Commensal Leucothoidae (Crustacea, Amphipoda) of the Ryukyu Archipelago, Japan. Part I: ascidian-dwellers

Abstract Commensal leucothoid amphipods have been collected from the branchial chambers of their ascidian hosts throughout the Ryukyu Archipelago, Japan. Seven new species are described in two genera with valuable location data and host records. An identification key to ascidian-dwelling 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. The family currently contains 139 species in five genera and can be divided into two clades (White 2011). The anamixid clade inhabits tropical to warm temperate waters and exhibits extreme sexual dimorphism.

Southern Ryukyu Islands
Other sampling efforts included isolating entire sponges in zip-lock plastic bags for dissection in the laboratory or capturing amphipods individually in situ, using a modified squirt bottle. Coral rubble samples were also taken, elutriated, and sieved on location using both saltwater and formalin washes. Samples were sorted immediately and amphipods were preserved as previously stated.
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 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.
Leucothoe amamiensis differs from these species in having two rows of dentition on the right mandibular lacinia mobilis, a heavily setose gnathopod 2 basis, and a telson with plumose facial setae and simple marginal setae, and truncate apex. Remarks. Leucothoe amamiensis has a pink-orange striped color pattern (Fig. 17B). This species appears to be endemic to Amami-oshima Island.
Etymology. After the Latin 'elegans', meaning tasteful, choice, fine, and referring to the elegant, elongate gnathopod 1 of males and females of this species.
Relationships. Leucothoe elegans is similar to Leucothoe germanalcyone Hirayama, 1992 in having an enlarged eye; similar to Leucothoe flammosa Thomas and Klebba, 2007 and Leucothoe uschakovi Gurjanova, 1951 in having an elongate gnathopod 1 with a centrally widened basis; and similar to L. nagatai in having short antennae, nar-row pereopod 5-7 bases, and a sinuous epimeron 3. Leucothoe elegans differs from these species in having a dentate right mandible lacinia mobilis, female gnathopod 1 with many posterior setae, and gnathopod 2 propodus with submarginal row of robust setae. Remarks. Leucothoe elegans is translucent ivory in color (Fig. 17C). This species has only been collected on Yakushima Island and from Shioya Bay, on the east coast of Okinawa-jima Island, Okinawa.
Etymology. Named for Nathan Stuart White, amphipod collector extraordinaire, who collected the type specimens of this species. Nathan has provided tremendous support and assistance throughout all sampling efforts in the Ryukyu Archipelago.
Relationships. Leucothoe nathani is similar to L. nagatai in having short antennae, an elongate maxilliped outer plate inner margin, a short gnathopod 1 dactyl, a heavily setose gnathopod 2 propodus medial surface, and narrow pereopod 5-7 bases. This species differs in having a smooth maxilliped outer plate inner margin, slenderner gnathopod 1 carpus (length 7.6 × width compared to length 6.6 × width in L. nagatai), and a longer telson (2.2 × longer than wide compared to 1.8 × longer than wide in L. nagatai).
Remarks. Leucothoe nathani is orange in color with robust dark orange stripes along pereonites (Fig. 17D). This species has been collected from only one location in February and April, two of the coldest months on Okinawa-jima Island.
Etymology. Named for "General" Masami Obuchi, who collected the type specimens of this species. Dr. Obuchi has provided invaluable sampling and logistical support for this research in the Ryukyu Archipelago.
Relationships. Leucothoe obuchii is similar to L. nagatai in having short antennae, an elongate tuberculate maxilliped outer plate inner margin, a short gnathopod 1 dactyl, and narrow pereopod 5-7 bases. This species differs in having a much less setose gnathopod 2 propodus medial surface. Leucothoe nagatai has robust tufts of mediofacial setae covering most of the proximal surface of the propodus compared to single mediofacial and submarginal setal rows in L. obuchii.
Remarks. Leucothoe obuchii is opaque ivory in color (Fig. 17E). In most collections of this species there was one specimen at the base of the branchial chamber of each ascidian collected. Rarely, there were one large and one small amphipod living together.
Remarks. Leucothoe trulla has faint pink-red stripes along pereonite edges and a slightly darker "saddleback" color in the middle (Fig. 17F). This species is endemic to the southern Ryukyu Islands.
Etymology. After the Latin 'vulgaris', meaning 'common, commonplace' and referring to the widespread distribution and the apparent lack of host specificity of this species.
Relationships. Leucothoe vulgaris is part of the "L. commensalis group" in the same aspects that L. trulla is similar (see 'Relationships' under L. trulla). Leucothoe vulgaris differs from these species in having a setose epistome (smooth in reports of all Leucothoe species), a dentate surface on right mandible lacinia mobilis, and a telson with a strong point (most Leucothoe species have a tridentate apex).
Remarks. Leucothoe vulgaris has a distinct red "saddleback" color pattern found in ascidian-dwelling leucothoids worldwide and yellow antennae (Fig. 17A). This species is widespread throughout the Ryukyu Archipelago, inhabiting many species of ascidians and sponges.
Etymology. Named for Dr. James Darwin Thomas in recognition of his contribution to amphipod taxonomy, particularly regarding the Leucothoidae. Dr. T has been a mentor and friend for the past 10 years and the first author is very grateful for all his support.
Relationships. Paranamixis thomasi is similar to P. aberro in having a cuspate anteroventral head margin and an enlarged gnathopod 2 coxa. Paranamixis thomasi differs in having a serrate ridge on the gnathopod basis instead of a large tubercle and in having plumose setae on the dactylus. Paranamixis thomasi is similar to P. misakiensis in having the head with a lateral ridge, maxilliped outer plate with a small cleft, an enlarged gnathopod 2 coxa, and a serrate ridge on the gnathopod 2 basis anterior margin. Paranamixis thomasi differs from this species in having a single cusp on the anteroventral head margin, a smaller serrate ridge on the gnathopod 2 basis, and smooth gnathopod 2 carpus and dactylus inner margins.
Remarks. Both anamorphs and leucomorphs are translucent with magenta-pink stripes along the pereonite edges (Figs 17G, H). When collected in an ascidian, one anamorph and two leucomorphs were collected together from one branchial chamber.

Discussion
The six Leucothoe species described here share the displaced gnathopod 2 propodus mediofacial setal row, a character common to ascidian-dwelling species worldwide. It is likely that this character is an artifact of convergent evolution in species adapting to feeding within similar hosts rather than evidence of relationships between species. Leucothoe amamiensis, L. elegans, and L. trulla each have a small accessory flagellum on antenna 1. This character is unusual in leucothoid species and apparently much more common in Pacific species than in Caribbean species. It is particularly interesting because these three species have limited distributions in the Ryukyu Archipelago. The currently recognized biogeographic boundaries (Hikida and Ota 1997;Ota 1998) do not appear to apply to leucothoid amphipods in the Ryukyu Archipelago despite their restricted distributions. There are some interesting distributional patterns evident in some species, while others, such as L. vulgaris and P. thomasi, are found throughout the entire archipelago. It is possible that these patterns in amphipod distributions are partly attributable to the ephemeral nature of their ascidian hosts. Numbers of ascidian species and individuals in the Ryukyu Archipelago appear to be much higher in the winter months than in the summer. A similar pattern is evident for leucothoid amphipods.
Interesting distributional patterns were observed in Leucothoe obuchii, which has been collected from Oura-wan Bay on the northeastern coast of Okinawa-jima Island as well as from two of the northernmost Ryukyu islands, Amami-oshima Island and Yakushima Island; and in Leucothoe elegans, which has only been collected from Yakushima Island and Shioya Bay on the northeastern coast of Okinawa-jima Island. Both Oura-wan and Shioya bays on the northeastern coast of Okinawa-jima Island are muddy and are ecologically very different from most of the environments on Okinawa-jima Island (Shokita et al. 2002;Naruse et al. 2009). These muddy bays also are very different from the coral reef habitats in the northern Ryukyu Islands that L. obuchii and L. elegans were collected from. laboratory space and logistical support. Reviews from Charles Oliver Coleman, Sara LeCroy, and an anonymous reviewer significantly improved the manuscript. Funding was provided by a Japanese Society for the Promotion of Science (JSPS) postdoctoral fellowship awarded to the first author (P10711). The second author was supported in part by the International Research Hub Project for Climate Change and Coral Reef/ Island Dynamics at the University of the Ryukyus.