Politolana impressa, (Harger, 1883)

Riseman, Sarah F. & Brusca, Richard C., 2002, Taxonomy, phylogeny and biogeography of Politolana Bruce, 1981 (Crustacea: Isopoda: Cirolanidae), Zoological Journal of the Linnean Society 134 (1), pp. 57-140 : 118-125

Redescription of P. impressa ( Harger, 1883)

( Figs 44–48 )

Synonymy. Cirolana impressa: Harger (1883) : 93–93, pl. 1, Figs 3, 3 (d), pl. 2, Figs 3, 3 (c); Verrill (1885): 559, pl. XXXVI, Fig. 165; Richardson (1900): 216 1901: 513 1905: 83, 97–99, Figs 78, 79; Schultz (1969): 184, Fig. 287; Watling et al. (1974): 347–349; Kussakin (1979): 203–205, Figs 81, 82; Jones (1979): 318–319. Politolana impressa: Bruce (1981a) : p.959; Kensley & Schotte (1989): 140–142, Fig. 63(a)–(b).

Type material. Adult female lectotype and three paralectotypes (1 nongravid female, 2 gravid females), herein designated from syntype series, MCZ # 3906a .

Type locality. East of Cape May , NJ, 38°21'50 N, 73°32'W, 361 m.

Additional material examined. USNM # 11189 : off Cape Hatteras , NC, 35°21.30 N, 74°52.00 W, col. by USFC, R/V Albatross, sta. 2306; 2 females (form a). USNM # 35321 : E Querean , Canada, 348 m, from halibut, col. by J.W. Collins, USFC, 2 June 1879; 2 females, 1 male (form a). USNM # 35930 : Georges Bank , off MA, 270 m 1872: 1 specimen. USNM # 236180 : off Palm Beach , FL, 73.2 m, col. by Thompson & Mcginty, R/V Triton, February 1950; 2 females (form a). USNM # 236182 : Bahamas, R/V Iselin, cruise Cr 8007, sta. 67, 21 September 1980; 2 females (form a) .

Series of specimens from off Martha’s Vineyard , MA, col. by USFC R/V Fishhawk— USNM # 36392 : 587 m, sta. 1095, 11 August 1882; 1 female (form a). USNM # 36404 : 551 m, sta. 1094, 11 August 1882; 1 gravid female (form a). USNM # 38127 : 287 m, sta. 943 1881; 4 females, 1 male, 2 juveniles (form a).

Series of specimens from Gulf of Mexico, col. by Marfin-Geryon cruise— GCRL # 1351 : 27°01 N, 84°56 W, 494 m, 6 May 1987; 3 females, 4 males (form a). GCRL # 1352 : 28°59.10 N, 88°25.33 W, 677 m, 2 August 1987; 15 + specimens (form a). GCRL # 1354 : 28°59.10 N, 88°25.33 W, 677 m, 2 August 1987; 50 + specimens (form b). GCRL # 1356 : 29°01.52 N, 88°27.04 W, 494 m, 2 August 1987; 15 + specimens (form b). GCRL # 1367 : 27°54.07 N, 85°15.19 W, 494 m, 7 May 1987; 15 + specimens (form a). GCRL # 1368 : 677 m, locality unknown; 11 specimens (forms a and b) .

Diagnosis. Body typically broad, pereon loosely articulating with and overlapping first two pleonites. Coxae 4–6 narrow, with distinct oblique impressions, coxae 7 with small punctations marking residual oblique impression. Cephalon anterior margin medially concave or straight; lateral margins of raised frontal ridge dividing eyes. Eyes large and round. Interocular furrow complete. Antennae reaching middle of second pereonite. Pereopods 1–3 with ischium and merus superior distal angles greatly produced, with meral lobe reaching midpoint of propodus, inferior margin of merus with row of narrow acute robust setae. Pereopods 5–7 posterior distal margin of ischium with studded-biserrate setae. Pereopod 6 with distal angles of merus and carpus bearing long distally biserrate setae. P6 propodus inferior margin with short acute distally biserrate setae. Pereopod 7 superior distal angle of merus with dense long slender plumose setae. Uropod endopod apex truncate; exopod shorter than endopod, peltate. Pleotelson posterior margin narrow, slightly subacute.

Description of lectotype. Female, 21 mm long; body broad, length about 3.5 Ⅹ width. Pereon cuticle highly polished; cream colour in alcohol with scattered, darkly pigmented chromatophores on pereon, pleon and pleotelson.

Cephalon: With light minute punctation. Anterior margin medially concave or straight, without distinct rostral point; lateral margins of raised frontal ridge extending over and dividing eyes. Eyes small, subquadrate and darkly pigmented; sometimes with midanterior notch of missing ommatidial facets where divided by frontal ridge. Interocular furrow complete. Frontal lamina narrow, length about 3.1 Ⅹ greatest width, with slight hourglass shape, reaching middle of antennule peduncles. Frontal lamina and clypeus with lateral margins raised as thickened ridges.

Antennule: Peduncle article 2 posterior distal angle with several short palmate setae. Flagellum composed of 11 articles, each with 4–7 long aesthetascs; first flagellar article longest with length subequal to width; width of subsequent articles about twice length.

Antenna: Reaching middle of second pereonite. Peduncle articles 3–5 progressively longer; article 3 length subequal to width, widening slightly distally; article 4 longer than wide, subquadrate, posterior distal angle bearing about two long stiff simple setae; article 5 approximately 2.5Ⅹ longer than wide, with width about half the basal width of article 4, posterior distal angle bearing a few short simple setae and several short circumplumose setae. Flagellum composed of 22 articles.

Mandible: Molar process dorsal surface with fine setae, concentrated in distinct patch on distal posterior margin ( Fig. 45C,E ); anterior margin with spines widely spaced, not touching at their bases; molar process with submarginal setal row of long lightly plumose setae extending from proximal cluster.

Maxilliped: Endite with distal cluster of approximately three robust circumplumose setae; right endite with two coupling hooks, left endite with one coupling hook.

Pereon: Body highly vaulted. Medially, pereonite 2 shorter than pereonite 1, pereonites 3–7 subequal in length, 6 and 7 slightly longer; pereonites 1–6 with dorsal medial round depression housing two minute pores, pores present but without depression on pereonite 7; in dorsal aspect, pereonite 1 narrowing markedly anteriorly to encompass cephalon; with distinct impression along lateral margin, anterolateral margins straight forming blunt anterior angle. Coxae 2 and 3 with impression parallel to lateral margin; coxae 4–6 narrow, with distinct oblique impressions, posterior margins oblique, progressively more extended posteriorly, with acute posterior angles; coxae 7 with small punctations marking residual oblique impression, length subequal to length of coxae 6, extended to pleonite 3 or 4. Posteroventral angle of pereonite 7 extending beyond posterior margin of respective coxae.

Pleon: First two pleonites loosely overlapped by pereonite 7; pleon cuticle less polished than pereon cuticle; dorso-ventrally narrower than pereon. Epimeres of pleonites 1–4 produced posteriorly and flared laterally; epimeres 2–4 bearing dense lateral setal fringe. Ventral flanges with ventral posterior angles rounded. Pereopods 1–3: Ischium superior distal angle produced into large scoop-shaped lobe; posterior face of ischium with two oblique rows of simple setae. Merus with superior distal angle produced into large recurved lobe reaching middle of propodus, with one (P1) or two (P2 and P3) giant apical setae; inferior margin of merus with row of long acute robust setae and row of adjacent short acute robust setae. Ischial and meral lobes of P1–P3 progressively less produced. Carpus of P2 and P3 subquadrate. Distal superior margin of propodus with short row of simple setae.

Pereopods 4–6: Ischium superior margin with sparse simple setae; distal margin of ischium with studdedbiserrate setae (absent on P4). Merus and carpus with robust setae arranged in transverse rows or clusters extending from inferior margin, progressively less setose on P5–P7. Carpus longer than wide. Pereopod 6 with distal angles of merus and carpus bearing long distally biserrate setae. Pereopod 6 propodus inferior margin with short acute distally biserrate setae and a robust studded-serrate seta on inferior distal angle.

Pereopod 7: Subequal in length to P6. Like P6 except: ischium width less than meral width; superior distal angle of merus with dense long slender plumose setae, long slender distally biserrate setae, and short acute robust setae; carpus distal margin expanded, about twice as wide as proximal propodal width.

Pleopods: Pleopod 1 peduncle subquadrate, or with length only slightly less than width, with six plumose coupling hooks; endopod width about one-half of exopod width. Pleopods 2 and 3 peduncle with five plumose coupling hooks. Pleopod 4 with four coupling hooks.

Uropod: Peduncle medial production distally acute, with 4–5 long plumose setae. Endopod apex flattened or truncate, typically with 3–4 very small robust setae, PMS and apical cluster of short simple setae (see variation section). Exopod shorter than endopod, peltate: broad, flat, with convex lateral margins; apex typically with very small robust seta (see variation section), distal medial margin with small robust setae.

Pleotelson: Faintly bent along dorsal midline; posterior margin narrow, slightly subacute, with PMS and with evidence of 2–4 small robust setae (abraded off specimen).

Sexual dimorphism. Males and females similar. Males with narrow appendix masculina, tapering evenly to narrow pointed apex; reaching beyond distal margins of pleopod 2 rami.

Variation. Larger animals may have more setation and more coupling hooks on the maxilliped endites. Antenna peduncle article 5 2–3Ⅹ longer than wide; peduncle articles 4 and 5 may have more setae than are present on the lectotype, but never with dense setae. The pleotelson apex is usually subacute, but in some cases may be more evenly rounded.

The distal margins of the uropod rami and the posterior margin of the pleotelson of the type specimens have had the setae abraded off. Non-type specimens have 2–4 small (difficult to discern with a light microscope) blunt robust setae on the pleotelson apex, 3–4 small robust setae and an apical cluster of short simple setae on the uropod endopod distal margin, and a minute apical robust seta on the uropod exopod.

Remarks. Politolana impressa is easily distinguished from P. polita , P. concharum and P. impostor by the presence of oblique impressions (after which this species was named) on the coxae of pereonites 4–6. Politolana impressa also has eyes that are typically rounder than in these other species. It differs from P. haneyi in the shape of the uropod endopod, which is more truncated in P. impressa , the shape of the pleotelson posterior margin, which is usually subacute in P. impressa and evenly rounded in P. haneyi , and the form of the pleon, which is dorso-ventrally narrower than the pereon in P. impressa and vaulted in P. haneyi .

The majority of P. impressa specimens that we have examined are of the same broad form as the lectotype and paralectotypes (marked ‘form a’ for some lots in the material examined list). However, as in P. eximia , some animals have a longer more tubular body form, which can be up to 5.5Ⅹ longer than wide (‘form b’). The latter were collected in the Gulf of Mexico and closely resemble P. wickstenae , having the longer body form and more ‘loosely’ articulated segments. ‘Form b’ P. impressa can be distinguished from P. wickstenae by the absence of eyes, and the presence of simple setae along the entire distal margin (ventral face) of the maxillipedal palp in the latter.

While the two body forms noted above appear quite different, there is a gradient between the two, and at least one lot contains animals of both types, leading us to conclude that this difference is intraspecific variation. In addition, the narrow form of P. impressa (form b) has been collected only in the Gulf of Mexico at depths where P. wickstenae co-occurs. At this point, we cannot conclusively rule out the possibilities that ‘form b’ individuals are the result of hybridization with P. wickstenae , or that P. wickstenae is in fact a blind morph of P. impressa form b.

In his original description of P. impressa, Harger did not fully illustrate the species, which shows considerable polymorphism hitherto unrecorded. We have documented two distinct morphs within the range of this polymorphism. Furthermore, this species grades almost imperceptibly into P. wickstenae of the Gulf of Mexico. Thus, for the sake of nomenclatural stability, we herein designate a lectotype for P. impressa .

Distribution. Politolana impressa has a broad Atlantic distribution, including the region south of Cape Hatteras, the mid-Atlantic sea board and the coldtemperate region north of Cape Cod, although it has been recorded only rarely from this latter area. In the region north of Cape Hatteras, P. impressa ’s distribution overlaps with those of the three west Atlantic cold-temperate species, P. polita , P. concharum and P. impostor . Politolana impressa is the deepest dwelling western Atlantic Politolana species , found at depths from 73 m to 610 m, but most commonly at depths greater than 300 m. In addition to its broad Atlantic distribution, P. impressa is also found within the north-west Gulf of Mexico, at depths of about 500– 700 m.

Figures

Figure 3. Unambiguous character change in Politolana sensu stricto, with branch lengths proportional to change. Branch numbers, referred to in the text, are indicated in the box to the left of the branch. Nonhomoplasious characters are marked with black bars, homoplasious characters are marked with white bars, and those characters that are homoplasious in the context of the entire phylogeny, but which are not homoplasious within Politolana sensu stricto, are marked by grey bars. The character number and direction of state change are given to the right of the bar and correspond to the character list (Appendix 1). This clade has a CI = 0.78, RI = 0.78 and RC = 0.61 (with uninformative characters removed).

Figure 44. Politolana impressa female lectotype: (A) lateral aspect; (B) dorsal aspect; (C) pleon detail; (D) frontal lamina; (E) antenna; (F) antennule. Politolana impressa male: (G) penes.

Figure 45. Politolana impressa female lectotype: (A) right maxilliped; (B) maxilla; (C) right mandible; (D) maxillule; (E) mandible molar process detail, anterior surface.

Figure 46. Politolana impressa female lectotype: (A–C) pereopods 1–3, respectively.

Figure 47. Politolana impressa female lectotype: (A, B) pereopods 4 and 5, respectively.

Figure 48. Politolana impressa female lectotype: (A, B) pereopods 6 and 7, respectively.