Cn Mundy

Coral recruitment: consequences of settlement choice for early growth and survivorship in two scleractinians

Settlement, early survivorship and growth of two species of reef-building corals were studied in order to assess the importance of these processes in determining the distribution of adult corals. Experiments were conducted using larvae of Platygyra sinensis (Edwards and Haime), a shallow water species with a massive morphology, and Oxypora lacera (Verrill), a shade-loving or deep water species with a laminar morphology. Settlement orientations in experimental field enclosures varied according to depth, Oxypora choosing more cryptic microhabitats than Platygyra. Laboratory settlement experiments utilising different light intensities indicated that light level was the variable responsible for changes in settlement orientation with depth. Total numbers of larvae settled at a range of depths did not correspond with adult distribution for either species, though there were significant variations in settlement with depth. For juveniles of each species, variations in growth and survivorship with depth were inconsistent with adult distributions. These results, in combination with the settlement experiments, suggest that pre-settlement factors may play an important role in determining reef-scale distribution patterns. Settlement orientation on experimental substrata influenced growth rates, which were highly correlated with survivorship. In the first few months after settlement, mortality was highest on highly sedimented upper surfaces. However, growth and survivorship were highest on these upper surfaces during the following 5 months, presumably because of higher light levels there than on undersurfaces. There was no correlation between settlement density on experimental surfaces and subsequent survivorship. This may be due to the fact that the susceptibility of newly settled corals to different sources of mortality changes over time. No single settlement orientation provided a means to optimise survivorship through the course of these changes.

Predictable and unpredictable spawning events: in situ behavioural data from free-spawning coral reef invertebrates

Summary We describe the spawning behaviour and some aspects of spawning periodicity in a diverse group of marine invertebrates, principally echinoderms, but including sponges, anthozoans, molluscs, and polychaetes. Our observations were made both opportunistically and on a systematic basis between 1978 and 1992 on the central and northern Great Barrier Reef. Spawning was predictable in some of the species observed, for example Bohadschia argus, Euapta godeffroyi, and Stichopus variegatus (Holothuroidea), which exhibited regular lunar and diel periodicity. Others, such as Holothuria coluber, Actinopyga lecanora, and Bohadschia graffei (Holothuroidea), Acanthaster planei (Asteroidea), Hyotissa hyotis and Arca spp. (Bivalvia) exhibited no clear lunar or diel periodicity in spawning behaviour. Mass heterospecific spawnings which involved several species, often from different phyla, were commonly observed. The species participating were usually those with unpredictable spawning patterns and while the species i...

Sperm Diffusion Models and In Situ Confirmation of Long-Distance Fertilization in the Free-Spawning Asteroid Acanthaster planci

This study was undertaken to compare fertilization rates of the sea star Acanthaster planci that were predicted using sperm diffusion models with those that were determined under natural conditions in the field. During experimentally induced spawnings, measured fertilization rates for broadcast eggs were high. More than 70% of the eggs were fertilized at distances as great as 8 m downstream from a single spawning male starfish, and more than 20% were fertilized at separations of more than 60 m. Fertilization was still measurable, at 5.8%, 100 m downstream. Lateral diffusion of sperm away from the axis of flow produced fertilization rates of 13.8% at 8 m normal to the flow and 32 m downstream. The large volumes of sperm released by male A. planci are the primary cause of high rates of fertilization for eggs derived from widely spaced individuals. Models of sperm diffusion using high sperm release rates such as those found in this starfish accurately confirmed the fertilization rates measured in situ for two populations of A. planci with widely differing rates of sperm release. We observed some changes in starfish density and degree of aggregation in the study population for spawning periods during two spawning seasons, though these were not striking. High levels of aggregation may not be necessary for fertilization success in this starfish, due to the potential for long-distance fertilization and the probability that, for any spawning starfish, the total number of zygotes formed will be greater at some distance from the point of spawning. Although fertilization rates in areas distant from the sperm source were relatively low, the total area for potential gamete encounters is much greater and may make a large contribution to net fertilization. We predict that other behaviors, such as migration to shallow water, commonly associated with spawning in A. planci and other marine invertebrates will have measurable impacts on fertilization success. The potential for high levels of fertilization in A. planci was realized during natural spawnings. Fertilization rates as high as 99% were recorded when levels of spawning synchrony were high.

Role of light intensity and spectral quality in coral settlement: Implications for depth-dependent settlement?

On coral reefs scleractinian corals show strong patterns of vertical zonation, yet the underlying mechanisms creating and maintaining vertical zonation are poorly understood. Here we examine the potential contribution of light-dependent settlement in scleractinian coral planulae to patterns of vertical zonation. The effect of intensity and spectral quality of light on the settlement of six species of scleractinian corals (Goniastrea favulus Dana, Goniastrea aspera Verrill, Acropora tenuis Dana, Oxypora lacera Verrill, Montipora peltiformis Bernard, and Platygyra daedalea Ellis and Solander) with contrasting depth distributions was examined in laboratory trials. Light-dependent settlement was shown by planulae from five of the six species examined. Planulae from individual species showed a response to either light quality or light quantity, but not both. Settlement patterns shown by planulae from all six species were consistent with the vertical distribution patterns of adults in the field. Settlement of planulae from con-generic species with similar adult distribution patterns did not respond to variation in light intensity or spectral quality in a uniform manner, indicating the optimal light environment for settlement is species specific. The settlement patterns shown by planulae from five of the six species examined were more complex than required for selection of cryptic or exposed micro-habitats at settlement. The ecological function of such complex responses to light at settlement may be to identify optimum habitats for adult survival.

An appraisal of methods used in coral recruitment studies

Abstract 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.

Genetic diversity and gene flow in collapsed and healthy abalone fisheries

Overexploitation of marine species invariably results in population decline but can also have indirect effects on ecological processes such as larval dispersal and recruitment that ultimately affect genetic diversity and population resilience. We compared microsatellite DNA variation among depleted and healthy populations of the black‐lip abalone Haliotis rubra from Tasmania, Australia, to determine if over‐fishing had affected genetic diversity. We also used genetic data to assess whether variation in the scale and frequency of larval dispersal was linked to greater population decline in some regions than in others, and if larval dispersal was sufficient to facilitate natural recovery of depleted populations. Surprisingly, allelic diversity was higher in depleted populations than in healthy populations (P < 0.05). Significant subdivision across hundreds of metres among our sampling sites (FST = 0.026, P < 0.01), coupled with assignment tests, indicated that larval dispersal is restricted in all regions studied, and that abalone populations across Tasmania are largely self‐recruiting. Low levels of larval exchange appear to occur at the meso‐scale (7–20 km), but age estimates based on shell size indicated that successful migration of larvae between any two sites may happen only once every few years. We suggest that genetic diversity may be higher in depleted populations due to the higher relative ratio of migrant to self‐recruiting larvae. In addition, we expect that recovery of depleted abalone populations will be reliant on sources of larvae at the meso‐scale (tens of km), but that natural recovery is only likely to occur on a timescale unacceptable to fishers and resource managers.

Evidence of genetic subdivision among populations of blacklip abalone (Haliotis rubra Leach) in Tasmania

The scale over which populations exchange individuals (migration) is central to ecology, and important for understanding recruitment and connectivity in commercial species. Field studies indicate that blacklip abalone (Haliotis rubra) have localised larval dispersal. However, genetic studies show differentiation only at large scales, suggesting dispersal over more than 100 km. Most genetic studies, however, have failed to test for subdivision at scales equivalent to field experiments. We used microsatellite DNA to investigate genetic structure at small scales (100 m to 10 km) in blacklip abalone in south-east Tasmania. We found significant subdivision (FST = 0.021; P < 0.05) among sites, and hierarchical FST analysis indicated 64% of genetic variation was at the smallest scale, supporting field studies that concluded larval dispersal is less than 100m. We also tested if genetic differentiation varied predictably with wave exposure, but found no evidence that differences between adjacent sites in exposed locations varied from differences between adjacent sites in sheltered populations (mean FST = 0.016 and 0.017 respectively). Our results show the usefulness of microsatellites for abalone, but also identify sampling scales as critical in understanding gene flow and dispersal of abalone larvae in an ecologically relevant framework. Importantly, our results indicate that H. rubra populations are self-recruiting, which will be important for the management of this commercial species.

Fifty years of sustained production from the Australian abalone fisheries

The sustained production of abalone from the five state-managed (Tasmania, Victoria, South Australia, New South Wales, and Western Australia) Australian abalone fisheries has contrasted with many of those elsewhere that exhibited rapid and sustained declines in production. Australian abalone fisheries are significant at local, regional, state, national, and international scales. Key attributes are (1) harvesting, processing, and reinvestment of profits occur away from major metropolitan centers; (2) they are among the most valuable wild-catch species in all states; (3) the combined Australian abalone harvest in 2011 (>4,500 t) had a landed value of ∼AU

The unprecedented 2015/16 Tasman Sea marine heatwave

200M and represented 15% of the Australian total wild-catch production; and (4) this level of production made these fisheries the dominant contributor (60%) to global wild-catch abalone production. Unlike many other abalone fisheries, total catches were controlled by limited entry, quotas, size limits, and geographic boundaries, overseen by stringent compliance regimes, early in their history. Subsequently, state-based research programs, explicitly tasked with providing scientific advice to support management decisions, undertook assessments to match harvests with stock productivity. This information upon which to base management decisions contributed to long-term (>20 years) stable harvests and enabled relationships among stakeholders to develop around consideration of the information and advice for management. In general, rights-holders developed stewardship for the resource, and this has led to numerous important outcomes, including evolving resource co-management and a nationally representative industry entity, the Abalone Council of Australia. The Abalone Council of Australia, state-based industry entities, and ongoing relationships among rights-holders, fishery managers, and researchers play vital roles in addressing and overcoming current and impending challenges for these fisheries. These difficulties include (1) urban encroachment into coastal regions (the so-called “sea change” phenomenon); (2) a growing interest in access to the abalone resource, reflecting the increasing, culturally diverse Australian population; (3) the ever-present threat of illegal fishing; (4) recent total allowable commercial catch reductions, particularly in Victoria and New South Wales, to facilitate stock rebuilding; (5) changing market conditions; (6) declining profitability from increasing operational costs and appreciation of the Australian dollar; and (7) environmental changes, such as prolonged drought and warmer seas associated with shifts in climate. Overall, this review demonstrates that abalone can be harvested sustainably over extended periods, despite aspects of their demography that suggest higher vulnerability to overexploitation, providing the management systems that control harvesting activities and external impacts that encompass several key underpinning elements. This review also identifies likely challenges to sustained production and shows that the future of these stocks and fisheries will require proactive strategies to mitigate current threats to sustainability and to maintain economically viable productivity.

THE EFFECTS OF SPERM DENSITY AND GAMETE CONTACT TIME ON THE FERTILIZATION SUCCESS OF BLACKLIP (HALIOTIS RUBRA; LEACH, 1814) AND GREENLIP (H. LAEVIGATA; DONOVAN, 1808) ABALONE

The Tasman Sea off southeast Australia exhibited its longest and most intense marine heatwave ever recorded in 2015/16. Here we report on several inter-related aspects of this event: observed characteristics, physical drivers, ecological impacts and the role of climate change. This marine heatwave lasted for 251 days reaching a maximum intensity of 2.9 °C above climatology. The anomalous warming is dominated by anomalous convergence of heat linked to the southward flowing East Australian Current. Ecosystem impacts range from new disease outbreaks in farmed shellfish, mortality of wild molluscs and out-of-range species observations. Global climate models indicate it is very likely to be that the occurrence of an extreme warming event of this duration or intensity in this region is respectively ≥330 times and ≥6.8 times as likely to be due to the influence of anthropogenic climate change. Climate projections indicate that event likelihoods will increase in the future, due to increasing anthropogenic influences.