An appraisal of methods used in coral recruitment studies

 A new method for attaching individual artificial settlement plates directly to the reef surface using small stainless steel base plates is described. Recruitment of corals to settlement plates attached to the reef substratum and to steel mesh racks is compared. The effects of differences in depth, settlement plate angle, and local topography on recruitment of corals were also investigated. No significant difference in mean recruit density was found between settlement plates deployed using the two attachment methods. Small differences in depth and plate angle among replicate plates explained less than 6% of the variability in coral recruitment on replicate settlement plates. The direct-attachment method is less obtrusive, more cost and time efficient, and settlement plates can be deployed at precise locations. Additionally, because settlement plates are deployed individually rather than grouped on racks or frames, the direct-attachment method avoids complications associated with assumptions of independence implicit in most statistical procedures. Accepted: 24 November 1999     

The biology of coral metamorphosis: molecular responses of larvae to inducers of settlement and metamorphosis

Like many other cnidarians, corals undergo metamorphosis from a motile planula larva to a sedentary polyp. In some sea anemones such as Nematostella this process is a smooth transition requiring no extrinsic stimuli, but in many corals it is more complex and is cue-driven. To better understand the molecular events underlying coral metamorphosis, competent larvae were treated with either a natural inducer of settlement (crustose coralline algae chips/extract) or LWamide, which bypasses the settlement phase and drives larvae directly into metamorphosis. Microarrays featuring > 8000 Acropora unigenes were used to follow gene expression changes during the 12 h period after these treatments, and the expression patterns of specific genes, selected on the basis of the array experiments, were investigated by in situ hybridization. Three patterns of expression were common—an aboral pattern restricted to the searching/settlement phase, a second phase of aboral expression corresponding to the beginning of the development of the calicoblastic ectoderm and continuing after metamorphosis, and a later orally-restricted pattern.     

Developmental fates of larval tissues after metamorphosis in the ascidian, Halocynthia roretzi

Cell lineages during ascidian embryogenesis are invariant. Developmental fates of larval mesodermal cells after metamorphosis are also invariant with regard to cell type of descendants. The present study traced developmental fates of larval endodermal cells after metamorphosis in Halocynthia roretzi by labeling each endodermal precursor blastomere of larval endoderm. Larval endodermal cells gave rise to various endodermal organs of juveniles: endostyle, branchial sac, peribranchial epithelium, digestive organs, peripharyngeal band, and dorsal tubercle. The boundaries between clones descended from early blastomeres did not correspond to the boundaries between adult endodermal organs. Although there is a regular projection from cleavage stage and larval stage to juvenile stage, this varies to some extent between individuals. This indicates that ascidian development is not entirely deterministic. We composed a fate map of adult endodermal organs in larval endoderm based on a statistical analysis of many individual cases. Interestingly, the topographic position of each prospective region in the fate map was similar to that of the adult organ, indicating that marked rearrangement of the positions of endodermal cells does not occur during metamorphosis. These findings suggest that fate specification in endoderm cells during metamorphosis is likely to be a position-dependent rather than a deterministic and lineage-based process. Received: 16 June 1999 / Accepted: 16 August 1999     

Nocturnal orientation to reefs by late pelagic stage coral reef fishes

 The nocturnal orientation behaviour of the late pelagic stages of two reef fish families (Apogonidae and Pomacentridae) was examined using behavioural cages deployed in the field. The behavioural cages enabled the fish to choose between swimming towards or away from the reef in response to natural cues. Overall, 55% of fish displayed a choice in the experiments, however, the proportion varied between the two families, with 67% of pomacentrids and 27% of apogonids displaying a choice. In both families, of the fish which displayed a choice, the proportion of fish swimming towards the reef was significantly greater than 50%, as random movement would predict (64% of pomacentrids and 67% of apogonids swam towards the reef). This proportion did not vary significantly among four field sites with different current regimes and geographic locations. The results suggest that the late pelagic stages of reef fish display nocturnal orientation behaviour, possibly in response to sound, which may aid in their settlement on reefs. Accepted: 18 August 1997     

The influence of adult density on larval settlement in a coral reef fish Coryphopterus glaucofraenum

 For marine species with open populations, patterns of larval settlement can have important consequences for performance and abundance at later life-stages. In this study, I tested whether larvae of a reef-dwelling goby (Coryphopterus glaucofraenum) settled differentially to reefs occupied by varying numbers of adults. I monitored settlement daily to reefs on which the density of adult gobies varied naturally, or was manipulated experimentally. Rates of settlement were constant across a broad range of adult densities, suggesting that larvae do not choose settlement sites based on the number of adults in their immediate vicinity. Accepted: 30 October 1998     

Developmental expression of Ultraspiracle proteins in the epidermis of the tobacco hornworm, Manduca sexta, during larval life and the onset of metamorphosis

 During the final two larval instars, a changing pattern of three Ultraspiracle (Usp) proteins (50.5, 52.5, and 57 kDa) was detected in immunoblots of the dorsal abdominal epidermis of the tobacco hornworm, Manduca sexta, by a monoclonal antibody against Drosophila Usp that was shown to detect MsUsp. The 57- and 52.5-kDa bands were present during the intermolt periods and the 50.5- and 52.5-kDa bands during the molting phases. The antibody detected a nuclear antigen present in epidermis, muscle, fat body, and the central nervous system from the time of hatching. In the epidermis Usp was present in all cell nuclei but was especially prominent in the tormogen and trichogen cells immediately after ecdysis in both the penultimate and final instars. This latter immunoreactivity disappeared within 12 h whereas the remainder of the epidermis retained high levels throughout the feeding period. During the molt immunostaining reappeared in the hair cell nuclei. During the wandering stage at the onset of metamorphosis and just before pupal ecdysis, immunoblots showed high levels of Usp, but nuclei showed little or no staining. This discrepancy is likely due to the loss of one Usp isoform from the nucleus and its dispersal in the cytoplasm in preparation for the appearance of the second isoform. Received: 10 June 1997 / Accepted: 22 August 1997     

Macroborers within coral framework at Discovery Bay, north Jamaica: species distribution and abundance, and effects on coral preservation

 Macroboring organisms are recognised as key agents of reef framework modification and destruction, and while recent studies in the Pacific have improved understanding of spatial variations in macroboring community structure, and rates of macroboring within individual reefs, comparable studies from the Caribbean are largely lacking. This study assesses the distribution of macroboring species and the degree of framework infestation across the reefs at Discovery Bay, north Jamaica. Although individual species of borers exhibit variable distributions across the reef, relative abundances of the main groups of macroborers (sponges, bivalves, worms) illustrate clear distributional trends. Sponges are dominant at fore-reef sites, while sipunculan and polychaete worms are only of importance at back-reef/lagoon and shallow fore-reef sites. Bivalves are locally important within back-reef and lagoon patch reef framework. Average percentages of internal bioerosion (macroboring) vary between sites, but are highest at back-reef and deep fore-reef sites. No systematic pattern of variation occurs within back-reef/lagoon samples, but a significant trend of increased macroboring is recognised with increased water depth on the fore-reef. In addition, significant differences in terms of the susceptibility of individual coral species are recognised. These factors are likely to result in biasing of the fossil record, with variable styles of preservation evident both between sites (i.e. with depth/environment) and within sites (i.e. between coral species). Accepted: 1 June 1998     

Digital analysis of multispectral airborne imagery of coral reefs

 The digital airborne sensor, CASI (Compact Airborne Spectrographic Imager) has considerable potential for mapping marine habitats. Here we present an account of one of the first coral reef applications. The CASI was flown over reefs of the Turks and Caicos Islands (British West Indies) and set to view 1 m pixels in 8 spectral bands. In addition, reef habitats were sampled in situ by visual assessment of percent cover in 1 m quadrats. Seagrass standing crop was assessed using a calibrated visual scale. Benthic habitats were classified using hierarchical cluster and similarity percentage analyses of the field survey data. Two levels of habitat discrimination were assessed: a coarse level (corals, algae, sand, seagrass) and a fine level which included nine reef habitats. Overall accuracies of CASI-derived habitat maps were 89% and 81% for coarse and fine levels of habitat discrimination, respectively. Accuracies were greatest once CASI data had been processed to compensate for variations in depth and edited to take account of generic patterns of reef distribution. These overall accuracies were significantly (P<0.001) better than those obtained from satellite imagery of the same site (Landsat MSS, Landsat TM, SPOT XS, SPOT Pan, merged Landsat TM/SPOT Pan). Results from CASI were also significantly better than those from interpretation of 1:10 000 colour aerial photographs of reefs in Anguilla (Sheppard et al. 1995). However, the studies may not have been entirely comparable due to a disparity in the areas mapped. Accepted: 26 May 1997     

Characterization of a natural inducer of coral larval metamorphosis

External chemical signals used by scleractinian corals to recognize suitable substrata for larval settlement and metamorphosis were identified from crustose coralline red algae (CCA). A fragment of coral rubble with CCA induced larval metamorphosis of the scleractinian coral Pseudosiderastrea tayamai. A natural inducer and compounds that enhanced its effect in larval metamorphosis were isolated from the methanol extracts of coral rubble with CCA. A bromotyrosine derivative, 11-deoxyfistularin-3 (10^− 7 M) isolated from the CCA, induced the metamorphosis of P. tayamai larvae (27.5 ± 24.0%). In the presence of fucoxanthinol (10^− 9 M) and fucoxanthin (10^− 9 M), the percentage of metamorphosis induced by the bromotyrosine derivative was further enhanced to 87.8 ± 13.0 and 88.4 ± 17.8%, respectively. Both carotenoids are also found in the coral rubble with CCA. These results suggest that bromotyrosine derivative and carotenoids have a synergistic effect in the metamorphosis of P. tayamai larvae. The synergistic effect provides a higher selectivity for recruitment than a single-component natural inducer for recognizing suitable substrata for larval metamorphosis. Thus, the effect might offer a survival advantage for benthic marine invertebrates.     

Chemical signals in gametogenesis, spawning, and larval settlement and defense of the soft coral Sinularia polydactyla

 Mass spawning strategies of hard and soft corals on the Great Barrier Reef, Australia have been described in recent years. Nonetheless, the applicability of those studies to corals on other reef systems has not been well documented. Here we describe the mass spawning behavior of the soft coral Sinularia polydactyla on coral reefs surrounding Guam; specifically we describe the events in an annual gametogenic cycle including steroidogenesis, spawning, settlement and early life history defense. The gametogenic cycle of female colonies lasted 12 months while male colonies produced viable sperm within 9 months. Sinularia polydactyla exhibited a split spawn between March and June that correlated with a significant reduction in tissue concentrations of progesterone and testosterone. Estradiol was released into the water column, apparently by female colonies, just prior to spawning. There was a trend for preferential larval settlement in the presence of the crustose coralline algae Hydrolithon reinboldii rather than coral rubble, a natural biofilm, or filtered seawater. The defensive compounds pukalide and 11β-acetoxypukalide were found in eggs and larvae at adult level and three-fold lower than adult-level concentrations, respectively. These compounds provided some predator deterrent and antimicrobial protection against an ecologically relevant omnivorous fish Canthigaster solandri and a sympatric microbe Vibrio sp. Accepted: 10 September 1998     

On natural metamorphosis inducers of the cnidarians Hydractinia echinata (Hydrozoa) and Aurelia aurita (Scyphozoa)

 Hydractinia echinata and Aurelia aurita produce motile larvae which undergo metamorphosis to sessile polyps when induced by external cues. The polyps are found at restricted sites, A. aurita predominantly on rocks close to the shore, H. echinata on shells inhabited by hermit crabs. It has been argued that the differential distribution of the polyps in their natural environment largely reflects the distribution of the natural metamorphosis-inducing cues. In the case of H. echinata, bacteria of the genus Alteromonas were argued to meet these conditions. We found that almost all substrates collected in the littoral to induce metamorphosis in H. echinata, and several bacterial strains isolated from the sea, including the common E. coli, induce metamorphosis efficiently. In A. aurita metamorphosis may be induced by the water–air interface, whereby metamorphosis precedes (final) settlement. Received: 7 December 1998 / Accepted: 8 July 1999     

Timing of larval release by Porites astreoides in the northern Florida Keys

 Annual and lunar patterns of larval release by the coral Porites astreoides in the Florida Keys were investigated. Corals were collected monthly and maintained in outdoor, flow-through systems to quantify nightly release of larvae. Planulae were released during a period from 10 days prior to the new moon through 11 days after the new moon, with a peak in release centered on the new moon. The average duration of larval release by a single colony was 9 nights. Although the correlation was low (r=0.292), there was a positive relationship between colony size and number of larvae released. The majority of larval release occurred in April and May, although smaller numbers of larvae were observed from June through as late as September. Larval release appeared to be correlated with the average water temperatures over the 30 days prior to the new moon, peaking when water temperatures were 24.5–27.5 °C. Accepted: 3 August 1998     

A 200-year coral stable oxygen isotope record from a high-latitude reef off Western Australia

 A core from a coral colony of Porites lutea was analysed for stable oxygen isotopic composition^*. A 200-year proxy record of sea surface temperatures from the Houtman Abrolhos Islands off west Australia was obtained from coral δ¹⁸O. At 29′S, the Houtman Abrolhos are the southernmost major reef complex of the Indian Ocean. They are located on the path of the Leeuwin Current, a southward flow of warm, tropical water, which is coupled to Indonesian throughflow. Coral δ¹⁸O primarily reflects local oceanographic and climatic variability, which is largely determined by spatial variability of the Leeuwin Current. However, coherence between coral δ¹⁸O and the current strength itself is relatively weak. Evolutionary spectral and singular spectrum analyses of coral δ¹⁸O demonstrate a high variability in spectral composition through time. Oscillations in the 5–7-y, 14–15-y, and quasi-biennial bands reflect teleconnections of local sea surface temperature (SST) to tropical Pacific climate variability. Deviations between local (coral-based) and regional (instrument) SST contain a cyclic component with a period of 15 y. Coral δ¹⁸O suggests a rise in SST by 0.6 ′C since AD 1944, consistent with available instrumental SST records. A long-term warming by 1.4 ′C since AD 1795 is inferred from the coral record. Accepted: 3 July 1998     

Induction of metamorphosis with potassium ions requires development of competence and an anterior signalling centre in the ascidian Herdmania momus

 Increased K⁺ concentration in seawater induces metamorphosis in the ascidian Herdmania momus. Larvae cultivated at 24°C exhibit highest rates of metamorphosis when treated with 40 mM KCl-elevated seawater at 21°C. At 24°C, H. momus larvae develop competence to respond to KCl-seawater and initiate metamorphosis approximately 3 h after hatching. Larval trunks and tails separated from the anterior papillae region, but maintained in a common tunic at a distance of greater than 60 μm, do not undergo metamorphosis when treated with KCl-seawater; normal muscle degradation does not occur in separated tails while ampullae develop from papillae-containing anterior fragments. Normal programmed degradation of myofibrils occurs when posterior fragments are fused to papillae-containing anterior fragments. These data indicate that H. momus settlement and metamorphosis only occurs when larvae have attained competence, and suggest that an anterior signalling centre is stimulated to release a factor that induces metamorphosis. Received: 15 May 1996 / Accepted: 19 September 1996     

Innervation regulates the metamorphic fates of larval abdominal muscles in the moth, Manduca sexta

 With the onset of metamorphosis, the abdominal muscles of the moth, Manduca sexta, follow one of three developmental fates: maintenance, respecification, or death. The maintained muscles retain their larval size and morphology throughout adult development. The respecified and dying muscles dedifferentiate, which involves regression, nuclear degeneration, and myofibril breakdown. Nuclei in both dying and respecified muscles also proliferate. The amount of nuclear degeneration is greater in the dying muscle fibers, and the amount of nuclear proliferation is greater in the respecified muscles. Four to ten days after pupation, the sizes of the respecified muscles stabilize while the dying muscles are lost. During regression, a subset of the respecified muscle fibers die. The surviving respecified muscle fibers grow and differentiate during the last half of adult development. In respecified muscles, denervation triggers an increased amount of nuclear degeneration and a decreased amount of nuclear proliferation. As a result, denervated respecified fibers experience increased muscle regression including an increased loss of muscle fibers and sometimes muscle death. Surviving respecified fibers still grow and differentiate yet are only 5 to 12% of the control size. Denervation triggers dedifferentiation in maintained muscles, resulting in fiber loss and occasionally muscle death. The percentage of fibers which dedifferentiate varies between different muscles. Denervation also triggers nuclear proliferation, with the amount of nuclear proliferation correlated with the extent of dedifferentiation of the individual muscle fibers. The dedifferentiated maintained fibers subsequently undergo differentiation in the absence of muscle growth. Received: 10 July 1997 / Accepted: 21 April 1998     

Morphological differences among three species of newly settled pocilloporid coral recruits

 Investigation of the life history of corals is hampered by an inability to identify early recruits. In this study, the pattern of formation and morphology of the juvenile skeletons of three laboratory-reared pocilloporids, Seriatopora hystrix, Stylophora pistillata and Pocillopora damicornis, were compared to determine whether they could be reliably distinguished. The pattern of skeleton formation, including the origin and structure of the septa, columella and corallite wall was similar in all species. Following the completion of the primary corallite wall after 4–5 days, these species could be identified by differences in the diameter of the primary corallite. The mean diameter (±SE) of each species differed markedly: S. hystrix 400 ± 2.7 μm, range 325–450 μm; S. pistillata 505 ± 3.5 μm, range 400–550 μm; P. damicornis 697 ± 7.5 μm, range 492–885 μm. Values for the primary corallite diameter overlapped in only 3% of samples, demonstrating the potential utility of this feature as a tool for classifying recruits obtained from the field. Accepted: 4 January 2000     

The ecdysone receptor and ultraspiracle regulate the timing and progression of ovarian morphogenesis during Drosophila metamorphosis

 Ecdysteroids regulate insect metamorphosis through the edysone receptor complex, a heterodimeric nuclear receptor consisting of the ecdysone receptor (EcR) and its partner ultraspiracle (USP). Differentiation in the Drosophila ovary at metamorphosis correlates with colocalization of USP and the EcR-A isoform in all but one of eight mesoderm-derived somatic cell types. The one exception is the larval terminal filament (TF) cells, in which only USP is detectable during cell differentiation. In cells destined to form the basal stalks and anterior oviduct, USP colocalizes with what appears to be the EcR-B2 isoform. Flies heterozygous for a deletion of the EcR gene exhibit several defects in ovarian morphogenesis, including a heterochronic delay in the onset of terminal filament differentiation. Flies heterozygous for a strong usp allele exhibit accelerated TF differentiation. Flies simultaneously heterozygous for both EcR and usp have additional phenotypes, including several heterochronic shifts, delayed initiation and completion of terminal filament morphogenesis and delayed ovarian differentiation during the first day of metamorphosis. Terminal filament morphogenesis is severely disrupted in homozygous usp clones. Our results demonstrate that proper expression of the ecdysone receptor complex is required to maintain the normal progression and timing of the events of ovarian differentiation in Drosophila. These findings are discussed in the context of a developmental and evolutionary role for the ecdysone receptor complex in regulating the timing of ovarian differentiation in dipteran insects. Received: 12 February 1998 / Accepted: 5 May 1998     

Interacting effects of predation risk and food availability on larval anuran behaviour and development

Age and size at metamorphosis are two important fitness components in species with complex life cycles. In anurans, metamorphic traits show remarkable phenotypic plasticity, especially in response to changes in growth conditions. It is also possible that the perception of risk directly determines changes in larval period and the size of metamorphs. This study examines how the perception of predation risk affects the timing of and size at metamorphosis in common frogs (Rana temporaria). I raised tadpoles at two risk levels (fish-conditioned water or unconditioned water) crossed with the availability or lack of food at night (all tadpoles had food available in the day). Tadpoles reacted to chemical cues from predatory fish by decreasing activity. A novel behavioural result was a predation×food interaction effect on refuge use, which also accounted for most of the predator main effect: predation risk only caused increased refuge use in the night-starved treatment. Despite these behavioural modifications, the perception of predation risk did not affect growth rate and mass at metamorphosis in a simple way: the effects of food regime on growth and size at metamorphosis were dependent on the level of predation risk as revealed by significant predation×food interaction effects. Tadpoles who had food withheld at night metamorphosed at the smallest size, suggesting a negative relationship between size at metamorphosis and refuge use. Tadpoles raised in fish-conditioned water had longer larval periods than those in unconditioned water, but these differences were significant only if food was available at night. These results conflict with the hypotheses that tadpoles should reduce their larval period or growth rates (and hence metamorphose at a smaller size) as the risk of predation increases. In contrast to predation risk, food availability strongly affected the length of the larval period: night-starved tadpoles metamorphosed relatively early with or without fish stimulus. Thus, early metamorphosis resulted from periods of low food availability, but not from a heightened ”perceived risk” of predation. This example counters the hypothesis of acceleration of the developmental rate (which shortens the time to metamorphosis) as a mechanism to escape a risky environment. Received: 18 August 1999 / Accepted: 10 January 2000