Melithaeidae of Japan (Octocorallia, Alcyonacea) re-examined with descriptions of 11 new species

Abstract Japanese melithaeid type material is re-examined and re-described. The sclerites of the different species are depicted using Scanning Electron Microscopy. All Japanese species of the family Melithaeidae treated here belong to the genus Melithaea and are endemic to Japanese waters. Old museum material and newly collected specimens from Japanese waters are identified after comparison with this type material. Acabaria modesta var. abyssicola is regarded a separate species, here named Melithaea abyssicola (Kükenthal, 1909). In addition, 11 new species are described: Melithaea boninensis sp. n., Melithaea doederleini sp. n., Melithaea isonoi sp. n., Melithaea keramaensis sp. n., Melithaea oyeni sp. n., Melithaea ryukyukensis sp. n., Melithaea sagamiensis sp. n., Melithaea satsumaensis sp. n., Melithaea suensoni sp. n., Melithaea tanseii sp. n., and Melithaea tokaraensis sp. n.. Pleurocorallium confusum Moroff, 1902, Pleurocoralloides formosum Moroff, 1902, Melitodes flabellifera Kükenthal, 1908, and Melitodes densa Kükenthal, 1908 are synonymized with Melithaea japonica (Verrill, 1865). We have designated a neotype for Melithaea mutsu Minobe, 1929. A key to the Japanese melithaeids is presented.


Introduction
Gorgoniam corals of the family Melithaeidae (Anthozoa: Octocorallia) are widespread and common on rocky sea bottoms of the Indo-Pacific Ocean (Bayer 1956, - Coenenchymal spindles slender, up to 0.04 mm wide .M. sagamiensis sp. n.
Remarks. Kükenthal (1909) probably made this a variety of Acabaria modesta Kükenthal, 1908 because the colonies and sclerites of these two species have the same color. However, morphologically the sclerites of these two species are completely different by M. modesta lacking clubs, double disks and disk spindles.
The species resembles Melithaea sagamiensis sp. n., but differs in having much smaller double disks, up to 0.05 mm long. BMNH 1921.10.26.24-2 (Fig. 1c) has been tentatively included in Melithaea abyssicola as it was collected together with BMNH 1921.10.26.5 by the same collector at the same locality, only at different depths. However, the specimen has clubs and disk spindles but lacks the double disks and shows sclerite damage caused by formalin (Figs 6, 7).
Description. The holotype is 8 cm long and 4.5 cm wide, branching is in two parallel planes, and with a holdfast (Fig. 9b). The stem is 5 mm wide, the end branches only 1 mm wide. The colony has many anastomoses. The polyps are situated all around the branches, the calyces are dome-shaped, and the polyps are retracted. Points with slightly bent spindles up to 0.15 mm long, distal end with leaves (Fig. 12a). Collaret with bent spindles up to 0.20 mm long, middle part with more tubercles or side branches (Fig. 12b). Tentacles with platelets, the larger ones crescent-shaped with irregular projections (Fig. 12c). These platelets are up to 0.10 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 12d). Coenenchyme with capstans ( Fig.  12e), unilaterally foliate spheroids (Fig. 12f), 0.05-0.10 mm long and small clubs of similar length (Fig. 12g); spindles (Fig. 13b) and unilaterally foliate spindles (Fig. 13a) are 0.10-0.20 mm long. The calyces with longer clubs, up to 0.15 mm long (Fig. 12h). Most coenenchymal sclerites have complex tubercles.
Color. The colony and sclerites are orange. Distribution. The species is only known from the Ogasawara Islands (= Bonin Islands) (Fig. 8).
Etymology. The species is named after the type locality, the Bonin Islands.
Remarks. This is the first record of Melithaea from this island group. The colony shape of paratype UMUTZ-CnidG-255, collected at the same time with G205, looks similar toZ-CnidG-205, but it has disintegrated sclerites. The colony is slightly smaller and brighter orange-colored (Fig. 9c). Its sclerites are similar to those of the holotype. This is the only species that looks like M. habereri (Kükenthal, 1908), having many anastomoses, but its sclerites are quite different, many with leaves, while Kükenthal described spiny sclerites for M. habereri.
Color. Colony red with yellow tentacles; tentacle and pharynx sclerites colorless, all others pink.
Remarks. The other syntype is in Hamburg, ZMH C3299. A number of specimens were included in M. corymbosa which show differences from the description above. Because of the limited material and rather small differences we refrain from describing them as new species. The specimens differ as follows: RMNH Coel. 41903 has a red colony color with white polyps (Fig. 14c), all coenenchymal sclerites are colorless, the axis sclerites are pink. It differs from M. corymbosa in having more capstans and derivatives of capstans (Figs 21,22). RMNH Coel. 41908 (Fig. 14d) and RMNH Coel. 41909 are white colonies with colorless sclerites. They differ from M. corymbosa in having many small clubs with rounded heads (Figs 23,24). RMNH Coel. 41910 and     RMNH Coel. 41911 (Fig. 14e) have more unilaterally spinose spindles than normal for M. corymbosa (Figs 25,26). Both colonies come from the same locality but have different color patterns. RMNH Coel. 41910 is orange with white calyces and polyps; sclerites of polyps and calyces colorless, others orange; RMNH Coel. 41911 is red with orange sclerites. ZMUC ANT-000646 has an orange colony with white polyps, sclerites yellow with colorless polyp sclerites. It differs from M. corymbosa in having more capstans and derivatives of capstans. In this respect it resembles RMNH Coel. 41903, from which it differs in having overall more tuberculate sclerites.    Description. Colony broken up, consisting of four fragments (Fig. 28a). Points with slightly bent spindles up to 0.25 mm long, distal end with leaves ( Fig. 29a). Collaret with bent spindles up to 0.30 mm long, middle part with more developed tubercles (Fig. 29b). Tentacles with platelets, the larger ones crescent-shaped with irregular projections (Fig. 29c). These platelets are up to 0.15 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 29d). Coenenchyme with predominantly capstans (Fig. 30a), and small clubs resembling flower buds (Fig. 30b), up to 0.10 mm long. Spindles, 0.10-0.20 mm long, with simple tubercles, are also present. (Fig. 30c). The calyces with additional clubs, up to 0.15 mm long (Fig. 30d).

Remarks.
The coenenchymal clubs of this species look like flower buds, similar to those described for Melitaea retifera Lamarck, 1916 by Ofwegen et al. (2000), but that species has unilaterally foliate spheroids, a type of sclerite not present in the present material.

Color. White with colorless sclerites.
Distribution. Only known from Hirado Strait, Nagasaki, East China Sea (Fig. 35). Remarks. The clubs and spinose spheroids with very spiny heads are characteristic for the species.

Material examined.
None, according to Kükenthal (1908) the material was deposited in Munich but it was not found there.
Remarks. According to Kükenthal (1908) this species mostly resembles M. undulata. From the description it most resembles M. corymbosa. One other Japanese melithaeid shows many anastomoses, namely M. boninensis sp. n. For differences see our discussion on that species.
Description. The holotype is 12 cm long and 11 cm wide, branching is in one plane and a holdfast is lacking (Fig. 28c). The stem is 10 mm wide, the end branches only 2 mm wide. The colony has no anastomoses. The polyps are situated biserially on the branches, the calyces are dome-shaped, and the polyps retracted.
Points with slightly bent spindles up to 0.20 mm long, distal end with leaves ( Fig.  33a). Collaret with bent spindles up to 0.20 mm long, middle part with more developed tubercles (Fig. 33b). Tentacles with platelets, the larger ones crescent-shaped with irregular projections (Fig. 33c). These platelets are up to 0.10 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 33d). Coenenchyme with capstans ( Fig. 34a), and unilaterally spinose spheroids, 0.05-0.10 mm long (Fig. 34b). Furthermore spindles are present, 0.10-0.25 mm long (Fig. 34c). All with simple and complex tubercles. The calyces with additional leaf clubs, up to 0.20 mm long (Fig. 33f). The axis has smooth and sparsely tuberculate rods (Fig. 33e).       Color. The colony is orange as are most sclerites; a few are yellow in color. Distribution. Only known from Okinawa Prefecture (Fig. 35). The material is probably collected during the Ryukyu (= Okinawa) expedition by K. Mitsukuri and I. Ikeda, in April, 1901. Etymology. The species is named after the late Prof. Naohide Isono who has worked on Japanese zoological history from the Edo to Meiji period, in appreciation of informing the first author about the collectors data in this publication.
Color. Red with yellow polyps. Variation: red (most colonies), pink, with yellow or white polyps, tentacle and pharynx sclerites colorless, all others orange; or colonies yellow with all sclerites yellow; or white with yellow polyps with polyp sclerites yellow and all others colorless. RMNH Coel. 41923 is rather unique in having a pale light brown colony, tentacle sclerites colorless, others colorless, partly white and partly yellow, or entirely yellow.
Despite the small remainder of the type we could link it with other species described from Sagami Bay. Apparently this is the most common shallow-water species of the region, Kükenthal (1908)  Melithaea densa is also reported to occur in shallow water. According to Kükenthal (1909) it resembles M. flabellifera very much but differs in having more spinose collaret and point sclerites, more densely, stronger ornamented coenenchymal sclerites, and the color always being red with yellow polyps. Later, he separated the two species with M. densa having no clubs (Kükenthal 1924). The ZMB 5801 colony examined by us (Fig. 36c) showed many disintegrated sclerites (Figs 45,46). As this mostly concerned the smaller sclerites we were unable to show the capstans and small clubs. But we found a few larger calyx clubs, apparently overlooked by Kükenthal (1909). Also most point sclerites were badly damaged. We also examined ZMB 5809 (Fig. 36d), which had its sclerites less disintegrated (Figs 47, 48). Kükenthal (1924: 53) referred Pleurocorallium confusum to Pleurocoralloides. Bayer and Cairns (2003: 222) suggested that the species belongs to Acabaria. Moroff (1902a, b), in his descriptions of the species, mentioned flattened branches; polyps on one side of the colony; sclerites straight or bent 0.25 mm long spindles; also plate-like sclerites and crosses present; colony red with yellow polyps. The type seems to be lost. Because of its flattened branches we consider P. confusum synonymous with M. japonica. Bayer and Cairns (2003: 222) suggested Pleurocoralloides formosum to belong to Acabaria. It was described as having polyps with seven spindles per point and six rows in the collaret; sclerites orange, tentacles ones yellow, axis orange. We consider it a synonym of M. japonica; the colonies are shown in Fig. 36a, its sclerites are depicted in Figs 49, 50.
Melithaea flabellifera has been described with two variations: M. flabellifera var. reticulata from Sagami Bay, 80-250 m depth. It differs in having many anastomoses, and no flattened branches, color orange red, sclerites bigger and more spinose. Depository of material is unknown.
M. flabellifera var. cylindrata from an unknown locality in Japan also lacks flattened branches, color red with yellow polyps, sclerites are less spinose. Two syntypes are reported to be in Frankfurt, SMF 1260 and 1262, but we could not find these specimens during a visit.
In northern Japan we found quite a number of specimens: RMNH Coel. 41915-41924. As an example we show the colony of RMNH Coel. 41922 (Fig.  37c) and its sclerites (Figs 51, 52). Here, the smallest colonies are white or yellow, only RMNH Coel. 41922 is red.
Three specimens were found with extremely slender sclerites: BIK-G224 ( Fig. 37a Re-description. Colony branched in one plane with few anastomoses (Fig. 56b). Points with slightly bent spindles up to 0.25 mm long, distal end with more developed tubercles (Fig. 59a). Collaret with bent, rather smooth spindles up to 0.30 mm long, middle part with tubercles (Fig. 59b). Tentacles with platelets, the larger ones crescentshaped (Fig. 59c). These platelets are up to 0.18 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 59d). Coenenchyme with spindles 0.10-0.25 mm long (Fig. 60), with simple or complex tubercles. The axis has smooth and sparsely tuberculate rods (Fig. 59e).
Distribution. Pacific side of Japan (Fig. 65); northern limit is Sagami Bay.
Remarks. The species is easily recognized by its white colony color, rather smooth polyp sclerites, and presence of spindles only.   Description. Colony broken up, consisting of three fragments, bushy with few anastomoses. The largest fragment is 7.3 cm long without holdfast (Fig. 56c). At the base the stem is 7 mm wide, the end branches are 2 mm wide. The polyps are arranged randomly, the calyces hardly project beyond the coenenchyme and most polyps are retracted. Points with slightly bent spindles up to 0.30 mm long, distal end with more tubercles (Fig. 63a). Collaret with bent spindles up to 0.30 mm long, middle part with tubercles (Fig. 63b). Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 63c). Coenenchyme with predominantly spindles, 0.10-0.25 mm long (Figs 63f, 64), with simple or complex tubercles. Capstans are also present, 0.05-0.10 mm long (Fig. 63e). The axis has smooth and sparsely tuberculate rods (Fig. 63d).        Color. Colony red, sclerites orange. Distribution. Only known from the northern tip of the main island of Japan, Honshu Is. (Fig. 65).

Melithaea mutsu Minobe, 1929
Remarks. The tentacle platelets were missing. The description and images provided by Minobe are inadequate to identify melithaeids and we were unable to find the depository of the material. As Minobe's material was collected near Sai, as is our material, we concluded that we have the same species and designated a neotype here. The species is similar to M. corymbosa, but lacks clubs in the calyces and disk spindles in the coenenchyme. Re-description. BMNH 89.5.27.117 (Fig. 66b): Points with slightly bent spindles up to 0.20 mm long, distal end with more developed tubercles (Fig. 67a). Collaret with bent spindles up to 0.25 mm long, middle part with more developed tubercles (Fig. 67b). Tentacles with platelets, the larger ones crescent-shaped with irregular projections (Fig. 67c). These platelets are up to 0.10 mm long. Pharynx with straight spiny rods, up to 0.035 mm long (Fig. 67d). Coenenchyme with spindles (Fig. 67e) and unilaterally spinose spindles (Fig. 67f), 0.08-0.23 mm long, with simple tubercles. The calyces with additional clubs, up to 0.20 mm long (Fig. 67g).

Melithaea nodosa Wright & Studer, 1889
Color. Reddish brown, polyps yellow, axis yellowish-red; reddish nodes. Sclerites yellow, those of the polyps a bit paler, the smallest tentacle and the pharynx sclerites colorless.
Variation. RMNH Coel. 41928 has similar sclerites and color. Distribution. Off the Izu Islands (Fig. 74) Etymology. The species is named after Mr. T.J.G.M. van Oyen (NBC) in appreciation of the many microscope slides he prepared for the second author.
Remarks. The species mostly resembles Melithaea abyssicola but has overall somewhat larger sclerites, a difference difficult to notice when not having both species at hand. However, the double disks of M. oyeni sp. n. are strikingly different, much wider than those of M. abyssicola (compare Fig. 70f with Fig. 3b). Material examined. Holotype UMUTZ-CnidG-32, Nakagusuku Bay, Okinawa Prefecture, Japan, diving, 16 April 1901; paratypes, UMUTZ-CnidG-254, same data as holotype; UMZC I.36300, S.W. Japan. T. Mizobuchi (purchased). Reg. Jan. 31.1903, 16. Description. The holotype is a 14.5 cm long fragment without holdfast (Fig. 66e). At the base the stem is 15 mm wide, the end branches are only 1 mm wide. The polyps are situated on one side of the colony, the calyces hardly project beyond the coenenchyme, and most polyps are retracted. Points with slightly bent spindles up to 0.10 mm long, distal end with more developed tubercles (Fig. 72a). Collaret with bent spindles up to 0.15 mm long, middle part with more developed tubercles (Fig. 72b). Tentacles with platelets, the larger ones crescent-shaped with irregular projections (Fig. 72c). These platelets are up to 0.10 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 72d). Coenenchyme with predominantly capstans (Fig. 73a, d), double disks (Fig. 73b) and disk spindles (Fig. 73c), 0.05-0.10 mm long, and small clubs with rounded heads, 0.05 mm long (Fig. 72f). Spindles are also present, 0.10-0.20 mm long, with complex tubercles (Fig. 73e). The calyces with additional clubs, up to 0.14 mm long (Fig. 72g). The axis has smooth and sparsely tuberculate rods (Fig. 72e).

Melithaea ryukyuensis
Color. Colony orange with yellow polyps. Part of calyx sclerites and all polyp sclerites yellow, all others orange.
Remarks. The material of UMUTZ-CnidG-32 was probably collected during the Ryukyu (= Okinawa) expedition by K. Mitsukuri and I. Ikeda, in April, 1901. UMZC I.36300 is dried material. According to us this is Melithaea sp. A of Aquilar-Hurtado et al. (2012) as the sclerite images given by them resemble ours. Melithaea ryukyuensis mostly resembles M. abyssicola, which has overall larger sclerites but much smaller spindles in the coenenchyme.         Description. The holotype (RMNH Coel. 41929) consists of a number of branches probably belonging to one colony broken up while collecting (Fig. 75a). The largest frag-ment is 2.8 cm long. A holdfast or anastomoses are not present. The polyps are situated on one side of the colony. The calyces are dome-shaped, about 0.5 mm high and 1 mm wide; most polyps are retracted. Points with slightly bent spindles up to 0.25 mm long, distal end with more developed tubercles (Fig. 76a). Collaret with bent spindles up to 0.40 mm long, middle part with more developed tubercles (Fig. 76b). Tentacles with platelets, the larger ones crescent-shaped (Fig. 76c). These platelets are up to 0.15 mm long and have almost no tuberculation. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 76d). Coenenchyme with capstans 0.05-0.07 mm long, unilaterally spinose spindles (Fig. 77b), small clubs of the same length as the capstans, and spindles 0.10-0.20 mm long (Fig. 77a); all with simple tubercles. The calyces with additional leaf clubs, up to 0.15 mm long (Fig. 76e).
Etymology. The species is named after the type locality, Sagami Bay. Remarks. We included BMNH 62.7.16.62(61?) in Melithaea sagamiensis sp. n. despite its slightly different sclerites (Figs 80, 81). We were not certain about the collection number, hence the "62(61)?". Description. The holotype is a 16.5 cm long colony with holdfast ( Fig. 75c). At the base the stem is 10 mm wide, the end branches are only 1 mm wide. On the lower half of the colony the polyps are situated on one side of the colony; the upper part has polyps all around the branches. The calyces are dome-shaped, and most polyps are expanded. Points with slightly bent spindles up to 0.20 mm long, distal end with more developed tubercles (Fig. 82a). Collaret with bent spindles up to 0.30 mm long, middle part with more developed tubercles (Fig. 82b). Tentacles with platelets, the larger ones crescent-shaped with irregular projections (Fig. 82c). These platelets are up to 0.15 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 82d). Coenenchyme with predominantly capstans (Fig. 82f), double disks (Fig. 82g), and unilaterally foliate spheroids (Fig. 83b), 0.05-0.15 mm long, and small clubs (Fig. 82h), up to 0.10 mm long. Spindles are also present, 0.10-0.25 mm long, with simple or complex tubercles (Fig. 83c). The calyces with additional clubs, up to 0.15 mm long (Fig. 83a). The axis has smooth and sparsely tuberculate rods (Fig. 82e).
Etymology. The species is named after the type locality, Satsuma, the old name of Kagoshima Prefecture.
Remarks. This species is unique by its unilaterally foliate spheroids and spindles with complex tubercles. Description. Colony branched in one plane, with slender branches (Fig. 75d). Points with slightly bent spindles up to 0.30 mm long, distal end with more developed tubercles or leaves (Fig. 84a). Collaret with bent spindles up to 0.35 mm long, middle part with more developed tubercles (Fig. 84b). Tentacles with platelets, the larger ones crescent-shaped with irregular projections (Fig. 84c). These platelets are up to 0.15 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 84d). Coenenchyme with capstans ( Fig. 84e) and disk spindles (Figs 84f, 85c), 0.05-0.15 mm long, and small clubs of similar length (Fig. 84g). Spindles are also present, 0.15-0.30 mm long, with simple or complex tubercles (Fig. 85b). The calyces with additional clubs, up to 0.20 mm long (Fig. 85a).

Melithaea suensoni
Color. White with orange calyces and polyps. Sclerites of calyces and polyps faint pink, all others colorless.
Distribution. Off Nagasaki, East China Sea (Fig. 86). Etymology. The species is named after the collector, E. Suenson, who belonged to the Telegraph company Great Nordic Ltd. (Store Nordiske), established in 1869 at Denmark (Matsumoto 2014(Matsumoto , 2015. Remarks. This species resembles M. sagamiensis sp. n., but differs by its thicker coenenchymal spindles.           ,Izu Isl,34°34.4640'N,139°17.8631'E,, coll. A.K. Matsumoto. Description. The holotype (RMNH Coel. 41937) consists of a large number of branches probably belonging to one colony broken up while collecting (Fig. 87a). The largest fragment, with the holdfast, is 6.5 cm long. The stem is 10 mm long and 3 mm wide; branching is in one plane. A few anastomoses are present. The polyps are situated on one side of the colony. The calyces are dome-shaped, about 0.5 mm high and 1 mm wide. Many polyps are expanded. Points with slightly bent spindles up to 0.20 mm long, distal end with more developed tubercles (Fig. 88a). Collaret with bent spindles up to 0.25 mm long, middle part with more developed tubercles (Fig. 88b). Tentacles with platelets, the larger ones crescent-shaped (Fig. 88c). These platelets are up to 0.15 mm long. Pharynx with straight spiny rods, up to 0.05 mm long (Fig. 88d). Coenenchyme with capstans ( Fig. 89a) 0.05-0.07 mm long, several slightly unilaterally foliate (Fig. 89b), unilaterally foliate spindles (Fig. 89c), small clubs of the same length as the capstans (Fig. 88e); spindles 0.10-0.25 mm long (Fig. 89d); all with simple tubercles. The calyces with additional clubs, up to 0.20 mm long (Fig. 88f).
Color. Colony white with the larger yellowish nodes shining through the coenenchyme. All sclerites colorless.

Melithaea tenuis
Color. Colony red, all sclerites orange. Distribution. Only known from off the Tokara Islands. (Fig. 98). Etymology. The species is named after the type locality, the Tokara Islands.
Remarks. The type specimen clearly has damaged sclerites, probably caused by formalin. Recently collected material shows less rounded sclerites (Figs 87e, 96, 97). Double disks are present but so poorly developed that they can hardly be recognized as such.
The species resembles M. tenuis but differs in having longer spindles (up to 0.30 mm long versus up to 0.18 mm long in M. tenuis), and lacking unilaterally spinose spheroids. It also resembles M. corymbosa, but that species has mostly more slender, shorter spindles. Moreover, M. undulata has poorly developed tentacle sclerites compared with the other two species.
Apparently the type material is mostly lost, only a small fragment remained (Fig.  87d) of a colony described as being 21 cm long (Kükenthal 1909).
Melithaea japonica, M. corymbosa and M. sagamiensis sp. n. were collected together. These species could not be separated on colony form but only based on their sclerites. Melithaea japonica mostly occurs in shallow water and is the only melithaeid species of which the spawning season and growth rate is known because of their accessibility for long-term study (formerly M. flabellifera in Matsumoto 2004). Reijnen et al. (2014) used eight Japanese specimens in their molecular study. At that time only RMNH Coel. 41942 was identified as M. tenuis. Here the other seven specimens are also identified or described as new species, and their affinities are examined. M. keramaensis sp. n. appears to be genetically identical to specimens from Indonesia, Vietnam, and Malaysia (Reijnen et al. 2014: Fig. 2). Of these it morphologically mostly resembles RMNH.Coel.41142 from Malaysia but clearly differs from that species in having disk spindles. Melithaea satsumaensis sp. n. is the only other species genetically somewhat different from the other Japanese specimens, which is supported by its sclerites looking like those belonging to the now abandoned genus Mopsella, while the other Japanese specimens in the tree could be abandoned genus Acabaria or valid genus Melithaea. The remaining six specimens did not differ genetically from each other while we identified them as three different species, M. corymbosa, M. japonica, and M. tenuis. This result could be explained by imagining deep water M. corymbosa and M. tenuis show different sclerites and colony shape from shallow water M. japonica, and the differences noted are merely reflecting ecophenotypic variation instead of interspecific variation. Ofwegen (1987: 20) and Ofwegen et al. (2000: 291) already reported species including specimens with different sclerites. The main sclerite difference between M. corymbosa and M. tenuis is in the presence of capstans and derivatives of those, many present in M. tenuis and only few in M. corymbosa. We can imagine this also reflecting variability and these two species actually being one and the same. If this is the case M. japonica, M. tenuis and M. corymbosa all could represent one and the same species. However, many specimens of these three species are not included in the molecular study and these complicate the above possibility of variability. The M. corymbosa specimen used in not typical of that species (see Remarks M. corymbosa), most specimens come from Sagami Bay and have slightly different sclerites. The M. japonica specimens used for the molecular work all came from the Pacific of northern Japan, with the deepest occurrence, while this species is most common in Sagami Bay, with slightly different sclerites than the northern specimens (see Remarks M. japonica).
Depth limitation in the Sea of Japan and the low species richness here, can probably be explained by the geographic history of Sea of Japan. The Sea of Japan is a marginal sea. It originated 15 million years ago during the last glacial maximum (LGM). It was almost totally closed off by a sea-level drop of ca.130 m. Approximately 12.000 years BP, the warm Tsushima Current, a branch of the warm Kuroshio current, started to flow into the Sea of Japan from the South (Oba et al. 1991, Ishiwatari et al. 2001, Yokoyama et al. 2000. It was a geographical barrier for the distribution of the marine organisms with a planktonic larvae stage, such as corals. Considering the highest species diversity depth range at Sagami Bay (100-200 m) the age of the Sea of Japan and the LGM barrier between 0-130 m could have restricted the warm Indo-Pacific species from the South to enter the Sea of Japan.
Four species have been synonymized and 11 new species described from Japanese waters. In total Japan now has 23 Melithaeidae species (including M. habereri) and two varieties.