A new species, Melithaea davidi sp. n., is described from the eastern coast of Oman, Oman Sea, in the northwestern Indian Ocean, where it differs from its congeners in lacking capstans and having predominantly spindles in the coenenchyme. A molecular phylogenetic analysis of mtMutS and 28S rDNA genes suggests that it is genetically distinct from similar species in the Red Sea. Furthermore, a species previously reported as Acabaria sp. from the Arabian Sea is now identified as Melithaea mabahissi (Hickson, 1940).

Persian Gulf, octocorals, Indian Ocean, Middle East, northwest Indian Ocean

Melithaeidae, one of the 49 presently recognized families of octocorals (Ofwegen and McFadden 2010; McFadden and Ofwegen 2012, 2013; Breedy et al. 2012), has only two valid genera (Reijnen et al. 2014) and numerous species distributed worldwide. They are sessile, benthic, and colonial organisms that require a hard substrate for settlement and anchorage. The melithaeid species are widespread in marine environments, from shallow to deep sea, but are most abundant in warm and tropical waters (Fabricius and Alderslade 2001). Their geographical distribution ranges from East Africa and the Red Sea (Grasshoff 2000), Indian Ocean (Thomson 1916; Ofwegen 1987, 1989; Williams 1992), Indo-West Pacific (Ofwegen 1987; Grasshoff 1999; Ofwegen et al. 2000; Hoeksema and van Ofwegen 2004, 2008; Matsumoto and Ofwegen 2015) to Hawaii (Bayer 1956). In spite of this vast distribution range, only a few species of melithaeids have been reported from the northwestern Indian Ocean (the coasts of the Arabian Sea, the Red Sea, the Gulf of Aden, the Oman Sea, and the Persian Gulf). These include Clathraria omanensis off Oman in the Oman Sea, Acabaria spec. indet. 2 from West India in the Arabian Sea (Ofwegen 1987), Acabaria spec. indet. 1 from Kenya and Somalia, and Acabaria mabahissi from Somalia (Hickson 1940). Although no melithaeid species have so far been recorded and sampled from the shallow coastal waters of the Persian Gulf (Samimi-Namin and van Ofwegen 2009, 2012), several colonies have been distinguished in video footage and photographs taken from mesophotic depths (> 40 m) of the Strait of Hormuz (pers. obs.).

Melithaeidae is an uncommon family in the north-western Indian Ocean, and its maximum depth extends beyond that of conventional diving activities. This might explain the scarce records and rarity of these species within this region where they have not been documented in major coral studies (see Sheppard and Salm 1988; Sheppard and Sheppard 1999; Sheppard et al. 2000; Claereboudt 2006; Samimi-Namin and van Ofwegen 2016a, b). In contrast, this family is relatively common in the shallow warm waters of the central Indo-Pacific and the Coral Triangle (Fabricius and Alderslade 2001; Hoeksema and van Ofwegen 2004, 2008).

It is known that the Arabian Sea and Oman Sea have a complex hydrography, mainly caused by seasonal monsoons. The summer southwest monsoon generates one of the five largest upwelling areas of the world (Bakun et al. 1998), whereas the winter northeast monsoon reverses the circulation pattern and increases the biological production of the whole northern Indian Ocean (Burkill 1999, Wilson 2000). In spite of these hydrographical and geological complexities which might induce high endemicity and diversity, there are still many species and habitats yet to be discovered and documented from shallow and deep waters of the north-western Indian Ocean. Traditionally six genera have been recognized in Melithaeidae based on the sclerite morphology of their coenenchyme: Acabaria Gray, 1859 and Asperaxis Alderslade, 2006, both with a variety of spindles and occasionally a few thorn-clubs; Clathraria Gray, 1859, with a complete layer of capstans, and double heads; Melithaea Linnaeus, 1758, with predominantly asymmetrical double-discs, a few leaf-clubs and thorn-clubs; Mopsella Gray, 1857, with leaf-clubs and thorn-clubs at the surface of the coenenchyme; and Wrightella Gray, 1870, with foliate spheroids (Hickson 1937; Ofwegen 1987, Grasshoff 2000). In the literature, numerous species have been described as being intermediate between these genera, with considerable variation and overlap in their sclerite morphology. Several researchers have pointed this out and have suggested a revised classification system for this family (Grasshoff 1999, 2000; Fabricius and Alderslade 2001; Reijnen et al. 2014). Although the latest study by Reijnen et al. (2014) considers all the genera in the classification proposed by Hickson (1937) as variations across geographical regions and suggests the merger of all these genera into one, i.e. Melithaea, the taxonomic situation of this group of octocorals has not been resolved satisfactorily and the family has to be formally revised. Based on the previous taxonomic system, an undescribed species discovered in the Oman Sea falls into a group with the coenenchyme predominated by spindles and occasionally thorn-clubs (Acabaria/Asperaxis). However, to avoid further confusion we follow Reijnen et al. (2014) regarding generic classification, considering two valid genera in the Melithaeidae: Melithaea, and Asperaxis, with the latter genus only reported from Australia.

In this paper, we describe a new species of the genus Melithaea from approximately 80 m depth, off the coast of Oman.

In situ observations and material collection was conducted in 2013, during a deep water dive in the Oman Sea (Fig. 1). In situ photographs were taken using a small compact underwater camera and the depth recorded using a dive computer. In total three colonies were collected and were preserved in ethanol. In order to identify the material, sclerites were obtained by dissolving the tissues in 10% sodium hypochlorite, followed by rinsing in fresh water. For scanning electron microscopy (SEM), the sclerites were carefully rinsed with double-distilled water, dried at room temperature, mounted on a stub with double-sided carbon tape, then coated with gold-palladium (AuPd), and examined using a Jeol 6480LV SEM operated at 10 kV.

Figure 1. 

Type locality of Melithaea davidi sp. n. in the Oman Sea, off Muscat governorate. RS = Red Sea, PG = Persian Gulf, GA = Gulf of Aden, OS = Oman Sea, AS = Arabian Sea.

All specimens are deposited at Naturalis Biodiversity Center, Leiden, the Netherlands (formerly Rijksmuseum van Natuurlijke Historie, Leiden, the Netherlands).

Molecular and phylogenetic results

Maximum likelihood and Bayesian analyses of mtMutS plus 28S rDNA recovered identical tree topologies (Fig. 2) and strong to moderate support for the same geographically structured clades found by Reijnen et al. (2014) in their analysis of a four-gene dataset. Specimen RMNH Coel. 42122 fell within a well-supported clade of species from the Red Sea including M. erythraea (Ehrenberg, 1834), M. sinaica (Grasshoff, 2000), and M. rubrinodis (Gray, 1859). It was well differentiated genetically from all three Red Sea species, differing from them by average genetic distance values (Kimura 2-parameter) ranging from 0.6–2.5% at mtMutS and 2.8–3.45% at 28S, values that are greater than those typically observed among conspecific octocorals (McFadden et al. 2014).

Figure 2. 

Maximum likelihood phylogeny of family Melithaeidae based on concatenated nucleotide sequences for mtMutS (486 nt) and 28S rDNA (670 nt) (sequence data for all but RMNH Coel. 42122 are from Reijnen et al. 2014). Well-supported clades of species from the Indo-Pacific, east and south Africa, and outgroup taxa (Annella, Chironephthya, Euplexaura, Siphonogorgia, Solenocaulon) have been collapsed to improve readability. Numbers above branches are maximum likelihood bootstrap percentages; numbers below branches are Bayesian posterior probabilities. Generic assignments follow the recommendations of Reijnen et al. (2014).

Reijnen et al. (2014) observed that melithaeid species appear to be grouped phylogenetically by geographical region, suggesting high regional endemicity in this family. Our re-analysis of their mtMutS and 28S sequence data reflects this pattern, with species from the western Indo-Pacific (Indonesia, Malaysia, Japan, Palau, etc.), east and south African coasts (Tanzania, South Africa), northern and western Indian Oceans (Seychelles, Maldives), and the Red Sea separated into distinct well-supported clades (Figure 2). Therefore, the likelihood of species having wide geographical ranges is low, and consequently we did not compare the new species with similar-looking species occurring in other geographical regions. The molecular phylogenetic analysis suggests that M. davidi is closely related to but distinct from several other species found in the Red Sea region for which we had sequence data for comparison. Although we did not have sequence data for Melithaea biserialis or Acabaria spec. indet. 2 reported by Ofwegen (1987), morphological differences support the distinction of those species from M. davidi.

We would like to thank Koos van Egmond (NBC) for curatorial assistance. We are grateful to the Ministry of Environment and Climate Affairs, Oman, for support. D. Mothershaw and R. Norman are thanked for collecting the material and donating it to the first author. The research at NBC and partial fieldwork was supported by grants from the Schure-Beijerinck-Poppingfonds (KNAW), Alida Buitendijkfonds, Jan Joost ter Pelkwijkfonds, and Martin-Fellowship programme (NBC) to the first author; Dr. B.W. Hoeksema is appreciated for his advice and support herein. The Alfred P. Sloan Foundation and the Census of Marine Life are gratefully acknowledged for the research grant provided to the first author; in this regard, Dr. M.R. Claereboudt (Sultan Qaboos University, Oman), Dr. N. D’Adamo (UNESCO, IOC, Perth), Dr. J.H. Ausubel (Rockefeller University) are greatly appreciated for their continued support and encouragement. Two anonymous reviewers are appreciated for their constructive comments and suggestions, which helped improve the manuscript.