No abstract

The effects of long-term exposure to elevated levels of carbon dioxide (CO2) on seagrass communities are still poorly understood. This study investigates the tropical subtidal seagrass communities at three shallow volcanic CO2 vents in Papua New Guinea. Seagrass cover and biomass increased threefold and fivefold, respectively, from control to medium and high pCO2 sites (average pH = 7.9, 7.7, and 7.5, respectively). The seagrass community composition differed significantly between the pCO2 sites: Cymodocea serrulata, Cymodocea rotundata, and Halodule uninervis were more abundant at high pCO2 sites, whereas Halophila ovalis, Thalassia hemprichii, and Syringodium isoetifolium occurred only at low and mid pCO2 sites. Cymodocea rotundata was the only species common among all pCO2 sites and locations. The δ13C in its leaves significantly declined with increasing pCO2, indicating that additional CO2 influenced seagrass carbon uptake, and specifically, that there was discrimination against the heavier isotope (13C) when carbon was more abundant. Size-specific leaf growth rates (i.e. leaf turnover) also significantly declined with increasing pCO2; however, leaf growth rates showed no consistent difference in response to elevated pCO2 in two of four surveys. Our study suggests that progressive ocean acidification may lead to higher cover and above- and below-ground biomass, but lower size-specific growth and altered species composition in tropical seagrass communities. The effects of co-limiting factors, such as light and nutrient availability, on early-responding parameters, such as growth rates, require further attention to improve projections.

Lutjanus erythropterus and L. malabaricus are sympatric, sister taxa that are important to fisheries throughout the Indo-Pacific. Their juveniles are morphologically indistinguishable (i.e. cryptic). A DNA metabarcoding dietary study was undertaken to assess the diet composition and partitioning between the juvenile and adult life history stages of these two lutjanids. Major prey taxa were comprised of teleosts and crustaceans for all groups except adult L. erythropterus, which instead consumed soft bodied invertebrates (e.g. tunicates, comb jellies and medusae) as well as teleosts, with crustaceans being notably absent. Diet composition was significantly different among life history stages and species, which may be associated with niche habitat partitioning or differences in mouth morphology within adult life stages. This study provides the first evidence of diet partitioning between cryptic juveniles of overlapping lutjanid species, thus providing new insights into the ecological interactions, habitat associations, and the specialised adaptations required for the coexistence of closely related species. This study has improved our understanding of the differential contributions of the juvenile and adult diets of these sympatric species within food webs. the diet partitioning reported in this study was only revealed by the taxonomic resolution provided by the DnA metabarcoding approach and highlights the potential utility of this method to refine the dietary components of reef fishes more generally.Reef fish communities are extraordinarily diverse 1 , and there is often partitioning of resources, particularly for food and habitat, between sympatric reef fishes 2-4 . This ecological process, called niche partitioning, is fundamental for the coexistence of species within an ecosystem 5 . Niche partitioning is especially relevant among species of the same genus, which may include cryptic and sympatric species, because the more similar the co-existing species are, the more intensively they presumably compete 2,4,6 . Niche partitioning within the same species is also a common strategy in reef fish in order to minimise intra-specific competition, and can be associated with ontogenetic movements relative to life history stages. Indeed, the life cycle of many reef fishes consists of a pelagic larval phase, followed by a demersal juvenile phase on shallow, low-relief substrate (e.g. mangrove and seagrass nurseries), and an adult phase on deeper, high-relief reefs 7,8 . Ontogenetic dietary shifts were previously identified in a number of snapper species (family Lutjanidae) 9-11 , suggesting that habitat partitioning between juvenile and adult fish was based not only on finding refuge from predation, but also for accessing food resources 12,13 .Lutjanus erythropterus (Bloch, 1790) and L. malabaricus (Bloch & Scheneider, 1801) are sympatric snapper species that co-exist in the tropical and subtropical Indo-Pacific region 14 . They are sister taxa 15 , and the juveniles are phenotypically cryptic 8 . Both species support imp...

Halophytes, such as seagrasses, predominantly form habitats in coastal and estuarine areas. These habitats can be seasonally exposed to hypo-salinity events during watershed runoff exposing them to dramatic salinity shifts and osmotic shock. The manifestation of this osmotic shock on seagrass morphology and phenology was tested in three Indo-Pacific seagrass species, Halophila ovalis, Halodule uninervis and Zostera muelleri, to hypo-salinity ranging from 3 to 36 PSU at 3 PSU increments for 10 weeks. All three species had broad salinity tolerance but demonstrated a moderate hypo-salinity stress response – analogous to a stress induced morphometric response (SIMR). Shoot proliferation occurred at salinities <30 PSU, with the largest increases, up to 400% increase in shoot density, occurring at the sub-lethal salinities <15 PSU, with the specific salinity associated with peak shoot density being variable among species. Resources were not diverted away from leaf growth or shoot development to support the new shoot production. However, at sub-lethal salinities where shoots proliferated, flowering was severely reduced for H. ovalis, the only species to flower during this experiment, demonstrating a diversion of resources away from sexual reproduction to support the investment in new shoots. This SIMR response preceded mortality, which occurred at 3 PSU for H. ovalis and 6 PSU for H. uninervis, while complete mortality was not reached for Z. muelleri. This is the first study to identify a SIMR in seagrasses, being detectable due to the fine resolution of salinity treatments tested. The detection of SIMR demonstrates the need for caution in interpreting in-situ changes in shoot density as shoot proliferation could be interpreted as a healthy or positive plant response to environmental conditions, when in fact it could signal pre-mortality stress.