Mysidella hoshinoi, a new species from Izu-Oshima Island, Japan (Crustacea, Mysidae, Mysidellinae)

Abstract A new mysid, Mysidella hoshinoi sp. n. is described from Izu-Oshima Island, Sagami Sea, central Japan. This species differs from its congeners in having a posterodorsal finger-like papilla on the eyestalk, five peculiar spines terminating in plumed seta on outer margin of carpopropodus of endopod of first thoracopod, and uropodal endopod bearing 27 spines on inner margin.


Mysidella nana
Our recent investigations yielded an undescribed species Mysidella from a marine benthic habitat of Izu-Oshima, Sagami Sea. Based on this material, a new species Mysidella hoshinoi sp. n. is described, and an updated identification key is provided to the known species of Mysidella.

Material and methods
Mysids were collected with sealable plastic bags (20 cm × 20 cm) by scooping seawater on a sea anemone beloninging to the family Haloclavidae by a local SCUBA diver. All specimens obtained were fixed and preserved in 80% ethanol. Each individual was dissected and prepared for observation by a light microscope (Nikon E600). The total length of individuals was measured from the end of the rostrum to the end of the telson excluding spines.
The terminology follows Murano (2002). The type specimens are deposited in the Kitakyushu Museum of Natural History and Human History, Japan (KMNH).
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.  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.
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.
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.
Description of the paratype female (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.
Variation. 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.
Color in life. 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.
Distribution and habitat. 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 Remarks. 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).
Etymology. 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.