organisms Idotea baltica and Gammarus crinicornis. Results from the sampling data confirm that the seaweed composition influences macrofaunal species composition and abundance: samples dominated by Sargassum muticum displayed higher densities but lower diversities compared to samples dominated by Ascophyllum nodosum and Fucus vesiculosus. Seaweed preference was also apparent from the multiple-choice experiments, but did not exactly match the results of the community analysis: (1) I. baltica had high densities in seaweed samples (SWS) dominated by F. vesiculosus and A. nodosum, while in the experiments, this isopod was most frequently associated with Enteromorpha sp. and F. vesiculosus, and fed mostly on S. muticum, A. nodosum and Enteromorpha sp.; (2) G. crinicornis had high densities in SWS dominated by F. vesiculosus, while in the experiments, this amphipod was most frequently associated with S. muticum, but fed most on A. nodosum and F. vesiculosus. It is clear from the laboratory experiments that preference for habitat (shelter) and food can differ among seaweed species. However, food and habitat preferences are hard to assess because grazer preference may change if choices are increased or decreased, if different sizes of grazers are used, or if predators or other grazers are added to the experiments. The effects of seaweed composition may also be blurred due to the obligate opportunistic nature of a lot of the associated macrofaunal species.     

Macro- and mesoherbivores prefer native seaweeds over the invasive brown seaweed <Emphasis Type="Italic">Sargassum muticum</Emphasis>: a potential regulating role on invasions

Herbivory has a strong impact on algal distribution, abundance and community structure and may influence the establishment and spread of introduced seaweed species. In this study, we assess the potential regulating role of herbivory on one of the most invasive brown seaweeds: Sargassum muticum. Multiple choice feeding experiments were conducted with 13 native seaweeds, S. muticum and 5 herbivore species from the Northwest, Southwest and South of Portugal. S. muticum was always the least or among the least preferred seaweeds and attained one of the highest growth rates of the tested seaweeds, with and without herbivores. The addition of herbivores increased the number of cases by 40% in which the invader had higher growth rates. Our results suggest that low grazing pressure on S. muticum by the recipient herbivore community may give the invader a competitive advantage over at least part of the native seaweed community, thereby contributing to the invasiveness of S. muticum along the Portuguese coast.     

Effects of coral transplantation in sites of varying distances and environmental conditions

Several scleractinian coral species with different growth forms and life history strategies were studied in terms of colony growth (expressed as projected linear increment) and survivorship over a range of distances and environmental conditions in the Philippines. The experimental design consisted of 1 m² plots grouped within a reef site, to several sites within reef systems separated by a distance of about 340 km. There were distinct differences among species, with submassive and massive forms displaying slower growth but better survival, confirming results of other studies. They probably play the role of framework builders of the reef. In contrast, the delicate branching and foliose species had higher growth rates but poor survivorship. This observation, plus their ease of fragmentation, suggests they act more as fillers of the reef matrix. There was high variability in colony increment of a species among the square meter plots, but not among sites within a reef system. Thus, more regular pattern could be observed at this level. In contrast to growth, survivorship differed significantly among sites, being lowest in the site which harbored the greatest amount of dead coral. Growth and survival, however, are not sufficient performance measures to evaluate the success of coral transplantation. Reproduction and subsequent recruitment must also be taken into account. It is recommended that coral transplant and restoration studies consider the broad environmental context of restoration and seek to develop assembly rules that will allow practitioners to match coral types and sequence of interventions to each unique context.     

Influence of growth on reproductive traits and its effect on fertility and gene diversity in a clonal seed orchard of scots pine, Pinus Sylvestris L

This study was carried out in a clonal seed orchard of scots pine (Pinus sylvestris L.), to determine the difference and interaction for reproductive and growth characters among clones and its impact on fertility variation and gene diversity Numbers of female and male strobili, and height and diameter at breast height were studied on six grafts chosen randomly in each of the 27 clones for the purpose. One-way analysis of variance revealed large differences in both reproductive and growth characters among clones. The differences were higher in growth characters than in reproductive traits. There was significant phenotypic correlation among growth and reproductive characters. So, growth characters had a greater effect on male and female fertility Estimates of total fertility variation (Sibling coefficient = 1.012), status number (26.8) and relative gene diversity (0.981) were computed. Fertility variation among clones was low, which caused a high relative population size (99% of census number). The positive phenotypic correlation between growth and reproductive characters showed that enhanced growth rate could be effective in improving fertility and gene diversity of seed orchard crop. The results of the study have implications in breeding and selection of plus tree and populations, establishment and thinning of seed orchards of the species.     

Life-history variation in three coexisting cheilostome bryozoan species of the genus Stylopoma in Panamá

Life-history variation was studied in three sympatric species of Stylopoma Levinsen [S. spongites (Pallas), S. projecta Canu and Bassler and Stylopoma n. sp. 15] in Panamá. Bryozoan colonies were collected from 27 reefs along 300 km of the Caribbean coast of Panamá. The distribution and abundance of each species were very patchy, but with broad overlap in occurrence among localities and depths. Nevertheless, species differed considerably in colony size, size at first reproduction and numbers of brooding larvae; implying that interspecific differences in life-history traits may contribute to their coexistence. To examine closely this variation in life-history patterns, we grew, in a common garden experiment, the offspring of the two most common species that were obtained from parent colonies collected from several reefs. There were highly significant differences in growth rates and the timing and extent of sexual reproduction which corresponded well to patterns observed in individuals from the field. Other factors, including size of larvae, extent of secondary zooidal calcification and numbers of avicularia were also correlated with differences in life histories. Despite this additional complexity, however, ecological consequences of trade-offs in life history among modular species such as Stylopoma spp. appear very similar to those among unitary species.     

Genetic Effects of Habitat Fragmentation on Common Species of Swiss Fen Meadows

Abstract:  The area of Caricion davallianae alliance in Switzerland has been considerably reduced and fragmented during the last 150 years. We assessed the genetic variability, inbreeding level, and among-population differentiation of two common habitat-specific plant species, Carex davalliana SM. and Succisa pratensis Moench, in 18 Caricion davallianae fen meadows subjected to fragmentation. We used a spatial field design of fen systems (six systems total), each consisting of one large habitat island and two small habitat islands. We used allozyme electrophoresis to derive standard genetic parameters ( A, P, H_O, H_E, F_IS, F_ST ). In Carex we identified a consistently lower A in isolated habitat islands; furthermore, H_E was lower in small habitat islands than in large habitat islands. In Succisa we identified a lower H_O in small habitat islands than in larger ones. Small habitat islands were marginally significantly differentiated (  F_ST ) from large islands for Succisa . For both species, no effects were evident for F_IS ; therefore, we argue that genetic drift rather than inbreeding is the main cause of the observed differences. The genetic structure of Carex and Succisa in small habitat islands differed from that in large habitat islands, but differences were small. It appears that the observed differences in genetic variability among fen meadows correspond to observed differences in fitness and demographic traits. We show that habitat fragmentation affects not only the rare species in an ecosystem but also reduces the survival probabilities of common species. One of the main goals of conservation should be to mitigate fragmentation of natural habitats in order to increase population sizes and connectivity.     

Variation in larval life-history traits among reef fishes across the Isthmus of Panama

We tested the hypothesis that regional differences in oceanic productivity have led to the evolution of predictable patterns of regional variation in life-history traits of pelagic larvae of tropical reef fishes. To do so we compared larval traits (egg and hatchling size, larval growth rate and duration, and size at settlement) among closely related reef fishes from the Atlantic and Pacific coasts of the Isthmus of Panama. This comparison provides a control for phylogenetic effects because those regions shared a common fauna prior to the rise of the Isthmus ˜3.5 million years ago, subsequent to which each fauna evolved independently under a very different productivity regime. We measured larval traits of 12 benthic-spawning damselfishes (Pomacentridae: Abudefduf, Chromis and Stegastes) and 13 pelagic-spawning wrasses (Labridae: Bodianus, Halichoeres and Thalassoma). These included members of each genus on each side of the Isthmus and four sets of transisthmian sister species of pomacentrids. Among the pomacentrids we found consistent transisthmian differences in hatchling size, but not in other larval traits. Essentially the reverse pattern occurred among the labrids – larval growth and duration differed consistently among congeners in the two regions, but without consistent differences in hatchling size or size at settlement. Neither relationship is predicted by the regional-productivity hypothesis. Most of the differences were quite small. Stronger phylogenetic effects on larval traits (inter- and intrageneric variation within regions) occur in both families and evidently overwhelm any effect of regional variation in productivity. Reassessment of data that takes into account such phylogenetic effects questions previous conclusions about the existence of regional differences in larval traits among damselfishes in the West Pacific and the Caribbean. Received: 19 January 2000 / Accepted: 26 September 2000     

Flexible habitat selection and interactive habitat segregation in the marine congeners <Emphasis Type="Italic">Idotea baltica</Emphasis> and <Emphasis Type="Italic">Idotea emarginata</Emphasis> (Crustacea,Isopoda)

Habitat segregation among competing species is widespread yet very little is know how this is achieved in practice. In a case study, we examined short-term effects of conspecific and congeneric density on habitat selection in two competing marine isopod species, Idotea emarginata and Idotea baltica. Under semi-natural conditions in large outdoor cylindrical tanks (4 m high; volume 5.5 m³), animal groups of different size and composition had the choice between a set of relevant habitat samples (surface-floating seaweed, the water column, seaweed on the bottom). Habitat selection in both I. baltica and I. emarginata proved to be largely independent of conspecific density (level of intraspecific competition). In single-species treatments, both species showed a similar and stable pattern of distribution, with a clear preference for seaweed on the bottom. In mixed-species treatments (MST), however, the species were largely separated by habitat. While the distribution of I. emarginata was completely unaffected by the mere presence of interspecific competitors, habitat selection of I. baltica changed notably when I. emarginata was present. The habitat use patterns observed in MST conformed to those realized in geographical areas where the two species overlap in distribution: I. emarginata is dominant among decaying seaweed on the sea floor, and I. baltica is the dominant species among surface-floating seaweed. Our findings suggest that habitat segregation between the two species is essentially interactive, resulting from rapid decision-making of I. baltica with respect to habitat selection. The underlying mechanism is discussed. I. emarginata is highly superior to I. baltica in interference competition and rapidly eliminates the latter from one-habitat systems which do not allow I. baltica to escape from this interaction. In more natural, heterogeneous environments, however, I. baltica seems to be able to coexist with the superior competitor due to its broader habitat niche, flexibility in habitat selection, and a behavioural disposition to avoid normally preferred habitats when these are occupied by I. emarginata.     

Nematocyst complements of nudibranchs in the genus <Emphasis Type="Italic">Flabellina</Emphasis> in the Gulf of Maine and the effect of diet manipulations on the cnidom of <Emphasis Type="Italic">Flabellina verrucosa</Emphasis>

Aeolid nudibranchs maintain functional nematocysts, which are sequestered from the nudibranchs’ cnidarian prey and provide protection against predators. Some species exhibit extensive variation in incorporated nematocysts, while others maintain a limited number of types. This study examines the apparent diversity in uptake and patterns of nematocyst incorporation among related species. Nematocyst complements were described for four Gulf of Maine nudibranch species in the genus Flabellina exhibiting a variety of feeding strategies and prey specificities. Diet manipulations were performed to examine the response to changing nematocyst availability using a generalist consumer, Flabellina verrucosa, to assess nematocyst uptake based on diet. The flabellinid species examined exhibited significant differences in nematocyst incorporation, reflecting differences in their specificity as predators and nematocyst types available in their natural prey. The nematocyst complement of F. verrucosa was the most variable and differed among collection regions. When diet was manipulated, nematocyst uptake depended on the prey the nudibranchs consumed, but when offered a variety of prey, F. verrucosa selectively preferred nematocysts from scyphistomae. The observed variation in nematocyst uptake among species and regions probably relates to environmental disparities among populations.     

Some ecological observations on fishes associated with drifting seaweed off Tohoku coast,Japan

Fishes associated with drifting seaweed were sampled in the Tohoku area, northwest Pacific, in late spring 1988, to analyse the community structure and ecological relationships of the dominant species.Seriola quinqueradiata Temminck et Schlegel was the dominant species and its escort was composed ofThamnaconus (Navodon)modestus, Sebastes inermis, Hyperoglyphe japonica andEnedrias nebulosus. E. nebulosus, which fed essentially on Gammarid crustaceans, was collected mainly in Sendai Bay; the other species, found in offshore water, were plankton feeders and did not exhibit any strong competition, indicating close cohabitation. The abundance of drifting seaweed in the Tohoku area peaked in May–June and decreased in July; they seemed to come from the nearest coast. JuvenileS. quinqueradiata (> 15 cm long) were collected with set nets in Sendai Bay during summer. They seemed to use drifting seaweed (along with the warm Kuroshio current) as a means of transportation during their migration from the southern spawning ground to northern Tohoku area. Drifting seaweed could be considered as a nursery, and the association of fishes with floating algae might have an influence on species recruitment.     

Mating systems in the sea anemone genus Epiactis

Four morphologically similar species in the sea anemone genus Epiactis exhibit overlapping distributions on the Pacific coast of North America; E. prolifera, E. lisbethae, E. ritteri and E. fernaldi. All brood their offspring up to the juvenile stage, but each has a different combination of internal versus external brooding and hermaphroditism versus gonochory (separate sexes). Specimens were collected from sites ranging from British Columbia to southern California between December 1988 and July 1992. Mating systems were inferred from genetic comparisons of mothers and offspring histological analyses of sex expression and observations on brooding and spawning behavior. Allozyme and multilocus DNA fingerprint analyses of the gynodioecious hermaphrodite E. prolifera showed that offspring were all identical to their mothers, a result consistent with either asexual reproduction, self-fertilization or extreme biparental inbreeding. In the gonochore E. lisbethae, mothers and offspring were also electrophoretically identical, but variation in DNA fingerprints indicated cross-fertilization. Similar DNA fingerprint differences between mother and offspring in the gonochore E. ritteri implied that cross-fertilization also occurs in this species. No mother-offspring comparisons were performed on E. fernaldi, as this species was not observed brooding offspring during this study. Although incomplete, the results of this study increase our knowledge of the very unusual combination of reproductive modes in the genus Epiactis, and argue for further investigations of the evolution and genetic consequences of mating systems in these species.     

Species boundaries in the starfish genus Linckia

 The genetic basis for species boundaries in the starfish genus Linckia was examined using variation observed in 613 base pairs (bp) of sequence from the cytochrome oxidase I gene of mtDNA and 16 allozyme loci. Five groups within Linckia were clearly genetically differentiated; L. columbiae, L. bouvieri, two clades within L. guildingi, and one clade with two sub-clades consisting of both L. laevigata and L. multifora. Genetic divergence among these groups is consistent with interspecific variation. The two clades within L. guildingi suggest the presence of a cryptic, partially sympatric, species. Genetic divergence between these two clades implies that they have been genetically distinct from each other for at least one million years. In contrast, genetic data suggest that L. laevigata and L. multifora are a single species, despite the fact that live individuals can be distinguished by their colour and colour pattern, number of madreporites and ratio of arm length to breadth. There are probably three closely related groups within the L. laevigata/L. multifora clade –L. multifora, and two groups in L. laevigata defined by biogeographic province. It is difficult to determine boundaries for these three entities, since genetic and morphological differences are complicated by phenotypic differences arising from both environmental variation and population genetic structure. The difficulties encountered in defining species boundaries in Linckia, particularly with respect to variation arising from the overlap of Indian and Pacific biogeographic provinces, may be a general issue for many marine organisms from this region. Received: 24 May 1999 / Accepted: 6 October 1999     

Variability of stable isotopes and maximum linear extension in reef-coral skeletons at Kaneohe Bay, Hawaii

Stable-isotope and growth records of coral skeletons are often used to reconstruct tropical paleoclimate, yet few surveys have systematically examined the natural variability in coral skeletal ¹³C, ¹⁸O and maximum linear skeletal extension (MLSE) across depth. Here, interspecific, intraspecific, and geographical variations in coral skeletal ¹³C, ¹⁸O, and MLSE were examined in the corals Porites compressa, P. lobata, and Montipora verrucosa grown at 1.7, 5.0, and 8.3 m depth from August 1996 to March 1997 at The Point Reef and Patch Reef #41 field sites in Kaneohe Bay, Hawaii. Coral skeletal ¹³C values significantly decreased with depth and differed between species, but did not vary between field sites. ¹⁸O values were not significantly different across depth within a species, but did differ among species and field sites. High-resolution analysis of the intra-annual variation in skeletal ¹³C and ¹⁸O in P. compressa at 2.0 m depth confirms that these isotopes reflect changes in solar irradiance and temperature, respectively. Changes in MLSE across depth were consistent within, but highly variable among, species. Peak MLSE occurred at 1.7, 5.0, and 8.3 m for P. lobata, P. compressa, and M. verrucosa, respectively. Such interspecific variation in MLSE patterns may be attributable to one or more of the following: increases in zooplankton in the diet, changes in metabolic processes, or changes in growth form with depth. Overall, these results imply that natural inter- and intraspecific variability in coral skeletal ¹³C, ¹⁸O, and MLSE should be considered when interpreting and comparing coral-based tropical paleoclimate data from various coral species, depths, and field sites. Received: 6 October 1998 / Accepted: 8 July 1999     

Effects of sheltering fish on growth of their host corals

Stony corals are the foundation species of tropical reefs, and their structures can harbor a diverse range of mutualist taxa that can confer important benefits, including provision of nutrients. Prominent among the associates of branching coral in the genus Pocillopora are groups of zooplanktivorous damselfishes that take refuge in the coral to avoid their predators. In field and laboratory experiments, we explored the effects of colonies of resident damselfishes on growth of their host corals. Laboratory studies revealed a positive relationship between biomass of fish and output of ammonium. In the field, levels of ammonium were significantly elevated in the water surrounding the branches of Pocillopora occupied by colonies of damselfish, particularly in time periods following active feeding by the fish. Experimental manipulation of the presence of fish on host corals during a month-long field experiment revealed that corals hosting fish grew significantly more than those that lacked fish, and coral growth was positively correlated with the biomass of resident fish. The Pocillopora colonies in the field experiment varied in the degree of openness of their branching structure, and dye studies indicated that this affected their ability to retain waterborne nutrients. Together with biomass of resident fish, colony openness explained 76% of the variation in coral growth rate during the experiment. Corals can exhibit considerable morphological variability, and mutualistic fish respond to colony architecture during habitat selection, with some species preferring more open-branched forms. This makes it likely that corals may face tradeoffs in attracting resident fish and in retaining the nutrients they provide.