Rhizophora mucronata var. alokii – a new variety of mangrove species from the Andaman and Nicobar Islands, India (Rhizophoraceae)

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.

Th e 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 km 2 in India, of which 604 km 2 occurs in the ANI. A total of 38 mangrove species has been recorded from the ANI. Th ese include fi ve 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, infl orescences and fl owers (Table 1) were examined for each taxon. Th e 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 diff ered signifi cantly between the two taxa. A key for the Rhizophora species of Andaman and Nicobar Islands has also been provided to facilitate identifi cation.

Results
Th e 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 fl owers and stamen number are signifi cantly diff erent (p < 0.05) between the two taxa (Table 1).
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 Defi cient" (DD), using the criteria of IUCN (2001).

Discussion
Rhizophora species are very similar and can be diffi cult to distinguish (Lo 2003). Th e key distinguishing characters of Rhizophora spp. in the ANI are given in Table 2. Th e identifi cation of R. apiculata is not problematic because it diff ers from the other species within its range in many characters, including having apiculate leaves with spinose mucronate tips, bi-fl owered infl orescences 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). Th e 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).
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 fl owered infl orescences 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 diff erentiate 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. Th e two species also diff er 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,