Four new species of splanchnotrophid copepods (Poecilostomatoida) parasitic on doridacean nudibranchs (Gastropoda, Opistobranchia) from Japan, with proposition of one new genus Author Uyeno, Daisuke Author Nagasawa, Kazuya text ZooKeys 2012 247 1 29 http://dx.doi.org/10.3897/zookeys.247.3698 journal article http://dx.doi.org/10.3897/zookeys.247.3698 1313-2970-247-1 Majimun gen. n. Diagnosis of adult female. Body elongate, comprising long prosome with 3 pairs of lateral processes and 3-segmented urosome. Prosome composed of anterior region (cephalosome), middle region (including first and second pedigerous somites), and posterior region (including third and fourth pedigerous somites). Cephalosome elongate. Middle region elongate, about twice as long as wide, without posterolateral processes. Posterior region elongate. Lateral processes long and slender. Urosome small; genital double somite cylindrical, narrower at mid region, bearing paired apertures with slightly prominent posterolateral corners; opercula bearing small shield-like structure with 2 spiniform processes. Caudal rami small, bearing 6 setae; apical seta styliform. Antennule 3-segmented bearing spiniform elements; proximal segment subdivided into basal part with 4 spines and distal part with 3 elements; middle segment bearing 3 elements; terminal segment bearing 11 elements. Antenna 3-segmented; coxo-basis broad, bearing 1 medial spiniform element; proximal segment of endopod bearing 1 inner spiniform element; terminal segment of endopod drawn out into strong apical claw, with 4 and 1 elements along outer and inner margins, respectively. Labrum bilobate, bearing paired extra lobes and small central, conical protrusion. Mandible spatulate, drawn out into blade with 3 dentiform processes. Maxillule not observed. Paragnath bulbous lobe. Maxilla 2-segmented; syncoxa unarmed; allobasis tapering into curved process, with seta. Maxilliped absent. Labium bearing single pointed process, small paired protrusions ornamented with spinules, and posterolateral patches of spinules. Legs 1 and 2 composed of protopod largely incorporated into ventral wall of prosome, with exopodal and endopodal lobes; protopod bearing outer basal seta, small protrusion at base of endopodal lobe of leg 1; exopodal lobe indistinctly 2-segmented, tapering into spiniform apical process; endopodal lobe cylindrical bearing apical process. Leg 3 represented by conical process with apical seta. Egg sacs cylindrical and spiral. Diagnosis of adult male. Body cyclopiform, composed of cephalothorax and 5 cylindrical somites. Cephalothorax large, bulbous, incorporating first and second pedigerous somites, bearing transverse constriction posterior to mouth parts and paired posterolateral outgrowth. Genital somite bearing paired apertures; opercula unarmed. Caudal rami conical, about as long as wide, bearing 6 setae; apical seta styliform. No marked sexual dimorphism in antennule and mouth parts. Shape of antenna as in female except terminal endopodal segment bearing 5 elements; inner margin bearing 2 of 5 elements. Mandible elongate, drawn out into spatulate apical blade with 3 dentiform processes. Legs 1 and 2 composed of round protopod with outer basal seta, indistinctly 2-segmented exopodal lobe drawn out into pointed process, and non-segmented endopodal segment with apical small process. Leg 3 represented by conical process with apical seta. Type and only species. Majimun shirakawai sp. n. by the present designation. Etymology. The generic name, "majimun" , refers to a dialect in Okinawa, which means demons. The gender is neuter. Remarks. Females of Lomanoticola and Splanchnotrophus differ from Majimun gen. n. in having a 2-segmented urosome comprising the genital double somite and the anal somite (vs. a 3-segmented urosome and the genital double somite separated from the abdomen) ( Huys 2001 ; present study). Females of Ismaila spp. share a 3-segmented urosome, which includes 1 postgenital somite (vs. 2 somites). There are also differences in the following female characters between Ismaila and the new genus: the antennule is 2-segmented (vs. 3-segmented); the mandible consists of a small rod tipped with a short tooth and a slender spine (vs. drawn out into a blade with dentiform processes); the paragnath is absent (vs. present); the maxillule is made of a lobe with 2 setae (vs. absent); and the maxilla has the allobasis drawn out into a multipinnate endite with 2 accessory elements (vs. with the allobasis drawn out into a curved process with 1 seta)(see Ho 1981 ; Haumayr and Schroedl 2003 ). Majimun gen. n. does not show distinct sexual dimorphism in the antennule, the antenna, and the mouth parts. The new genus also possesses a 3-segmented antennule, the mandible, and the paragnath in both sexes and 2 postgenital somites in females. These characters are not shared with Arthurius (see Huys 2001 ). All of these characters are shared with Ceratosomicola , except the 3-segmented antennule. However, Ceratosomicola differs from the new genus by having the following characters in both sexes: the antennule is composed of 4 distinct segments (vs. 3 segments); the antenna is conical (vs. elongate); the mandible is covered with numerous spinules (vs. spatulate bearing a blade with dentiform processes around apex); and the maxilla possesses a lanceolate process without armature (vs. with process and 1 seta) (see Huys 2001 ; present study).