Two new species of eyeless amphipods from a coastal area in Japan (Crustacea: Amphipoda: Hadziidae, Melitidae), with reinstatement of the genus Paraniphargus Tattersall, 1925

ABSTRACT Two new species of eyeless amphipods are described from coastal Japan. Dulzura projecta sp. nov. (Hadziidae) was collected under large stones and in coarse sand from Osaka to Mie Prefectures. Dulzura projecta can be distinguished from the other Dulzura species by the distinct projection on article 1 of the male pleopod 3 inner ramus and the very long carpus of male gnathopod 1. Paraniphargus shiosai sp. nov. (Melitidae) was collected in coarse sand from Mie Prefecture, and can be differentiated from the other two species in the genus by the dorsal teeth on the pleonites, the smaller coxa 4 with shallow excavation and the shorter antenna 1 flagellum. Paraniphargus is reinstated as a distinct genus, following observation of the gnathopods, which revealed sexual monomorphism between males and females.


Introduction
The order Amphipoda is a large group in Crustacea comprising approximately ten thousand described species (Ahyong et al. 2011). Amphipod species display a high plasticity in morphological characters; for example, eyes vary among species from a total absence of ommatidia to truly enormous hypertrophy with complex visual systems (Bellan-Santini 2015). In the terrestrial aquatic environments, many eyeless amphipods inhabit caves or subterranean water, and several comparative studies on morphology and behaviour of the amphipods have been carried out (e.g. Culver et al. 2010;Fišer et al. 2016). However, in marine species, knowledge of the eyeless species is poorly documented. Thurston and Bett (1993) suggested that 17.8% of marine amphipods are eyeless with most of the eyeless species being correlated positively with depth and latitude; however, formal ecological studies of eyeless amphipods are scarce. Although 'cobble or beach interstitial' species were excluded in their analysis, many eyeless amphipods do occur in shallow and also temperate waters. In Japan, 14 eyeless species were recorded from shallow seas (< 100 m in depth) including Bogidiellidae: Bollegidia takedai Ariyama, 2012; Eriopisidae: Paraflagitopisa excavata Ariyama, 2015; Psammogammarus mawatarii Tomikawa et al., 2010;Psammogammarus Material and methods Specimens were collected from coastal areas, intertidal to 4 m depth, from Osaka, Wakayama and Mie Prefectures. The main collecting sites were benthic sediments in the subtidal zone. Approximately one litre of the sediment was collected using a net while snorkelling. Small amounts of the sediment and seawater were put into a plastic beaker. Then the water was stirred vigorously, and the suspended material was immediately decanted through a fine-meshed sieve, with this process being repeated three to five times. Sieved samples were fixed in a 5-10% formalin-seawater solution. Samples were also collected by hand from the underside of large stones in the intertidal zone. Specimens were dissected and illustrated using a phase-contrast microscope. Body length was measured from the apex of the rostrum along the dorsal margin to the distal end of the telson. Type material is deposited in the Osaka Museum of Natural History, Japan (OMNH).

Remarks
Springthorpe and Lowry (2009) synonymized Protohadzia with Dulzura for the reason that distinctions between them have never been strong. However, Lowry and Myers (2013) left Protohadzia unsynonymized.

Coloration in life
Whole body faintly yellowish white.

Etymology
From the Latin projecta (= projected), referring to the shape of male pleopod 3.

Remarks
In Dulzura projecta sp. nov. the inner ramus of male pleopod 3 has a projection on article 1 which differentiates it from other species in the genus. Such sexual dimorphism in pleopod 3 is also known for D. lobata, where the male pleopod 3 has a swelling on articles 1 in both rami (Stock and Vonk 1991). The very long carpus of male gnathopod 1 is also unique to D. projecta with other species of Dulzura having a shorter carpus. In D. projecta the bases of pereopods 5 to 7 are lobate, which is also seen in D. lobata, and the male epimeron 3 has a sharp projection on the posterodorsal corner which is shared with D. hamakua and D. paucispinosa.

Distribution
From Osaka Prefecture to Mie Prefecture in Japan.

Habitat
Under large stones and in coarse sand, probably interstitial; from the lower intertidal zone to 4 m depth.

Remarks
Four species have been described in the genus Paraniphargus: P. annandalei Tattersall, 1925 from a freshwater stream in the Andaman Islands; Paraniphargus ruttneri Schellenberg, 1931 from a freshwater lake in Java; Paraniphargus leleuporum Monod, 1970 from groundwater in the Galapagos Islands; Paraniphargus vermiamicus Bamber, 2003 from marine waters in Hong Kong. Paraniphargus leleuporum was subsequently transferred to a new genus, Galapsiellus Barnard, 1976 by Barnard (1976), and P. vermiamicus was transferred to Tegano Karaman, 1982 by Horton andLowry (2012), leaving only two species in the genus.
Several studies have dealt with the difference between Paraniphargus and the related genus, Melita Leach, 1814. Schellenberg (1931 pointed out that they differ only in the 1articulated outer ramus of the uropod 3, the lack of the anteroventral cusp on the head, the weak mandibular palp, and the naked medial margin of the inner lobes of both maxillae. Barnard and Barnard (1983) stated that many species of Melita have lost article 2 on the uropod 3 outer ramus, and that Paraniphargus differs from Melita in the loss of eyes and medial maxillary setae. Stock and Ilife (1995) suggested that there were only slight differences between Paraniphargus, Melita and a third genus Josephosella Ruffo, 1985. More recently, Sawicki et al. (2005) synonymized Paraniphargus with Melita citing a lack of distinction in the setation of maxilla 2 and the number of articles in the uropod 3 outer ramus between these genera. The present study recognizes the sexually monomorphic gnathopod 2 as a distinctive generic character of Paraniphargus separating it from Melita species that have a sexually dimorphic gnathopod 2 ( Barnard and Barnard 1983;Jarrett and Bousfield 1996). Although the original description of P. annandalei and P. ruttneri by Tattersall (1925) and Schellenberg (1931), respectively, do not include details of the presence or absence of sexually dimorphic characters, Barnard (1976) proposed the absence of sexual dimorphism in the gnathopods without stating the basis for this observation. The medial margin of the maxilla 2 inner plate is quite bare of setae in all species of Paraniphargus including P. shiosai sp. nov., whereas in the Melita species, number of setae on the medial margin varies from very many to a few but never without setae. Based on the above characters Paraniphargus is here reinstated as a distinct genus from Melita.
Gnathopod 1 (Figure 8(a)), coxa longish rhomboid, anterior margin with short seta, ventral margin with 1 medium and 2 short setae; basis almost straight, anterior margin with 2 long setae; ischium setose on posterodistal margin; merus setose on posterior margin and posteromedial surface; carpus bearing several long setae on anterodistal corner and posterior margin, anterodistal surface with many short setae medially; propodus about 0.9 times as long as carpus, anterodistal corner and posterior margin setose, posterodistal corner with several short setae (not robust), anteroproximal surface with many short setae medially; dactylus short, curved. Gnathopod 2 (Figure 8(b)), coxa longish rhomboid, anterior margin with short seta, ventral margin with 1 medium and 2 short setae, posterior margin bearing medium seta, gill small; basis relatively stout, anterior and posterior margins with 4 and 1 short setae, respectively; merus slightly projected posterodistally; propodus long, about 2.1 times as long as carpus, posterior margin gradually curved, distal half of anterior margin bearing several setae, palm defined by 1 long and 1 short robust seta, with many short setae, posterior margin except palm bearing sparse setae; dactylus slender, with several short setae.

Coloration in fixed specimen
Whole body white.

Etymology
Shiosai is the name of a famous Japanese novel by Yukio Mishima. The setting for the novel is Kamishima Island, the type locality of this new species.

Remarks
Paraniphargus shiosai sp. nov. differs from P. annandalei and P. ruttneri in (1) the dorsal teeth on pleonites, (2) the smaller coxa 4 with shallow excavation, (3) the 3-articulated accessory flagellum, and (4) in having a shorter antenna 1 flagellum. Paraniphargus shiosai can also be distinguished from P. annandalei by the smooth posteroventral margins of pleonal epimera. In addition, P. shiosai was collected from marine habitats whereas P. annandalei and P. ruttneri are freshwater species.

Distribution
Only Kamishima Island in Mie Prefecture, Japan.

Habitat
In coarse sand, interstitial; 2.5 m depth. This species occurs together with Dulzura projecta sp. nov.