Fecundity and the demographic strategies of coral morphologies.

Understanding species differences in demographic strategies is a fundamental goal of ecology. In scleractinian corals, colony morphology is tightly linked with many demographic traits, such as size-specific growth and morality. Here we test how well morphology predicts the colony size-fecundity relationship in eight species of broadcast-spawning corals. Variation in colony fecundity is greater among morphologies than between species with a similar morphology, demonstrating that colony morphology can be used as a quantitative proxy for demographic strategies. Additionally, we examine the relationship between size-specific colony fecundity and mechanical vulnerability (i.e., vulnerability to colony dislodgment). Interestingly, the relationship between size-specific fecundity and mechanical vulnerability varied among morphologies. For tabular species, the most fecund colonies are the most mechanically vulnerable, while the opposite is true for massive species. For corymbose and digitate colonies, mechanical vulnerability remains relatively constant as fecundity increases. These results reveal strong differences in the demographic tradeoffs among species of different morphologies. Using colony morphology as a quantitative proxy for demographic strategies can help predict coral community dynamics and responses to anthropogenic change.

The role of light in mediating the effects of ocean acidification on coral calcification.

We tested the effect of light and PCO2 on the calcification and survival of Pocillopora damicornis recruits settled from larvae released in southern Taiwan. In March 2011, recruits were incubated at 31, 41, 70, 122 and 226 µmol photons m(-2) s(-1) under ambient (493 µatm) and high PCO2 (878 µatm). After 5 days, calcification was measured gravimetrically and survivorship estimated as the number of living recruits. Calcification was affected by the interaction of PCO2 with light, and at 493 µatm PCO2 the response to light intensity resembled a positive parabola. At 878 µatm PCO2, the effect of light on calcification differed from that observed at 493 µatm PCO2, with the result that there were large differences in calcification between 493 µatm and 878 µatm PCO2 at intermediate light intensities (ca. 70 µmol photons m(-2) s(-1)), but similar rates of calcification at the highest and lowest light intensities. Survivorship was affected by light and PCO2, and was highest at 122 µmol photons m(-2) s(-1) in both PCO2 treatments, but was unrelated to calcification. In June 2012 the experiment was repeated, and again the results suggested that exposure to high PCO2 decreased calcification of P. damicornis recruits at intermediate light intensities, but not at lower or higher intensities. Together, our findings demonstrate that the effect of PCO2 on coral recruits can be light dependent, with inhibitory effects of high PCO2 on calcification at intermediate light intensities that disappear at both higher and lower light intensities.

Six years of demography data for 11 reef coral species.

Scleractinian corals are colonial animals with a range of life-history strategies, making up diverse species assemblages that define coral reefs. We tagged and tracked ~30 colonies from each of 11 species during seven trips spanning 6 years (2009-2015) to measure their vital rates and competitive interactions on the reef crest at Trimodal Reef, Lizard Island, Australia. Pairs of species were chosen from five growth forms in which one species of the pair was locally rare (R) and the other common (C). The sampled growth forms were massive (Goniastrea pectinata [R] and G. retiformis [C]), digitate (Acropora humilis [R] and A. cf. digitifera [C]), corymbose (A. millepora [R] and A. nasuta [C]), tabular (A. cytherea [R] and A. hyacinthus [C]) and arborescent (A. robusta [R] and A. intermedia [C]). An extra corymbose species with intermediate abundance, A. spathulata was included when it became apparent that A. millepora was too rare on the reef crest, making the 11 species in total. The tagged colonies were visited each year in the weeks prior to spawning. During visits, two or more observers each took two or three photographs of each tagged colony from directly above and on the horizontal plane with a scale plate to track planar area. Dead or missing colonies were recorded and new colonies tagged to maintain ~30 colonies per species throughout the 6 years of the study. In addition to tracking tagged corals, 30 fragments were collected from neighboring untagged colonies of each species for counting numbers of eggs per polyp (fecundity); and fragments of untagged colonies were brought into the laboratory where spawned eggs were collected for biomass and energy measurements. We also conducted surveys at the study site to generate size structure data for each species in several of the years. Each tagged colony photograph was digitized by at least two people. Therefore, we could examine sources of error in planar area for both photographers and outliners. Competitive interactions were recorded for a subset of species by measuring the margins of tagged colony outlines interacting with neighboring corals. The study was abruptly ended by Tropical Cyclone Nathan (Category 4) that killed all but nine of the more than 300 tagged colonies in early 2015. Nonetheless, these data will be of use to other researchers interested in coral demography and coexistence, functional ecology, and parametrizing population, community, and ecosystem models. The data set is not copyright restricted, and users should cite this paper when using the data.

Effects of diurnally oscillating pCO2 on the calcification and survival of coral recruits.

Manipulative studies have demonstrated that ocean acidification (OA) is a threat to coral reefs, yet no experiments have employed diurnal variations in pCO(2) that are ecologically relevant to many shallow reefs. Two experiments were conducted to test the response of coral recruits (less than 6 days old) to diurnally oscillating pCO(2); one exposing recruits for 3 days to ambient (440 µatm), high (663 µatm) and diurnally oscillating pCO(2) on a natural phase (420-596 µatm), and another exposing recruits for 6 days to ambient (456 µatm), high (837 µatm) and diurnally oscillating pCO(2) on either a natural or a reverse phase (448-845 µatm). In experiment I, recruits exposed to natural-phased diurnally oscillating pCO(2) grew 6-19% larger than those in ambient or high pCO(2). In experiment II, recruits in both high and natural-phased diurnally oscillating pCO(2) grew 16 per cent larger than those at ambient pCO(2), and this was accompanied by 13-18% higher survivorship; the stimulatory effect on growth of oscillatory pCO(2) was diminished by administering high pCO(2) during the day (i.e. reverse-phased). These results demonstrate that coral recruits can benefit from ecologically relevant fluctuations in pCO(2) and we hypothesize that the mechanism underlying this response is highly pCO(2)-mediated, night-time storage of dissolved inorganic carbon that fuels daytime calcification.

An Indo-Pacific coral spawning database.

The discovery of multi-species synchronous spawning of scleractinian corals on the Great Barrier Reef in the 1980s stimulated an extraordinary effort to document spawning times in other parts of the globe. Unfortunately, most of these data remain unpublished which limits our understanding of regional and global reproductive patterns. The Coral Spawning Database (CSD) collates much of these disparate data into a single place. The CSD includes 6178 observations (3085 of which were unpublished) of the time or day of spawning for over 300 scleractinian species in 61 genera from 101 sites in the Indo-Pacific. The goal of the CSD is to provide open access to coral spawning data to accelerate our understanding of coral reproductive biology and to provide a baseline against which to evaluate any future changes in reproductive phenology.

Deciphering the nature of the coral-Chromera association.

Since the discovery of Chromera velia as a novel coral-associated microalga, this organism has attracted interest because of its unique evolutionary position between the photosynthetic dinoflagellates and the parasitic apicomplexans. The nature of the relationship between Chromera and its coral host is controversial. Is it a mutualism, from which both participants benefit, a parasitic relationship, or a chance association? To better understand the interaction, larvae of the common Indo-Pacific reef-building coral Acropora digitifera were experimentally infected with Chromera, and the impact on the host transcriptome was assessed at 4, 12, and 48 h post-infection using Illumina RNA-Seq technology. The transcriptomic response of the coral to Chromera was complex and implies that host immunity is strongly suppressed, and both phagosome maturation and the apoptotic machinery is modified. These responses differ markedly from those described for infection with a competent strain of the coral mutualist Symbiodinium, instead resembling those of vertebrate hosts to parasites and/or pathogens such as Mycobacterium tuberculosis. Consistent with ecological studies suggesting that the association may be accidental, the transcriptional response of A. digitifera larvae leads us to conclude that Chromera could be a coral parasite, commensal, or accidental bystander, but certainly not a beneficial mutualist.

Coral larvae are poor swimmers and require fine-scale reef structure to settle.

Reef coral assemblages are highly dynamic and subject to repeated disturbances, which are predicted to increase in response to climate change. Consequently there is an urgent need to improve our understanding of the mechanisms underlying different recovery scenarios. Recent work has demonstrated that reef structural complexity can facilitate coral recovery, but the mechanism remains unclear. Similarly, experiments suggest that coral larvae can distinguish between the water from healthy and degraded reefs, however, whether or not they can use these cues to navigate to healthy reefs is an open question. Here, we use a meta-analytic approach to document that coral larval swimming speeds are orders of magnitude lower than measurements of water flow both on and off reefs. Therefore, the ability of coral larvae to navigate to reefs while in the open-ocean, or to settlement sites while on reefs is extremely limited. We then show experimentally that turbulence generated by fine scale structure is required to deliver larvae to the substratum even in conditions mimicking calm back-reef flow environments. We conclude that structural complexity at a number of scales assists coral recovery by facilitating both the delivery of coral larvae to the substratum and settlement.

The reproductive season of Acropora in Socotra, Yemen.

Determining when corals reproduce has clear management and economic implications. Here we document the reproductive condition of corals in the genus Acropora on the island of Socotra in Yemen during February 2014. Twenty percent of colonies (n = 143) contained mature gametes and 28% had immature gametes indicating that spawning will occur in both February and March in 2014, confirming previous anecdotal reports of coral spawning at this time in Socotra. Acropora typically reproduce in synchrony with many other broadcast spawning scleractinian corals, and we therefore predict that many other species are reproductively active at this time of year.

A pre-zygotic barrier to hybridization in two con-generic species of scleractinian corals.

Hybridization is often cited as a potential source of evolutionary novelty in the order Scleractinia. While hybrid embryos can be produced in vitro, it has been difficult to identify adult hybrids in the wild. Here, we tested the potential for hybridization between two closely related species in the family Fungiidae. We mixed approximately 5000 eggs of Ctenactis echinata with sperm from C. crassa. No hybrid embryos were produced. This observation adds to a growing body of evidence for pre-zygotic barriers to hybridization in corals and challenges the claim that hybridization is a major source of evolutionary novelty in the order.

Chromera velia is endosymbiotic in larvae of the reef corals Acropora digitifera and A. tenuis.

Scleractinian corals occur in symbiosis with a range of organisms including the dinoflagellate alga, Symbiodinium, an association that is mutualistic. However, not all symbionts benefit the host. In particular, many organisms within the microbial mucus layer that covers the coral epithelium can cause disease and death. Other organisms in symbiosis with corals include the recently described Chromera velia, a photosynthetic relative of the apicomplexan parasites that shares a common ancestor with Symbiodinium. To explore the nature of the association between C. velia and corals we first isolated C. velia from the coral Montipora digitata and then exposed aposymbiotic Acropora digitifera and A. tenuis larvae to these cultures. Three C. velia cultures were isolated, and symbiosis was established in coral larvae of both these species exposed to all three clones. Histology verified that C. velia was located in the larval endoderm and ectoderm. These results indicate that C. velia has the potential to be endosymbiotic with coral larvae.