Macrochiridothea estuariae, Poore, Gary C. B., 2009

Poore, Gary C. B., 2009, A new species of Chaetiliidae (Crustacea: Isopoda: Valvifera) from the Río de la Plata estuary, Argentina-Uruguay, and reconsideration of Macrochiridothea and Chiriscus, Zootaxa 2119, pp. 51-65 : 54-59

Macrochiridothea estuariae new species

( Figs 1–3 )

Material. Holotype: Río de la Plata estuary (34°49' to 36º10'S, 55°50' to 57º5'E), Argentina, oblique plankton tows from bottom to surface (nets of 330 and 550 µm mesh aperture) from RV Capitán Cánepa, 5–16 m depth, March 1998, MACN-In 37496 (adult female, 9.4 mm).

FIGURE 2. Macrochiridothea estuariae n. sp. a1, a2, antennae 1 and 2; p1, p2, p4–p7, pereopods 1, 2, 4–7 (with ends of propodi and dactyli of pereopods 2, 4, 6, without propodal setae) from paratype, adult male, 4.5 mm, NMV J55675 .

Paratypes: collected with holotype, MACN-In 37497 (adult male, 6.0 mm, adult male, 7.0 mm, 2 adult females, 6.0, 7.5 mm), NMV J55675 (adult male, 4.5 mm), NMV J55676 (ovigerous female, 5.2 mm), NMV J55677 (male, 4.5 mm), NMV J55678 (male, 3.7 mm, and juvenile male, 2.5 mm).

Diagnosis. Total body length of male 7 mm; female 9.4 mm. Dorsal surface of head, pereon and pleotelson smooth. Lateral lobes on head rounded, separated by broad angles. Coxa 6 reaching posteriorly to pleonite 3. Antenna 1 peduncle, article 2 without anterodistal prolongation. Antenna 2 peduncle, article 4, twice as long as greatest width; article 5, 5 times as long as greatest width. Pereopods 2 and 3 dactyli minute, hook-like. Pereopod 5 ischium without a prominent hook on upper margin. Pleopod 2, appendix masculina straight, apex laterally denticulate.

Description. Total body length of male 7 mm, of female 9.4 mm. Body of male 2.2 times as long as greatest width, of female 2.4 times as long as greatest width. Body smooth.

Head of male 2.6 times as wide as length in midline, of female 2.2 times as wide as length in midline; with lateral incisions, depth 3% of greatest head width; anterolateral lobe broadly rounded, separated by almost right-angled notch from linguiform, subacute posterolateral lobe, both lobes less pronounced in female than male; lateral lobes with minutely crenulate margin, anterolateral lobe with 16 marginal setae, alternating long and short, and group of 3 ventral setae near anterior margin; posterolateral lobe with 11 marginal setae, 1 ventral seta near notch, and submarginal row of 7 ventral setae. Eye of about 4 ommatidia.

Pereon of male widest at pereonite 1, tapering posteriorly; pereonite 1 with strong ‘shoulders’; each pereonite with more or less vertical lateral plates defined by obtuse longitudinal ridge; coxa 1 small but evident laterally; coxae 2–4 visible only in lateral view; coxa 5 with a free dorsal plate, subrectangular in lateral view; coxa 6 thorn-like, posteriorly directed to reach beyond posterior margin of pleonite 2; coxa 7 with small triangular dorsal plate. Pereon of female widest at pereonite 3, pereonite 1 without obvious ‘shoulders’; otherwise pereonites and coxae as in male.

Pleon 2.1 times as long as width of pleonite 2; pleonite 1 extremely short and almost concealed by adjacent segments; pleonites 2 and 3 similar, with rounded epimera; pleonite 4 defined posteriorly by complete transverse suture (female) or defined posteriorly as partial lateral sutures, not present middorsally (male); pleotelson without any sculpture.

Antenna 1 of male and female similar; peduncle article 1 with triangular dorsolateral lobe bearing 4 setae; article 2 without anterodistal prolongation, setose along anterior and posterior margins; article 3 half as long as article 2, with few setae; flagellum with major article plus 2 minute articles, 0.8 length of peduncle article 3, with 2 pairs of aesthetascs and setae on anterior margin, 3 aesthetascs at tip of major article. Antenna 2 peduncle with linear articles; article 4 twice as long as greatest width, setose along anterior margin especially; article 5, 5 times as long as greatest width, marginally setose; flagellum of 8 articles, 0.9 times length of peduncle article 5.

Pereopod 1 basis twice as long as wide; ischium twice as long as wide; carpus bluntly quadrate, with apical spiniform seta and 5 finer short setae (both sexes); propodus of male 1.5 times as long as broad, palm barely concave, with obtuse angle subdistally, with submarginal setae; propodus of female 1.2 times as long as broad, palm convex; dactylus clearly falcate (male) or almost straight, curved only at tip (female). Pereopod 2 basis about twice as long as greatest width, with about 35 long setae along proximal two-thirds of anterior margin; ischium with groups of 8 and 6 setae distally; merus with abundant setae along posterior margin, anterolateral angle produced to overlap base of propodus, with about 12 long setae; carpus without free anterior margin, posterodistal angle produced to freely overlap propodus by about 0.2 its length, with abundant long setae; propodus about 4 times as long as greatest width, with 7 setae along anterior margin and numerous setae around rounded apex; dactylus minute, hook-like. Pereopod 3 essentially similar to pereopod 2. Pereopod 4 basis 2.4 times as long as wide, with about 30 long setae along proximal two-thirds of anterior margin; ischium with groups of 8 and 6 setae distally; merus with abundant setae along posterior margin, anterolateral angle barely overlapping carpus, with about 6 long setae; carpus subrectangular, distal margin almost square, with submarginal row of 5 long setae on lateral face; propodus half width of carpus, 5 times as long as wide, with few marginal setae, 16 setae distally; dactylus short, cylindrical, with a short terminal seta. Pereopod 5 basis 2.3 times as long as wide, with about 30 long setae along proximal two-thirds of anterior margin; ischium with 2 oblique rows of 10 and 6 setae on inner face, in addition to marginal and distal setae; ischium without a prominent hook on upper margin; merus expanding distally, with setae concentrated distally; carpus 1.7 times as long as wide, with abundant setae along distal margin; propodus 5 times as long as wide, with 6 setae along anterior margin, clusters posteriorly, and circlet of 10 setae distally; dactylus tapered, with single apical seta as long as propodus. Pereopod 6, 1.4 times as long as pereopod 5; basis 2.7 times as long as wide with about 26 long setae along most of anterior margin; ischium and merus with clusters of setae on lower margin and row on upper margin; carpus 3.5 times as long as wide, with clusters of setae on lower margin and row on upper margin, with about 15 setae along distal margin; propodus 10 times as long as wide, marginally setose, with circlet of 8 long setae distally; dactylus narrow, cylindrical, with distal seta, together 0.7 length of propodus. Pereopod 7 about as long as pereopod 5; basis 3.6 times as long as wide; carpus 2.8 times as long as wide; propodus 6 times as long as wide; dactylus short, cylindrical, with a terminal seta, together 1.3 length of propodus; setation much as in pereopods 5 and 6.

Pleopods 1–3 similar, rami with setose margins; pleopod 4 exopod 2-articled, with apical seta; pleopod 5 exopod 2-articled, without setae. Pleopod 2 of male, endopod extending beyond exopod, with 10 plumose setae along distal margin; exopod with 34 plumose setae along all margins; appendix masculina exceeding end of endopod by half its total length, straight, apex laterally denticulate (with 20 saw-like teeth).

Uropodal exopod oval, half length of exopod, with 16 marginal plumose setae; exopod triangular, with 11 marginal setae concentrated distally.

Penes contiguous, subtriangular lobes posterior to margin of ventral coxal plate 7, each with 3 long setae.

Oostegites 1 broad, overlapping, longer than broad; oostegites 2–4 almost circular, overlapping; oostegites 5 linguiform, reaching midline.

Distribution. Río de la Plata estuary, Argentina.

Remarks. The overall body shape (dorsal view) of the new species is sexually dimorphic. In males, pereonite 1 is clearly the widest with strong ‘shoulders’ laterally. The female is more oval, widest at pereonite 3. Neither sex has strong lateral incisions on the head but in males the head is shorter and the incisions between the anterior and posterior lobes more pronounced. In six other described species the illustrated specimen is a female and in five a male (one not stated). We are able to confirm that M. setifer is sexually dimorphic in the same way. Moreira (1973) noted sexual dimorphism in body shape (and pereopod 1) for M. marcusi but not in a way seen in the new species. Sexual dimorphism in other species is unknown. Males of M. lilianae , M. marcusi and M. setifer are similar to that of the new species but so too is the female holotype of M. kruimeli . The female of the new species is oval like most females.

Macrochiridothea estuariae has much less developed incisions on the head than any other species of the genus. It shares a fine toothed appendix masculina with M. lilianae , M. marcusi , M. stebbingi , M. kruimeli and M. setifer but differs from these in many features. The new species shares linear distal articles on antenna 2 with M. marcusi , M. setifer and M. kruimeli and minute dactlyi on pereopods 2 and 3 with M. marcusi , M. setifer , M. lilianae and M. robusta .

We report for the first time the nature of the oostegites in Macrochiridothea . We confirmed the same arrangement in M. setifer (NMV material) and M. uncinata (S. Ahyong, NIWA, pers. comm.). The penes of M. setifer are similar to those of M. estuariae but those of M. uncinata appear more separate and with a single shorter seta (S. Ahyong, NIWA, pers. comm.).

All specimens of the new species were collected in the Río de la Plata estuary using plankton samplers (at 5−16 m depth). Species of Macrochiridothea and Chiriscus are typically benthic animals from subtidal sand to depths over 300 metres.

Figures

FIGURE 1. Macrochiridothea estuariae n. sp. a, dorsal habitus of male; b, c, dorsal and lateral habitus of female; d, ventral view of left head and pereonite 1; e, oblique view of coxae 6 and 7 of male. a, d, e from paratype, adult male, 4.5 mm, NMV J 55675. b, c from paratype, ovigerous female, 5.2 mm, NMV J 55676. Scale bar applies to a – c.

FIGURE 3. Macrochiridothea estuariae n. sp. a, sternite and coxae of pereonite 7 and penes; pl 2, pleopod 2 with detail of appendix masculina; ur, uropodal rami; t, telson apex from paratype, adult male, 4.5 mm, NMV J 55675. b, pereopod 1 from paratype, ovigerous female, 5.2 mm, NMV J 55676.