Limnoria segnoides Menzies, 1957

Yoshino, Hiroki & Ohsawa, Takeshi A., 2019, Description of new species of algal-boring Limnoria (Crustacea, Isopoda, Limnoriidae) from Japan and redescription of Limnoria segnoides Menzies, 1957 and L. nagatai Nunomura, 2012, Zootaxa 4550 (2), pp. 236-250 : 242-244

Limnoria segnoides Menzies, 1957

Figs 5–6

Limnoria segnoides Menzies, 1957: 184 –186 fig. 38

Material examined. Holotype: non-ovigerous female 2.7 mm (with 3 slides), Misaki, Japan, low tide level, Dr. Th. Mortensen, 16 May 1914 (ZMUC CRU-8050 ). Type locality. Misaki , Kanagawa prefecture, Japan.

Description. Body oblong and pale yellow in 70% ethanol. Most of dorsal surface of pereonite, pleon and pleotelson covered with small pores. Pleon with 5 distinct segments. Pereonal segments 1–4 nearly the same length, segments 4–7 thereafter becoming shorter. Posterior pereonal segments becomes shorter than anterior ones among segments 4–7. Coxal plates of pereonal segments 2–4 rectangular in shape and those of posterior segments prolonged acutely at posterior angle.

Pleonites 1–4 each with transverse row of many small setae. V-shaped carinae dorsomedially on pleonite 5. Pleonite 5 fringed with scale spikes and weak tubercle posteromedially.

Pleotelson medially with 1 large median tubercle followed by 1 pair of subparallel strongly defined carinae on which scale spikes form line. Lateral crests and posterior margin of pleotelson margined with sets of about 3–4 directed upward tubercles ( Fig. 5A ), which is described as a fringe of dorsally directed spike-like setae in Menzies (1957). Posterior edge of pleotelson with a fringe of thick setae and short thin setae ( Fig. 5B ).

Secondary unguis on pereopods 4 and 5 undivided ( Fig. 5C, D ). Secondary unguis on pereopod 6 slightly bifid ( Fig. 5E ). Secondary unguis on left pereopod 7 undivided, with 2 ventral spinules and right pereopod 7 undivided, with 3 ventral spinules ( Fig. 5F ).

Pleopod 2 with plumose setae up to 0.8 times length of exopod ( Fig. 6B ). Endopod of pleopod 5, oval, anterior to exopod, 0.9 times as long as endopod of pleopod 2 ( Fig. 7E ).

Uropod peduncle without lateral tubercles, 1.3 times as long as endopod; exopod without tubercles 0.3 times as long as peduncle ( Fig. 6F ).

Substrate. Collected from the red alga Corallina by washing.

Distribution. Known only from the type locality.

Remarks. Menzies (1957) did not provide sufficient description of this species for modern comparisons. Only one specimen of Limnoria segnoides is in the collection as holotype at CZM. Unfortunately, some parts were missing: antenna, mouth parts, head, pereopod 1–3, thick setae on posterior margin of pleotelson. Fortunately, the shape of maxilliped, dactylus of first pereopod, outer surface of left mandible and lacinia mobilis and setal row of right mandible were written in Menzies (1957), “Epipod of maxilliped clavate, widest at mid-region, about three times as long as wide and extending to articulation of palp with endopod. Mandibular palp lacking but is replaced by a stout long seta. […] Lacinioid seta of right mandible composed of two simple marginally serrated setae which are similar to those in setal row.” Menzies’s sketch suggested that anterior lobe of the lacinia mobilis of right mandible was slightly shorter than posterior lobe, secondary unguis on pereopod 1 is undivided, and mandibular incisours lack rasp and file.

We were able to provide additional information such as on the dactylus of pereopods 4–7, lateral and posterior margin of pleotelson, and pleopod 5, based on holotype specimen. We were not able to determine whether there are sheathed setae on posterior margin of pleotelson because all thick setae were broken and Menzies (1957) did not draw them.

There are 7 species where the mandibular palp is replaced by a single stout seta: L. furca sp. nov., L. bacescui , L. bituberculata , L. nagatai , L. segnoides , L. uncapedis and L. zinovae ( Cookson 1991, Pillai 1957, Kussakin 1963, Menzies 1957, Nunomura 2012, Ortiz & Lalana 1988). L. bacescui and L. zinovae differ from L. segnoides by shape of uropod, sculpturing of pleonite 5 and pleotelson and no branched lacinia mobilis of right mandible ( L. segnoides has 2 branched lacinia mobilis). L. bituberculata , L. segnoides and L. uncapedis share similar shape of lacinia mobilis of right mandible, but L. segnoides is separated from other 2 species by lacking the accessory spinules on the secondary unguis on pereopod 1 and by the distinctive sculpturing on pleonite 5 and the pleotelson. L. segnoides has undivided secondary unguis on pereopod 1 and V-shaped carina on pleonite 5, while L. furca sp. nov. has bifid or trifid secondary unguis on pereopod 1 and Y-shaped carina on pleonite 5 and L. nagatai has bifid secondary unguis on pereopod 1 and Y-shaped carina on pleonite 5.

Figures

FIGURE 5. Limnoria segnoides Holotype female, ZMUC CRU-8050. A, lateral margin of pleotelson. B, posterior margin of pleotelson. C, pereopod 4. D, pereopod 5. E, pereopod 6. F, pereopod 7. Scale bar: A, B = 0.1 mm, C–F = 0.05 mm.

FIGURE 6. Limnoria segnoides Holotype female, ZMUC CRU-8050. A, pleopod 1. B, pleopod 2. C, pleopod 3. D, pleopod 4. E, pleopod 5. F, uropod. Scale bar: A–F = 0.1 mm. Setae omitted. Outline of folliculinid protozoans attached to some setae also shown.

FIGURE 7. Limnoria nagatai Female KMNH IvR 500790. Dorsal view of pleonite 5, pleotelson. Scale bars: 0.1 mm.