Overview of the genus Briareum (Cnidaria, Octocorallia, Briareidae) in the Indo-Pacific, with the description of a new species

Abstract The status of Indo-Pacific Briareum species (Cnidaria, Octocorallia, Briareidae) is reviewed by presenting their sclerite features and habitus descriptions. Following the re-examination of type material, museum specimens and newly collected specimens, a species identification key is provided. The species distributions are discussed and updated distribution ranges are depicted. Moreover, a new taxon, Briareum cylindrum sp. n. is described and depicted, whereas Briareum excavatum (Nutting, 1911) is synonymised with Briareum stechei (Kükenthal, 1908). Briareum hamrum (Gohar, 1948) is recorded from the Persian Gulf and Oman Sea for the first time. Consequently, in total four Briareum species are recognized in the Indo-Pacific; Briareum hamrum from the western Indian Ocean, and Briareum cylindrum sp. n., Briareum stechei, and Briareum violaceum from the central and eastern Indo-Pacific region.


Introduction
Briareum Blainville, 1830 is the only genus in the family Briareidae with a wide distribution, occurring in both the Atlantic and the Indo-West Pacific (Fabricius and Alderslade 2001). It is zooxanthellate and therefore restricted to shallow, well-illuminated waters. It Here, the sclerite features and descriptions of Briareum species are presented based on the re-examination of type specimens, museum material, and newly collected material from the Indian Ocean and Indo-Pacific region, much of which is from the centre of maximum marine species richness, the Coral Triangle (Hoeksema 2007). An identification key to the presently recognized Indo-Pacific species is provided, a new taxon is described and two species are synonymised. Moreover, we show the variability of the sclerites among examined material and point out the difficulties, uncertainties and potential topics for further research. A distribution map of the examined material is also provided, together with all published species for the Indian Ocean and Indo-Pacific region (Figure 1). This study can be used in molecular and biochemical studies and may help coral researchers to identify Briareum material.

NBC
Naturalis Biodiversity Center, Leiden, The Netherlands; previously National Museum of Natural History (NNM); formerly Rijksmuseum van Natuurlijke Historie (RMNH) OCDN/OPHG Numbers used by the Coral Reef Research Foundation, Palau RMNH Rijksmuseum van Natuurlijke Historie, currently NBC

UNESCO-IOC United Nations Educational, Scientific and Cultural Organization-Intergovernmental Oceanographic Commission UNHAS
Universitas Hasanuddin, Makassar, Indonesia ZMA Zoological Museum Amsterdam, Amsterdam, The Netherlands ZMB Zoologisches Museum Berlin, Berlin, Germany

Material and methods
All studied material is deposited in the Naturalis Biodiversity Center. All Briareum specimens deposited in the RMNH coelenterate collection were examined, including misidentified material. Additional specimens collected by the Coral Reef Research Foundation, Palau, were also examined. In order to identify the material, sclerites were obtained by dissolving the tissues in 10% sodium hypochlorite, followed by rinsing in fresh water. Due to variation in size and shape of the sclerites, it is recommended to use all parts of the colony. For example, missing calyces might result in finding shorter sclerites. For scanning electron microscopy (SEM), the sclerites were carefully rinsed with double-distilled water, dried at room temperature, were 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.  Roule, 1908: 165 Solenopodium Kükenthal, 1916a Diagnosis. Colonies lobate, digitate or encrusting, normally with a whitish outer layer and magenta inner layer, but completely magenta or white colonies also occur. Polyps monomorphic, retractile, and without sclerites. Protruding false calyces appear in varying degrees of prominence or are not present at all. Surface layer with straight or curved spindles. Medulla with sclerites shaped like those of the surface layer but larger and coarser, and with additional branching sclerites, which can be fused. Zooxanthellate.      Description. The holotype consists of several fragments of an encrusting colony, the largest being 4 by 1.5 cm in diameter ( Figure 2A) with white surface and magenta underside. Calyces hardly projecting.

Morphological descriptions and systematic account
The calyces contain colourless, flattened rods with prominent simple tubercles ( Figure 3A, B). These rods are up to 0.20 mm long. The cortex contains colourless spindles, cylinders, and tripoids ( Figure 3C). All these forms have complex tubercles, often arranged in girdles. These sclerites can be up to 0.60 mm long but most are only 0.30 mm long. The medulla contains magenta spindles and branched spindles with simple or complex tubercles ( Figure 4). These sclerites are 0.20-0.60 mm long. They can be fused into small clumps.
Remarks. Briareum cylindrum mostly resembles B. stechei but differs in having many cylinders with complex tubercles in the coenenchyme.
Remarks. Alderslade (2000) referred Clavularia hamra Gohar, 1948 to Briareum, consequently the species name had to be changed to hamrum. Gohar (1948: 10) compared his Clavularia hamra with both Sympodium punctatum May, 1898 and S. splendens Thomson & Henderson, 1906, and noticed their close resemblance. According to Gohar (1848), S. punctatum differs in having sclerites up to 0.266 mm long while they are up to 0.35 mm long in C. hamra. Sympodium splendens differs in having two rows of pinnules on either side of the tentacles, each row consisting of 20-24 pinnules, while in C. hamra there is only one row of 16-22 pinnules, which are much longer. However, an odd second row of 1-3 pinnules can be present in C. hamra. Furthermore, C. hamra has no triradiate or tetraradiate sclerites, described for S. splendens. Next to the radiates Thomson and Henderson (1906) described the sclerites to be straight and curved spindles, up to 0.4 mm long. From our material and findings of Prof. Y. Benayahu (see Alderslade 2000: 246) it seems only one Briareum species is present in the Red Sea and the western Indian Ocean. Consequently, the correct name should be the oldest available, Briareum punctatum May, 1898, but the type material of B. punctatum is missing. As we had no material from its type locality, Zanzibar, we could not designate a proper neotype yet. As the species was never again found in Zanzibar we still have some doubts about its identity and thus defer to B. hamrum for the moment. Notably, also the type material of Sympodium splendens, Alcyonium (Erythropodium) contortum and Briareum hamrum seems to be missing. This is the first record of a Briareum species from the Persian Gulf, and Oman Sea (see van Ofwegen 2009, 2012).
Morphological variation. RMNH Coel. 41407 ( Figure 2F) from the Persian Gulf differs from the above described Red Sea specimen. It has longer sclerites (up to 0.40 mm long; Figure 9B) and more slender interior branched bodies ( Figure 9C). RMNH Coel. 41410 ( Figure 11A) from Oman has even longer sclerites than the Persian Gulf specimen (up to 0.45 mm long; Figure 10); it is the only specimen having long calyces. RMNH Coel. 41409 ( Figure 11B), also from Oman, has sclerites ( Figure  9D-F) with the same size as the Red Sea specimen, but the slender interior branched bodies as the Persian Gulf and other Oman specimen. RMNH Coel. 41412 has completely colourless sclerites, however, the colour of live specimens was similar to others.
The shape of the colonies in the examined material showed variation, from completely encrusting to somewhat having branches and an undulated surface.
Colour. The living colonies were cream with magenta tints in some parts of the colony. Polyps were dark green to brown, brown pinnules, white oral disk and white line that continues along the tentacles ( Figure 26A-B) Distribution. Red Sea, East Africa, Oman Sea, Arabian Sea, Persian Gulf.

Figure 18. Briareum stechei
Remarks. The sclerites are most like those of B. violaceum but in that species many spindles are longer than the longest of B. stechei. Nutting (1911) apparently was not aware of Kükenthal's earlier (1908) description of Erythropodium stechei; actually, at first he did not compare his new species with any previously described one. Later, Kükenthal (1919) noticed the resemblance with his E. stechei, now in the genus Solenopodium Kükenthal, 1916a, and put it in the synonymy of that species with a question mark as he did not re-examine Nutting's material. Stiasny (1937) re-examined type material of both S. stechei and S. excavatum and kept them as separate species. According to him, S. excavatum differs in having higher calyces ( Figure  11G), and by lacking calyx sclerites and "dendritic" sclerites in the interior. Verseveldt (1940: 37) was the last to compare these two species and noted no less than six aspects of difference between them, however, he did not re-examine the type material of B. stechei. We present sclerites images of the types of the two species (Figures 12-13). We consider the differences mentioned by previous authors as intraspecific variation, similar to that as observed in B. hamrum, and therefore we synonymize B. excavatum with B. stechei. Kükenthal (1908) described Erythropodium stechei from Banda only. In the Berlin Museum, ZMB 5828 (Fig 11C), material from Banda, and ZMB 5816 ( Figure 11D), material from Ambon, are present, both labelled type. It looks like these specimens represent the same material. It is puzzling to us why Ambon is now mentioned as the locality of ZMB 5816.
Solenopodium stechei var. novaepommeraniae is also represented by two collection numbers in Berlin, ZMB 5016 ( Figure 11E) and ZMB 5854 ( Figure 11F), here obviously the original material was split into two.
Morphological variation. To show the enormous variation in sclerites we have made SEM images of two specimens from Palau (RMNH Coel. 40023) collected at the same locality. One of them shows almost smooth spindles (Figs 14-15) while the other, like the type, has none at all (Figs 16-17). RMNH Coel. 41421 has peculiar bent and smooth sclerites ( Figure 18).

Discussion
All Briareum specimens deposited at the RMNH coelenterate collection were examined, from more than 50 localities around the world. The status of the Indo-Pacific Briareum species is reviewed and additional information provided. Moreover, a new taxon, B. cylindrum is described, and B. excavatum (Nutting, 1911) synonymised with B. stechei (Kükenthal, 1908). In total four Briareum species are recognized in the Indo-Pacific region; one recorded from the western Indo-Pacific, and the rest from the central and eastern Indo-Pacific.
The development in molecular and chemical studies, which reliably discriminate species, has been a challenge in cnidarians. Mitochondrial genes evolve slower than nuclear genes in anthozoans (Chen et al. 2009), therefore mitochondrial markers are invariant within and among genera (Shearer and Cofforth 2008). In octocorals, an extended mitochondrial barcode of COI plus the octocoral-specific mitochondrial gene mtMutS is usually diagnostic at the genus level and narrows species down to a small number of candidate sister taxa (McFadden et al. 2014). McFadden et al. (2011McFadden et al. ( , 2014 included five Briareum specimens from Palau  and one specimen from the Red Sea (ZMTAU CO34187) in their molecular studies using this marker. They distinguished three different species, two from Palau and one from the Red Sea. All Palau specimens were examined by us and proved to be indeed two species, B. stechei and B. violaceum. The one from the Red Sea identified by Prof. Benayahu represents B. hamrum. Miyazaki and Reimer (2014), who used other DNA markers and examined specimens from southern Japan, found three different morphological types of Briareum which seemed to be similar genetically and the authors suggested further analysis to reveal the phylogenetic relationships of these three types. Probably their material now can be identified with the morphological findings presented here.
This study shows variability in sclerite morphology among the examined material which is in agreement with the previous studies. Considering this fact, we decided not to complicate the situation with introducing more new species than necessary. Instead we grouped the species together based on major differences in sclerite shape and variability. Several examples in our examined specimens have somewhat different sclerite shapes, and they are considered as intraspecific variation.
Based on the examined underwater photographs, the polyp shape and colour pattern in the examined material of B. hamrum were consistent, having distinguishable pinnules with dark green to brown colour, white oral disk and white line along the tentacles ( Figure 26A-B). The pinnules in this species were also noticed by Gohar (1948). In B. stechei, the pinnules were not distinguishable and in B. violaceum they were very small. There was no underwater in situ photograph of B. cylindrum available to us. These characters were not reported before, therefore their importance and consistency is yet to be understood.
Briareum shows a wide distribution range with one Atlantic and four Indo-West Pacific species. Our results showed that B. hamrum occurs only in the western and north-western Indian Ocean (Figure 1). This area consists of several sub-regions including East Africa, Seychelles, central Indian Ocean (Maldives and Chagos Archipelago), northwestern Indian Ocean (Arabian Sea, Oman Sea), Red Sea, and the Persian Gulf. The recent larval dispersal modelling suggests that the Red Sea and the Persian Gulf have the highest isolation in larval sources (Wood et al. 2014). This perhaps could explain the high number of endemic species described from these areas (Sheppard and Sheppard 1991;Sheppard et al. 2010;Samimi-Namin and van Ofwegen 2009), and suggests that the majority of the coral population maintained by high levels of selfseeding. B. hamrum clearly can tolerate high environmental fluctuations that exist in the Persian Gulf (Sheppard et al. 2010), and the Red Sea. Briareum species have not yet been recorded from the central Indian Ocean, Chagos Archipelago (Reinicke and van Ofwegen 1999), Maldives (Vennam and van Ofwegen 1996), and south west India (Herdman 1905;Thomson and Simpson 1909); however, it is expected to be found in these areas. The rest of the Briareum species have overlapping distribution in the central Indo-Pacific, which is expected due to its high levels of larval connectivity (Wood et al. 2014). More sampling efforts and examination of more material is necessary to clarify the distribution boundaries.
At present there are still uncertainties about the total number of Briareum species and their distribution boundaries, especially in the central Indo-Pacific. Further examination of newly collected material, together with in situ photographs (see e.g. Hoeksema and van Ofwegen 2004) and genetic material will eventually reveal the species characters and their variation along environmental gradients.