Keith A. McGuinness

Above- and below-ground biomass, and allometry, of four common northern Australian mangroves

Mangrove forests are an integral part of tropical coastal ecosystems, particularly in northern Australia. In the Northern Territory, studies have determined the extent and species diversity of these associations but little is known of biomass or productivity. We sampled the above- and below-ground biomass of the four most abundant species, Avicennia marina, Bruguiera exaristata, Ceriops australis and Rhizophora stylosa, developed allometric relationships and examined partitioning. Unlike many other studies, we sampled below-ground biomass, which constituted a substantial proportion (0.29-0.57) of the total dry weight. Our results should be valuable in modelling potential changes in carbon allocation resulting from small- and large-scale ecosystem changes.

Dispersal, establishment and survival of Ceriops tagal propagules in a north Australian mangrove forest

Abstract Studies of the ecology of mangroves show that a wide variety of factors, including salinity, desiccation, disturbance, competition and predation, may affect the distribution and abundance of species. Field studies were done to examine the relative importance of several of these factors in the establishment and early survival of Ceriops tagal, a species common in mid-to high-shore regions of mangrove forests in northern Australia. The fate of marked and tethered propagules was followed to estimate the range of dispersal and the intensity of predation. Propagules were artificially planted under different thicknesses of shade cloth (none, 30%, 80%) and in different habitats (clearing, forest, clearing-forest fringe) to examine the effects of light and soil conditions on survival and growth. Results suggested that dispersal was very limited: only 9% of marked propagules were ever found more than 3 m from the parent tree. Losses to predators were great, with 83% of tethered propagules being damaged or consumed within 3 months. On average, 56% of planted propagules survived for at least 6 weeks and 76% of these initiated growth. Survival in clearings was lower than in other habitats, with 29% fewer surviving six weeks and 48% fewer surviving 15 months. The growth of seedlings was correlated with soil temperature, but the effects of treatments were complex. Overall, results indicated that poor dispersal and establishment were the main factors likely to limit the colonisation and population growth of this species.

Seed predation in a tropical mangrove forest: a test of the dominance-predation model in northern Australia

Studies of predation on propagules of the mangroves Avicennia marina, Bruguiera exaristata, Ceriops tagal and Rhizophora stylosa were made in a forest in northern Australia to test the generality of the dominance-predation model. This model states that an inverse relationship exists between the dominance of a species in the canopy of mangrove forests and the rate of predation on the propagules of that species Significant differences in predation were found among the four species, and among patches of forest dominated by the different species. Predators attacked more than 50% of the propagules of all species except R stylosa, so are likely to significantly affect forest structure. The intensity of predation did not, however, vary as the dominance-predation model predicted. Instead, predation on the propagules of a species appeared to depend on the availability of propagules of other, more highly preferred, species.

Environmentally influenced variability in the morphology of Cinachyrella australiensis (Carter 1886) (Porifera : Spirophorida : Tetillidae)

The influence of environmental variability on body form and tissue structure of Cinachyrella australiensis is reported for populations from three sites within Darwin Harbour, Northern Territory, Australia, that varied considerably in hydrological conditions. External morphology of these sponges differed among sites ranging from typical spherical shapes to flattened forms. A large proportion of dry weight consisted of inorganic matter, i.e. silica spicules, varying between 62.9% and 78.2%. Sites with highest water velocity and sediment size were significantly correlated with sponge populations having the greatest inorganic content and lowest organic cellular content and the thickest oxea. Thicker oxea may in part account for the higher structural content of sponges at these sites. There was no significant difference in oxea length among sites. It is concluded that sponges subjected to highly perturbed environs with large water flow and sedimentation regimes may devote more energy to spicule reinforcement relative to organic content. These robust sponges have the potential to make an important structural contribution to their habitats.

A Comparison of Methods for Estimating Relative Abundance of Grapsid Crabs

Grapsid crabs are one of the most common, and potentially important, elements of the mangrove fauna but relatively little information is available on patterns in their distribution and abundance. In part, this may be due to difficulties in estimating the abundance of burrowing species. By not having reliable methods of estimating changes in distribution and abundance of crabs, ecological impacts of crabs may be greatly underestimated. We tested several methods for estimating the apparent abundance of eight species of grapsids in a north Australian mangrove forest. These methods included continuous and instantaneous visual counts at two distances, two types of pitfall traps and photography. We also excavated crabs to test the reliability of the best of these methods. Overall, pitfall traps equipped with funnels proved most useful, although these did preferentially capture larger crabs. An exception was the large crab, Neosarmatium meinerti, which was rarely captured and more reliably estimated by burrow counts. Traps proved to be most useful in this study and may be for long term studies of grapsid species, however, the selection of a method should be made after careful evaluation of the questions and relevant information required for any particular study. Finally, these methods may also prove to be useful in other environments such as salt marshes.

Born to be bad: Agonistic behaviour in hatchling saltwater crocodiles (Crocodylus porosus)

Detailed observations on groups of captive saltwater crocodile (Crocodylus porosus) hatchlings revealed sporadic periods of intense agonistic interactions, with 16 highly distinctive behaviours, in the morning (06:00–08:00 h) and evening (17:00–20:00 h) in shallow water. Ontogenetic changes in agonistic behaviour were quantified by examining 18 different groups of hatchlings, six groups each at 1 week, 13 weeks and 40 weeks after hatching. Agonistic interactions between hatchlings at 1 week of age (mean 7.3 ± 0.65/night) were not well-defined and varied in intensity (low, medium, high), number of individuals that were aggressive, and the outcome, while most interactions involved contact (94.5%). There were also clutch specific differences in the frequency of agonistic interactions. At 13 and 40 weeks, a more hierarchal dominance relationship appeared to be established which primarily involved aggression–submission interactions. Agonistic interactions were more frequent (13 weeks 9.7 ± 0.61/night; 40 weeks 22.2 ± 0.61/night) and intense (medium, high), but shorter in duration, in which the subordinate individual fled in response to an approach by a dominant animal that often gave chase but did not make contact. While the full repertoire of behaviour was displayed by hatchlings at 1 week of age, a smaller subset based on dominance status was displayed among 13- and 40-week-old hatchlings. Agonistic behaviour occurs in C. porosus shortly after hatching and is important in establishing and maintaining dominance hierarchies that are characterised by aggression–submission interactions. This type of interaction appears typical for C. porosus both in the wild and in captivity, and may be important in preventing serious injury in a species equipped with formidable armoury. Dispersal by hatchling C. porosus at around 13 weeks of age appears to be driven by a growing intolerance of conspecifics, while territoriality is apparent at an early age. Consequently, agonistic behaviour and social status may be major contributors to the observed differences in growth rates and survival in captivity.

The transcriptome and proteome are altered in marine polychaetes (Annelida) exposed to elevated metal levels

Polychaetes are often used in toxicological studies to understand mechanisms of resistance and for biomarker detection, however, we know of only a few genetic pathways involved in resistance. We found the marine polychaete Ophelina sp.1 (Opheliidae) in sediment containing high copper levels and investigated this phenomenon by measuring metal accumulation in the worms and changes in gene and protein expression. We sequenced the transcriptome of Ophelina sp.1 from both the impacted and reference sediments using 454-sequencing and analysed their proteomes using differential in gel electrophoresis (DIGE). We used the sequenced transcriptome to guide protein identification. Transcripts coding for the copper chaperone, Atox1, were up-regulated in the worms inhabiting the high copper sediment. In addition, genes coding for respiratory proteins, detoxification proteins and cytoskeletal proteins were significantly altered in metal-exposed worms; many of these changes were also detected in the proteome. This dual approach has provided a better understanding of heavy metal resistance in polychaetes and we now have a wider range of suitable indicator genes and proteins for future biomarker development.

Experimental evaluation of the use of hatchery-reared juveniles to enhance stocks of the topshell Trochus niloticus in Australia, Indonesia and Vanuatu

Stock enhancement is an attractive option for the management of fisheries but needs to be carefully evaluated before large-scale implementation. We advocate an experimental approach to test the hypothesis that reseeded stocks will yield greater harvests than unseeded controls. We tested the effectiveness of releasing hatchery-reared juveniles to enhance stocks of the topshell Trochus niloticus in Australia, Indonesia and Vanuatu using a rigorous design enabling direct comparisons among countries. Trochus were released at different sizes (6–12, 16–27 mm) and densities (2–8 m2) at replicated sites in each region. Populations at those sites were subsequently compared with those at unseeded controls. Temporal variation in success was also assessed by repeating the experiment in Indonesia. Initial survival was monitored by tagging the juveniles with aluminium foil and recapturing them using underwater metal detectors. The majority of juveniles released in Australia and Indonesia died or disappeared within a month of release and stocks were not enhanced relative to controls. Predation, especially by fish, was considered likely to be the most important cause of loss, but further investigation is required. Losses in Vanuatu were also substantial, but up to 4.4% of 10–27 mm juveniles were recaptured alive after 12 months. Results from Australia and Vanuatu suggested that survival may be improved by releasing juveniles in greater densities and at greater sizes. We discuss additional strategies for improving survival of free-released juveniles in Vanuatu and suggest alternative approaches for Australia and Indonesia.

Differences in regeneration between hurricane damaged and clear-cut mangrove stands 25 years after clearing

The effect of human disturbance on mangrove forest may be substantially different from the effects of natural disturbances. This paper describes differences in vegetation composition and structure of five vegetation types in two mangrove areas near Darwin, Australia, 25 years after disturbance. The vegetation in clear-felled forest showed more adult Avicennia marina than in the hurricane-affected forest, and a virtual absence of A. marina juveniles and saplings. This indicates that A. marina will be replaced by other species in the canopy, showing a multi-phase vegetation development in mangrove forest after human disturbance. The mechanism of disturbance and the conditions after clearing therefore affects the vegetation composition for at least 25 years after this disturbance took place.

Influence of re-orientation on alignment to flow and tissue production in a Spongia sp. (Porifera:Demospongiae:Dictyoceratida)

Spongia individuals on intertidal reefs in Darwin Harbour displayed a distinct tendency to orientate towards a strong uni-directional water flow, their longest axis facing across the water current. Individuals rotated by 90° re-orientated tissue to face across the prevailing currents. There were significant differences in growth between re-orientated sponges, and both moved control and undisturbed control treatments. Compared to mean growth rates of −1.27 cm year−1 (from undisturbed controls) and −1.68 cm year−1 (from moved controls), re-orientated individuals had significantly higher growth rates (46.15 cm year−1). Increased volumetric growth of re-orientated individuals was not a result of re-arrangement of existing tissue but a consequence of the production of more sponge tissue, evident by an increase in sponge volume. Increased tissue production identified in this study may be beneficial to researchers growing sponges for aquaculture purposes.