Limnoria rhombipunctata, Yoshino, Hiroki, Watabe, Hikari & Ohsawa, Takeshi A., 2017

Yoshino, Hiroki, Watabe, Hikari & Ohsawa, Takeshi A., 2017, A new species of seagrass-boring Limnoria (Limnoriidae, Isopoda, Crustacea) from Japan, Zootaxa 4232 (2), pp. 251-259 : 252-256

Limnoria rhombipunctata , sp. nov.

Figs. 1–3

Material examined. Holotype: male, 4.0 mm (1.0 mm wide pleotelson, with 21 slides), Choshi-shi, Chiba Prefecture, Japan, 35°41’ N 140°51’ E, subtidal zone, Phyllospadix iwatensis rhizomes, Ohsawa A. Takeshi, Yoshino Hiroki, and Watabe Hikari, 20 April 2015 ( KMNH IvR 500862).

Paratypes: male, 4.0 mm ( KMNH IvR 500867); male, 4.4 mm ( KMNH IvR 500868); non-ovig. female, 3.9 mm ( KMNH IvR 500863); non-ovig. female, 5.1 mm ( KMNH IvR 500864); ovig. female, 4.5 mm ( KMNH IvR 500865), same data as holotype.

Type locality. Choshi-shi , Chiba Prefecture, Japan

Description. Body oblong and dark yellow in 70% ethanol. Head almost globular. Eyes black in color, each with 8 ocelli. Most of dorsal surface of pereonite, pleonite and pleotelson covered with many small setae. Pleonite composed of 5 distinct segments. Pereonal segment 1 longest, approximately 2.2 times longer than segment 2 with 2 obvious lateral grooves. Segments 2–4 nearly equal. Posterior pereonal segments become shorter than anterior ones between segments 4–7.

Pleonite 5 0.6 times as long as pleotelson. Dorsomedial region of pleonite 5 with single elevated tubercle, expanding anteriorly towards border of pleonite; with posterior pair of tubercles. Lateral crests well raised. Tubercles and lateral crests of pleonite 5 covered with small spikes.

Pleotelson with 1 strong median tubercle anteriorly, followed posteriorly by 2 pairs of carinae, and laterally with 1 more pair of short carinae not connected to tubercle. Opposing central carina pair longest and sub-parallel; joined anteriorly to more lateral parts of central carinae that widen posteriorly towards lateral sides. Lateral crests emerge anteriorly, not connected to tubercle. Many solitary scale spikes on tubercles, carinae, and lateral crests. Pleotelson margin with irregular row of dorsally directed spike-like setae from lateral crests to posterior margin of pleotelson, and with fringe of long stout setae, short unsheathed setae and spike-like setae.

Antennae similar in length. Antenna 1 with 3 flagellar articles; second article with 4 aesthetascs. Flagellum of antenna 2 with 4 articles.

Mandibular palp with 3 articles; second article longest, with 1 apical seta and 2 plumose setae; distal article shortest with 4 plumose setae. Mandibular incisors lack rasp and file. Lacinia mobilis of right mandible long and apically bifurcate. Spine row of right mandible composed of 9 stout and serrated setae, progressively increasing in length from first to last. Inner seta on inner lobe of maxilla 2 almost straight.

Epipod of maxilliped laterally-concaved triangular, approximately 4 times as long as wide, reaching past palp articulation; epipod lacking simple setae.

Secondary unguis of pereopods 1, 2, 3, 6, and 7 bifid, slightly bifid on pereopods 4 and 5. Ventral comb seta on merus of pereopod 7 and carpus of pereopods 5–7.

Pleopod 2 with plumose setae up to 0.5 times length of exopod. Appendix masculina long, reaching beyond endopod tip, articulating near midlength of endopod. Endopod of pleopod 5 shorter than exopod, oval, and 0.6 times as long as endopod of pleopod 2. Peduncles with coupling hook sequence 32220. Peduncle of pleopod 5 with simple seta laterally.

Uropod exopod with 4 simple setae and laterally recurved apical claw. Row of simple setae on endopod only placed apically. Few lateral small tubercles present on uropod peduncle. Uropod endopod 0.7 times as long as peduncle and exopod 0.3 times as long as peduncle.

Substrate. Phyllospadix iwatensis rhizome.

Distribution. Known only from the type locality.

Etymology. From the Latin rhombus, and the Latin puncta, referring to the rhombus shape constituted of 4 tubercles (puncta), 3 on pleonite 5 and 1 on the pleotelson.

Remarks. Few records exist of algal-borer limnoriids in Japan, including L. segnoides and L. zinovae . L. segnoides was recorded from Honshu Island, Japan, while L. zinovae feeds on the rhizomes of P. iwatensis around the Sea of Okhotsk and the Sea of Japan. Both species share the mandibular palp being reduced to a seta, which was also found on the algal-borers L. bacescui , L. bituberculata , L. nagatai , and L. uncapedis ( Cookson 1991, Pillai 1957, Nunomura 2012, Ortiz & Lalana 1988). The mandibular palp of L. rhombipunctata sp. nov. has 3 articles, which is a common trait among the Limnoria . L. zinovae has a Y-shaped carinae on pleonite 5 and no branched lacinia mobilis of right mandible, while L. rhombipunctata sp. nov. has 3 tubercles and no carina on pleonite 5, and a branched lacinia mobilis of right mandible.

L. magadanensis is distributed in the Sea of Okhotsk and the Sea of Japan ( Jesakova 1961, Kussakin 1979). Similarly to L. rhombipunctata sp. nov. the epipod of maxilliped reaches the palp articulation, there are 3 articles on the mandibular palp, body is covered in small setae, lateral small tubercles on the uropod peduncle, and 4 flagellar articles of antenna 2. However, L. magadanensis has semi-triangular epipod of maxilliped, 2 low transverse ridges on pleonite 5, pleotelson with 3 tubercles, and lacinia mobilis with two or three branched spinulate projections at the apex of the right mandible, while L. rhombipunctata sp. nov. has laterally-concaved triangular epipod of maxilliped, pleonite 5 with 3 tubercles, pleotelson with 1 tubercle, and 2 branches on the lacinia mobilis of right mandible. L. rhombipunctata sp. nov. is distinguished from L. borealis , L. japonica , and L. tripunctata by the sculpture of pleonite 5 and pleotelson, lacinia mobilis of right mandible, second antenna with 4 flagellum articles, posterior margin of pleotelson, and laterally-concaved triangular epipod of maxilliped.

Figures

FIGURE 1. Limnoria rhombipunctata sp. nov. Holotype, male, KMNH IvR 500862: A, dorsal view; B, pleotelson; C, antenna 1; D, antenna 2; E, maxilla 1; F, maxilla 2; G, maxilliped; Scale bars: A = 0.5 mm, B = 0.2 mm, C – D, G = 0.1 mm, E – F = 0.05 mm.

FIGURE 2. Limnoria rhombipunctata sp. nov. Holotype, male, KMNH IvR 500862: A, left mandible; B, lacinia mobilis of right mandible; C, apical region of pereopod 1; D, pereopod 1; E, pereopod 2; F, pereopod 3; G, pereopod 4; H, pereopod 5; I, pereopod 6; J, pereopod 7. Scale bars: A, C – J = 0.1 mm, B = 0.05 mm.

FIGURE 3. Limnoria rhombipunctata sp. nov. Holotype, male, KMNH IvR 500862: A, pleopod 1; B, pleopod 2; C, pleopod 3; D, pleopod 4; E, pleopod 5; G, posterior margin of pleotelson. Paratype, male, KMNH IvR 500867: F, penes; H, left uropod. Scale bars: A – E = 0.25 mm, F, H = 0.1 mm. G = 0.02 mm.