Limnoria aspera Yoshino & Kambara & Kajiwara & Ohsawa 2021, sp. nov.

Limnoria aspera Yoshino & Ohsawa, sp. nov.

Figs 1–8

Material examined. Holotype: male 2.6 mm, Kamogawa-shi, Chiba prefecture, Japan, 35°07’N, 140°10’E, subtidal zone (0–1 m in depth), Sargassum ringgoldianum holdfasts, Takeshi A. Ohsawa and Hiroki Yoshino, 9 September 2020 (CBM-ZC 16564).

Paratypes: male 2.8 mm, Kamogawa-shi, Chiba prefecture, Japan, 35°07’N, 140°10’E, subtidal zone (0–1 m in depth), Sargassum ringgoldianum holdfasts, Yuriko Kambara, 14 July 2014 (CBM-ZC 16565). male 2.7 mm, Kamogawa-shi, Chiba prefecture, Japan, 35°07’N, 140°10’E, subtidal zone (0–1 m in depth), Sargassum siliquastrum holdfasts, Takeshi A. Ohsawa and Hiroki Yoshino, 9 September 2020 (CBM-ZC 16566). male 3.2 mm, Kamogawa-shi, Chiba prefecture, Japan, 35°07’N, 140°10’E, subtidal zone (0–1 m in depth), Sargassum siliquastrum holdfasts, Takeshi A. Ohsawa and Hiroki Yoshino, 9 September 2020 (CBM-ZC 16567).

Type locality. Kamogawa-shi, Chiba Prefecture, Japan.

Description. Body oblong, dark yellow to light brown in 70% ethanol (Figs. 1, 2A). Head almost globular. Eyes black in color, each with 8 ommatidia. Pleonite composed of 5 distinct segments. Pereonal segment 1 longest, approximately 1.5–1.7 times longer than segment 2 (Fig. 1), with 1 thinly V-shaped groove. Segments 2–4 subequal length. Posterior pereonal segments 4–7 progressively shorten. Coxal plates of pereonal segments 2–4 rectangular in shape and those of posterior segments prolonged acutely at posterior angle. Pereonites 6, 7 and pleonites 1 to 4 each with transverse row of many small setae.

Pleonite 5 approximately 0.6 times as long as pleotelson (Figs 2, 3A). Pleonite 5 dorsomedially with Y- or Ishaped indistinct longitudinal carina on which few scale spikes form line (Figs 2B, 3A). Pleonite 5 covered with fused scales, which make tile mosaics (Fig. 2C).

Pleotelson 0.6–0.8 times as long as wide, medially with 1 tubercle on which few scale spikes follow its line (Figs 2, 3A). Pleotelson covered with fused scales, which present tiled pattern on surface (Fig. 2C). Lateral crests and posterior margin of pleotelson margined with sets of about 2–5 directed upward scale spikes. Posterior edge of pleotelson with fringe of long sheathed setae and many short setae (Fig. 3B).

Antenna 1 with 4 flagellar articles; second article with 7 aesthetascs (Fig. 3D). Flagellum of antenna 2 with 4 articles (Fig. 3E).

Mandibular palp lacking, replaced by single long stout seta (Figs 4A, B). Mandibular incisors lack rasp and file (Figs 4A–B, 8A). Lacinia mobilis of right mandible branched at intermediate point, branches gradually curving 90-degrees and serrated on anterior side (Fig. 4B). Posterior branch almost same length as anterior branch. Lacinia mobilis of left mandible with 1 serrated seta.

Epipod of maxilliped, clavate, approximately 3 times as long as wide, reaching articulation of palp, with simple true setae (Fig. 4E).

Secondary unguis on pereopod 1 undivided, without spinule or with 1–7 spinules (Fig. 5A). Secondary unguis on pereopods 2–6 undivided (Figs 5B, D, 6A–B) or slightly bifid (Fig. 5C). Secondary unguis on pereopod 7 undivided with 4–7 spinules (Fig. 6C). Ventral comb seta absent on merus and present on carpus of pereopods 6 and 7. Propodus of pereopods 3–5 with prominent barbed projection opposing secondary unguis, projection reduced on pereopod 2 (Figs 5, 6, 8B).

Pleopod 2 with plumose setae up to 0.9 times length of exopod (Fig. 7B). Appendix masculina long, reaching beyond endopod tip, articulating near midlength of endopod. Endopod of pleopod 5, oval (Fig. 7C). Peduncle of pleopod 5 with simple seta laterally. Peduncles with coupling hook sequence 32220.

Uropod exopod with laterally recurved apical claw (Figs 3C, 8C). Row of simple long setae on endopod placed apically. Uropod peduncle about 1.1 times as long as endopod, with many short simple setae and row of plumose setae. Exopod about 0.4–0.5 times as long as peduncle.

Molecular data. We deposited the nucleotide sequences of COI region of mitochondrial genes (Acc. no. LC610781, LC610782) and 28S rRNA of nuclear genes (Acc. no. LC612562, LC612563) obtained from the holotype and paratype in GenBank.

Substrate. Sargassum ringgoldianum, S. siliquastrum holdfasts.

Distribution. Known only from the type locality.

Etymology. From the Latin aspera (= rough; Gender feminine), referring to the rough exoskeleton on pleonite 5 and pleotelson.

Remarks. Mandibular palp of Limnoria aspera sp. nov. is reduced to a seta. This feature is also found in 7 species: L. bacescui, L. bituberculata, L. furca, L. nagatai, L. segnoides, L. uncapedis and L. zinovae (Cookson 1991; Pillai 1957; Kussakin 1963; Menzies 1957; Nunomura 2012; Ortiz & Lalana 1988; Yoshino & Ohsawa 2019). Cookson 1991 stated that “ L. uncapedis, L. segnoides and L. bituberculata all share the following features: broad maxillipedal epipod, loss of mandibular palp, similar shape of the lacinia mobilis of the right mandible, and modification of the secondary unguis on pereopod 1”, and Yoshino & Ohsawa (2019) said that the species of non- mandibular-palp group are algal-feeding and shared the broad epipod of maxilliped, except seagrass-feeding L. zinovae, the shape of epipod of maxilliped of which is unknown. L. aspera sp. nov. fits into this group, as an algalfeeding species that also has the broad epipod of maxilliped (Table 1).

L. bacescui, L. segnoides, and L. zinovae differ from L. aspera sp. nov. by the number of flagellar articles of antenna 2, the secondary unguis on the pereopods, lacinia mobilis of right mandible, and the sculpturing of pleonite 5 and pleotelson.

L. aspera sp. nov. is similar to L. uncapedis in the number of flagellar articles of antennae 1 and 2, secondary unguis of pereopod 1, lacinia mobilis of right and left mandible, and pereopods 2–5 with barbed projection on the propodus. However, L. uncapedis is pale yellow and has no carinae on pleonite 5 and pleotelson, while L. aspera sp. nov. is dark yellow and has Y- or I-shaped carina on pleonite 5 and 1 tubercle on pleotelson.

Similarly to L. aspera sp. nov., L. bituberculata has 4 flagellar articles of antenna 1, spinules on the secondary unguis of pereopod 1, and a dorsomedial longitudinal carina between 2 longitudinal grooves on pleonite 5. However, L. bituberculata is pale yellow and has 2 prominent tubercles on the pleotelson, while L. aspera sp. nov. is dark yellow and has 1 medial tubercle on the pleotelson.

L. aspera sp. nov. seems to be most similar to L. furca, and L. nagatai. L. aspera sp. nov. is distinguished from L. furca, and L. nagatai by dark yellow body color (L. furca, and L. nagatai pale yellow), undivided or slightly bifid secondary unguis of pereopods (L. furca bifid or trifid, L. nagatai bifid), and the medial tubercle on the pleotelson lacks the attachment of two inverted V-shaped carinae (L. furca, and L. nagatai with medial tubercle followed by a pair of subparallel carinae).