The Thai-Malay Peninsula separates the Andaman Sea from the Gulf of Thailand. This barrier is known to affect species distribution and genetic diversity of marine organisms. Biodiversity studies need a correct species taxonomy to interpret their biogeography; this is especially relevant in marine algae where morphological characters are few or cryptic. We address species distributions of the important macroalgal genus Halimeda. The combination of morphological data and chloroplast tufA gene sequences were used to identify species, and to reveal the variation among Halimeda species around the Thai-Malay Peninsula. The morpho-anatomical and molecular analysis showed eight species of Halimeda in Thai and Malaysian waters: Halimeda borneensis, H. discoidea, H. gigas, H. macroloba, H. micronesica, H. minima, H. opuntia, H. renschii, and two undescribed species. Three species, H. minima, H. renschii and H. micronesica, were reported for the first time from the area. Species variation was much greater in the Andaman Sea than in the Gulf of Thailand. Our results show great variation in species composition and genetic variation between the two sides of the peninsula, which could be due to differences in sea-surface currents and environmental differences between the two sides of this important marine barrier.
Plasticity gives plants the ability to adapt their morphology and growth to various environments. Environmental factors can lead to changes in the growth forms of algae. Halimeda opuntia was used as a model organism to determine the effects of highly irradiated and shade conditions in the field and its responses to low light conditions in the laboratory, which are known to be some of the main factors driving its morphological plasticity. To investigate the morphological and anatomical characteristics of the species, a total of 61 characteristics and 54 samples from Thailand, Japan and Indonesia were examined. In addition, we also tested the response under different light conditions in the laboratory. A phylogenetic tree was constructed using the tufA marker to confirm the species identification. We found that the thallus forms, branching patterns and segment shapes and sizes responded to the different light conditions. A brittle thallus network was found under shade conditions with low light intensity, while a compact thallus form was found under highly irradiated conditions. The H. opuntia segment shape was reniform under highly irradiated conditions, but its segments were adapted to be deeply trilobed in the lower thallus and tripartite shape in the upper thallus under shade conditions. The segment size also adapted; the segments in shaded areas were longer than those in sunny areas, which might have helped them increase their thallus height to ensure exposure to the high light habitat. This morphological variation and plasticity expanded our understanding of the range of H. opuntia plasticity present in the field and could help explain why this species is common and successfully colonizes various habitats worldwide.
Halimeda macroloba is a common widespread and morphologically variable species in the Indo-Pacific Ocean. A series of morphometric data (16 morphological and 46 anatomical variables) and the chloroplast-encoded tufA gene were examined to explore the morphological and genetic variations within the species and to better determine species boundaries in H. macroloba in Thai waters. Moreover, the environmental conditions, which may affect morphological variations in H. macroloba, especially of Haplotype I were examined. Our results showed that H. macroloba has more morpho-anatomical variability and broader species boundaries than previously thought in several characters: holdfast type, segment shape and size, node height with differences in pore size, the presence of two additional patterns of peripheral utricle as seen in surface view, and additional layers of utricles (up to seven layers). Additionally, two new variable characteristics have been reported for the first time, i.e. shape and margin of peripheral utricles. Water depth and light correlated with thallus morphology, larger thalli and segments with a greater number of utricle layers were associated with subtidal plants, while smaller thalli and segments were found in the intertidal or high light intensity areas. The high plasticity of this species may explain why in the Indo-Pacific Ocean this species is common and dominant.
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.
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