Subtropical Marine Ecosystems Research Articles

Molecular analyses of carangid fish diets reveal inter-predation, dietary overlap, and the importance of early life stages in trophic ecology.

Carangid fishes are commercially important in fisheries and aquaculture. They are distributed worldwide in both tropical and subtropical marine ecosystems. Their role in food webs is often unclear since their diet cannot be easily identified by traditional gut content analysis. They are suspected to prey on pelagic and benthic species, with clupeiform fishes being important dietary items for some species, though it is unknown whether carangids share food resources or show trophic segregation. Here, we used metabarcoding to overcome traditional challenges of taxonomic approaches to analyze the diet of seven carangid species caught as bycatch in the Brazilian southwest Atlantic sardine fishery. Stomach contents were processed from the following species: Caranx crysos, Caranx latus, Chloroscombrus chrysurus, Hemicaranx amblyrhynchus, Oligoplites saliens, Selene setapinnis, and Trachinotus carolinus. Identified diets were dominated by teleost fishes. The C. latus diet was the most distinct among the seven species, preferentially consuming Engraulis anchoita, but H. amblyrhynchus, O. saliens, and S. setapinnis also showed a trend of predominantly consuming small pelagic fishes. Finally, we found evidence of inter-predation in carangids, especially strong between S. setapinnis and C. crysos, suggesting that consumption of early life stages may result in indirect competition through reduced recruitment in these fishes. These findings provide unprecedented insights into the biodiversity in marine ecosystems, especially the poorly known diet of carangid fishes.

A 2 °C difference affecting the spatiotemporal distribution of small demersal fish assemblages in shallow tropical and subtropical waters of Western Taiwan

Two OR3 research vessel cruises were conducted at seven nearshore sites from north to south in the western coastal waters off Taiwan in May (late spring) and November (late autumn) 2019 in order to gain insights into the 2 °C difference in the tropical-subtropical fish fauna. Totally, 37 families, 72 genera, and 99 taxas were recorded for the 1809 fishes. Three fish assemblages, the North, South, and Fall-Zhuoshui River (ZRf) groups, were identified as eurythermal, stenothermal, and croaker communities. Their dominants, in rank order, were Tarphops oligolepis, Liachirus melanospilosa, Ostorhinchus fasciatus, and Trachinocephalus myops for the Northern eurythermal group, Arnoglossus tenuis, Eubleekeria splendens, and Ostorhinchus pleuron for the Southern stenothermal group, and Johnius taiwanensis, Chrysochir aureus, and Pennahia macrocephalus for the croaker-ZRf group. Their distribution was markedly correlated with the bottom water temperature, seafloor grain size, and concentration of suspended solids. The influence of a 2 °C difference on the tropical and subtropical demersal fish could indicate how rising temperatures due to climate change are shaping fish communities. The flatfish, Liachirus melanospilos, was distributed northward by 0.5oN and is suggested to be considered as an ecological indicator of the tropicalization of subtropical marine ecosystems in the future.

Habitat limits the poleward establishment of anemonefishes in a climate change hotspot

Climate change is modifying marine species' distribution with consequences for the structure and function of ecosystems. The warming and strengthening East Australian Current (EAC) is driving the tropicalisation of subtropical marine ecosystems through changing overwinter survival of tropical species. However, this can be complex for obligate symbionts, such as anemonefishes. Here, we documented fine-scale temporal changes in the density of anemonefishes and assessed the influence of latitude, temperature, and host sea anemone availability. Juvenile, subadult and adult Amphiprion akindynos and Amphiprionlatezonatus occurred at all study locations: North Solitary Island (equatorward), South Solitary Island, and Fish Rock (poleward). While the sea anemone Heteractis crispa was readily available at Fish Rock, we found that a lack of another sea anemone Entacmaea quadricolor hindered the establishment of adult A. akindynos and A. latezonatus populations. Minimum water temperature was also a bottleneck for the tropical anemonefish A. akindynos but not for the subtropical A. latezonatus. Our findings suggest anemonefishes are capable of contributing to the tropicalisation of the east coast of Australia. However, poleward establishment is currently limited by the absence of the preferred host sea anemone.

Genetic population structure and demography of an apex predator, the tiger shark Galeocerdo cuvier.

Population genetics has been increasingly applied to study large sharks over the last decade. Whilst large shark species are often difficult to study with direct methods, improved knowledge is needed for both population management and conservation, especially for species vulnerable to anthropogenic and climatic impacts. The tiger shark, Galeocerdo cuvier, is an apex predator known to play important direct and indirect roles in tropical and subtropical marine ecosystems. While the global and Indo‐West Pacific population genetic structure of this species has recently been investigated, questions remain over population structure and demographic history within the western Indian (WIO) and within the western Pacific Oceans (WPO). To address the knowledge gap in tiger shark regional population structures, the genetic diversity of 286 individuals sampled in seven localities was investigated using 27 microsatellite loci and three mitochondrial genes (CR,COI, and cytb). A weak genetic differentiation was observed between the WIO and the WPO, suggesting high genetic connectivity. This result agrees with previous studies and highlights the importance of the pelagic behavior of this species to ensure gene flow. Using approximate Bayesian computation to couple information from both nuclear and mitochondrial markers, evidence of a recent bottleneck in the Holocene (2,000–3,000 years ago) was found, which is the most probable cause for the low genetic diversity observed. A contemporary effective population size as low as 111 [43,369] was estimated during the bottleneck. Together, these results indicate low genetic diversity that may reflect a vulnerable population sensitive to regional pressures. Conservation measures are thus needed to protect a species that is classified as Near Threatened.

Ecological assessment of heavy metals in the grey mangrove (Avicennia marina) and associated sediments along the Red Sea coast of Saudi Arabia

Summary Mangroves play an integral role as a metal accumulator in tropical and subtropical marine ecosystems. Twenty-one sets of sediment samples and portions of mangroves were collected along the Saudi Arabian coast of the Red Sea to assess the accumulation and ecological risks of heavy metals. Results showed that the following mean concentrations of heavy metals in sediments: Cr (46.14 μg g−1 ± 18.48) > Cu (22.87 μg g−1 ± 13.60) > Ni (21.11 μg g−1 ± 3.2) > Pb (3.82 μg g−1 ± 2.46) > Cd (0.75 μg g−1 ± 0.87). The maximum concentrations of the studied metals were above the threshold effect level, indicating a limited impact on the respective ecosystems. The maximum concentration of Cd exceeded its toxic effect threshold, revealing a harmful risk to biota in the sediments. Based on metallo-phytoremedation, biological concentration factors were >1, suggesting that Avicennia marina can accumulate heavy metals, especially Cr and Pb. The translocation factor was above the known worldwide average. The geo-accumulation index revealed that sediments in mangrove areas ranged from moderately to heavily contaminated with Cd at Al-Haridhah and moderately contaminated at South Jeddah, Rabigh, Duba, and the wastewater treatment station near Jazan. The ecological risk index revealed that Cd could pose a relatively very high risk to the mangrove ecosystem. The present study emphasized the possibility of establishing a framework for the management of the coastal aquatic ecosystems along the Red Sea coast of Saudi Arabia.

Unusual caudal spines in the surgeonfish Zebrasoma scopas

Surgeonfishes (Acanthuridae) are found in tropical and subtropical marine ecosystems and typically comprise a core component of coral reef fish assemblages (Randall 1955). The surgeonfishes are disparate in terms of their morphology, feeding habits and behavioural tendencies, yet all share the distinct, unifying feature of at least one pair of scalpel-like modified scales on their caudal peduncle (Randall 1955) which can be used for defensive or territorial interactions (Schober and Ditrich 1992). Indeed, the common name ‘surgeonfishes’ is derived from the possession of these scalpel-like caudal spines.

Microbiome of Trichodesmium Colonies from the North Pacific Subtropical Gyre.

Filamentous diazotrophic Cyanobacteria of the genus Trichodesmium, often found in colonial form, provide an important source of new nitrogen to tropical and subtropical marine ecosystems. Colonies are composed of several clades of Trichodesmium in association with a diverse community of bacterial and eukaryotic epibionts. We used high-throughput 16S rRNA and nifH gene sequencing, carbon (C) and dinitrogen (N2) fixation assays, and metagenomics to describe the diversity and functional potential of the microbiome associated with Trichodesmium colonies collected from the North Pacific Subtropical Gyre (NPSG). The 16S rRNA and nifH gene sequences from hand-picked colonies were predominantly (>99%) from Trichodesmium Clade I (i.e., T. thiebautii), which is phylogenetically and ecologically distinct from the Clade III IMS101 isolate used in most laboratory studies. The bacterial epibiont communities were dominated by Bacteroidetes, Alphaproteobacteria, and Gammaproteobacteria, including several taxa with a known preference for surface attachment, and were relatively depleted in the unicellular Cyanobacteria and small photoheterotrophic bacteria that dominate NPSG surface waters. Sequencing the nifH gene (encoding a subcomponent of the nitrogenase enzyme) identified non-Trichodesmium diazotrophs that clustered predominantly among the Cluster III nifH sequence-types that includes putative anaerobic diazotrophs. Trichodesmium colonies may represent an important habitat for these Cluster III diazotrophs, which were relatively rare in the surrounding seawater. Sequence analyses of nifH gene transcripts revealed several cyanobacterial groups, including heterocystous Richelia, associated with the colonies. Both the 16S rRNA and nifH datasets indicated strong differences between Trichodesmium epibionts and picoplankton in the surrounding seawater, and also between the epibionts inhabiting Trichodesmium puff and tuft colony morphologies. Metagenomic and 16S rRNA gene sequence analyses suggested that lineages typically associated with a copiotrophic lifestyle comprised a large fraction of colony-associated epibionts, in contrast to the streamlined genomes typical of bacterioplankton in these oligotrophic waters. Additionally, epibiont metagenomes were enriched in specific genes involved in phosphate and iron acquisition and denitrification pathways relative to surface seawater metagenomes. We propose that the unique microbial consortium inhabiting colonies has a significant impact on the biogeochemical functioning of Trichodesmium colonies in pelagic environments.

Distribution and diversity of bacterioplankton communities in subtropical seawater around Xiamen Island, China

Marine bacterioplankton communities have profound impact on global biogeochemical cycles and ecological balances. However, relatively little is known about the bacterioplankton communities and the factors shaping their spatial distribution in subtropical island. Here, the bacterioplankton communities around a typical subtropical island, Xiamen Island, were revealed by analyzing bacterial 16S rRNA gene through quantitative PCR (qPCR) and 454 pyrosequencing methods. The qPCR results indicated that the abundance of 16S rRNA gene ranged from 2.07×107 to 2.13×108copiesmL−1 in surface seawater among eight sampling sites (S1–S8) around Xiamen Island, and the nitrogen and phosphorus-rich sites (S5 and S8) were detected with higher 16S rRNA gene abundance. Pyrosequencing evidenced that a total of 267 genera of 47 classes in 26 different phyla (or candidate phyla) and some unclassified bacteria were obtained from seawater around Xiamen Island. The most dominant phylum was Proteobacteria (49.62–76.84% among sites), followed by Bacteroidetes (6.64–20.88%), Actinobacteria (2.58–9.20%), Firmicutes (0.03–13.30%), Verrucomicrobia (0.23–2.67%) and Planctomycetes (0.14–2.20%). Among eight sites, the nitrogen and phosphorus-rich sites (S5 and S8) exhibited higher proportions of Gammaproteobacteria, Epsilonproteobacteria, Firmicutes and lower proportions of Alphaproteobacteria and Planctomycetes than other sites. S5 and S8 also had more similar β-diversity, and sampling site near the estuary (S8) showed the highest bacterial diversity. Redundancy analysis (RDA) confirmed that total nitrogen and total phosphorus significantly (P<0.05 and P<0.01, respectively) influenced the bacterioplankton communities around Xiamen Island. These results will provide insights into bacterial abundance, diversity and distribution patterns, as well as their controlling factors, in subtropical marine ecosystems.

Assessing the relationship between seagrass health and habitat quality with wasting disease prevalence in the Florida Keys

Marine pathogens of the genus Labyrinthula have been identified as the cause of wasting disease in seagrass systems in both temperate and subtropical regions. An understanding of the association between environmental factors and the prevalence of wasting disease in seagrass meadows is important for elucidating plant–pathogen interactions in coastal environments. We conducted a survey of 7 turtle grass-dominated beds within the Florida Keys National Marine Sanctuary to assess the relationship between environmental and biological parameters on seagrass health. All sites contained Labyrinthula spp.; the most pathogenic strain was obtained from an anthropogenically impacted site. Leaf and total biomass, in addition to root/rhizome carbon content, canopy light and % light transmitted, all displayed strong negative correlations with a wasting index (WI). It was noted that many of the same environmental measurements that showed negative correlations with WI also displayed strong positive correlations with canopy light levels. These data suggest that light availability may be an important factor that has previously been understated in the seagrass disease literature yet warrants more attention with respect to turtle grass susceptibility to infection. Studies such as this are important because they identify gaps in our understanding of plant–pathogen interactions in subtropical marine ecosystems. Furthermore, the relationships identified in this study may offer insight into which factors are most useful in identifying “at-risk” meadows prior to the onset of larger scale die-off events.

Laser Optical Plankton Counter and Zooscan intercomparison in tropical and subtropical marine ecosystems

Two recently developed instruments, the Laser Optical Plankton Counter (LOPC) and the Zooscan, have been applied to study zooplankton biomass size spectra in tropical and subtropical marine ecosystems off Brazil. Both technologies rely on optical measurements of particles and may potentially be used in zooplankton monitoring programs. Vertical profiles of the LOPC installed in a 200 µm ring net have been obtained from diverse environmental settings ranging from turbid and nearshore waters to oligotrophic open ocean conditions. Net samples were analyzed on the Zooscan and counted under a microscope. Particle biovolume in the study area estimated with the LOPC correlated with plankton displacement volume from the net samples, but there was no significant relationship between total areal zooplankton biomass determined with LOPC and the Zooscan. Apparently, normalized biomass size spectra (NBSS) of LOPC and Zooscan overlapped for particles in the size range of 500 to 1500 µm in equivalent spherical diameter (ESD), especially at open ocean stations. However, the distribution of particles into five size classes was statistically different between both instruments at 24 of 28 stations. The disparities arise from unequal flow estimates, from different sampling efficiencies of LOPC tunnel and net for large and small particles, and possibly from the interference of non‐zooplankton material in the LOPC signal. Ecosystem properties and technical differences therefore limit the direct comparability of the NBSS slopes obtained with both instruments during this study, and their results should be regarded as complementary.

Contaminants in oysters in Kaneohe Bay, Hawaii

Despite past and present concerns about the toxicity and persistence of various environmental contaminants (heavy metals and pesticides), relatively few studies have documented their concentrations in tropical or sub-tropical marine ecosystems. In this investigation, a ‘mussel watch’ approach was applied in Kaneohe Bay, Hawaii, to assess the present levels of potential contaminants in the Pacific oyster, Crassostrea gigas. Geographical gradients of these contaminants were observed and related to what is known about past and present inputs. Concentrations of lead, copper, chromium and zinc were elevated in oyster tissues near stream mouths in the southern watersheds of Kaneohe Bay. Dieldrin and chlordane concentrations in oysters from one of these sites exceeded the US Environmental Protection Agency's screening levels to protect human health, and were much higher than in oysters from the east and Gulf coasts of the temperate mainland USA.