The effect of habitats, densities and seasons on morphology, anatomy and pigment content of the seagrass Halophila ovalis (R.Br.) Hook.f. at Haad Chao Mai National Park, Southern Thailand

• H. ovalis characters and pigments varied greatly among habitats and densities.
• Desiccation could be the main driving forces causing the leaves variations.
• Rhizome internode was longer in rainy season; lacunar was larger in tidepool.
• Anthocyanin was greater in rainy season, unexpected result as much lower light.

Halophila ovalis is a small marine angiosperm that is widely distributed throughout the world. In this study, the morphology, the anatomy and pigment content of H. ovalis leaves from two habitat types (sand vs. tide pool) on the upper shore, each from high density and ca. 50% of high density locations, which represents the differences in emersion times, were investigated at Haad Chao Mai National Park, Southern Thailand, during the dry (December–April, 2011) and rainy seasons (May–November, 2011). Although the average light intensity and temperature were significantly higher during the dry season, they did not differ between the sand and tide pool habitats. Leaf length, width and area as well as petiole length, were significantly greater in high density beds of the sand habitat. The rhizome internode length was significantly longer in the tide pool habitat during the rainy season. The size of the air lacunars was significantly smaller during the dry season in the low density sand habitat. The amount of chlorophyll a and total chlorophyll were significantly lower in high density tide pool populations during both seasons. Anthocyanin, as indicated by a high amount of red colour in the leaves, was greatest in the rainy season sand habitat. This study is provides information on a range of H. ovalis variations and helps to better understand the H. ovalis response to changes in exposure conditions in the upper intertidal area.