An updated inventory of sea slugs from Koh Tao, Thailand, with notes on their ecology and a dramatic biodiversity increase for Thai waters.

Research in recent years have provided rapid advances in biogeographic and taxonomic documentation of sea slugs around the world. However, efforts are lacking in surveying most coastlines and habitats in South-East Asia. Recent studies from the Gulf of Thailand have indicated that a wealth of unexplored sea slug diversity and ecology may be gained from an investigation of soft sediment habitats beyond the reef slopes. Additionally, the waters of Koh Tao have been found to host regionally high levels of sea slug diversity with several species awaiting taxonomic clarification. In this work the initial findings of an expanded survey effort from the waters around Koh Tao are provided, with the identity of two soft sediment-associated sacoglossan species in the family Plakobranchidae being investigated. By integrating morphological and molecular analyses, the species Plakobranchus noctisstellatussp. nov. and Elysia aowthaisp. nov. are described and species complexes surrounding Plakobranchus ocellatus van Hasselt, 1824 and Elysia japonica Eliot, 1913 are discussed. The topics of morphological variability and the cryptic species problem are also discussed.

Habitat specificity plays a pivotal role in forming community patterns in coral reef fishes, yet considerable uncertainty remains as to the extent of this selectivity, particularly among newly settled recruits. Here we quantified habitat specificity of juvenile coral reef fish at three ecological levels; algal meadows vs. coral reefs, live vs. dead coral and among different coral morphologies. In total, 6979 individuals from 11 families and 56 species were censused along Ningaloo Reef, Western Australia. Juvenile fishes exhibited divergence in habitat use and specialization among species and at all study scales. Despite the close proximity of coral reef and algal meadows (10's of metres) 25 species were unique to coral reef habitats, and seven to algal meadows. Of the seven unique to algal meadows, several species are known to occupy coral reef habitat as adults, suggesting possible ontogenetic shifts in habitat use. Selectivity between live and dead coral was found to be species-specific. In particular, juvenile scarids were found predominantly on the skeletons of dead coral whereas many damsel and butterfly fishes were closely associated with live coral habitat. Among the coral dependent species, coral morphology played a key role in juvenile distribution. Corymbose corals supported a disproportionate number of coral species and individuals relative to their availability, whereas less complex shapes (i.e. massive & encrusting) were rarely used by juvenile fish. Habitat specialisation by juvenile species of ecological and fisheries importance, for a variety of habitat types, argues strongly for the careful conservation and management of multiple habitat types within marine parks, and indicates that the current emphasis on planning conservation using representative habitat areas is warranted. Furthermore, the close association of many juvenile fish with corals susceptible to climate change related disturbances suggests that identifying and protecting reefs resilient to this should be a conservation priority.

Anchored fish aggregating devices (AFADs) have been widely used for fishing in Thai waters. However, abandoned, lost, and discarded fishing gears (ALDFGs), including lost AFADs, may cause environmental impacts. Fishing gear marking (FGM) is considered as a tool to help identification of ALDFGs. The main objective of this study is to compare the durability represented by the percentage of remaining condition (R-value) of three material types of FGM applied for AFADs, i.e., stainless steel (SS), colored acrylic (CA), and polypropylene (PP). This study was carried out using 50 AFADs deployed in the Gulf of Thailand (GOT) and the Andaman Sea (ANS) between July and October 2020 in cooperation with 10 fishers. The AFADs were deployed in similar habitat (bottom depth and type) between the GOT and the ANS. The three material types of FGM were assumed to be sufficiently durable to last for the lifespan of the AFADs in both the GOT and the ANS (within 2 months and 3.5 months, respectively) though some FGMs in the ANS were detached from cable ties or broken before AFADs were lost. The loss of AFADs and FGMs was mainly caused by adverse weather condition (rough sea). Only data from the ANS was included in comparative analysis due to the insufficient variance data obtained from the GOT. The analysis revealed that SS had the higher durability than CA and PP when the AFADs lasted for less than 3.5 months. As a result of our study, some recommendations were made. For example, the cable ties can be replaced by ropes or threads to improve the installation method. This study serves as a basis to develop FGM and to support responsible fisheries. Beneficiaries of the study include fisheries policy makers, managers, and fishers.

Genetic mixed-stock analysis of short mackerel, Rastrelliger brachysoma, catches in the Gulf of Thailand: Evidence of transboundary migration of the commercially important fish

Sources and mass inventory of sedimentary polycyclic aromatic hydrocarbons in the Gulf of Thailand: Implications for pathways and energy structure in SE Asia

The Gulf of Thailand is ideal for studying eustatic sea level fluctuations in Southeast Asia due to its shallow basin and tectonic stability. However, our understanding of how this region’s relative sea level (RSL) has fluctuated over the Holocene epoch is far from complete. In this study, we used lithostratigraphy, loss on ignition, grain size, and pollen analyses to reconstruct the environmental changes in the Sam Roi Yot wetland, which was significantly influenced by seawater intrusion, driven by fluctuations in RSL in the Gulf of Thailand. Therefore, the analyzed pollen records of the sediment core from the wetland reflected variabilities in the RSL in the Gulf of Thailand. Subsequently, we found that after a sea level highstand prior to 4000 cal y BP, the RSL gradually fell with two significant regressions at c. 2950 and 1850–1450 cal y BP before rising at 1450–1050 cal y BP and declining after that. The inconsistency between RSL reconstruction based on our results and the global sea level changes simulated by the Glacial Isostatic Adjustment (GIA) model further suggests that long–term El Niño Southern Oscillations (ENSO) variabilities may have played a significant role in sea level changes in the Gulf of Thailand over the Late-Holocene period. Thus, during extended El Niño or La Niña conditions, the sea level would have been consistently lower or higher than expected from eustatic and isostatic processes alone. Overall, this study emphasizes the importance of considering regional factors such as ENSO to understand sea level changes in Southeast Asia.

Results of a detailed palaeomagnetic study of Cretaceous-age volcanic, intrusive and sedimentary rock formations from southern Vietnam (24 sites, 163 core samples) are presented. The palaeomagnetic and supplementary rock magnetic studies indicate that magnetite and titanomagnetite are the predominant magnetic carriers in the volcanic and intrusive rock samples, whereas hematite is the principal carrier in the red-beds. The mean palaeomagnetic direction of twenty-one sites from southern Vietnam yields D = 14.5°, I = 33.3°, α 95 = 6.3°, k s / k g = 1.04, which corresponds with a VGP at λ = 74.2°N, φ = 171.1°E, A 95 = 5.9°. Comparison of the pole with the Eurasia mean Cretaceous palaeopole shows that relative to Eurasia southern Vietnam has experienced a southward displacement of 6.5° ± 5.1°, but with insignificant rotation since the Cretaceous. Previously reported Cretaceous palaeomagnetic data, combined with new palaeomagnetic data from this study and analysis of regional structural trends, indicate that Sundaland can be divided into several fault-bounded tectonic domains (Shan-Thai, Indochina, offshore Sundaland), each with a different rotation and/or translation history. Such differential motion might explain, for example, Oligocene transtension and basin formation in the Gulf of Thailand and central onshore Thailand (between the Shan-Thai and Indochina blocks). Our data combined with previously acquired palaeomagnetic data across Southeast Asia, also suggest that, during the Cenozoic, Indochina and parts of Sundaland underwent complex internal deformation and did not behave as a rigid block.

Sediment Properties along Gradients of Siltation in South-east Asia

Community similarity is the proportion of species richness in a region that is shared on average among communities within that region. The slope of local richness (alpha diversity) regressed on regional richness (gamma diversity) can serve as an index of community similarity across regions with different regional richness. We examined community similarity in corals at three spatial scales (among transects at a site, sites on an island, and islands within an island group) across a 10 000-km longitudinal diversity gradient in the west-central Pacific Ocean. When alpha diversity was regressed on gamma diversity, the slopes, and thus community similarity, increased with scale (0.085, 0.261, and 0.407, respectively) because a greater proportion of gamma diversity was subsumed within alpha diversity as scale increased. Using standard randomization methods, we also examined how community similarity differed between observed and randomized assemblages and how this difference was affected by spatial separation of species within habitat types and specialization of species to three habitat types (reef flats, crests, and slopes). If spatial separation within habitat types and/or habitat specialization (i.e., underdispersion) occurs, fewer species are shared among assemblages than the random expectation. When the locations of individual coral colonies were randomized within and among habitat types, community similarity was 46-47% higher than that for observed assemblages at all three scales. We predicted that spatial separation of coral species within habitat types should increase with scale due to dispersal/extinction dynamics in this insular system, but that specialization of species to different habitat types should not change because habitat differences do not change with scale. However, neither habitat specialization nor spatial separation within habitat types differed among scales. At the two larger scales, each accounted for 22-24% of the difference in community similarity between observed and randomized assemblages. At the smallest scale (transect-site), neither spatial separation within habitat types nor habitat specialization had significant effects on community similarity, probably due to the small size of transect samples. The results suggest that coral species can disperse among islands in an island group as easily as they can among sites on an island over time scales that are relevant to their establishment and persistence on reefs.

Biodiversity of sea slugs and shelled relatives (Mollusca: Gastropoda) of the Cretan Archipelago (Greece), with taxonomic remarks on selected species

Carbon cycling in the tropical margin is more extensive compared with other regions of the world. The goal of this study was to better understand the origins, transport, and burial of sedimentary organic matter (SOM) in the Gulf of Thailand (GOT) from the coastal margin of Southeast (SE) Asia, which serve as a major depository of fine‐grained sediments and the associated organic carbon (OC). The results revealed a variety of organic matter (OM) inputs and the selective transport of fine sediment, resulting in preferential dispersal of terrigenous SOM in the GOT. Bulk OC indices with low carbon/nitrogen ratios and enriched stable carbon isotope ratios (−24.2‰ to −20.4‰, and mean −21.4 ± 0.56‰) are likely related to the presence of marine‐derived OM and anthropogenic interference. A binary mixing model further clarified the significant contributions of terrestrial derived‐OM within the upper and central GOT. The n‐alkane compositions and principal component analysis indicated that a majority of the terrigenous SOM settles within the estuary in the upper GOT, while a selective dispersal of land‐based SOM through long‐distance transport toward the modern depocenter in the lower GOT. The characteristics of lower molecular weight n‐alkanes also suggest anthropogenic OM input from petroleum‐related contributions. Altogether, the depositional patterns and spatial heterogeneity of the SOM indicated by both the bulk and molecular signatures reveal the important roles of source variability and the selective dispersal of land‐based OM on the supply and accumulation of OC in the tropical coastal margin.

Rastrelliger brachysoma (Bleeker, 1851), the short mackerel, is a dietary staple and of significant economic demand in Southeast Asia and Thailand. However, the demand for short mackerel has precipitated an overfishing crisis, leading to a depletion of fish stocks. Overfishing, coupled with parasitism, may result in a decline in the population of R. brachysoma. Digenetic trematode infection is prevalent in marine fish and has a considerable impact on the overall health of the fish. Here, to identify digenetic trematodes infecting R. brachysoma, we aim to determine the identity, prevalence, and intensity of digenean infections in R. brachysoma from the Gulf of Thailand. A total of 194 short mackerel were obtained from Chon Buri Province, where digeneans were isolated and identified. The molecular identity of the digeneans was confirmed using the nuclear 28S rRNA gene. Of the 194 short mackerel, 100% were found to be infected with digeneans, comprising of Lecithocladium, Prodistomum, Opechona, and Aphanurus. Lecithocladium was the most prevalent (98%) and had the highest intensity of infection (37 mean intensity), followed by Prodistomum (75% prevalence and 17 mean intensity). Our study thus presents the first evidence of digeneans infecting the economically important short mackerel from the Gulf of Thailand. The high infection rate of digenetic trematodes may have implications on the health of R. brachysoma, further driving their population decline. These data underscore the importance of safeguarding fisheries resources in the Gulf of Thailand, and downstream conservation efforts are crucial for evidence-based management decisions to safeguard the long-term sustainability of fish resources.

More than two decades that the Gulf of Thailand (GOT) has been installed with petroleum hydrocarbon production platforms, currently over 400 platforms were installed and operated. Since mercury is a common contaminant in petroleum hydrocarbon production in Southeast Asia, minimal risk and environmental integrity should be concerned. Mercury concentration in surface sediment collected from the Gulf of Thailand in 2003 (89 stations), 2012 (174 stations) and 2013 (45 stations). Sedimentological characteristics, readily oxidizable organic carbon and calcium carbonate were determined. All analyses were carried out in our laboratory using cold vapor atomic absorption spectroscopy. The results show an increase in trace amount of mercury in the Gulf's sediment. Average concentrations of mercury in surface sediments in the lower GOT collected in 2003, 2012 and 2013 were 24.4±9.00, 34.9±21.5 and 41.4±15.3 μg/kg dry weight (carbonate free basis). It is coincident to an increment in the number of platforms for natural gas exploration and production in the Gulf of Thailand. Spatial distribution of mercury in the sediments indicates a clearly linked to the exploration, development, production, and processing in petroleum and gas operation. Although the elevation of mercury level in the GOT's sediment does not showed high risk yet, treating and recycling of mercury contaminated substances generated during production are required in order to minimize the health risk in consumption of seafood collecting from the GOT.

Sampling larval fishes with a nightlight lift-net in tropical inshore waters

Southeast Asia has experienced rapid ocean warming and increased frequency of major thermal events in recent decades. However, the spatial and temporal variability of Marine Heatwaves (MHWs) across the region remains understudied. Here, we examine historical MHWs in Southeast Asia from 1982 to 2023 using optimally interpolated sea surface temperature data. In addition to analyzing broader regional trends, we also focused on three sub-regions (Karimata Strait (KS), Gulf of Thailand (GT), and Celebes Sea (CS)) that represent different geographic and oceanographic conditions. Our analyses aim to 1) evaluate the spatial and temporal variability in MHWs frequency and duration; 2) identify seasonal patterns of MHWs occurrence; and 3) investigate the mechanisms driving MHWs formation through heat budget analysis. Results show a significant increase in both the frequency and duration of MHWs over the past 42 years, with the highest frequency observed in the eastern and northern parts of Southeast Asia. MHW frequency increased from fewer than two events per year in the 1980s to five events per year in the last decade, with a sharp rise after 2003 (a 108% increase in frequency and a 63% increase in duration). This warming has caused an increase in both the mean and cumulative intensity of MHWs in the region. Among the sub-regions, CS experienced the largest increase in frequency, while KS showed notable increases in MHW duration and intensity. Seasonally, MHWs are most frequent from June to September, with maximum duration and cumulative intensity from December to March. Heat budget analysis during MHW days highlights that net heat flux under clear skies is the primary driver of surface warming across Southeast Asia, while horizontal advection contributes significantly in some coastal areas. Furthermore, vertical structure analysis suggests that surface warming associated with MHWs penetrates the subsurface, with distinct regional patterns: shallow, surface-intensified MHWs dominate in KS and GT, while subsurface-intensified MHWs are prevalent in CS. Our findings provide valuable insights into the historical variability of MHWs in Southeast Asia, providing a foundation for future projections and implications for marine and coastal ecosystems, particularly on coral reefs.