Enhalus acoroides

Appearance: The leaves are very long and ribbon-like (30-150cm long, approximately 1-2cm wide) with many parallel veins and air spaces, generally dark green in colour and thick. The inrolled leaf margins make the leaves tough (hard to tear)

Distribution: Widely distributed in the tropical parts of the Indian Ocean and the Western Pacific and are very common in the Indo-Malay Archipelago and in the Philippines

Environment/habitat: Common in shallow intertidal areas with sandy and muddy substrata, but can extend down to 4m depth

Rhizome: Has thick rhizomes (underground stems) that are massive (1.5 cm in diameter), branching monopodially when a new shoot is formed. The rhizomes are densely covered with long black fibrous bristles which are the remnants of a leaf sheath. They have coarse, cord-like and hairless roots which have wide air-channels. The roots are 10-30cm long and 3-5mm thick arising from the axillary buds of the ventral leaves.

Propagule dispersal: The fruits are round and large (4-6cm in diameter) with dark, ribbed skin and 6-7 white seeds. When the ripe fruit bursts, the seeds are released and float for only about 5 hours before they start to sink. The seeds are estimated to be able to travel 42 km.  When the seeds settle, roots develop rapidly and the seeds germinate quickly. Enhalus acoroides spreads mostly by vegetative reproduction.

Reproduction: Flowers only in habitats where the flowers are exposed at low tides because this species undergoes aerial surface pollination. The seed upon liberation from the fruit germinates immediately after it sinks to the muddy bottom. Enhalus acoroides has white flowers- male flowers are tiny while female flowers are larger. The male Enhalus acoroides bears a single pedunculate inflorescence containing numerous flowers whilte the female Enhalus acoroides bear single uniflorous inflorescences. Flowering is more or less continuous over the year, and represents the investment of around 20% of above-ground production

Importance/Value: Enhalus with long strap-like leaves form good wave breakers and extensive beds give some protection to shorelines exposed to strong waves. Such thick vegetation also provides good hiding places for small species and the young of other organisms. It is a common food for the dugong. Tiny algae often grows on the leaves of this seagrass, providing food for grazing creatures such as snails. The fruits are sold as human food in the market and the seeds are eaten raw by coastal dwellers. Enhalus is highly productive and contribute greatly to oxygenation of the seas as well as carbon sequestration, leading to reduction in the effects of global warming.

Thalassia hemprichi

Appearance: The leaves are usually sickle-like hooked/curved shaped and are dark green. With short black bars of tannin cells in their leaf blades, they are around 10-40cm long although in Singapore, leaves are only about 10 to 20cm long.

Structure: They have thick rhizomes, of up to 5mm, with air channels and conscpicous scars between shoots. With 10-17 longitudinal leaf veins and leaf sheaths of 3-7cm long, they are firmly anchored.

Distribution: They are widely distributed in the coastal waters of the Indian Ocean and the western Pacific, they are often found on shallow reef flats and in large beds. They are common on reef flats where it may form dense meadows. Moreover, they can also be found colonizing muddy substrates, especially at low tide.

Environment/ Habitat: They are often found on shallow reef flats, where it may form dense meadows. Moreover, it is common to see them in large beds and also be found colonizing muddy substrates, especially at low tide.

Reproduction: The inflorescence on the flowers is usually single, each having a separate spathe on short shoots. The male flower releases pollen grains in long chains within their mucilage tubes while the female flower has papillate stigma have papillate stigmata with surface pellicles that undergo esterase activity and their cuticle accumulated with acid phosphatase. Thalassia hemprichi usually produce relatively larger fruits.

This is a paper published by the The Raffles Bulletin of Zoology, Raffles Museum of Biodiversity. The marine lab in NUS did a study of the effects of the impact of visitors on the 4 taxa in the intertidal region of Labrador Park.

54rbz-labrador-beach.pdf