There are 6 main sections in this key. To use it, just check for the characteristics in the same order listed below and narrow down to the species by elimination. So basically, Just start with asking whether the plant has: A) No Upright Stem(s); B) Compound Leaves; C) Alternate Simple Leaves; or D) Opposite Simple Leaves. If it has opposite leaves, and each leaf is longer than 2cm, then check whether the leaves has: E) Rounded Tips or F) Pointed Tips. Click on the links on the plant names for more details, but note that this guide can only be used in Singapore, as some plant species may have different morphs in other countries.
A) No Upright Stem(s)
1. Long leaflets(60-130cm) with parallel veins.
Nipah Palm (Nypa fruticans) - The only true mangrove palm in Singapore. It is a monocot, and has huge compound leaves up to 9m long. The palm lacks a trunk, but instead, has thick underground rhizomatous forking stems.
2. Mature leaf has blunt tips, sometimes with a small point.
Piai Raya (Acrostichum aureum) - This is a mangrove fern which can grow as tall as 4m. It prefers sunny sites. Like other ferns, they reproduce from spores, and large sporangia cover the undersides of fertile fronds.
3. Mature leaf has elongated pointed tips.
Piai Lasu (Acrostichum speciosum) - This mangrove fern grows up to about 1.5m, much shorter than A aureum. It prefers shady areas. Like other ferns, they reproduce from spores, with the sporangia on the undersides of fertile fronds.
B) Compound Leaves
4. Leaflets with rounded tips.
Nyireh Bunga (Xylocarpus granatum) - This tree can also be recognised by its thin and flaking barks. The fruits are round, about the size of a small coconut.
5. Leaflets with pointed tips.
Nyireh Batu (Xylocarpus moluccensis) - Unlike X. granatum, this tree has fissured bark. The fruits are round, about 10cm across.
C) Alternate Leaves
6. Leaves with silvery undersides.
Dungun (Heritiera littoralis) - When in bloom, it can be identified by its clusters of numerous tiny cup-shaped flowers. The fruits are woody with a keel on one side.
7. Leaves with pointed tips.
Buta-buta (Excoecaria agallocha) - Also called "Blind-your-eyes", it has poisonous white sap in its leaves and branches. It bears tiny flowers on catkin-like, elongated clusters. The tree is deciduous.
8. Alternate/subopposite leaves, obovate with prominent yellowish/light green veins.
Kacang-kacang (Aegiceras corniculatum) - This small tree bears white flowers with protruding anthers and a pinkish style. The fruits ranges from green to pinkish to red in colour, and are strongly curved.
9. Leaves mostly drop-shaped (broader at the tip) and red flowers.
Teruntum Merah (Lumnitzera littorea) - This tree has pretty red flowers, and even the young branches are reddish in colour. The small and somewhat corky fruits may have a reddish tinge as well.
10. Leaves mostly obovate (broadest about 2/3 towards the tip) and white flowers.
Teruntum Putih (Lumnitzera racemosa) - The leaves of this tree are usually of a fresher green compared to L. littorea. The small fruits are fibrous and compressed sideways, with a prominent persistent style.
D) Opposite Leaves
11. Small leaves not more than 2cm long.
Mentigi (Pemphis acidula) - This small shrub/tree bears pretty small white flowers. The fruits are small, turning reddish brown as they mature. The tip of the leaves may be pointed or rounded.
E) Opposite Leaves with Rounded Tips
12. Apical bud compressed and rounded.
Chengam (Scyphiphora hydrophyllacea) - This small tree/shrub may have one or multiple stems. The white flowers occur in dense clusters. The small fruits are cylindrical and green, ribbed along its length.
13. Apical bud compressed and twisted, enclosed in stipules.
Pisang-pisang (Kandelia candel) - The flowers are white and occur in clusters, dichotomously branched. The seedling's hypocotyl has a somewhat pointed tip, and is capped by the persistent sepals whose tips bend backwards towards the stalk.
14. Apical bud compressed and enclosed in stipules. Fruit with textured patterns and seedling with red collar.
Ceriops zippeliana - The seedlings of this tree generally hang in various directions. The small flowers are white and turn brown quickly.
15. Apical bud compressed and enclosed in stipules. Fruit plain and seedling with white collar.
Tengar (Ceriops tagal) - The small flowers are white and turn brown quickly, and have 5 thick and claw-like sepals. Unlike C. zippeliana, the seedlings usually occur hanging downwards.
16. Leaves dark green on top, and light brown and velvety below.
Api-api Bulu (Avicennia rumphiana) - Like other Avicennia species, it has pencil-like aerial roots (pneumatophores). The fruits are broadly ovate, covered with short hair, giving it a furry appearance. Flowers are small and yellow, occurring in clusters.
17. Leaves dark green on top and light green below.
Api-api Ludat (Avicennia officinalis) - Like other Avicennia species, it has pencil-like pneumatophores. The fruits are broadly ovate with a short beak, covered with short hair, and yellowish-brown in colour. Flowers are small and yellow, occurring in clusters.
18. Leaves broadly drop-shaped/pear-shaped.
Perepat (Sonneratia alba) - Like other Sonneratia species, it has conical pneumatophores. The flower has small and white petals, and numerous white stamens. The fruits are somewhat pear-shaped with persistent sepals whose tips bend back towards the stalk.
19. Leaves broadly ovate, oval or almost round.
Gedabu (Sonneratia ovata) - Like other Sonneratia species, it has conical pneumatophores. The flower has numerous white stamens but no petals are absent. The fruits are round, with the calyx enveloping the berry.
F) Opposite Leaves with Pointed Tips
20. Leaves glossy green on top and whitish below. Fruits the shape of elongated inverted teardrops.
Api-api Putih (Avicennia alba) - Sometimes confused with A. marina, it is best to confirm the species by looking at the fruits, which are in the shape of elongated inverted teardrops and are whitish/greyish green in colour. Like other Avicennia species, it has pencil-like pneumatophores. It has small yellow flowers, occurring in clusters.
21. Leaves green (with a yellowish tinge) on top and yellow or whitish green below. Young twigs have a squarish cross-section. Fruits the shape of fat inverted teardrops.
Api-api Jambu (Avicennia marina) - Easily confused with A. alba, this species is best confirmed with the fruits, which are in the shape of a fat inverted teardrops and are light green in colour. Like other Avicennia species, it has pencil-like aerial roots. It has small yellow flowers, occurring in clusters.
22. Leaves have a minute, recurved point at the tip.
Berembang (Sonneratia caseolaris) - Like other Sonneratia species, it has conical pneumatophores. The flowers have red petals and numerous stamens. The filaments are either all white, or white in the upper part and red in the lower part. The fruit is persimmon-like with a flattened calyx tube.
23. Narrowly elliptic leaves, undersides covered with small black dots. Apical buds enclosed in thin, pointed, red stipules.
Bakau Minyak (Rhizophora apiculata) - Like other Rhizophora species, prop roots and stilt roots can be seen at the lower part of the tree. The stalks of the flower heads are short, about 1cm long, and each stalk bears 2 stalkless flowers. Mature seedlings have a red collar.
24. Elliptic leaves less than 6.5-12.5cm long, undersides covered with small black dots. Green stipules. Flower has long style, 4-6 mm long.
Bakau Pasir (Rhizophora stylosa) - Like other Rhizophora species, prop roots and stilt roots can be seen at the lower part of the tree. Mature seedlings are dispersed by water and have a greenish collar. Leaves are generally shorter but proportionally broader than R. apiculata.
25. Broadly elliptic leaves more than 11-23cm long, undersides covered with small black dots. Green or red stipules. Flower has short style, 0.5-1.5 mm long.
Bakau Kurap (Rhizophora mucronata) - Like other Rhizophora species, prop roots and stilt roots can be seen at the lower part of the tree. It has red or light green stipules. Mature seedlings are dispersed by water and have a light green or whitish collar. The leaves are much bigger and proportionally broader than the other 2 Rhizophora species.
26. Small flowers with yellow petals. Short and straight calyx lobes.
Lenggadai (Bruguiera parviflora) - Like other Bruguiera species, the bark is covered with small lenticels, and the tree has knee roots.
27. Small flowers with white petals. Calyx lobes bend backwards on seedlings.
Bakau Putih (Bruguiera cylindrica) - Like other Bruguiera species, the bark is covered with small lenticels, and the tree has knee roots.
28. Flowers with white or light orange petals. Calyx lobes extend away from seedling and bend downwards perpendicularly.
Berus Mata Buaya (Bruguiera hainesii) - Like other Bruguiera species, the bark is covered with small lenticels, and the tree has knee roots.
29. Calyx ranges from pinkish to bright red. Petals have bristles at the tip.
Tumu Merah (Bruguiera gymnorhiza) - Like other Bruguiera species, the bark is covered with small lenticels, and the tree has knee roots.
30. Calyx varies from bright yellow to light green to pinkish. Petals have no bristles or at most 1 or 2 short bristles.
Tumu Putih (Bruguiera sexangula) - Like other Bruguiera species, the bark is covered with small lenticels, and the tree has knee roots.
As mentioned, only the 30 true mangrove species are covered in this key. Hopefully I can find time to put up information on the mangrove associates in future :)
Hope this will be useful for other amateur mangrove lovers like me! :P