BackgroundThe Indo-Pacific region has the largest number of seagrass species worldwide and this region is considered as the origin of the Hydrocharitaceae. Halophila ovalis and its closely-related species belonging to the Hydrocharitaceae are well-known as a complex taxonomic challenge mainly due to their high morphological plasticity. The relationship of genetic differentiation and geographic barriers of H. ovalis radiation was not much studied in this region. Are there misidentifications between H. ovalis and its closely related species? Does any taxonomic uncertainty among different populations of H. ovalis persist? Is there any genetic differentiation among populations in the Western Pacific and the Eastern Indian Ocean, which are separated by the Thai-Malay peninsula? Genetic markers can be used to characterize and identify individuals or species and will be used to answer these questions.ResultsPhylogenetic analyses of the nuclear ribosomal internal transcribed spacer region based on materials collected from 17 populations in the Western Pacific and the Eastern Indian Ocean showed that some specimens identified as H. ovalis belonged to the H. major clade, also supported by morphological data. Evolutionary divergence between the two clades is between 0.033 and 0.038, much higher than the evolutionary divergence among H. ovalis populations. Eight haplotypes were found; none of the haplotypes from the Western Pacific is found in India and vice versa. Analysis of genetic diversity based on microsatellite analysis revealed that the genetic diversity in the Western Pacific is higher than in the Eastern Indian Ocean. The unrooted neighbor-joining tree among 14 populations from the Western Pacific and the Eastern Indian Ocean showed six groups. The Mantel test results revealed a significant correlation between genetic and geographic distances among populations. Results from band-based and allele frequency-based approaches from Amplified Fragment Length Polymorphism showed that all samples collected from both sides of the Thai-Malay peninsula were clustered into two clades: Gulf of Thailand and Andaman Sea.ConclusionsOur study documented the new records of H. major for Malaysia and Myanmar. The study also revealed that the Thai-Malay peninsula is a geographic barrier between H. ovalis populations in the Western Pacific and the Eastern Indian Ocean.

Marine macrophytes help protect coastal habitats by modifying the energy of their environment, attenuating wave action, and stabilizing the sediment. The aim of this research was to examine the effect of the above ground parts of different seagrass and seaweed species on the deposition, retention, physical (dry bulk density and grain size), and chemical properties (organic carbon content and organic carbon density) of sediment. Six seagrass and seaweed communities with different above ground morphologies were studied between the dry (April 2015) and wet seasons (August 2015). The results suggested that the highest canopy complexity index was in the green alga, Halimeda macroloba Decaisne patches, which had the highest canopy score. High canopy complexity patches were likely to retain more sediment and store more organic carbon than lower canopy complexity patches and bare sand, respectively. The sediment retention and percent of organic carbon content were also affected by seasons. The wave action in the wet season affected the sediment through higher sediment deposition and movement, which caused increased sediment instability and led to changes in sediment properties. In addition, shoot density in the dry season was lower than that in the wet season, most likely due to damage from long exposure during low tide. Our results indicate that the canopy complexity of marine macrophytes and seasons influenced both sediment retention and properties of the sediment.

The common seagrass, Halophila ovalis (R. Brown) J.D. Hooker, is highly variable morphologically. It adapts well to various environmental conditions rendering the various forms unclear taxonomically. Halophila species were collected along the coast of southern Thailand. The morphology was quantified according to different parts of the leaf and the ages of leaves. Some samples had significantly different characters from H. ovalis: the lengths of their leaves ranged from 11.7-29.4 mm, and the widths from 5.6-14.8 mm; there were 9-18 cross veins. Phylogenetic analyses based on ribosomal internal transcribed spacer sequences divided them into two groups: one agrees with H. ovalis and the other with H. major. We suggest that leaf size at maturity (age iii-iv) and the ½ ratio between the leaf width and the space between the intra-marginal vein and lamina margin are important characters that distinguish Halophila species.

In the tropical ecosystem, sea cucumbers are associated with seagrass meadows in various ways, often forming a network of ecological interactions. From this myriad of interactions, the trophic relationship between the seagrasses and sea cucumbers has received recent attention with the advent of analytical techniques. However, little is understood about the exact mechanism by which seagrasses are sustaining the sea cucumber populations in the food chain, considering the high number of refractory components in seagrasses and the lack of digestive enzymes among sea cucumbers. This manuscript aims to review existing concepts in ecology concerning the association between tropical seagrasses and sea cucumbers to provide directions for research and management of this vital resource. We searched literature from electronic databases and identified key concepts concerning sea cucumber and seagrass communities based on geographic distribution, nutrient compositions, seagrass decomposition process, and trophic enrichments in the food chain. A conceptual model was then developed detailing the factors influencing the association between the seagrass meadows and sea cucumbers. Despite the limited published information on the seagrass–sea cucumber association, a synthesis of the current understanding of this topic is provided to address the declining sea cucumber populations in the tropical seagrass meadows. We suggest that the successful restoration of sea cucumber fisheries requires a thorough understanding of the seagrass decomposition process, which is vital to the diet of sea cucumbers.

Coastal areas have high potential to store carbon from both terrestrial and marine materials due to the interconnectivity among the habitats. We investigated the contributions of the various sources to the sedimentary organic carbon by examining the δ13C, δ15N, %TOC (total organic carbon) and %TN (total nitrogen) of several marine macrophytes, which included three seagrass species: Cymodocea rotundata Ascherson & Schweinfurth, Cymodocea serrulata (R. Brown) Ascherson & Magnus, and Halophila ovalis (R. Brown) J.D. Hooker; three seaweed species: Halimeda macroloba Decaisne, Halimeda borneensis W.R. Taylor, and Halimeda opuntia (Linnaeus) J.V. Lamouroux; and one coral species, Porites sp., over a vertical gradient at Tangkhen Bay, Thailand. The distribution of organisms differed significantly between distances from the shore and seasons (p < .001), with seagrass occupying the area of 150–350 m from the shore, while Halimeda was most abundant from 150 to 400 m from the shore. The %TOC and %TN in the sediment were significantly different along the shore (p < .001), with the highest values at 300 m from the shore where the highest organism variability was recoded. Seagrass and Halimeda played major roles in sediment organic matter and carbon contributions, contributing up to 42% of the sedimentary organic carbon, while allochthonous sources such as ocean and terrestrial sources had lower contributions. This study elucidates the importance of macrophytes as organic carbon contributors to sedimentary carbon storage, especially in small semi‐enclosed bays.