“Coperonus comptus, new species
Holotype. Copulatory male, bl 2.6 mm, ambulatory pereopods and antennae missing, USNM 227052.
Paratypes. Preparatory female, bl 2.8 mm, USNM 227053. Preparatory female, copulatory male, ZMUC. Preparatory female, copulatory male, MNHN Is.1813. Brooding female, copulatory male, BMNH 1985:417. 89 individuals, some dissected for description, SIO.
Type-Locality. WHOI 236, 36°27.0'-28.1'S, 53°31.0'-32.3'W, 497-518 m, collected 11 March 1971 during R/V Atlantis cruise number 60.
Other Material. WHOI 239, 37 individuals. WHOI 237, 36 individuals. WHOI 245, 2 fragments.
General Distribution. Argentine Basin in the southwestern Atlantic Ocean, 497-2707 m.
Derivation of Name. Comptus means "elegant" in Latin.
Diagnosis. Apex of cephalon only slightly convex, neither linear nor strongly convex. Pleotelson posterior margin in dorsal view smoothly curving. or heart-shaped. Male antennular article 3 shorter than article 2. Maxillipedal epipod distal tip pointed, not rounded. Pereopod VI only slightly shorter than pereopod V, pereopod VI length 0.94 pereopod V length. Male pleopod I distal tips concave in ventral view, with broadly angular inner and outer lobes.
Description. Adult body length 2.5-2.8 mm (5 inds), length 1.9-2.1 width (4 inds).
Body setation: Natasome with tiny setae on dorsal and lateral surfaces; other dorsal surfaces with only scattered fine setae.
Cephalon: Dorsal length 0.31 width, length 0.42 height. Ventral margin at posterior articulation of mandible with distinct indentation or notch.
Antennula: In males, length 0.35-0.36 (2 inds) body length; in females, 0.23-0.26 (2 inds). Male antennula with 14 articles and approximately 6 aesthetascs distally; female antennula with 10-11 articles and 2-5 aesthetascs distally. Article 1 medial length 1.1 width in male, 0.75-0.78 (2 inds) in female; medial lobe of both sexes with approximately 6 denticulate setae having long sensilla and 3-4 denticles on distal tips. Articles 2 and 4 with broom setae. Articles 2 and 3 only slightly geniculate at articulation. Article 2 slightly shorter than article 1 medial lobe in females, length 0.7 medial-lobe length in males. Article 3 length 0.61 article 2 length in male, 0.71 (2 inds) in females.
Mandible: Normally developed. Both mandibles with 3 distinct cusps on incisor processes. Lacinia mobilis reaching to tip of incisor process, with 4 cusps. Left spine row with 7 members. Molar process distal end with low circumgnathal denticles, lacking large pointed cusp on ventral margin; posterior margin with 3 flattened setulate setae; triturating surface with approximately 4 sensory pores. Condyle length 0.27 mandibular body length. Palp second article length 0.49 mandibular body length.
Maxillula: Normally developed. Inner endite width 0.74 outer endite width.
Maxilla: Normally developed. Outer lobes shorter than inner lobe.
Maxilliped: Basis with 4 receptaculi and 4 fan setae distally, medial fan seta more robust, with fewer and broader branches than 3 lateral fan setae. Endite length 0.53 total basis length. Palp article 2 width greater than 2 times endite width, lateral length 1.6 medial length. Palp article 3 lateral length 0.19 medial length. Epipod short, narrow, and distally pointed; length 0.81 basis length; length 2.9 width.
Pereopodal Bases: Bases I-IV length-body length ratios in male holotype 0.22, 0.24, 0.23, 0.26; all similarly robust. Bases V-VII in brooding female shorter than bases I-IV; length-body length ratios 0.11, 0.18, 0.19.
Pereopod I: Sexually dimorphic. In males, pereopod I length 0.67 body length, with robust basis and ischium, and with tuft of setae on proximal venter of ischium; ischium length 0.48 basis length. In females, pereopod length 0.63 body length, with thin basis and ischium, and lacking tuft of setae on ischium; ischium length 0.43 basis length.
Natatory Pereopods: Natapods heterogeneous in form: pereopod V strongly natatory; pereopod VII resembling walking leg but with slightly broadened carpus and propodus, and with reduced plumose setae; pereopod VI intermediate in form. Bases, propodi, and dactyli increase in length posteriorly; ischia, meri, carpi, and natatory setae on carpus and propodus decrease in length posteriorly; widths of carpus and propodus decrease posteriorly. Pereopod V-VII length-body length ratios 0.70, 0.66, 0.60. Carpi V-VII length-width ratios 1.1, 1.3, 3.5. Propodi V-VII length-width ratios 1.6, 2.5, 5.6. Dactylus V tiny, with no distal claw (or unguis); dactyli VI-VII much longer, with claw.
Male Pleopod I: Pleopod widest proximally, abruptly narrowing midlength. Length 2.9 width; dorsal orifice 0.09 total length from distal tip. Distal tips bibbed, rounded in lateral view, slightly concave in ventral view. Fine setae on distal third of ventral surface, and 2 paired groups of setae on distal tip.
Male Pleopod II: Protopod broad proximally, narrowing to small rounded lobe distal to exopod; length 1.5 width. Small plumose setae on distolateral margin of protopod. Stylet short, half length of protopod; sperm duct opening at stylet midpoint. Exopod bare, without tuft of fine setae.
Female Pleopod II: Keel deep, sharply defined from lateral fields. Dorsal surface with scattered setae; distolateral margins with small plumose setae. Length 0.81 width; depth 0.47 length. Apex anterior to length midpoint, lacking large seta.
Pleopod III: Exopod extending to distal tip of endopod, with 2 long plumose setae, and 1 simple seta on distal tip.
Uropod: Protopod medial length 0.61 distal width. Exopod 0.69 endopod length. Endopod 0.76 medial length of protopod. Distal margin of protopod with group of unequally bifid setae on medial lobe, and few setae laterally.
Remarks. Coperonus has 3 described species and 5 undescribed species known to me. C. comptus can be identified among these only by using a combination of characters: the cephalic and pleotelson form, the male antennulae and pleopods, the comparative sizes of pereopods V and VI, and the maxillipedal epipod. It is restricted to the Argentine Basin from just below the shelf break to below 2000 m.”
(Wilson, 1989: 28-32)
