PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 109(3):512-516. 1996. Siriella tuberculum, a new species (Crustacea: Mysidacea: Mysidae) from Akajima Island, Ryukyu Islands, Japan Kouki Fukuoka and Masaaki Murano (KF) Department of Aquatic Biosciences, Tokyo University ofFisheries, 4-5-7 Konan, Minato-ku, Tokyo 108, Japan; (MM) Institute of Environmental Ecology, METOCEAN co. LTD., Riemon 1334-5, Oigawa-cho, Shida-gun, Shizuoka 421-2, Japan — Abstract. A new species ofMysidae, Siriella tuberculum, is describedfrom the Ryukyu Islands. This species is distinguished from other species of the genus Siriella by the rounded rostrum and the armature on the uropod and telson in both sexes, and particularly by the presence ofa single, medial, dorsal protuberance on the carapace in the adult female. In the genus Siriella sexual dimorphism mm), 1 immature female (4.5 mm) and 3 is frequently observed in the carapace, an- juveniles (2.2-3.6 mm); Aka Harbor, sur- tennal scale and telson in addition to sec- face towing, 23 Jun 1990, collected by M. ondary sexual characters that include mar- Murano, T Ishimaru, K. Koike and K. Ko- supium, penis, antennule and pleopods. ike. — During a study of Mysidacea in vicinity of Description. Carapace anteriorly pro- the Ryukyu Islands, an undescribed species duced into low triangular rostral plate with which shows sexual dimorphism in the car- narrowly rounded apex, not extending to apace was collected from Akajima Island. base ofantennularpeduncles, lateral margin Females of this mysid have a dorsal tuber- of rostrum slightly concave (Fig. lA, B); clejust anteriorto the cervical sulcus which posterior margin of carapace emarginate, is lacking in male specimens. The only oth- leaving last 3 thoracic somites exposeddor- er species with a similarcharacterisSiriella sally; female, small, with an obtusely an- nodosa, the females of which have two tu- gled pre-cervical protuberance present on bercles compared to only one in the unde- median dorsal surface (Fig. IC). scribed species. In this paper we presentthe Eye short, expanded, as long as broad, description of this new species of Siriella. cornea occupying half of eye, globular, The type specimens are deposited in the wider than eyestalk; eyestalk without pa- National Science Museum, Tokyo (NSMT). pilliform process (Fig. lA, B). Antennularpeduncle ofmale morerobust Siriella tuberculum, new species than that offemale, first segment as long as Figs. 1-2 third, third segment armed with 1 seta on inner margin and 2 setae at inner distal cor- Type series.—YioXoXyipQ (NSMT-Cr 11747), ner (Fig. lA); female, first segment 1.2 adult male (6.9 mm); allotype (NSMT-Cr times longer than third, second segment 11748), adult female (6.3 mm); paratypes armed with 1 long seta at inner distal cor- (NSMT-Cr 11749), adult male (6.8 mm) ner, third segment armed with 1 long seta and adult ovigerous female (6.4 mm); Aka on inner margin and 2 long setae at inner Harbor, Akajima Island, Ryukyu Islands, 8 distal comer (Fig. IB). m, bottom trap, 24 Jun 1990. Antennal scale ofmale extending to mid- Other material: 1 immature male (4.6 dle ofthird segmentofantennularpeduncle. VOLUME NUMBER 109, 3 513 Fig. 1. Siriella tuberculum, new species. A, D, F-N: holotype; B, C, E: allotype. A, Anterior end, dorsal view; B, anterior end, dorsal view; C, carapace, lateral view; D, antenna; E, antennal scale; F, labrum; G, mandibular palp; H, first thoracic limb; I, second thoracic limb; J, endopod ofthird thoracic limb; K, endopod offourth thoracic limb; L, seventh thoracic limb; M, eighth thoracic limb; N, penis. 514 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON 3.4 times as long as broad, with indistinct Endopod of uropod extending beyond suture near distal end, distal segment as telson for Va of its length, tapering, armed long as broad, lateral margin naked and on inner ventral margin from statocyst re- straight, distal lobe 4 times as long as spi- gion to apex with 23 prominent barbed nous process terminating external margin spines, which are increasing regularly in (Fig. ID); female scale nearly 3 times lon- size towards apex and not arranged in se- ger than broad, distal lobe only slightly lon- ries; exopod ofuropod longerthan endopod ger than terminal spinous process of exter- ofuropod, divided by obscure suture at dis- nal margin (Fig. IE). Antennal peduncle tal third, proximal segment with external extending to distal fourth of scale, second margin naked except for distal Vs armed segment longest, about twice as long as with 4 spines, distal segment 1.7 times as broad and twice as long as third segment long as broad, setose all round (Fig. 2F, G). (Fig. ID). Telson 1.2 times longer than sixth ab- Labrum with forwardly directed long dominal somite, about 2.5 times as long as spine (Fig. IF). Mandibular palp with sec- broad, not reaching articulation of exopod ond segment about twice as long as broad, of uropod; lateral margin armed with 2 third segment Vs as long as second segment large spines near base, followed by un- (Fig. IG). Maxilla with second segment of armed section which occupies Va- ofthe mar- endopod more than 3 times as long as gin, distal half armed with continuous row broad. of 16-18 spines increasing in length distal- First and second thoracic endopods short, ly; distal margin armed with pair of long terminating in a strong nail. Third to eighth stout spines between which 3 tiny spines thoracic endopods slender, carpopropodus and pair of plumose setae are present (Fig. undivided; third limb with merus armed 2G). with long plumose seta at outer distal cor- Marsupium composed of3 pairs ofbrood ner, fourth to eighth limbs without such seta laminae. — on merus. Thoracic exopods 10-segmented Remarks. Siriella tuberculum clearly in first and eighth limbs, 11-segmented in belongs to the thompsoni subgroup (li second to seventh limbs; basal plates offirst 1964) because the pseudobranchial rami on to seventh exopods with small tooth on out- the second to fourth male pleopods are spi- er distal comer (Fig. IH-M). rally coiled, the male pleopods are without Penis 2.6 times as long as broad, distal modified setae, and the exopod of the uro- margin rounded, armed with 1 long plu- pod has spines confined to the distal part of mose seta at anterior comer and 3 naked the outer margin of the proximal joint. setae on apex (Fig. IN). Siriella tuberculum shows prominent Abdomen consisting of 6 somites, first sexual dimorphism in the carapace. Adult somite % length of second somite, second females have a small protuberance just an- to fifth somites subequal, sixth somite 1.5 terior to the cervical groove. This character times longer than fifth. is not present in males and relates S. tuber- Male pleopods developed; first pleopod culum to S. nodosa Hansen, 1910. In the with 10-segmented exopod, endopod pres- latter species, however, adult females con- ent as bilobed and straight pseudobranchial tain two protuberances, pre-cervical and process; second to fifth pleopods with post-cervical, along the median line of the 11-segmented exopods, endopods subequal carapace. In immature specimens of S. no- in length to exopod, 11-segmented, with bi- dosa only a single protuberance is found, lobed pseudobranchial rami which are spi- but its position is posterior to the cervical rally coiled in second to fourth pairs and groove (Hansen 1910, Tattersall 1936). Fur- straight in fifth pair; no modified setae on thermore, S. tuberculum is distinguished all pleopods (Fig. 2A-E). from S. nodosa in the following points: The VOLUME NUMBER 109, 3 515 Fig. 2. Siriella tuberculum, new species. A-G: holotype. A, First pleopod; B, second pleopod; C, third pleopod; D, fourth pleopod; E, fifth pleopod; F, endopod ofuropod, ventral view; G, uropod and telson, dorsal 516 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON telson has two spines on the lateral margin of Fisheries, Mr. K. Koike, Kitazato Uni- near the base as opposed to one in S. no- versity, and Mrs. K. Koike for their coop- dosa; and, the median margin of the endo- eration in collecting the present material. pod of the uropod is furnished with spines We also thank Dr. Richard F. Modlin, Uni- that become longer distally, while these versity of Alabama in Huntsville, for his spines are arranged in a series oflonger and critical reading the manuscript. shorter ones in—S. nodosa. Etymology. Siriella tuberculum is Literature Cited named after the presence ofa small tubercle Hansen, H. J. 19—10. The Schizopoda of the Siboga just anterior to the cervical groove of the expedition. Siboga-Expeditie 37:1-123. carapace. li, N. 1964. Fauna Japonica, Mysidae (Crustacea). Biogeographical Society of Japan, Tokyo, 610 pp. Acknowledgments Tattersall, W—. M. 1936. Mysidacea and Euphausi- acea. British Museum (Natural History), The authors wish to express our sincere Great Barrier ReefExpedition 1928-29, Scien- thanks to Dr. T. Ishimaru, Tokyo University tific Reports 5:143-176.