Restricted gene flow in the Caribbean staghorn coral Acropora cervicornis: implications for the recovery of endangered reefs

Coral reef conservation requires information about the distance over which healthy reefs can rescue damaged reefs through input of coral larvae. This information is desperately needed in the Caribbean where the 2 dominant shallow water corals Acropora cervicornis and Acropora palmata have suffered unprecedented declines. Here we compare the population genetic structure in the staghorn coral A. cervicornis across the greater Caribbean using DNA sequence data from 1 mitochondrial and 3 nuclear genes. Data from 160 individuals from 22 populations and 9 regions show that A. cervicornis exhibits significant population genetic structure across the greater Caribbean in both the mitochondrial (Phi(st) = 0.130) and nuclear data (Phi(st) = 0.067). The highest population structure was observed in the species' own, native mtDNA haplotypes (Phi(st) = 0.235). Introgressed alleles from A. palmata tempered higher population structure in A. cervicornis over regional scales but in some cases generated highly localized "introgression hot spots" and fine-scale genetic structure among reefs separated by as few as 2 km. These data show that larval dispersal over moderate or long distances (>500 km) is limited for this threatened species and in some cases locally limited as well. Thus, the endangered Caribbean staghorn corals require local source populations for their recovery and targeted conservation efforts over spatial scales much smaller than the hundreds to thousands of kilometers usually proposed for marine reserves.     

Propagation of the threatened staghorn coral Acropora cervicornis: methods to minimize the impacts of fragment collection and maximize production

Coral reef restoration methods such as coral gardening are becoming increasingly considered as viable options to mitigate reef degradation and enhance recovery of depleted coral populations. In this study, we describe several aspects of the coral gardening approach that demonstrate this methodology is an effective way of propagating the threatened Caribbean staghorn coral Acropora cervicornis: (1) the growth of colonies within the nursery exceeded the growth rates of wild staghorn colonies in the same region; (2) the collection of branch tips did not result in any further mortality to the donor colonies beyond the coral removed for transplantation; (3) decreases in linear extension of the donor branches were only temporary and donor branches grew faster than control branches after an initial recovery period of approximately 3–6 weeks; (4) fragmentation did not affect the growth rates of non-donor branches within the same colony; (5) small branch tips experienced initial mortality due to handling and transportation but surviving tips grew well over time; and (6) when the growth of the branch tips is added to the regrowth of the fragmented donor branches, the new coral produced was 1.4–1.8 times more than new growth in undisturbed colonies. Based on these results, the collection of small (2.5–3.5 cm) branch tips was an effective propagation method for this branching coral species resulting in increased biomass accumulation and limited damage to parental stocks.     

Physiological effects of heat and cold exposure in the common reef coral Acropora millepora

Reef-forming corals are under threat globally from climate change, leading to changes in sea temperatures with both hot and cold events recorded and projected to increase in frequency and severity in the future. Tolerance to heat and cold exposure has been found to be mutually exclusive in other marine invertebrates, but it is currently unclear whether a trade-off exists between hot and cold thermal tolerance in tropical corals. This study quantified the changes in physiology in Acropora millepora from the central Great Barrier Reef subjected to three temperature treatments; sub-lethal cold, ambient and sub-lethal heat (23.0 °C, 27.0 °C and 29.5 °C, respectively). After 10 weeks, pigment content and Symbiodiniaceae density increased in cold-treated corals but decreased in heat-treated corals relative to corals at ambient conditions. Heat-treated corals gained less mass relative to both ambient and cold-treated corals. These results indicate that the physiological condition of A. millepora corals examined here improved in response to mild cold exposure compared to ambient exposure and decreased under mild heat exposure despite both these temperatures occurring in situ around 15% of the year. The energetic condition of corals in the hotter treatment was reduced compared to both ambient and cooler groups, indicating that corals may be more resilient to mild cold exposure relative to mild heat exposure. The results indicate that the corals shifted their resource allocation in response to temperature treatment, investing more energy into skeletal extension rather than maintenance. No evidence of thermal tolerance trade-offs was found, and cold thermal tolerance was not lost in more heat-tolerant individuals. An enhanced understanding of physiological responses of corals at both ends of the thermal spectrum is important for predicting the resilience of corals under projected climate change conditions.

     

Variation in susceptibility among three Caribbean coral species and their algal symbionts indicates the threatened staghorn coral,Acropora cervicornis,is particularly susceptible to elevated nutrients and heat stress

Coral Reefs - Coral cover is declining worldwide due to multiple interacting threats. We compared the effects of elevated nutrients and temperature on three Caribbean corals: Acropora cervicornis,...     

Atypically low rate of cytochrome b evolution in the scleractinian coral genus Acropora

Unexpectedly low levels of mitochondrial DNA (mtDNA) cytochrome b sequence divergence are found between species of the scleractinian coral genus Acropora. Comparison of 964 positions of the cytochrome b gene of two out of the three Caribbean Acropora species with seven of their Pacific congeners shows only 0.3-0.8% sequence difference. Species in these biogeographic regions have been evolving independently for at least three million years (since the rise of the Isthmus of Panama) and this geological date is used to estimate nucleotide divergence rates. The results indicate that the Acropora cytochrome b gene is evolving at least 10-20 times slower than the 'standard' vertebrate mtDNA clock and is one of the most slowly evolving animal mitochondrial genes described to date. The possibility is discussed that, unlike higher animals, cnidarians may have a functional mtDNA mismatch repair system.     

Ultra short-term heart rate recovery after maximal exercise: relations to aerobic power in sportsmen

The main aim of the study was to investigate whether different levels of aerobic power influence heart rate (HR) responses during the first minute of recovery following maximal exercise in athletes. Thirty-two young male soccer players were recruited for the study during the final week of their training prior to [corrected] the competition. Following the maximal exercise on treadmill the participants were placed supine for 60 s of HR recording. The time between exercise cessation and the recovery HR measurement was kept as short as possible. At the end of exercise (i.e., the start of recovery), HRs were [corrected] was similar in both trials. At both 10 s and 20 s of recovery period, the players characterized by high aerobic power (> 60 ml/kg/ min) revealed significantly lower HR as compared to their sub-elite counterparts (< 50 ml/kg/min; P < 0.05). No differences between the groups were found at later stages of the analyzed post-exercise HR. The data suggest that the athletes characterized by high aerobic capacity could be better adapted to maximal exercise with faster recovery HR immediately following an exercise test. These results generally suggest that the aerobic power along with autonomic modulation might have played a role in the ultra short-term cardiovascular responses to all-out exercise.     

Mechanical properties,spectral vibrational response,and flow-field analysis of the aragonite skeleton of the staghorn coral (Acropora cervicornis)

Coral Reefs - Understanding the structural and mechanical properties of coral skeletons is important to assess their responses to natural and anthropogenic challenges and to predict the long-term...     

The evolution of nuclear receptors: evidence from the coral Acropora

We have amplified and sequenced PCR products derived from 10 nuclear receptor (NR) genes from the anthozoan cnidarian Acropora millepora, including five products corresponding to genes not previously reported from the phylum Cnidaria. cDNAs corresponding to seven of these products were sequenced and at least three encode full-length proteins, increasing the number of complete cnidarian NR coding sequences from one to four. All clear orthologs of Acropora NRs either lack an activation domain or lack a known ligand, consistent with the idea that the ancestral nuclear receptor was without a ligand. Phylogenetic analyses indicate that most, and possibly all, presently identified cnidarian NRs are members of NR subfamily 2, suggesting that the common ancestor of all known nuclear receptors most resembled members of this subfamily.     

Ecological and genetic data indicate recovery of the endangered coral Acropora palmata in Los Roques, Southern Caribbean

The rapid decline of Acropora cervicornis and Acropora palmata has often been linked with coral reef deterioration in the Caribbean; yet, it remains controversial whether these species are currently recovering or still declining. In this study, the status of ten populations of A. palmata in Los Roques National Park (LRNP), Venezuela is presented. Six of these populations showed signs of recovery. Ten 80 m² belt-transects were surveyed at each of the ten reef sites. Within belt-transects, each colony was measured (maximum diameter and height) and its status (healthy, diseased or injured) was recorded. Populations in recovery were defined by a dominance of small to medium-sized colonies in densities >1 colony per 10 m², together with 75% undamaged colonies, a low prevalence of diseases (<10%), and a low density of predators (0.25 snails per colony). Based on allozyme analysis of seven polymorphic loci in four populations (N = 30), a moderate to high-genetic connectivity among these populations (F _ST = 0.048) was found with a predominance of sexual over asexual reproduction (N* : N = 1; N _go : N = 0.93–1). Both ecological and molecular data support a good prognosis for the recovery of this species in Los Roques.     

Calcification rate and the effect of temperature in a zooxanthellate and an azooxanthellate scleractinian reef coral

Calcification rates, normalized to skeletal mass, in the zooxanthellate Galaxea fascicularis and the azooxanthellate Dendrophyllia sp. were similar over the whole temperature range of 18–29 °C. Calcification was measured by Ca⁴⁵ incorporation in corals that were naturally acclimated to the prevailing seawater temperature. In both species maximum calcification rate occurred at about 25 °C and calcification rates can be fitted to a Gaussian distribution with respect to temperature. The similarity in temperature dependence of the zooxanthellate and azooxanthellate coral suggests that temperature affects some fundamental process of calcification that is independent of light effects. It is shown that two different populations of Galaxea fascicularis have distinctly different ratios of tissue protein to skeletal mass per polyp. This indicates that tissue protein may not be suitable for normalizing calcification rates in individual coral polyps, both within and between species. Intra- and interspecific comparisons of calcification rates may be better made on the basis of skeletal mass when polyps are similar in size and shape.Communicated by Topic Editor C. Barnes     

Skeletal development in Acropora cervicornis: I. Patterns of calcium carbonate accretion in the axial corallite

Summary Scanning electron microscopy and serial petrographic thin sections were used to investigate skeletal elongation and mineralization in the perforate coral, Acropora cervicornis. The axial corallite extends by the formation of randomly oriented fusiform crystals which are deposited on its distal edge. Aragonitic needle-like crystals grow in random directions from the surface of these fusiform crystals. Only those needle-like crystals growing toward the calicoblastic epithelium (i.e. crystals whose growth axis is perpendicular to the plane of the calicoblastic cell membrane) continue to elongate. Groups of these growing crystals join to form well-defined fasciculi which make up the primary skeletal elements comprising the septotheca. The resulting skeleton is highly porous with all surfaces covered by the continuous calicoblastic epithelium. This cell layer is separated by thin mesoglea from the flagellated gastrodermis which lines the highly ramified coelenteron. Porosity and permeability of the skeleton decrease with distance from the tip. Density correspondingly increases due to the addition of aragonite to the fasciculi whose boundaries become less distinct as channels fill with calcium carbonate.     

Shifting ecological baselines and the demise of Acropora cervicornis in the western North Atlantic and Caribbean Province: a Pleistocene perspective

 In recent years, marine scientists have become increasingly alarmed over the decline of live coral cover throughout the Caribbean and tropical western Atlantic region. The Holocene and Pleistocene fossil record of coral reefs from this region potentially provides a wealth of long-term ecologic information with which to assess the historical record of changes in shallow water coral reef communities. Before fossil data can be applied to the modern reef system, critical problems involving fossil preservation must be addressed. Moreover, it must be demonstrated that the classic reef coral zonation patterns described in the early days of coral reef ecology, and upon which “healthy” versus “unhealthy” reefs are determined, are themselves representative of reefs that existed prior to any human influence. To address these issues, we have conducted systematic censuses of life and death assemblages on modern “healthy” patch reefs in the Florida reef tract that conform to the classic Caribbean model of reef coral zonation, and a patch reef in the Bahamas that is currently undergoing a transition in coral dominance that is part of a greater Caribbean-wide phenomenon. Results were compared to censuses of ancient reef assemblages preserved in Pleistocene limestones in close proximity to each modern reef. We have determined that the Pleistocene fossil record of coral reefs may be used to calibrate an ecological baseline with which to compare modern reef assemblages, and suggest that the current and rapid decline of Acropora cervicornis observed on a Bahamian patch reef may be a unique event that contrasts with the long-term persistence of this taxon during Pleistocene and Holocene time. Accepted: 19 May 1998     

The coral Acropora: what it can contribute to our knowledge of metazoan evolution and the evolution of developmental processes

The diploblastic Cnidaria form one of the most ancient metazoan phyla and thus provide a useful outgroup for comparative studies of the molecular control of development in the more complex, and more often studied, triploblasts. Among cnidarians, the reef building coral Acropora is a particularly appropriate choice for study. Acropora belongs to the Anthozoa, which several lines of evidence now indicate is the basal class within the phylum Cnidaria, and has the practical advantages that its reproduction is predictable, external and accessible and that the base content of its genome is not strongly biased. The Acropora system has already provided insights into ancestral linkages of homeobox genes and the evolution of the Pax genes, and has the potential to provide further new perspectives on the age, role in development, and evolution of these and other gene families.     

Pseudogenes contribute to the extreme diversity of nuclear ribosomal DNA in the hard coral Acropora

One characteristic of Indo-Pacific Acropora spp. is extremely high diversity in rDNA sequences at both the species and individual levels. In order to test the hypothesis that pseudogenes may contribute to this diversity, three kinds of analyses were conducted. First, for A. millepora (the species containing the most diverse suite of rDNA types), RT-PCR was used to determine which 5.8S rDNA types are expressed. Second, as previous studies have indicated that interspecific hybridization has occurred in the genus Acropora and silencing of rDNA loci via nucleolar dominance has been shown in some cases to involve methylation, patterns of variation were examined at methylation-susceptible sites. Third, patterns of substitution at conserved sites (including those that are likely to contribute to secondary structure in rRNA) in the 5.8S rDNA were examined. These analyses consistently indicated that one rDNA sequence type present in a broad range of Indo-Pacific Acropora species is likely to consist predominantly of pseudogenes. Patterns of variation also suggest that species may differ with respect to which rDNA sequence types have been silenced and which are active. These pseudogenes are likely to have arisen as a consequence of the introduction of highly divergent rDNA types into single genomes by interspecific hybridization events, and we attribute the extreme rDNA diversity characteristic of many Acropora species to both the independent evolution of these silenced rDNA types and to the suppressive effects of high sequence diversity on homogenization processes acting on functional loci.