A new species of Enterognathus (Copepoda, Cyclopoida, Enterognathidae) collected from the Seto Inland Sea, western Japan

Abstract A new species of the endoparasitic copepod Enterognathus (Cyclopoida, Enterognathidae) is described from a crinoid host in the Seto Inland Sea, western Japan. This is a third species of the genus and its first occurrence in the Pacific Ocean. The new species is distinguished from two previously known congeners by the morphology of the body somites, caudal rami, antennae and legs. Crinoid parasites belonging to Enterognathus and the closely related genus Parenterognathus have a broad distribution from the northeastern Atlantic through the Red Sea to the West Pacific.


Introduction
The cyclopoid copepod family Enterognathidae is a compact group accommodating only four genera and six species Halsey 2004, Ohtsuka et al. 2010). All members are symbionts on Deuterostomia including crinoids, holothuroids and hemichordates (Boxshall and Halsey 2004). Two genera, Enterognathus Giesbrecht, 1900 andParenterognathus Ohtsuka, Kitazawa andBoxshall, 2010 are endoparasites of crinoids, and have a wide distribution in the northeastern Atlantic and the Indo-West Pacific regions (Ohtsuka et al. 2010).
During a research cruise in the Seto Inland Sea, western Japan in 2011, we found an undescribed species of the genus Enterognathus in a benthic sample. The genus has hitherto consisted of only two species, E. comatulae Giesbrecht, 1900 from the northeastern Atlantic and E. lateripes Stock, 1966 from the Red Sea (Giesbrecht 1900, Stock 1966, Illg and Dudley 1980, Boxshall and Halsey 2004. This is its first occurrence in the Pacific Ocean. The present paper provides a detailed description of the new species with some zoogeographical notes on the genus.
Terminology follows Huys and Boxshall (1991). The type specimen is deposited at the Kitakyushu Museum of Natural History and Human History (KMNH IvR). The host crinoid is deposited at the Atmosphere and Ocean Research Institute of the University of Tokyo.   Holotype. ♀, partly dissected, with appendages on 5 slides and body in a vial (KMNH IvR 500,539).
Description. Female. Body (Fig. 1A) 5.17 mm long, from anterior tip of rostrum to caudal ramus excluding caudal setae, flattened dorso-ventrally, weakly sclerotized, elongate, but tagmosis clearly defined. Cephalosome ca. 1.2 times wider than long; rostrum ( Fig. 1B) defined basally, slightly asymmetrical with 2 pairs of hair-sensilla. First to fifth pedigerous somites about 2.5, 1.6, 1.6, 1.2 and 1.7 times wider than long, respectively; fourth pedigerous somite (slightly twisted toward right side in Fig. 1A) exhibiting maximum width; genital double-somite protruded laterally into triangular process; each genital opening ( Fig. 1C) covered with operculum representing leg 6 and armed with minute seta; single copulatory pore possibly located on posteroventral surface as in Enterognathus comatulae (see Fig. 4 in Giesbrecht 1900) and Parenterognathus troglodytes (see Fig. 2M in Ohtsuka et al. 2010), but not clearly seen due to damage. First post-genital somite expanded anterolaterally; second and third (anal) free abdominal somites nearly as long as wide. Caudal rami (Fig. 1A, D) symmetrical, slightly curved outward, about 3.7 times as long as wide; caudal setae I to III rudimentary, IV slender, V thick and VI positioned subterminally.
Male unknown. Etymology. The new specific name "inabai" is named in honor of the late emeritus Professor Akihiko Inaba (Hiroshima University) who made great contributions to the faunistic surveys of the Seto Inland Sea (Inaba 1983(Inaba , 1988. Comparison. The present new species is more closely related to Enterognathus lateripes from the Red Sea than to E. comatulae from the northeastern Atlantic in sharing synapomorphies such as reductions in segmentation and setation: (1) only one developed seta on the caudal ramus (2 developed setae in E. comatulae); (2) 2-segmented antenna lacking a basal seta (3-segmented, with a single seta on the first segment); (3) a single element on the maxillary basis (2 elements); (4) fewer elements on the distal endopodal and exopodal segments of leg 1 (more elements); (5) 3 developed setae on the fifth leg (4 developed setae).
However it is readily distinguished from E. lateripes in the following features: (1) pedigers 2-5 wider than long (longer than wide in the latter); (2) the first post-genital somite much wider than long (about as long as wide); (3) the second and third postgenital somites about as long as wide (longer than wide); (4) the caudal ramus with 6 setae (4 setae); (5) the terminal seta of the antenna shorter than the second segment (longer); (6) the fifth leg armed with 3 developed setae and 1 minute setule (3 developed setae only); (7) the shape of the distal endopodal segments of legs 1 and 2-4 bulbous and spatulate, respectively (more or less irregular-shaped).
Members of the Enterognathidae have been characterized by the possession of a maximum of 4 setae on the female caudal ramus (see Boxshall and Halsey 2004). However the new species clearly bears 6 setae on each caudal ramus. It is probable that highly reduced setae such seta I have been overlooked in previous works. In addition, the endopods of legs 1-4 of E. lateripes seem to have been misinterpreted by Stock (1966). Stock (1966) interpreted a projection between both rami as originating from the basis, but it is revealed in the present study that it comes from the first endopodal segment. Giesbrecht's (1900) descriptions of adults and copepodid stages of E. comatulae are so elaborate that we can provide some morphological and evolutionary comments on the genus. A copulatory pore is located ventrally in the middle of the genital doublesomite in this species (Fig. 4 in Giesbrecht 1900). Although the area around the copulatory pore of the new species is damaged, it seems to be located as in E. comatulae, in consideration of the configuration of the paired gonopores. As already pointed out by Stock (1966), 4 post-genital somites are clearly illustrated in E. comatulae (Ab2-5 in Fig. 1 in Giesbrecht 1900). Giesbrecht (1900) seems to have misinterpreted his "second abdominal" somite (Ab2) as a real somite, possibly due to a clear suture line separating it from the genital somite. If a separate genital somite is retained, this might indicate that a reversal or secondary separation of the first post-genital somite from the genital somite has occurred as known in some other copepods (Huys and Boxshall 1991). The caudal ramus of the third copepodid stage of E. comatulae (Fig. 3 in Giesbrecht 1900) is similar to that of E. inabai, suggesting that this character might be neotenic.
Zoogeography. Ohtsuka et al. (2010) pointed out that the genera Enterognathus and Parenterognathus whose hosts are shallow-to deep-water crinoids are distributed in the northeastern Atlantic through the Red Sea to the West Pacific, and that their origin could have dated back to the early late Paleocene. The present discovery shows that the genus Enterognathus alone shows this broad distributional pattern in the Indo-West Pacific plus the northeastern Atlantic. These endoparasitic genera seem to have originated from warm, shallow waters along the southern Eurasian coast of the Tethys Sea in the Paleocene when the North American continent was already separated and located far from Eurasian continent, and simultaneously expanded their horizontal distribution to both the northeastern Atlantic and the Indo-West Pacific since then, as inferred by Ho (1988) for the commensal harpacticoid Sunaristes Hesse, 1867 which exhibits a similar modern distribution pattern. In addition, it may be that the ancestor of enterognathids had vertically colonized new frontiers or deep waters since then, because the monotypic Parenterognathus was collected from depths of 775-787 m (cf. Ohtsuka et al. 2010). Various symbioses between copepods and invertebrate hosts could have been newly established in shallow-to deep-waters since the Paleocene.

Key to species of Enterognathus (females only)
1 Two well developed setae on the caudal ramus; antenna 3-segmented with a single seta on first segment; 4 developed setae on fifth leg; body length <5 mm (