Description of a zooxanthellate scleractinian coral ( Blastomussa angulares sp. nov.) from Lakshadweep, India

One species zooxanthellate scleractinian coral i.e. Blastomussa angulares sp. nov. is described from Lakshadweep atoll for the first time based on the morphometric characteristics. The significant characters which differentiate this presently described species from previously known species are shape of angular corallites, length of the corallite diameter, number of septa and their cycles, highly exsert septa, structure of columella and septa like extra projection from the basal plate. Blastomussa angulares sp. nov. represents close structural affinities with B. loyae Head, 1978 which is briefly narrated here to establish the presently designated species.


Introduction
The genus Blastomussa was previously categorized under the family Mussidae Ortmann, 1890 while the recent classification and systematic arrangement emphases a considerable amount of doubt for this previous rank of hierarchy based on the molecular as well as microstructural studies along with revisionary works and presently tiered this genus as accepted one while the distal genetically derived relationship with the species under the family Plerogyridae Rowlett, 2020 (the genus was also categorized under Family Lobophylliidae Dai & Horng, 2009) which were previously known as Mussidae corals from Indo-Pacific realm [1][2][3][4][5][6][7] . The genus Blastomussa represents five valid and extant species viz. B. merleti (Wells, 1961), B. wellsi Wijsman-Best, 1973, B. loyae Head, 1978, B. omanensis (Sheppard & Sheppard, 1991)

Materials and Methods
Inclusive surveys were carried out to investigate the scleractinian species of Lakshadweep from October 2019 to February 2020 within the depth limit of 40 m by employing SCUBA diving and the specimen was sampled for detailed taxonomic studies by the handpicking method. In-situ photographic digitization was made with the help of a Canon 1x Mark II camera along with housing. The sample was kept in fresh water at room temperature for one week for the dilution of the algal parts then carefully cleaned to remove all the algal parts by running tap water and dried under sunlight. Ex-situ studies were carried out under a Leica stereo zoom microscope (M 205 A) to compare the morphological characteristics while digitization was made by an attached DFC 500 camera. Morphological characteristics of all the five previously described species under the genus Blastomussa were compared in conjunction with the available literature of Wells 11 , Wijsman-Best 12 , Head 13 , Sheppard and Sheppard 14 , Veron 15 , and Benzoni et al. 5 . On completion of the morphometric analysis, the specimen was deposited as National Zoological Collections in Zoological Survey of India, Port Blair.

Results
One new species of scleractinian coral under the Blastomussa is described here. The details of the species are cited below.

Diagnostic description:
The colony is encrusting (Figure 2a). Corallites are with strong angular structures and pentagonal while the developmental pattern is sub-ceriod to plocoid (Figs. 2b&c). The largest diameter of the corallites ranges between 6-10 mm. Mostly two order of septa is visible and while 3rd order may be seen sometimes (S1≥S2>S3) (Figure 2c). Second-order septa are not showing structural symmetry and even can be seen as incomplete (Figure 2c). The first-order septa (S1=6) are mostly equal in length and usually reach the columella whereas second-order septa (S2≤6) are 2/3 in thickness in comparison with first-order septa and extensively irregular to reach columella (Figs. 2b&c). The third order septa (S3~2) very thin, narrow, and incomplete if present and does not reach columella (Figure 2c). The distal arrangement of the septa creates a deep hollow or gap in between inter septa space (Figs. 2b&c). Septa are highly exserted (Figure 2d). The height of exsert septal region is ranged between 2.4 to 2.9 mm for the first order septa and 0.6 to 1.8 mm for secondary septa from the colony surface (Figure 2e). The exsert septal availability can be seen as a spiny appearance for the colony while the polar view of the exsert septa is comparatively thick and the side view is fan alike with a smooth margin (Figure 2e). Small triangular-shaped granular spines are arranged in radiating manner on the septal side which is prominent on the exsert septal region (Figure 2e). One separate septum like structure is projecting from the basal disc of some of the corallites (Figs. 2b&c). Columella is solid, narrow and pointed (Figures  2a-c).

In-situ observation:
The color of the live colony is violet to blue color while the septal margins are green and columella is brown to deep green. Mantle of the corallites are giving bold leathery appearance to the corallite (Figures 3a&b).

Etymology:
The species name is angulares (Latin angulares = angular) after the corallites shape. The primarily defining structure of the corallites of presently described species is strongly angular and pentagonal which clearly make differentiation with all the other five valid and extant species.

Closely related species:
This species is closely related to Blastomussa loyae Head, 1978. But the size of the corallite shape and diameter, septal number and structure as well as the columella is representing differentiation with B. loyae (Table 1).

Zoogeographical range:
This species is only reported from Bangaram Island of Lakshadweep, India.

Discussion
Among the described five species under the genus Blastomussa, two categorizations can be drawn based on the corallite diameter such as corals within the diameter 8mm (B. loyae, B. merleti and B. omanensis) while the others with the diameter of more than 9mm (B. wellsi and B. vivida) 5 while the presently described species share the corallite diameter within the range of 6 to 10 mm whereas the presence of smooth septal characterization considers this new species i.e. Blastomussa angulares among the second cluster. This currently described species characterize some closely related morphometry with B. loyae while some features showed remarkable differences to establish this one as new species. In B. angulares sp. nov. the corallites are angular with a diameter of 6 to 10mm while circular to oval corallites with 4 to 8mm of diameter are seen among B. loyae. The only S1 of B. angulares sp. nov. is with complete structure and cycle among the three orders while B. loyae showed only S3 as incomplete. The septa are extensively exsert with the height of 2.4 to 2.9 mm for S1 and 0.6 to 1.8 mm for S2 in B. angulares whereas the mean height of the exsert septa of B. loyae is near about 1-2 mm. The descriptions, studies, and comparative analysis made by Wells 11&16 , Wijsman-Best 12 , Head 13  while molecular studies can be considered as the defining factor for the validation of this species in near future. The description of a new species from Lakshadweep atoll intensely suggests favorable biogenic habitat for the survival and development of scleractinian corals which is required to be investigated in extensive ways for improvement of proper conservatory measures through the sustainable means.  Figure 1: Locality of the Holotype specimen-Blastomussa angulares sp. nov.  In-situ images of the live colony with violet to blue colour corallites, green septal margin, brown to deep green columella; leathery appearance of corallites