Commensal Leucothoidae (Crustacea, Amphipoda) of the Ryukyu Archipelago, Japan. Part III: coral rubble-dwellers

Abstract Commensal leucothoid amphipods have been collected from coral rubble samples throughout the Ryukyu Archipelago, Japan. Seven new species are described in two generawith valuable location data. A new locality is presented for Paranamixis misakiensis Thomas, 1997. An identification key to all described Leucothoidae of the Ryukyu Archipelago is provided.


Introduction
The Leucothoidae are a family of marine gammaridean amphipods that can be found inhabiting sessile invertebrate hosts worldwide. Including the species described here, the family contains 163 species in five genera and can be divided into two clades, the anamixid clade and the leucothoid clade after a wide revision of the group made by White (2011). Table 1 lists the diagnostic characters for each clade. Leucothoids are typically found as endocommensal associates of sponges, ascidians, and bivalve mollusks, where they utilize the feeding current produced by their hosts to feed (White 2011;White and Reimer 2012a).
Collection of amphipods throughout the Ryukyu Archipelago has increased the number of leucothoid species known from Japan from seven to 32, with 25 of these occurring in the Ryukyu Archipelago (White and Reimer 2012a, b). Seven new species are described, six from the genus Leucothoe and one from the genus Anamixis. The currently recognized biogeographic boundaries for terrestrial organisms within the Ryukyu Archipelago (see Figure 1 in White and Reimer 2012a) due to the past connection of the island chain to the Eurasian continent by land bridges Oshiro 1977, 1980;Hikida and Ota 1997;Ota 1998;Ota et al. 2004) do not appear to apply to these marine species (White and Reimer 2012a, b).
One 12 liter bucket was filled with colorful, sponge-filled coral rubble ( Figure 16) at each location. The bucket contents were then elutriated and sieved on location using both saltwater and formalin washes. Samples were sorted immediately. Amphipods were preserved in 2% seawater buffered formalin for morphological analysis and 99% ethanol for molecular studies.
Specimens used for morphological analyses were transferred to glycerin, dissected, mounted on slides, and illustrated using a Nikon® Y-IDT drawing tube attached to a Nikon® Eclipse 50I compound microscope. Pencil drawings were scanned and digitally inked in Adobe® Illustrator using a Wacom® Tablet, following the methods of Coleman (2003).
Descriptions are of males unless noted with sexually dimorphic characters described in a separate section. Terminology used in descriptions follows White and Thomas (2009) with 'proximal margin' of the carpus and dactylus referring to the margins closing on the propodus. Setae nomenclature follows Oshel and Steele (1988) where possible without having SEM images for the specimens described here. All setae are simple, unless noted. Type material is deposited in the University of the Ryukyus Museum (Fujukan), with the prefix RUMF for museum numbers. Additional material has been deposited in the National Museum of Nature and Science in Tokyo, with the prefix NSMT for museum numbers.

Ecology.
Host unknown, presumably living in sponges in coral rubble. Relationships. Anamixis sentan sp. n. fits into the Anamixis bazimut Thomas, 1997, Anamixis kateluensis Thomas, 1997, and Anamixis moana Thomas, 1997 group introduced by Thomas (1997). These species share several characters, including a truncate anterior head margin with the ventral cephalic keel extending below the distal head margin and similar shape and morphology of gnathopod 2. Anamixis sentan sp. n. shares the cleft maxilliped inner plates with A. bazimut and A. kateluensis. Anamixis sentan sp. n. is distinct from all other Anamixis species in having serrate maxilliped inner plates in the anamorph; gnathopod 1 coxa anterodistal margin bi-cuspidate, distal margin excavate, and propodus palm smooth with 8 sets of 2 slender and 1 robust proximal setae in the leucomorph.
Remarks. Anamorph males of Anamixis sentan sp. n. are white in color with 1 robust magenta-pink stripe on each pereonite segment ( Figure 15A). Leucomorph males and females are translucent with very faint pink stripes along pereonite edges; some specimens appear to be translucent pink in color ( Figure 15B). This species appears to be endemic to the central Ryukyu Islands. The anamorph and leucomorph of this species have not been collected together directly from their host, yet more than one year of collecting has revealed only two anamorph morphologies and two leucomorph morphologies. One anamorph and leucomorph morphology belong to Paranamixis thomasi White and Reimer, 2012a. The other anamorph and leucomorph morphology are described here as Anamixis sentan, although there is a very slight chance that this leucomorph belongs to a different anamorph counterpart that has not been collected.
Etymology. After the Japanese words 'akai', meaning 'red' and 'sen', meaning 'line' and referring to the red stripes along the antenna 1 peduncle (pronounced ah-ky-sen).
Remarks. Leucothoe akaisen sp. n. is very unique in coloration. This species has deep red stripes along the peduncle of antenna 1, a large yellow eye, a yellow "saddleback" pattern, and a deep red dot on coxa 4 ( Figure 15D). This species is endemic to the Ryukyus Archipelago, having been collected throughout the island chain. The color pattern of this species resembles that of an undescribed species collected in Guam from polychaete worm tubes (Thomas, pers.com.).
Etymology. After the Japanese word 'akuma', meaning 'devil' and referring to the type locality, also known as 'Devil's Cove' (pronounced ah-koo-ma).
Ecology. Host unknown, presumably in sponges in coral rubble. Relationships. Leucothoe akuma sp. n. is similar to Leucothoe togatta White and Reimer, 2012b, Leucothoe hashi White and Reimer, 2012b, and Leucothoe amamiensis White and Reimer, 2012a in having a rounded anterodistal head margin and a ventral cephalic keel with an excavate anterior margin and a produced, quadrate anteroventral margin. Leucothoe akuma sp. n. differs from L. hashi in the overall shape of gnathopod 1 and from the other species in having mandibular palp article 2 with fewer distal setae, lower lip with setose inner lobes, gnathopod 2 carpus longer and distally truncate, and gnathopod 2 propodus medial surface with two rows of submarginal setae. This last unique character is shared only with L. enko sp.n.
Remarks. Leucothoe akuma sp. n. is white in color with magenta-pink stripes along pereonite edges ( Figure 15H). This species is endemic to the central Ryukyu Islands.
Remarks. Leucothoe chiisainame sp. n. is white in color with a very small yelloworange eye ( Figure 15F). This species is endemic to Yakushima Island. Although there is only one specimen available for analysis, the authors are confident that this is a new species based on the diagnostic characters.
Remarks. Leucothoe enko sp. n. is deep yellow in color with a small red eye ( Figure  15C). This species is endemic to the central Ryukyu Islands. Upon further examination, this group of species may prove to be a part of a cryptic species complex, with different species inhabiting similar niches at different locations.
Remarks. Leucothoe kebukai sp. n. is translucent yellow in color with faint pink stripes along pereonite edges ( Figure 15E). This species is endemic to the central and northern Ryukyu Islands.
Leucomorph (juvenile and sexually dimorphic characters). Unknown. Ecology. Host unknown, presumably in sponges in coral rubble.
Remarks. Anamorph males of Paranamixis misakiensis is white in color with pink stripes along pereonite edges ( Figure 15G). This species was collected from Yakushima Island, extending its known range approximately 1000 kilometers. The specimen from Yakushima closely agrees with Thomas' 1997 description, differing slightly in the ventral cephalic keel, which is excavate with a mid-distal projection, versus excavate in Thomas' 1997 material. Distribution. East China Sea: Yakushima Island (Kagoshima), Japan. Pacific Ocean: Misaki, Miura Peninsula (Kanagawa), Japan.

Discussion
Leucothoe akaisen sp. n. and Leucothoe chiisainame sp. n. share a displaced mediofacial setal row, which is typically found in ascidian-dwelling species worldwide, suggesting that these species may also inhabit ascidian hosts, or that this character may be homologous among Leucothoe species and not an artifact of convergent evolution, as noted in White and Reimer (2012a). Three species described here have a small accessory flagellum on antenna 1 (Leucothoe akuma sp. n., Leucothoe chiisainame sp. n., and P. misakiensis). This character is unusual among leucothoid species, but apparently is much more common in Pacific species than in Caribbean species. A range extension is reported for Paranamixis misakiensis which was previously known from only Misaki, Kanagawa prefecture, Japan. Both Thomas' (1997) report and this research have found this species from coral rubble, without more specific host data. Leucomorphs are currently unknown for this species, highlighting the need for further investigation into connecting life stages for the anamixid clade. Both P. thomasi and Anamixis sentan sp. n. anamorphs are connected to their leucomorph counterparts due to diligent collecting and recording of important host data. However, in most cases this specialized collecting is not undertaken, leaving morphologically different life stages unknown.
Leucothoe chiisainame sp. n. is reported from only one island, while all other new species here have been collected from at least two islands. Leucothoe akaisen sp. n. has been collected from throughout the entire Ryukyu Archipelago, Leucothoe kebukai sp. n. has been collected from the central and northern islands, Anamixis sentan sp. n. has been collected from the southern and central islands, and all other species described here are reported from only the central islands.
The currently recognized biogeographic boundaries of the Ryukyu Archipelago Oshiro 1977, 1980;Hikida and Ota 1997;Ota 1998;Ota et al. 2004) do not appear to apply to all leucothoid amphipods in this region. Of the 25 leucothoid species collected in the Ryukyu Archipelago, two species were collected from only above the Watase Line at the Tokara Strait, two species were collected from only below the Hachisuka Line at the Kerama Gap, and eight were collected within these two boundaries (see map of boundaries in White and Reimer, 2012a). The other 14 species ranges cross these boundaries. It is possible that further collections will reveal that the boundaries do not apply to any of these species or that host specialization limits the ranges of certain species and not of others.
The very high diversity of new leucothoid species discovered in the Ryukyu Archipelago to date supports the observation of Roberts et al. (2002), stating that Indo-Pacific reefs and in particular southern Japan and Taiwan include some of the most diverse areas in the world with high levels of endemicity. Taking this into account, this research noticed a higher species richness among leucothoids when comparing with a recent survey done in the Great Barrier Reef. This study reports 25 leucothoid species in 3 genera from 108 samples at 47 locations spanning approximately 1000 kilometers compared with 17 leucothoid species in 3 genera from 225 samples at 43 locations spanning approximately 1100 kilometers at Lizard Island, Australia and including samples from Orpheus and Heron Islands. Perhaps the incredible Leucothoidae diversity found in the Ryukyu Archipelago will apply to other amphipod families. It is clear that further sampling and research is needed in this region for a better understanding of the Amphipoda diversity.