Meridiosignum disparitergum, Doti & Roccatagliata, 2009

Doti, Brenda L. & Roccatagliata, Daniel, 2009, On the South American species of the genus Meridiosignum (Crustacea: Isopoda: Asellota: Paramunnidae), Journal of Natural History 43 (17 - 18), pp. 1115-1138 : 1124-1130

Meridiosignum disparitergum n. sp.

( Figures 7–11 )

Material examined

Holotype: brooding ♀ (MACN-In 37529), R / V Aldebarán 2003–02, Sta. R1 , off the Río de la Plata estuary; 36 ° 46.99S, 54 ° 15.29W to 36 ° 46.69S, 54 ° 14.79W; 129 m, 6 Nov 2003 . Paratypes (same data as for holotype): 6 brooding ♀♀ (MACN-In 37530, MNHNM 1805 ) .

Diagnosis

Head anterior margin convex in the middle, with a concavity on each side and projecting lateral corners. Eyestalk with a few ommatidia. Lateral margins of pereonites dissimilar: 1 and 4 produced into single processes, 2–3 bilobed (anterior lobes similar to processes of pereonites 1 and 4), 5–6 rounded, 7 with small tooth (present only on one side in some specimens). Coxal plates 5–7 rounded. Lateral margins of pleotelson with nine strong denticles. Pereopod I propodus with one robust seta on ventral margin. Pereopod II carpus without robust setae, propodus with one robust seta on ventral margin. Uropod exopod with two terminal simple setae.

Description of brooding female

(Body description based on the holotype, appendages on paratype MACN-In 37530- a). Length: 1.44 mm ( Figure 7A, 7B ). Body width 0.62×length, widest at pereonite 3. Head length 0.38×width; anterior margin convex in the middle, with a concavity on each side and projecting lateral corners; labrum distinctly surpassing head anteriorly ( Figures 7A, 7C , 11A ). Eyestalks long axis angling forward at approximately 30 °, with four ommatidia. Lateral margins of pereonites dissimilar: 1 and 4 produced into single processes; 2, 3 bilobed (anterior lobes similar to processes of pereonites 1 and 4); 5, 6 rounded; and 7 with small tooth (present only on one side in some specimens) ( Figures 7A, 7B , 11A, 11B ). Both 1–4 and 5–7 pereonite series increasing in length backwards, pereonites 1 and 5 subequal in length (relative length ratios 1.0:1.5:2.3:2.6:1.0:1.1:1.6). Coxal plates rounded, visible in dorsal view on pereonites 5–7 only. Pleotelson width 0.9×length, lateral margins with nine strong denticles.

Antennula ( Figure 8A ), article 1 length 0.65×article 2 length, with one broom and three simple setae; article 2 longest, 1.2×length of articles 4–6 combined, with four broom and three simple setae; article 3 length 0.34×article 2 length, with one simple seta; article 4 shortest, with one broom and one simple setae; article 5 glabrous; article 6 subequal to 5, with one aesthetasc, one broom and five simple setae.

Antenna ( Figure 8B ), articles 1 and 2 glabrous; article 3 slightly longer than article 5, with six simple setae; article 4 with two simple setae; article 5 length 0.86×article 6 length, with two broom and three simple setae; article 6 longest, with five broom and four simple setae; flagellum of six articles.

Right mandible ( Figure 8C ), incisor process with five blunt cusps; setal row with five setae; molar process with one row of teeth on distal edge, and one accessory row of teeth and one cuticular comb close to distal edge. Left mandible ( Figure 8D ) as right except for: incisor process with a four-cusped lacinia mobilis and a setal row with four setae; molar process lacking accessory row of teeth. Setae on distal edge of molar process apparently absent in both mandibles.

Maxillula ( Figure 8E ) as in M. undulatum n. sp. but mesial lobe with three distinct setulate setae.

Maxilla ( Figure 8F ) as in M. undulatum n. sp. but mesial lobe with five simple setae (with rounded tip).

Maxilliped ( Figure 9A ), endite with two coupling hooks, chaetotaxy as in Meridiosignum undulatum n. sp. Epipod subovate, width 0.52×length.

Pereopod I ( Figure 9B ), basis longest article with three simple setae. Ischium 0.65×basis length, with four simple setae and one cuticular comb. Merus approximately as long as wide, with five simple setae and two cuticular combs. Carpus 0.8×ischium length; dorsal margin with two simple setae; ventral margin with two robust setae, four simple setae and one hyaline lamella; anterior surface with one cuticular comb. Propodus subequal in length to carpus; dorsal margin with four simple setae (one halfway along article and three distal); ventral margin with one robust seta, three simple setae and two hyaline lamellae; anterior surface with one simple seta and cuticular combs. Dactylus with four simple setae near distal end and two simple setae between unguis and ventral claw, unguis slightly longer than dactylus, ventral claw 0.5×unguis length.

Pereopod II ( Figure 9C ), basis longest article, with three simple setae. Ischium 0.85×basis length, with five simple setae. Merus distodorsal corner with two simple setae, ventral margin with four simple setae. Carpus slightly longer than ischium, dorsal margin with one broom and three simple setae, ventral margin with four simple setae. Propodus 0.86×ischium length, dorsal margin with one broom and three simple setae, ventral margin with one robust seta and four simple setae. Dactylus, setae arrangement as in pereopod I, unguis longer than dactylus, ventral claw weak, 0.3×unguis length.

Pereopod VII ( Figure 9D ), basis longest article with three simple setae. Ischium 0.83×basis length, with four simple setae. Merus with five simple setae. Carpus 1.15×ischium length, dorsal margin with one broom and two simple setae, ventral margin with four simple setae, anterior surface with cuticular combs. Propodus 0.91×ischium length, dorsal margin with one broom and two simple setae, ventral margin with one small robust seta and three simple setae. Dactylus as in pereopod II.

Operculum ( Figure 10A, 10B ) ovoid, width 0.73×length, with four to five simple setae on ventral surface, two at distal end, and many on lateral margins.

Pleopod III ( Figure 10C ), endopod width 0.55×length, with three plumose setae distally. Exopod articles not fully separated; with one long simple seta, which extends beyond tip of endopod setae.

Pleopod IV ( Figure 10D ), endopod width 0.48×length, exopod reaching distal end of endopod, with fringe of tiny setae on lateral margin.

Pleopod V ( Figure 10E ) width 0.4×length.

Uropod ( Figures 10F , 11C ), exopod 0.31×endopod length, with two simple setae distally; endopod with three simple setae subapically and six broom setae distally.

Size range

Brooding females: 1.35–1.45 mm.

Distribution

Only known from the type locality ( Figure 1 ).

Etymology

From the Latin dispar meaning ‘‘different’’ and tergum meaning ‘‘back’’, alluding to the unequal lateral margins of the pereonites.

Remarks

Meridiosignum disparitergum n. sp. differs from the other species of the genus by its three types of tergal lateral margins, i.e., produced into a single process, bilobed and rounded. Meridiosignum disparitergum n. sp. is most similar to M. undulatum n. sp., but it can be easily distinguished from the latter by the following characters (features dealing with M. undulatum n. sp. are in parentheses): anterior margin of head convex in the middle, with a concavity on each side and projecting lateral corners (vs. nearly straight), antenna flagellum of six articles (vs. seven articles), lateral margins of pereonites 2 and 3 deeply bilobed (vs. concavity less pronounced), lateral margins of pereonites 5 and 6 rounded (vs. produced into single processes), pleotelson with nine strong denticles on each side (vs. 12–14 small denticles on each size), exopod of uropod with two simple setae (vs. one simple seta).

Figures

Figure 1. Distribution of the South American species of the genus Meridiosignum: (Z) M. undulatum n. sp.; (O) M. disparitergum n. sp.; (m) previous records; (n) new records of M. menziesi (Winkler, 1994).

Figure 7. Meridiosignum disparitergum n. sp. Holotype brooding ♀ (MACN-In 37529). (A, B) habitus in dorsal and lateral views; (C) head in ventral view. Scales: (A, B) 0.5 mm; (C) 0.2 mm. A and B share the same scale bar.

Figure 8. Meridiosignum disparitergum n. sp. Paratype brooding ♀ (MACN-In 37530-a). (A) antennula; (B) antenna; (C) right mandible; (D) left mandible, detail of the incisor process; (E) maxillula; (F) maxilla. Scales: (A, B) 0.1 mm; (C, E, F) 0.05 mm.

Figure 9. Meridiosignum disparitergum n. sp. Paratype brooding ♀ (MACN-In 37530-a). (A) maxilliped; (B–D) pereopods I, II and VII. Scales: 0.1 mm.

Figure 10. Meridiosignum disparitergum n. sp. Paratype brooding ♀ (MACN-In 37530-a). (A) pleotelson in ventral view; (B–E) pleopods II-V; (F) uropod. Scales: (A, B) 0.2 mm; (C–E) 0.1 mm; (F) 0.05 mm. D and E share the same scale bar.

Figure 11. Meridiosignum disparitergum n. sp. SEM photographs. Paratype brooding ♀ (MACN-In 37530-b). (A, B) habitus in dorsal and lateral views; (C) uropod. Scales: (A, B) 0.2 mm; (C) 0.02 mm. A and B share the same scale bar.