Mysidella typica G. O. Sars, 1879 (by original designation and monotypy).

Holotype. Adult ♂ (4.0 mm) (KMNH IvR 500893), 34°47'N, 139°24'E, Akino-hama, Izu-Oshima Island, Sagami Sea, Japan, 23 August 2014, 35 m.

Paratypes. Adult ♀ (4.0 mm) (KMNH IvR 500894), immature ♀ (3.0 mm) (KMNH IvR 500895), immature ♂ (3.1 mm) (KMNH IvR500896), data same as holotype; adult ♀ (3.4 mm) (KMNH IvR 500897), immature ♀ (3.0 mm) (KMNH IvR 500898), immature ♂ (2.7 mm) (KMNH IvR 500899), 34°47'N, 139°24'E,Akino-hama, Izu-Oshima Island, Tokyo, Japan, 16 August 2014, 35 m.

Eyestalk with posterodorsal finger-like papilla; carpopropodus of endopod of first thoracopod with five peculiar spines terminating in plumed seta on outer margin; terminal claw of carpopropodus of endopod of first thoracopod with one short seta and suture distinct; uropodal endopod with 27 spines on inner margin.

Carapace (Fig. 2A): anterior margin produced into short rounded rostral plate and covering basal part of eyestalks; anterolateral corner produced; posterior margin emarginated, leaving last thoracic somite exposed. Eye (Fig. 2A, C) developed; cornea well-pigmented, globular, wider than eyestalk, occupying nearly half of eye; eyestalk with posterodorsal finger-like papilla.

Antennula (Fig. 2A, H): first segment of antennular peduncle longest, 1.3 times as long as third article, with anterolateral corner produced laterally and tipped with three plumose setae, and with two short projections anterodorsally bearing some plumose setae apically; second article shortest, with short projection anterodorsally bearing four plumose setae apically and one simple seta distomedially; third article slightly wider than long, small appendix masculina on ventral side, with short projection anterodorsally bearing some short stout setae and two plumose setae apically, and with six simple setae distomedially.

Antenna (Fig. 2A, J): antennal scale setose all round, extending beyond distal margin of antennular peduncle for 0.3 of its length, 3.2 times as long as width, distal suture distinct; outer margin slightly concave; inner margin convex. Antenna peduncle 3-articlulate: first segment shortest; second and third segments subequal in length.

Labrum (Fig. 3A) rounded apically, produced posteriorly into two unequal lobes; right lobe broadly rounded posteriorly, with fine teeth on margin; left lobe smaller; both lobes with fine teeth on posterior margin.

Left mandible (Fig. 3B) without teeth; molar portion trapezoidal; first article of mandibular palp shortest; second article longest, with seven setulate setae distally; third article slightly curved, with several setae. Right mandible (Fig. 3C) without teeth and molar portion, slightly curved medially, mandibular palp similar in shape than the left one.

Maxillula (Fig. 3D): inner lobe broad, 2.6 times as wide as outer lobe, with three plumose and one simple setae; outer lobe with 12 stout setae distally.

Maxilla (Fig. 3E): exopod with nine plumose setae on margin; first article of endopod with two plumose setae distally; second article with many plumose and some simple setae on margin; bilobulate basal endites each with ten plumose setae distally; coxal endite with six plumose setae distally and four plumose setae medially.

Endopod of first thoracopod (Fig. 3F) robust: basis with two plumose setae; preischium triangular, with four plumose setae distally; ischium 1.1 times as long as basis, with five plumose setae distally; merus 0.6 times as long as ischium, with two plumose setae and one simple seta; carpopropodus 1.4 times as long as merus, twice as long as width, with five peculiar spines (Fig. 3G) terminating in plumed seta on outer margin; terminal claw nearly straight, 1.1 times as long as carpopropodus, with one short setae, suture distinct.

Endopod of second thoracopod (Fig. 3H): ischium 0.8 times as long as basis; merus longest, 1.6 times as long as ischium; carpopropodus 0.7 times as long as merus, with two rows of setae distally; dactylus small, with one long, setulate seta apically and several short setae. Endopod of third thoracopod (Fig. 4A): preischium trapezoidal; ischium 3.0 times as long as preischium; merus 1.1 times as long as ischium; carpopropodus divided into two subsegments, 0.8 times as long as merus; dactylus small, with strong terminal claw. Endopod of fourth thoracopod (Fig. 4B): preischium trapezoidal; ischium 3.0 times as long as preischium; merus 0.9 times as long as ischium; carpopropodus divided into two subsegments, 0.7 times as long as merus; dactylus small, with strong terminal claw. Endopod of fifth thoracopod (Fig. 4C): preischium triangular; ischium 6.1 times as long as preischium; merus half as long as ischium; carpopropodus divided into three subsegments, 0.8 times as long as merus; dactylus small, with strong terminal claw. Endopod of sixth thoracopod (Fig. 4D): preischium triangular; ischium 5.7 times as long as preischium; merus half as long as ischium; carpopropodus divided into three subsegments, 0.8 times as long as merus; dactylus small, with strong terminal claw. Endopod of seventh thoracopod (Fig. 4E): preischium triangular; ischium 4.2 times as long as preischium; merus 0.7 as long as ischium; carpopropodus divided into three subsegments, 0.8 times as long as merus; dactylus small, with strong terminal claw. Endopod of eighth thoracopod (Fig. 4F): preischium triangular; ischium 3.9 times as long as preischium; merus 0.7 as long as ischium; carpopropodus divided into three subsegments, 0.8 times as long as merus; dactylus small, with strong terminal claw.

Exopod of first thoracopod with 8-segmented flagellum. Exopods of second to seventh thoracopods (Figs 3H, 4A–E) similar in shape and size, with 7-segmeted flagellum; basal plate with rounded outer corner. Exopod of eighth thoracopod (Fig. 4F) with 7-segmented flagellum; basal plate narrower than those of anterior six thoracopods.

Penis (Fig. 4G) cylindrical, 6.2 times as long as width, without setae.

Abdomen: first four somites decreasing in length posteriorly; second and fifth segments subequal in length; sixth somite 1.3 times as long as fifth somite.

All pleopods (Fig. 5A–E) reduced to unsegmented lobe, not modified. First pleopod as long as second pleopod; second pleopod to fifth pleopod increasing in length; fifth pleopod 1.3 times as long as fourth pleopod.

Uropod (Fig. 2G): endopod of uropod extending to apex of apical spines of telson, 2.1 times as long as width, with large statolith and 27 spines on inner margin; exopod of uropod 3.9 times as long as width.

Telson (Fig. 2D) tapering posteriorly, 1.3 as long as maximum width; lateral margins each with three pairs of anterior spiniform setae, seven posterior spiniform setae on left side and six posterior spiniform setae on right side, and three pairs of terminal spiniform setae; cleft shallow and narrow, 0.08 times as deep as telson length, with six short spines on margin.

(KMNH IvR 500894).Antennula (Fig. 2B, I): first segment of antennular peduncle as long as third article, with anterolateral corner produced laterally and tipped with three plumose setae, and with two short projections anterodorsally bearing some plumose setae apically; second article shortest, with short projection anterodorsally bearing four plumose setae apically and one plumose and one simple setae distomedially; third article slightly wider than long, with short projection anterodorsally bearing some short stout setae and two plumose setae apically, and with two plumose setae medially and six simple setae distomedially.

All thoracopods and pleopods (Fig. 5F) similar to holotype male in morphology and chaetotaxy.

Telson (Fig. 2E): lateral margins each with two pairs of anterior, six pairs of posterior and three pairs of terminal spiniform setae; cleft with four spines on margin.

Marsupium composed of two pairs of developed oostegites on seventh and eighth thoracopods.

Some variations (N = 7: holotype and 6 paratypes) were recognized in the number of spiniform setae on telson (Fig. 2F). Lateral margins each with two or three pairs of anterior, four to seven pairs of posterior spiniform setae; cleft with one to four spines on margin.

Body (Fig. 1) dark to light read, with or without light brownish marbled pattern. Cornea of eye light orange; posterodorsal finger-like papilla on the eyestalk white. Antennular flagella transparent with white and red stripes.

The new species has so far been found only the type locality, 35 m depth, Akino-hama, Izu-Oshima Island, Sagami Sea, central Japan. According to the sampling notes by Mr. O. Hoshino, a number of individuals hovered above and around oral disc and tentacles of Haloclavidae sp. at the bottom. The mysids sometimes perched on the tentacles of the sea anemone. The new species live in ectocommensal association with sea anemones of the family.

Mysidella hoshinoi sp. n. differs from all the congeners in having a posterodorsal finger-like papilla on the eyestalk.

The arrangement of the spines of the telson links the new species to Mysidella incisa Wang, 1998, from the northern area of the South China Sea (Wang, 1998) and the Timor Sea (Murano, 2002). Mysidella hoshinoi is distinguished from M. incisa by the following characters (those of M. incisa in parentheses): cornea occupying nearly half of eye (nearly one third); eyestalk with posterodorsal finger-like papilla (without papilla); uropodal endopod 2.1 times as long as width (2.5–2.7 times as long as width), with 27 spines on inner margin (with 22–24 spines).

This species is named after Mr. O. Hoshino, who gave me the present material for taxonomic study. The specific name thus is a noun in the genitive singular.