Colopisthus Richardson, 1902

MOORE, WENDY & BRUSCA, RICHARD C., 2003, A monograph on the isopod genus Colopisthus (Crustacea: Isopoda: Cirolanidae) with the description of a new genus, Journal of Natural History 37 (11), pp. 1329-1399 : 1337-1341

Colopisthus Richardson, 1902 Synonymy. Colopisthus Richardson, 1902: 289–290 , pl. 38, figures 33–36; Richardson, 1905:

137, figure 119; Monod, 1952: 813–816; Menzies and Glynn, 1968; Kensley and Schotte,

1989: 146–147, figure 66a; Müller, 1993: 212–218, figures 72–96.

Type species. Colopisthus parvus Richardson, 1902 , by monotypy. Type material deposited at USNM (Acc. No. 37555, Lot No. 24867) .

Diagnosis. Sexual dimorphism not pronounced. Body length 3–4 mm. Coxae 4–7 visible dorsally. Pereon seven times pleon length. Body length 2.5–2.6 times greatest width; pereonite 1 longest with lateral margins forming an abrupt shelf-like angle. Cephalon with interocular depression. Antenna 1 short; peduncle composed of four articles, distal article minute; flagellum two-articulate or three-articulate. Antenna 2 short, peduncle five-articulate, proximal article minute (not easily dissected, but visible with SEM; see figure 4A), four to seven flagellar articles. Maxilliped with five-articulate palp, many palp setae jointed (of two parts), endite extended to third palp article, with one to three sharply curved coupling setae and four or five large circumplumose setae. Mandibular incisor middle tooth with minute flask-like inclusion. Mandibular palp with robust biserrate setae on articles two and three. P-1 propodal palm with two large complex setae and four or five connate spines. P-1 to P-7 dactylus often with thin, acute accessory seta and a secondary unguis. Coxal plates become progressively larger posteriorly; coxae of pereonite 7 largest and extended well beyond anterior margin of pleotelson. Penes simple, well-developed, rounded (not flattened), one and a half times longer than wide, separated by distance equal to one half width of one penis. Pleon with two or three free (unfused) pleonites, or one free and two partly fused pleonites; without epimeres. Pl-2 appendix masculina longer than pleopodal rami, flask-like (narrows abruptly distally), grooved, and articulating sub-basally on endopod; jointed with subovate basal portion and long flask-like distal region, tapering to ridged subacute apex. Pleopods with PMS decreasing in abundance posteriorly, present on all rami except endopod of P1-5; endopods never as wide as exopods; Pl-1 peduncle subquadrate. Uropodal endopod dorsal surface with two clusters (one distal and one proximo-lateral) of four or five palmate setae; peduncle with two robust setae arising from ventral surface. Pleotelson with distinct dorso-medial, longitudinal carina; apex narrowly round with fine slender setae and two stout setae.

Description

Head. Cephalon with interocular depression, without tubercles. Eyes round, well developed, partially hidden laterally by pereonite 1; ommatidia arranged in rows. Frontal margin of cephalon not medially concave, with distinct small rostral process separating first antennae. Frontal lamina not fused to rostrum, without ventral projection, anterior end expanded, anterior margin rounded with slight medial depression, posterior end moderately stemmed and abutting clypeus. Clypeus triangular; anterior margin rounded, directed anteriorly, set at less than 25° angle to frontal lamina.

Antennae. Antennae 1 short, reaching just beyond lateral margin of cephalon; peduncle four-articulate, articles 1–3 with setae (slender and/or palmate), article 1 subquadrate, not sexually dimorphic, article 2 subquadrate, subequal or shorter than article 3; article 3 longer than wide, article 4 minute, with or without setation; flagellum two-articulate or three-articulate, the first of which is elongate (twice width), the remaining being short (width greater than or equal to length), all with jointed aesthetascs, usually with slender setae, terminal article often with one uniplumose or biplumose seta. Antennae 2 peduncular article 1 minute (generally not visible except with SEM), articles 2–3 subequal, article 5 subequal to or slightly longer than 4; four to seven flagellar articles, all with slender setae.

Maxilliped. Endite well developed and reaching third palp article; with one to three coupling setae and four or five large lateral circumplumose setae. Palp article 1 (proximal article) subquadrate; article 2 wider than long (expanded on lateral border), may be partly fused with article 3; article 3 largest; articles 3–5 subquadrate. Palp article 1 with one slender seta on medial angle, articles 2–4 with slender setae on medial margin and usually on lateral margin, most dense on distal margin; articles 4 and 5 also with uniserrate setae on distal margin. Many palp setae jointed (of two parts).

Maxillae. Maxilla 1 with medial and lateral lobes distinct, gnathal surface of lateral lobe with five to seven forked setae and three or four uniserrate setae; inner lobe with three stout biplumose setae and one or two slender or plumose setae. Maxilla 2 with apical setae on all three lobes; lateral lobe with three uniserrate setae; medial lobe with two to four uniserrate setae; gnathal lobe with one large circumplumose seta and three to five stout biplumose setae and/or slender setae.

Mandible. Incisor broad, with three large teeth and a small medial accessory tooth; with minute flask-like inclusion on large middle tooth. Molar process with 11–20 small acute spines on anterior margin, dorsal surface smooth, without setae; spine row well developed, with 9–14 large flexible spines without intermediate, slender setae; without a lacinia mobilis. Mandibular palp of three articles, article 2 longest with biserrate (some unequally biserrate) and slender setae; distal article well developed, two times longer than wide, with 7–10 biserrate setae on lateral margin.

Pereon. Smooth, without ornamentation; fine setae present on dorsal surface; surface of cuticle with shingle sculpturing (visible with SEM). Pereonite 1 longest (1.3–1.6 times longer than pereonite 2); with lateral margins forming an abrupt shelf-like angle; extended forward on lateral margins to partly hide eyes. Coxae 3–7 or 4–7 visible dorsally; coxae become progressively larger posteriorly; coxae 2–3 with longitudinal ridge, 4–7 with oblique ridge; posterolateral angles of coxae 7 extend beyond anterior margin of pleotelson.

Pereopods. P-1 to P-7 dactylus often with thin, acute accessory seta and a secondary unguis lying alongside primary unguis, and one to four slender setae on superior margin; basis with weak anterior ridge. P-1 to P-3 propodus expanded and ovate; P-4 to P-7 long and ambulatory.

Pereopods 1–3. Unique, complex pattern of setae and spines as follows: P-1 merus superior-distal angle not produced (superior margin of carpus is free), inferior margin with stout molariform setae, inferior distal angle with two stout setae; carpus with two to four connate spines, propodus inferior margin with four or five connate spines and two large complex setae; superior distal angle of P-1 propodus with two stout setae. P-2 merus inferior margin (and occasionally ischium) with molariform setae; propodus (and occasionally carpus) with connate spines; propodus with two complex setae and one or two connate spines. P-3 inferior margin of ischium, merus (and occasionally carpus or propodus) with molariform setae; occasionally carpus and/or propodus with connate spines; propodus with one (occasionally two) complex setae.

Pereopods 4–7. Basis-propodus articles not greatly flattened, without connate spines or complex setae; superior distal angles of ischium, merus and carpus with one to four robust setae; merus with superior-distal angle expanded. P-5 carpus anterior-distal margin with one biplumose seta; carpus of P-6 with a row of two or three biplumose setae; carpus of P-7 with a row of five to 11 biplumose setae. P-5 to P-7 propodus with one or two robust setae on inferior margin and two or three robust setae at inferior distal angle.

Pleon. Pleon with two or three free pleonites, or with one free and two fused pleonites; without epimeres. Penes simple, well-developed, rounded (not flattened), one and a half times longer than wide, separated by distance equal to one half width of one pene.

Pleopods. Without digitate respiratory branchiae or accessory lamellae. Peduncle of Pl-1 to Pl-3 with three coupling setae on medial margin. Peduncle of Pl-4 with three or four coupling setae on medial margin. Pl-1 peduncle subquadrate; exopod and endopod elongate, not operculate. Pleopod exopods approximately 1.5 times wider than endopods; endopod and exopods subequal length on Pl-1, endopod increasingly shorter than exopod on Pl-2 to Pl-5. Endopods and exopods with PMS, decreasing in abundance posteriorly, absent on endopod of Pl-5; endopods with PMS restricted to distal margins; exopods with PMS on distal and lateral margins. Exopods of Pl-3 to Pl-5 with transverse suture. Pl-2 appendix masculina longer than pleopodal rami, flask-like (narrows abruptly distally), grooved and articulating subbasally on endopod; jointed with subovate basal portion and long flask-like distal region, tapering to ridged subacute apex.

Uropod. Uropodal endopod and exopod extend slightly beyond pleotelson apex. Peduncle with two robust ventral setae arising from mediodistal margin; disto-lateral angle not produced, with one to three slender setae; disto-medial angle produced, subacute, with one large, apical, circumplumose seta. Endopod margins and apex with slender setae; medial margin with two large robust setae; lateral margin with one robust seta; apex broadly rounded; dorsal surface with two clusters of four or five long palmate setae, one distal and one proximo-lateral. Exopod medial margin with slender setae and two robust setae; apex round, with long slender setae; lateral margin with slender setae and occasionally one robust seta.

Pleotelson. Subtriangular, with large dorso-medial longitudinal carina; apex narrowly rounded, with fine slender setae and a pair of stout setae.

Remarks

Colopisthus is a rarely collected genus that is widely distributed in the tropical Atlantic and Caribbean, but unknown elsewhere. Specimens have been collected only from the intertidal zone, usually associated with algae. Male specimens are rare, being known from only a few individuals of three species ( C. parvus , C. ronrico n. sp., C. tresesquinas n. sp.). Members of the genus Colopisthus have the most reduced pleon of all cirolanid species. The pleon is reduced from the plesiomorphic state of five free pleonal segments to two or three free segments.