Haimormus shimojiensis, a new genus and species of Pseudozeuxidae (Crustacea: Tanaidacea) from a submarine limestone cave in Northwestern Pacific

We establish a new pseudozeuxid genus Haimormus gen. nov. based on a new species Haimormus shimojiensis sp. nov. which was collected from a submarine limestone cave with the entrance at 35 m depth, in the Northwestern Pacific Ocean. H. shimojiensis differs from the other confamilial members, Pseudozeuxo belizensis Sieg, 1982 and Charbeitanais spongicola Bamber & Bird, 1997, in having the pleonite 1 without the pleopod, the pereopods 2 and 3 propodus with a ventral spiniform seta, and the pereopods 4–6 propodus with one long and two short dorsodistal setae. A key to females of species of Pseudozeuxidae is presented. This is the first tanaidacean report from submarine caves around Japan.


MATERIALS AND METHODS
All tanaidaceans were collected by a scuba diving from a submarine limestone cave "Akuma-no-Yakata (Devil's Hole)" on September 1, 2017. This cave is located on a reef slope at Shimoji-jima Island, Miyako Island Group, southwestern Japan (24 49′22.51″N, 125 08′07.84″E; Fig. 1A), and its entrance lies at a depth of about 35 m; detailed information of the cave is included in Anker & Fujita (2014). From the inside of the cave, one brittle-star and more than 14 decapod species have been found so far (Fujita, Naruse & Yamada, 2013;Anker & Fujita, 2014;Okanishi & Fujita, 2018). Mud deposited in holes or gaps of the cave wall was collected by using a "yabby pump (commercial suction pump)", from a totally dark point at 18 m depth, ca. 80 m from the cave entrance. Tanaidaceans were sorted from the mud sample, and fixed and preserved in 99% ethanol. The studied specimens were deposited in the crustacean collection of the National Museum of Nature and Science, Tsukuba (NSMT), Japan and in the University Museum Fujukan, University of the Ryukyus (RUMF), Japan. The methods used for dissection, preparation of slides, light microscopy, scanning electron microscopy (SEM), and drawing were as described by Kakui & Angsupanich (2012). DNA extraction was attempted for one paratype specimen (NSMT-Cr 26837), but subsequent PCR was not successful. Orientation and terminology here follow Larsen (2003), except that the term "plumose sensory seta(e)" (PSS; Bird, 2011) is used instead of "broom seta(e)." We proposed one setal term, "triserrate seta(e)," for the setae found in the dorsodistal region of the pereopods 4-6 propodus; the seta expands distally and bears three (dorsal, ventral, and outer/inner) rows of serrations (Fig. 2). Body length (BL) was measured from the tip of the eyelobe to the tip of the pleotelson, and body width at the widest portion of the carapace (CW, carapace width). Appendages were measured only in the holotype specimen. Measurements were made axially: dorsally on the body, antennules, antennae, and uropods; laterally on the pereopods. The suffixes in the newly proposed Japanese names, viz., "-ka" and "-zoku" represent the taxonomic ranks family and genus, respectively, in the Japanese language.
The electronic version of this article in portable document format will represent a published work according to the International Commission on Zoological Nomenclature (ICZN), and hence the new names contained in the electronic version are effectively published under that Code from the electronic edition alone. This published work and the nomenclatural acts it contains have been registered in ZooBank, the online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix http://zoobank.org/. The LSID for this publication is: urn:lsid:zoobank.org:pub:3019D534-3E50-49D3-AC6E-8EB5A80DD1BB. The online version of this work is archived and available from the following digital repositories: PeerJ, PubMed Central, and CLOCKSS.
Etymology. The genus name (masculine) is named in honor of Michitaka Shimomura who greatly contributes to the taxonomy of tiny crustaceans from submarine caves around Japan in recent years (an anagram of his last name).

Figures 2-6
Diagnosis. Same as for the genus.
Etymology. The specific name is an adjective referring to the type locality.  (Fig. 3); body wall not heavily calcified. Cephalothorax 0.16 times as long as BL, 1.27 times as long as wide, acorn-shaped in dorsal view, with two pairs of lateral simple setae (one seta lost); rostrum triangular; eye present. Pereonites 1-6 with length ratio of 1.00:1.49:1.92:2.04:1.86:1.45; pereonites 1, 2, 6 wider than long, pereonites 3-5 longer than wide; pereonite 1 with pair of dorsolateral simple setae; pereonites 2-5 with pair of lateral simple setae. Pleon 0.10 times as long as BL. Pleonites narrower than pereonite 6; all wider than long, similar in shape, but the width gradually narrower from pleonites 1-5; pleonite 5 with pair of dorsolateral simple setae. Pleotelson 0.56 times as long as wide, almost same width to pleonite 4, pentangular in dorsal view, with pair of subdistal simple setae, pair of distal simple setae and pair of distal PSS. Antennule (Fig. 4C) as long as cephalothorax; articles 1-3 with length ratio of 1.00:0.31:0.48. Article 1 with one middle and one distal simple setae, and several PSS; several fine setae on proximal margin (Fig. 4C, arrowheads). Article 2 with distal simple seta and two distal PSS. Article 3 with six distal simple setae, two distal PSS, and distal aesthetasc. Antenna (Fig. 4D) 0.72 times as long as antennule; articles 1-6 with length ratio of 1.00:1.97:1.22:4.64:2.85:0.31. Article 1 naked. Articles 2 and 3 each with dorsodistal spiniform seta. Article 4 with one subdistal and two distal simple setae and two distal PSS. Article 5 with distal simple seta and PSS. Article 6 with five distal simple setae. Labrum (Fig. 4E) not projected anteriorly, setulate. Mandibles (Figs. 4F and 4G) with molar well developed; masticatory region broad. Incisor of left mandible (Fig. 4F) with several teeth; lacinia mobilis with several small and one large teeth. Incisor of right mandible (Fig. 4G) bifurcate distally, with subdistal anterior crenulation and subdistal posterior process. Labium (Fig. 4H) bilobed; inner and outer lobes setulate. Maxillule (Fig. 5A) with endite bearing eight distal single-tipped and one distal bifurcate spines and fine setules; palp biarticulate, with two distal simple setae. Maxilla (Fig. 5B) oval, naked. Maxillipeds (Fig. 5C) with bases not fused medially, each bearing simple seta at insertion of palp. Endites not fused medially, reaching beyond distal margin of palp article 1, each with long simple outer seta and three short (one inner most one relatively shorter and more blunt than other two) spiniform setae in ventrodistal region, and two long and one short spiniform setae in inner dorsodistal region; outer margin setulate. Palp article 1 naked; article 2 with several fine setae, three distal simple setae, and distal bipinnate seta in inner region; article 3 with three proximal fine setae and six simple setae in inner region; article 4 with one mid-dorsal and seven distal simple setae. Epignath lost during dissection. Cheliped (Fig. 5D) with triangular articulation with cephalothorax via sclerite. Basis longer than wide, with free posterior portion just shorter than anterior, latter with dorsal simple seta. Merus with two ventral simple setae. Carpus slender, more than twice as long as wide, with one mid-dorsal, one dorsodistal, and three ventral simple setae. Chela as long as carpus, slender, 2.5 times as long as wide; propodal palm longer than fixed finger, with one outer and two inner simple setae at insertion of dactylus; fixed finger with two simple setae on ventral margin, three outer simple setae on cutting surface, and pointed claw; dactylus-unguis slightly longer than fixed finger, with inner proximal simple seta; unguis pointed. Pleopods absent. Uropod ( Fig. 5K) with basal article naked. Endopod 2.60 times as long as basal article, uniarticulate; length/width ratio 3.10; pseudoarticulation present ( Fig. 5K, arrowhead); with one subdistal and five distal simple setae and one middle and one distal PSS. Exopod uniarticulate, 0.25 times as long as endopod with one middle and two distal simple setae.
Distribution. So far known only from the type locality.

DISCUSSION
The reduced pleon, characterized by the pleonites narrower than the pereonite 6 and the combined length of pleonites 1-5 shorter than the pereonite 6, is one of diagnostic characteristics of Pseudozeuxidae found in both sexes. Haimormus gen. nov. shares the above character state with the other confamilial genera (Pseudozeuxo Sieg, 1982 andCharbeitanais Bamber &Bird, 1997), but differs in lacking the pleopod 1 and having the propodus of pereopods 2 and 3 with one ventral spiniform seta (Sieg, 1982;Bamber & Bird, 1997). As no males of H. shimojiensis sp. nov. were found in the sample, the degree of sexual dimorphism in antennules, chelipeds, and maxillipeds could not be checked in this genus. We found a curious morphological difference in the uropod among three observed specimens. Two specimens (holotype and one paratype) have a uniarticulate uropodal endopod but another has a triarticulate one (Figs. 5K, 5L and 6). Although we cannot reject the possibility that the latter is not conspecific to the other two specimens, we consider that, at this moment, all three must be conspecific as the other morphological features are shared among them. Such intraspecific variation in the article number of the uropodal endopod has been reported between sexes in some tanaidaceans (e.g., Siphonolabrum fastigatum Sieg, 1986(Sieg, 1986; Paratanais tara Bird, 2011(Bird, 2011), but as the case among females in Paratanaoidea (except Leptocheliidae), this is the first case for us. The female uropodal endopod with three or more articles is hitherto restricted in Leptocheliidae and Heterotanoididae (Bird & Larsen, 2009;Bird, 2012). Although Pseudozeuxo and Charbeitanais do not share this character state (in the two genera, the uropodal endopod is uni-or biarticulate), Bird & Larsen (2009), who illustrated phylogenetic relationships within Paratanaoidea with a parsimonious method using more than 100 morphological characters, suggested the close relation of Pseudozeuxidae to Leptocheliidae and Heterotanoididae (Pseudozeuxidae was nested within the Leptocheliidae-Heterotanoididae clade in their strict consensus tree; Bird & Larsen, 2009). The triarticulate state found in H. shimojiensis may support the above hypothetical phylogenetic relationship, and it should be tested by molecular phylogenetic approaches in the future. García-Herrero et al. (2018) summarized the ecological information of tanaidaceans with reports from marine subterranean environments (e.g., submarine caves and anchialine pools), and showed that most species were not restricted to such environments, i.e., they have reports from open marine environments. In the Pacific, three of four species with report(s) from submarine caves were also collected from the outside of caves (Bamber, 1997;García-Herrero et al., 2018). This study reported H. shimojiensis sp. nov. as the fifth tanaidacean species from submarine caves in Pacific. But, as this species lacks remarkable troglomorphic features in the external morphology (such as the elongation of locomotory and sensory appendages; cf. García-Herrero et al., 2018), this species may also be found in open marine environments.
Key to females of species of Pseudozeuxidae

CONCLUSION
We establish a new pseudozeuxid genus, Haimormus, based on the new species H. shimojiensis which was collected from a submarine limestone cave in the Northwestern Pacific. H. shimojiensis is the first pseudozeuxid lacking the pleopod 1, and this feature easily distinguishes the species from the other two confamilial members. This is the first tanaidacean report from submarine caves around Japan, and just a first step to reveal tanaidacean diversity there. We still have specimens not yet examined, and there are numerous unstudied submarine caves in Japanese waters. Further investigations will undoubtedly find many previously undiscovered species from submarine caves around Japan.