Yury V. Deart

The nudibranch genus Pteraeolidia has traditionally been treated as taxonomically simple, but recent molecular studies suggest that it includes hidden species diversity across the Indo-Pacific. In this study, the Pteraeolidia semperi species complex was investigated from the coastal waters of Vietnam using an integrative approach. Thirty-five specimens from five localities were analysed using mitochondrial (COI, 16S rRNA) and nuclear (H3) markers, together with external and internal morphological data and haplotype network analysis. Our results revealed two deeply divergent clades (Clades A and B) occurring sympatrically in Vietnam. These clades correspond to several molecular operational taxonomic units and show strong geographic structure across the Indo-West Pacific. One of these units (MOTU20), which falls within Clade B, corresponds to Pteraeolidia ianthina (Angas, 1864), a valid species restricted to its type locality, Sydney Harbour, New South Wales; it is treated here as separate from the P. semperi complex. Nuclear data support the separation of the two clades as independent evolutionary lineages, whereas relationships among mitochondrial lineages within each clade remain unresolved. The two clades also differ in external morphology, especially in body colouration, pigmentation pattern, and ceratal arrangement, while internal anatomy is largely similar. Mitochondrial divergence in Pteraeolidia is exceptionally high, including within clades, and exceeds values usually reported between species. This unusual pattern may reflect accelerated mitochondrial evolution, possibly linked to the photosymbiotic lifestyle of these nudibranchs. Our data show that Vietnamese populations of Pteraeolidia include at least two species within a broader and highly diverse species complex. These findings confirm that Pteraeolidia semperi represents a complex of cryptic or semi-cryptic taxa and highlight the need for genomic data to resolve species limits more precisely.

This study presents the first targeted survey of six marine lakes located on karst islands of the Van Don Archipelago (Bai Tu Long Bay, northern Vietnam). The aim of the study was to describe the morphometric, hydrological, and biological characteristics of these lakes and to evaluate differences associated with their degree of connectivity to the sea. Field investigations revealed a gradient of hydrological conditions ranging from tidal lagoons with direct seawater exchange to highly isolated anchialine lakes connected to the sea only through subterranean pathways. Lakes with direct marine connections exhibited relatively homogeneous salinity and temperature profiles, lacked persistent stratification, and supported communities dominated by typical coastal marine species. In contrast, more isolated lakes showed pronounced vertical stratification, increasing salinity with depth, and distinct biological assemblages adapted to brackish and stratified conditions. The most isolated lake displayed meromictic characteristics, including a warm stratified water column, a hydrogen sulfide-rich bottom layer, and a sharp decline in macrofaunal abundance in deeper waters. The results indicate that hydrological connectivity is the primary factor controlling environmental conditions and biological community structure in the marine lakes of the Van Don Archipelago. These lakes encompass a wide range of ecological states despite their close geographic proximity and represent valuable natural systems for future studies of isolation, stratification, and biodiversity in tropical coastal ecosystems.

It is known that the giant barrel sponge (GBS) Xestospongia spp. may provide shelter or a micro-environment for multi-species coral colonies both in the Indo-Pacific and Atlantic regions. An assessment of such interactions between Indo-Pacific GBS Xestospongia testudinaria and stony corals performed for two groups of small Vietnamese islands in the Gulf of Thailand revealed at least 12 species of scleractinians associated with GBS. An average of 21.7% of all observed GBSs were found to interact with stony corals. The phenomenon of positive GBS–coral interactions without any restrictions on coral development can be regarded as a form of ecological facultative commensalism and warrants further investigation.

The impact of the introduced red king crab (RKC), Paralithodes camtschaticus, in the Barents Sea was mostly studied through its direct predation on native species. This study uses stable isotope analysis of carbon and nitrogen to characterize major trophic groups (including grazing sea urchins) and specifically looks at RKC's trophic position and niche partitioning with the native hermit and spider crab species. To achieve this, we sampled organisms from the food web of a small fjord‐like Zelenaya Inlet on the Murmansk coast in the southwestern Barents Sea and the open sea coast, just outside the inlet for comparison. There, macrophyte‐derived carbon is the major source of organic matter along with the phytoplankton. In general, the fucoid source of primary production was more prominent within the inlet food web compared to the nearby open coastal zone. Isotopic trophic niches calculated using SIBER analysis showed partition between the native hermit and spider crabs suggesting, some degree of trophic segregation between coevolved species. RKCs are clustered together in the center of native decapod species' niches. There were some distinct differences between the feeding habits of RKCs within the inlet and the open sea coastal waters. The subadult RKCs that are present in the inlet throughout the year have a narrower (in the area and specifically in δ¹⁵N dimension) niche than the adults from the open sea. The latter have a wider variety of food items due to their migration and may prey on food items from different trophic levels. Sea urchins are an important trophic link transferring the macrophyte carbon to RKC. However, P. camtschaticus may also directly consume plant residues. Despite generally similar calculated trophic levels of decapod species, our isotopic data and literature data on the food composition of Paralithodes camtschaticus in the Barents Sea do not indicate significant RKC's competition with, and predation on, native species of crabs.

Coryphellina rubrolineata (Gastropoda: Nudibranchia: Flabellinidae) was believed to be a widespread tropical species demonstrating high diversity in external and internal morphological traits. In this paper, we perform an integrative analysis of the C. rubrolineata species complex based on samples collected in Vietnam waters, combined with available data from other localities of the Indo-West Pacific. The methods of the study include morphological analysis of external and internal traits using light and scanning electron microscopy and the molecular analysis of four markers (COI, 16S, H3, and 28S). The phylogenetic hypothesis was performed using Bayesian and maximum likelihood approaches, and the species delimitation analyses included ASAP, GMYC, and bPTP. Our results support the validity of the genus Coryphellina as a distinct taxon and confirm that Coryphellina rubrolineata is restricted to the type locality and adjacent waters, while in the Indo-West Pacific, it represents a complex of pseudocryptic species. Based on our integrative analysis, we describe four new species: Coryphellina pseudolotos sp. nov., Coryphellina pannae sp. nov., Coryphellina flamma sp. nov., and Coryphellina aurora sp. nov. For the first time, Coryphellina lotos is reported in Vietnam waters. All five species differ in combination of coloration and other external traits and show minor differences in internal morphology.

Based on the data of benthic surveys of 2010–2012, three benthic communities: a shallow-water community with the dominance of Scoloplos spp., Nucula tennuis, and Scalibregma inflatum; a deep-water...

To assess whether closely related host species harbour similar symbiotic communities, we studied two polychaetes,Chaetopterussp. (n = 11) andChaetopteruscf.appendiculatus(n = 83) living in soft sediments of Nhatrang Bay (South China Sea, Vietnam). The former harboured the porcellanid crabsPolyonyxcf.heokandPolyonyxsp., the pinnotherid crabTetriassp. and the tergipedid nudibranchPhestillasp. The latter harboured the polynoid polychaeteOphthalmonoe pettiboneae, the carapid fishOnuxodon fowleriand the porcellanid crabEulenaios cometes, all of which, exceptO. fowleri, seemed to be specialized symbionts. The species richness and mean intensity of the symbionts were higher inChaetopterussp. than inC.cf.appendiculatus(1.8 and 1.02 species and 3.0 and 1.05 individuals per host respectively). We suggest that the lower density ofChaetopterussp. may explain the higher number of associated symbionts observed, as well as the 100% prevalence (69.5% inC.cf.appenciculatus). MostChaetopterussp. harboured two symbiotic species, which was extremely rare inC.cf.appendiculatus, suggesting lower interspecific interactions in the former. The crab and nudibranch symbionts ofChaetopterussp. often shared a host and lived in pairs, thus partitioning resources. This led to the species coexisting in the tubes ofChaetopterussp., establishing a tightly packed community, indicating high species richness and mean intensity, together with a low species dominance. In contrast, the aggressive, strictly territorial species associated withC.cf.appendiculatusestablished a symbiotic community strongly dominated by single species and, thus, low species richness and mean intensity. Therefore, we suggest that interspecific interactions are determining species richness, intensity and dominance, while intraspecific interactions are influencing only intensity and abundance. It is possible that species composition may have influenced the differences in community structure observed. We hypothesize that both host species could originally be allopatric. The evolutionary specialization of the symbiotic communities would occur in separated geographical areas, while the posterior disappearance of the existing geographical barriers would lead to the overlapped distribution.