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Research Article
Rhizophora mucronata var. alokii – a new variety of mangrove species from the Andaman and Nicobar Islands, India (Rhizophoraceae)
expand article infoP. Ragavan, P.M. Mohan, R.S.C. Jayaraj§, K. Ravichandran|, S. Saravanan
‡ Pondicherry University, Port Blair, India
§ Department of Environment and Forest, Arunachal Pradesh, Itanagar, India
| Department of Environment and forest, ANI, Port Blair, India
¶ Institute of Forest Genetics and Tree Breeding, Coimbatore, India
Open Access

Abstract

Rhizophora mucronata var. alokii (Rhizophoraceae), a new variety of Rhizophora from the Andaman and Nicobar Islands, India, is described and illustrated. The new variety is remarkable in having four stamens, laterally folded leaves, a short peduncle, thick leathery petals, and a four-sided ovary with a sessile style. A key for the species of Rhizophora of the Andaman and Nicobar Islands is also provided.

Keywords

Rhizophoraceae , Rhizophora mucronata var. alokii , new variety, Andaman and Nicobar Islands, India

Introduction

The genus Rhizophora is the most common mangrove genus worldwide. Two species (R. mangle L. and R. racemosa G. Mey) and one natural hybrid (R. × harrisonii Leechm.) are restricted to the Atlantic-East Pacific Region, three species (R. apiculata Blume, R. mucronata Lam., and R. stylosa Griff.) and four named natural hybrids (R. × annamalayana Kathiresan, R. × lamarckii Montrouz, R. × selala (Salvoza) Toml., and R. × tomlinsonii Duke) are restricted to the Indo-West Pacific (IWP) region, and one species (R. samoensis (Hochr.) Salvosal) extends into both regions (Duke and Bunt 1979, Duke 1992, Duke et al. 1998, Duke 2002, Duke 2010). In addition, Ng et al. (2013) recognized an unnamed hybrid between R. mucronata and R. stylosa through molecular studies. All the IWP taxa except R. samoensis and R. × selala are known from India (Ragavan et al. 2011).

The mangroves of the Andaman and Nicobar Islands (ANI) are denser and more diverse compared to other mangrove habitats in India (Mandal and Naskar 2008). According to the latest estimate by the Forest Survey of India (FSI 2013), the total mangrove area is approximately 4,628 km2 in India, of which 604 km2 occurs in the ANI. A total of 38 mangrove species has been recorded from the ANI. These include five Rhizophora taxa (R. apiculata, R. mucronata, R. stylosa, R. × lamarckii and R. × annamalayana; Ragavan et al. 2011).

During a recent botanical excursion, an interesting population of Rhizophora was encountered in the mangrove forest of Austin Creek, North Andaman. Specimens were collected and did not match any of the known species of the genus and hence have been described and illustrated here as new.

Materials and methods

To better assess the taxonomic placement and distinguishing characteristics of the new taxon, a morphometric analysis of the Rhizophora taxa present in ANI was performed. Seventeen attributes of leaves, inflorescences and flowers (Table 1) were examined for each taxon. The dataset was used for cluster analysis with Primer-e software (Version 6). Results of the cluster analysis were then used to select the taxon morphologically most similar to the new entity. T-tests were used to determine which attributes differed significantly between the two taxa. A key for the Rhizophora species of Andaman and Nicobar Islands has also been provided to facilitate identification.

Table 1.

Characters used for classification analysis of Rhizophora species of the ANI; average value (range) in cm for each taxon. Where no range is included, the values for the taxon showed no variation. Values that differ significantly (p < 0.05) between the varieties of R. mucronata are marked with an asterisk.

Characters R. apiculata R. mucronata var. mucronata R. mucronata var. alokii R. stylosa R. × annamalayana R. × lamarckii
Leaf length 13
(8.5–16.2)
13.55*
(8.5–18)
11.29*
(7–13)
11.1
(8–13)
12.39
(10–16)
13.08
(8–16)
Leaf width 5.9
(4–8.5)
8.47
(5.7–11.3)
6.71
(4–8.5)
5.68
(4–6.3)
7.4
(6–12)
6.45
(4.5–8.5)
Leaf length width ratio 2
(1.7–3.12)
1.6*
(1.4–1.8)
1.69*
(1.43–1.79)
2.02
(1.8–2.8)
1.67
1.4–1.7
2
(1.79–2.2)
Leaf mucro length 0.4
(0.4–0.5)
0.5
(0.4–0.6)
0.45
(0.4–0.5)
0.5
(0.4–0.6)
0.34
(0.3–0.5)
0.45
(0.4–0.5)
Petiole length 1.8
(1.4–2.5)
2.61*
(1.5–3)
2.22*
(1.5–3)
3.35
(2–3.5)
2.17
(1.8–2.5)
2.39
(1–3)
Petiole width 0.2
(0.2–0.3)
0.31
(0.3–0.5)
0.4
(0.3–0.5)
0.23
(0.3–0.4)
0.3
(0.3–0.4)
0.3
(0.3–0.4)
Number of flowers per inflorescences 2 5*
(2–8)
4*
(2–6)
5
(2–8)
2
(2–4)
2
(2–4)
Bud length 1.2
(1–1.6)
1.47
(1.2–1.6)
1.48
(1.4–1.6)
1
(0.7–1.2)
1.5
(1.4–1.6)
1.65
(1.5–1.7)
Bud width 1
(0.9–1)
0.8
(0.8–1)
0.79
(0.7–0.9)
0.43
(0.3–0.6)
1
(0.8–1.1)
0.8
(0.7–0.8)
Bud length width ratio 1.2
(0.9–14)
1.81
1.69–2.23
1.87
(1.74–2.28)
2.39
(1.81–2.51)
1.68
(1.2–1.81)
2.06
(1.79–2.32)
Peduncle length 1
(0.8–1)
3.15*
(1.5–6)
2.72*
(2–3.5)
3.9
(2.5–5.5)
1.3
(1.2–1.5)
1.85
(1–2.5)
Peduncle width 0.5
(0.4–0.6)
0.3
(0.3–0.5)
0.4
(0.3–0.5)
0.2
(0.2–0.3)
0.5
(0.4–0.5)
0.4
(0.3–0.4)
Petal length 0.8
(0.7–1)
0.8
(0.8–1)
1
(0.9–1.1)
0.8
(0.7–0.9)
1.2
(1–1.2)
1
(0.9–1.1)
Petal width 0.2
(0.2–0.3
0.3
(0.3–0.4)
0.4
(0.3–0.4)
0.3
(0.2–0.4)
0.4
(0.3–0.4)
0.3
(0.2–0.3)
Stamen number 12
(9–14)
8* 4* 8 12
(8–16)
12
(8 – 16)
Stamen length 0.8
(0.8–1.1)
0.7
(0.7–0.9)
0.7
(0.5–0.7)
0.5
(0.4–0.6)
0.8
(0.4–1)
0.6
(0.4–0.8)
Style length 0.1
(0.06–0.12)
0.1
(0.08–0.12)
0.1
(0.08–0.12)
0.4
(0.3–0.5)
0.12
(0.08–0.15)
0.3
(0.28–0.41)

Results

The morphometric analysis shows that R. mucronata var. alokii has closest similarity with R. mucronata than to other Rhizophora taxa (Fig. 1). However, attributes such as leaf length, length-width ratio, petiole length, peduncle length, number of flowers and stamen number are significantly different (p < 0.05) between the two taxa (Table 1).

Figure 1. 

Cluster dendrogram (group average) showing similarity among the Rhizophora species of the ANI.

Taxonomic treatment

Rhizophora mucronata var. alokii P.Ragavan, var. nov.

Material

India. North Andaman: Austin Creek, mangrove forest (Fig. 2A), 12°52'36.9"N, 92°50'40.2"E, 3 April 2014, leg. P. Ragavan, PBL 31001 and 31002 (holotype: PBL).

Figure 2. 

Rhizophora mucronata var. alokii (A) habit (B) stem base with stilt roots (C) bark (D) branches (E) leafy branch end with flowers (F)leaf apex with mucro (G) inflorescence (H) minute bract at dichotomous inflorescence branch (I) mature bud with minute bracteole below calyx (J) cross section of bud (K) mature propagules (L) thick leathery petal (M) stamens (N) flower (O) pistil showing four-sided ovary (P) flower with one petal removed (Q) pear-shaped fruit (R) stamens with pollen.

Tree: columnar to spreading, height to 20 m, evergreen (Fig. 2A). Bark: dark brown, friable, fissured horizontally (Fig. 2C). Roots: both stilt roots and aerial roots growing from lower branches, stilt roots are highly conspicuous arching above ground to 2 m (Fig. 2B). Leaves: simple, opposite, green to dark green, elliptical to broadly elliptical (Fig. 2D, E), laterally folded, underside with numerous dark spots, 7–13 × 4–8.5 cm, length to width ratio averaging 1.69 (not greater than 1.8), apex obtuse with pointed mucro, 0.4–0.5 cm long (Fig. 2F), base cuneate, margin entire; petiole green, 1.5–3 × 0.3–0.5 cm. Inflorescences: axillary, 2–6 flowered (Fig. 2G); bract and bracteoles minute (Fig. 2H); peduncle 2–3.5 × 0.3–0.5 cm; pedicel stout; Mature flower: ellipsoidal, creamy white (Fig. 2I), 1.4–1.6 × 0.7–0.9 cm, length to width ratio ca. 1.87, cross section slightly four-sided (Fig. 2J); calyx lobes 4, thicker than R. mucronata, yellowish white, apex acute; petals 4, thick, leathery, folded laterally, creamy white, velvety and hairy on the margin (Fig. 2L, N), 0.9–1.1 × 0.3–0.4 cm; stamens 4, 0.5–0.7cm long (Fig. 2M, N); style bilobed (Fig. 2O), 0.8–0.12 cm long, seated on four sided domed ovary (Fig. 2P). Mature fruits: pear-shaped, brown, 4–5 × 2.5–3.5 cm, calyx persistent with erect lobes (Fig. 2Q). Mature hypocotyls: 40–60 cm long, green, tip pointed, 1.5–1.7 cm wide at widest point (Fig. 2K); plumule green, 2–3 cm long.

Distribution

Rhizophora mucronata var. alokii is currently known only from Austin Creek, North Andaman, India.

Habitat and ecology

It grows in a mangrove forest along the banks in an intermediate estuarine position in association with Rhizophora apiculata, R. mucronata and Ceriops tagal.

Phenology

Flowering December to March; fruiting April to July.

Etymology

Named in honour of Dr. Alok Saxena (Principal Chief Conservator of Forests) for his inspiration and his outstanding contribution to mangrove conservation in the ANI.

Conservation status

Rhizophora mucronata var. alokii was collected only from Austin Creek (North Andaman Islands). At this site ca. 15 individuals were observed and hence it is assumed to be rare. At present, until further areas can be sampled the species can be accessed as “Data Deficient” (DD), using the criteria of IUCN (2001).

Discussion

Rhizophora species are very similar and can be difficult to distinguish (Lo 2003). The key distinguishing characters of Rhizophora spp. in the ANI are given in Table 2. The identification of R. apiculata is not problematic because it differs from the other species within its range in many characters, including having apiculate leaves with spinose mucronate tips, bi-flowered inflorescences borne on short peduncles below the leaves, short styles and a swollen, corky, brown bract below the calyx. However, dark spots are present on the leaf undersides of R. apiculata from India to southeast Asia and northern Papuasia; they are absent in southern Papuasia and northern Australia (Duke et al. 2002). The number of calyx lobes varies geographically; throughout most of the species range there are four lobes but in Australia there are three to six lobes (Duke et al. 2002).

Table 2.

Diagnostic characters of Rhizophora species of the ANI. The hybrids do not produce seeds so hypocotyl characters are not present in them and therefore not included in the table.

Component Attributes R. apiculata R. mucronata var. mucronata R. mucronata var. alokii R. stylosa R. × annamalayana R. × lamarckii
Leaves Leaf shape narrowly elliptic ovate, broader at base elliptic narrowly obovate broader at apex broadly elliptic narrowly elliptic
Leaf apex acute acute obtuse obtuse acute acute
Leaf base cuneate broadly acute to rounded cuneate cuneate cuneate attenuate to cuneate
Inflorescences Position relative to leaves matures below matures within matures within matures within matures within mature within
Flower number 2 2–8 2–6 2–8 2–4 2–4
Juncture number 1 1 to 3 1 to 3 1 to 3 1 to 2 1 to 2
Bract condition corky smooth, minute smooth, minute smooth, conspicuous smooth, swollen smooth swollen
Mature flower bud(closed) Bud length 1–1.6 cm 1.2–1.6 cm 1.4–1.6 cm 0.7–1.2 cm 1.4–1.6 cm 1.5–1.7 cm
Bud width 0.9–1 cm 0.8–1 cm 0.7–0.9 cm 0.3–0.6 cm 0.8–1.1 cm 0.7–0.8 cm
Shape x-section rounded rounded slightly four- sided rounded four-sided rounded
Bud length /width ratio 1.2 1.81 1.87 2.39 1.68 2.06
Petal x-section flat enclose stamens thick folded enclose stamens curved curved
Petal margin glabrous Hairy velvety hairy hairy slightly hairy slightly hairy
Style length 0.08–0.12 cm 0.08–0.12 cm 0.08–0.1 cm 0.3–0.4 cm 0.08–0.12 cm 0.2–0.4 cm
Stamen number 9 to 14 8 4 8 8–16 in two whorls 8–16 in one whorls
Mature hypocotyls Expanded fruit cork -like pear- like pear- like pear-like
Hypocotyl length 20–40 cm 50–80 cm 40–60 cm 21–35 cm
Distal shape bluntly pointed narrowly pointed narrowly pointed narrowly pointed

Rhizophora hybrids are recognized by intermediate morphology and absence of advanced reproductive stages (Tomlinson 1986). Both R. × lamarckii and R. × annamalayana are distinguished from R. apiculata by their smooth green bract and 2-4 flowered inflorescences within the leaf axils. Rhizophora × annamalayana is distinguished from R. × lamarckii by its broader leaves (length: width ratio <1.8 vs >1.8), and shorter style (<1.5 mm vs. > 1.5 mm) and stamens in two whorls vs. usually in one single whorl.

Distinguishing R. mucronata and R. stylosa is often problematic. Style length is the main feature used to differentiate these taxa; Ragavan et al. (2011) showed that in R. mucronata the style is short and the ovary elongate and tapering, similar to that in R. apiculata, whereas in R. stylosa the style is long and ovary is short, although intermediates are found. The two species also differ in that R. stylosa has prominent, two-lobed bracts and bracteoles, smaller buds, obovate leaves, smaller fruits and shorter propagules.

All previously described Rhizophora species have eight or more stamens, whereas R. mucronata var. alokii has four stamens. Rhizophora mucronata var. alokii closely resembles R. mucronata var. mucronata in its minute bract and bracteoles, bark texture, and bud shape, but can be distinguished not only by stamen number but also by its dense foliage, laterally folded leaves, thick leathery petals with dense hairs, shorter peduncle, and four-sided ovary. It can be difficult to distinguish var. alokii from var. mucronata without the presence of flowers. Differences in flowering time is likely to make this taxon reproductively isolated. A key to the ANI species of Rhizophora is given below.

Key to Rhizophora spp. of ANI

1 Peduncle shorter than petiole 2
Peduncle as long as or longer than petiole 4
2 Mature flower bud and fruits below the leaves; inflorescences two-flowered; bract corky, brown; hypocotyl present R. apiculata
Mature flower buds within the leaves; inflorescences 2-4-flowered; bract smooth and green; hypocotyls not present 3
3 Leaves broadly elliptical; styles 0.8–1.2 mm long; stamens in two whorls, inner shorter; mature flower bud four-sided in cross-section R. × annamalayana
Leaves narrowly elliptical; styles 2–3 mm long; stamens in one whorl; mature flower bud rounded in cross-section R. × lamarckii
4 Stamens 4, petals thick and leathery, densely hairy R. mucronata var. alokii
Stamens 8; petals thin, hairy at margin 5
5 Bract and bracteoles minute; style 1 mm long, seated on elongate, tapering ovary; hypocotyls 50–80 cm long R. mucronata var. mucronata
Bract and bracteoles prominent, forming two-lobed, cup-like structure; style 3-4 mm, seated on short ovary; hypocotyls 20–40 cm long R. stylosa

Acknowledgements

We are extremely grateful to the Principal Chief Conservator of Forests, Andaman and Nicobar Islands for his guidance and ensuring we had the necessary support in the field. We also appreciate the cooperation and support provided by the CCF (Research and Working Plan), Territorial Circle and all the Divisional Forest Officers and their staff in the Department of Environment and Forests, Andaman and Nicobar Administration. Thanks are due to Dr. N. Krishnakumar, IFS, Director, Institute of Forest Genetics and Tree Breeding, Coimbatore, for his support and encouragement. Special thanks are extended to Mr/Mrs/Miss/Ms DFO Mayabunder for constant support.

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