Prosaetes rhinodontis ( Wright, 1876 )

Tang, Danny, Yanagisawa, Makio & Nagasawa, Kazuya, 2010, Redescription of Prosaetes rhinodontis (Wright, 1876) (Crustacea: Copepoda: Siphonostomatoida), an enigmatic parasite of the whale shark, Rhincodon typus Smith (Elasmobranchii: Orectolobiformes: Rhincodontidae), Zootaxa 2493, pp. 1-15 : 4-8

Prosaetes rhinodontis ( Wright, 1876)

( Figs 2–4 )

Stasiotes rhinodontis Wright, 1876: 584 ; Wilson 1907: 439; Wilson 1944: 533; Yamaguti 1963: 102; Heegaard 1972: 316; Boxshall & Halsey 2004: 848.

Prosaetes rhinodontis ( Wright, 1876) : Wilson 1907: 439; Yamaguti 1963: 128; Heegaard 1972: 316; Kabata 1979: 212; Kazachenko 2001: 16; Boxshall & Halsey 2004: 847.

nec Dysgamus atlanticus Steenstrup & Lütken, 1861 : Wilson 1944: 531; Williams & Williams 1986: 209.

Dysgamus rhinodontis ( Wright, 1876) : Wilson 1944: 533; Yamaguti 1963: 102; Heegaard 1972: 316.

Echthrogaleus pectinatus Kirtisinghe, 1964: 89 ; Kabata 1979: 220; Pillai 1985: 196; Boxshall & Halsey 2004: 798. [New synonymy]

Material examined. A total of 121 adult females from 1 whale shark, Rhincodon typus Smith , captured alive on 7 November, 2007 in the western North Pacific Ocean, offshore from Umino Fishing Port, Chinen-son (26°11΄N, 127°48΄E), Okinawa-jima Island, Okinawa Prefecture, Japan, and then held in captivity from 8 November, 2007 to 9 May, 2008 by the Okinawa Churaumi Aquarium in the East China Sea, offshore from Yamagawa Port (26°40΄N, 127°52΄E), Motobu-cho, Okinawa-jima Island, Okinawa Prefecture, Japan: 53 adult females from live host on 12 April, 2008; 32 adult females from live host on 30 April, 2008; 36 adult females from dead host on 9 May, 2008. Twenty adult females ( NSMT –Cr 21025) from the 12 April, 2008 sample are deposited in the Crustacea collection at the National Museum of Nature and Science ( NSMT), Tokyo, Japan. The remaining specimens are kept in the personal collection of the authors.

Redescription of adult female. Total body length (excluding setae on caudal rami) 5.90–6.70 (6.14). Prosome ( Fig. 2 A) composed of dorsoventrally flattened cephalothorax (incorporating first pedigerous somite) and 1 free thoracic somite comprised of second and third pedigerous somites fused together. Paired frontal plates ( Fig. 2 A) well-developed, lacking lunules, and ventrally fused to anterior margin of cephalothorax but demarcated dorsally by posterior suture line. Cephalothorax, large, representing about 60% of total body length, wider than long [3.5–3.9 (3.7) × 3.9–4.3 (4.11)], flattened dorsoventrally, slightly indented on mid-lateral margin, and dorsally divided into 3 zones (2 lateral and 1 cephalic-thoracic) by pair of longitudinal suture lines; each lateral zone with prominent posterior lobe typically extending to posterior margin of free thoracic somite; ornamented with weak marginal membrane, sensilla (not figured) on dorsal surface, adhesion pad ( Fig. 2 B) on anterolateral corners of ventral surface, and row of serrations ( Fig. 2 B) along ventrolateral surface. Free thoracic somite wider than long [0.95–1.05 (1.01) × 2.22–2.55 (2.41)], with well-developed lateral plate, sensilla (not figured) on dorsal surface, and adhesion pad ( Fig. 2 C) on ventrolateral margin of each lateral plate. Urosome ( Fig. 2 A) comprised of fourth pedigerous somite, genital complex, and 1 free abdominal somite. Fourth pedigerous somite relatively small, wider than long [500–600 µm (545 µm) × 1.15–1.25 (1.21)], with sensilla (not figured) on dorsal surface and strongly serrated posterodorsal plate (4 large and 3 small teeth on left side; 6 large, 2 small, and 2 minute teeth on right side); latter also grooved along posterior midline and typically extending over proximal margin of genital complex. Genital complex suborbicular, wider than long [1.85–2.15 (1.95) × 2.25–2.57 (2.41)], dorsoventrally compressed, with small rounded posterolateral lobes, sensilla (not figured) on dorsal surface, transverse row of 6 denticulate patches ( Fig. 2 D) arranged in crescentic pattern at midline of anteroventral surface, and genital apertures ( Fig. 2 E) situated ventrally near junction of abdomen. Abdomen ( Figs 2 A, E) short, wider than long [375–400 µm (382.5 µm) × 387.5–425 µm (410 µm)], concealed partially or entirely by genital complex, with several groups of pores and sensilla on dorsal and ventral surfaces. Caudal ramus ( Fig. 2 E) longer than wide [225–262.5 µm (240 µm) × 87.5–112.5 µm (102.5 µm)], with 6 setae (seta I absent) and mid-dorsal sensillum; setae III–VI highly sclerotized; seta IV longest; all setae naked. Egg sacs (Fig. 1E) uniseriate, straight, not concealed by genital complex.

Antennule ( Fig. 2 F) 2-segmented; proximal segment longer than distal segment, armed with 28 spinulate and highly sclerotized setae (26 on ventrodistal surface; 2 on dorsodistal surface); distal segment slim, bears aesthetasc and 12 setae (seta on posterodistal corner with bifurcate tip). Antenna ( Fig. 2 G) 3-segmented, comprising coxa, basis, and 1-segmented endopod incorporating distal claw. Coxa unarmed but with inner distal sclerotized ridge; basis stout, unarmed; terminal segment uncinate, long, bears 1 shorter seta proximally and 1 longer seta about halfway and fine surface ridges ( Fig. 2 H) along inner distal margin. Postantennal process ( Fig. 3 A) rudimentary, represented by small obtuse sclerite on ventral surface of cephalothorax, with 4 setulate papillae located on adjacent surface. Oral cone ( Fig. 3 B) elongate, extending slightly beyond maxillary bases, bears several serrated membranes at tip. Mandible ( Fig. 3 C) modified into elongate stylet bearing outer distal tooth and 12 inner distal teeth. Maxillule ( Fig. 3 D) small, unilobate, with palp completely fused to praecoxal endite; former represented by 3 outer naked surface setae and latter with 2 unequal apical elements. Maxilla ( Figs 3 E F), brachiform, 2-segmented, composed of elongate, unarmed syncoxa and slender basis; latter with inner subapical rounded, spinulate crista plus short, thumb-shaped clavus adjacent to its base and long apical calamus and short apical canna; calamus and canna each furnished with finely serrated membranes. Maxilliped ( Fig. 3 G) large, subchelate, 3-segmented (comprising protopod (corpus) and subchela consisting of free endopodal segment (shaft) and claw), and located on prominent pedestal. Corpus robust, with slightly raised protrusion on proximal region of posterior surface and well-developed spiniform process on myxal region; shaft short, with naked distal seta; claw elongate, recurved, with striated flange along inner margin and fine surface ridges (not figured) on anterodistal surface.

Legs 1–4 biramous ( Figs 3 H–I, 4A–B); leg 1 with bimerous rami; legs 2 and 3 with trimerous exopod and bimerous endopod; leg 4 rami incompletely 2-segmented. Armature on rami of legs 1–4 as follows (Roman numerals = spines; Arabic numerals = setae):

Leg 1 ( Fig. 3 H) intercoxal sclerite naked and elongate. Protopodal segments with several denticulate patches; outer and inner setae on basis spiniform. Exopod longer than endopod, with setules along inner margin of both segments and relatively strong denticles along proximolateral margin of distal segment; all spines with serrulate margins. First endopodal segment with denticulate patch on anterior surface; terminal segment with relatively strong denticles along outer margin and at insertion point of middle seta. Leg 2 ( Fig. 3 I) intercoxal sclerite subquadrate, with numerous fine short spinules along distal margin. Coxa with denticles along outer margin, fine spinules on anterior surface, and large adhesion pad near inner margin. Basis with fine setules along inner distal margin; outer seta naked. Exopod with setules along inner margin of first two segments; proximal segment with minute triangular hyaline process on mid-lateral margin, denticles along outer margin of both anterior and posterior surfaces, and lateral row of setules along posterior surface; last two segments with strong denticles along outer margin; all spines with serrulate margins. Proximal endopodal segment with lateral setules and distolateral group of strong denticles; distal segment with mid-lateral indentation, setules along inner margin, and strong denticles along outer margin and at insertion point of second outermost seta.

Leg 3 ( Fig. 4 A) intercoxal sclerite large, with several rows of long stiff spinules along distal margin. Coxa with large denticles along distolateral margin and 2 sets of adhesion pads and fine spinules on anterior surface. Basis with numerous rows of progressively longer stiff setules along inner distal margin. Exopod with minute hyaline knob on anterior surface of proximal segment and setules along inner margin of first two segments; all segments with strong denticles along outer margin and on posterior surface; all spines with serrulate margins. Endopod ornamented as in leg 2, except without denticles at insertion point of second outermost seta. Leg 4 ( Fig. 4 B) lacking intercoxal sclerite and proximally fused to ventrolateral surface of somite. Coxa with strong denticles near distolateral margin and patch of fine spinules and nipple-like protuberance near inner margin. Basis with fine spinules near inner margin and strong denticles on inner distal corner. Exopod 1.5 times longer than endopod, with larger denticles along outer margin of both segments and smaller denticles near insertion point of most spines on terminal segment. Endopod with small denticles along outer margin of both segments and scattered on anterior surface of terminal segment. Most spines on rami serrulate.

Leg 5 ( Figs 2 A, 4C) vestigial, unsegmented, situated on posteroventral surface of genital complex, bears stout spine and 3 naked setae apically. Leg 6 (not figured) rudimentary, represented by unarmed genital operculum at gonopore opening.

Adult male. Unknown.

Variability. Specimen No. 1 with additional denticles between insertion point of each ramus on right leg 1, highly deformed exopod on left leg 2, and additional small patch of spinules on posterior surface of terminal endopodal segment of left leg 3. Specimen No. 2 lacking inner adhesion pad and 2 spinular patches on coxa of right and left leg 3 and having additional patch of denticles along proximal inner margin of basis and row of strong denticles along outer distal corner of first exopodal segment of right and left leg 1, additional short inner spine on proximal exopodal segment of right and left leg 4, and only 4 spines on terminal endopodal segment of left leg 4. Specimen No. 3 with 7 patches of denticles on anteromedian surface of genital complex, 8 teeth (6 large, 1 small, and 1 apically bifurcate) on left side and 8 teeth (5 large, 3 small) on right side of posterodorsal plate of fourth pedigerous somite, additional patch of denticles along proximal inner margin of basis of right and left leg 1, additional row of strong denticles along outer distal margin of first exopodal segment of right leg 1, only 3 spines and 4 setae on terminal exopodal segment of right leg 2, vestigial innermost seta on terminal endopodal segment of right leg 3, and additional short inner spine on proximal exopodal segment of right leg 4.

Attachment site. The copepod uses its antennae and maxillipeds to attach to the surface of the host’s sieve-like gill rakers.

Figures

FIGURE 2. Prosaetes rhinodontis (Wright, 1876), adult female. (A) habitus, dorsal; (B) adhesion pad and serrations along left anterolateral margin of cephalothorax, ventral; (C) adhesion pad on right anterolateral margin of first free thoracic somite, ventral; (D) spinules on anteromedian surface of genital complex, ventral; (E) genital apertures, abdomen, and caudal rami (socket of missing seta indicated by arrowhead), ventral; (F) left antennule, with detail of elements near anterodistal margin of proximal segment and apical margin of distal segment, ventral; (G) right antenna, posterior; (H) tip of right antenna, ventral. Scale bars: A = 1.00 mm; B, C = 50 µm, D, F = 100 µm; E, G = 200 µm; H = 25 µm.

FIGURE 3. Prosaetes rhinodontis (Wright, 1876), adult female. (A) left postantennal process (setulate papillae indicated by arrowheads), ventral; (B) oral cone, anterior; (C) left mandible, with detail of distal end, anterior; (D) left maxillule, ventral; (E) left maxilla, posteroventral; (F) detail of clavus (cl) and crista (cr) from left maxilla, dorsal; (G) right maxilliped, posterior; (H) left leg 1, anterior; (I) right leg 2, anterior. Scale bars: A, C, D = 50 µm; B, E, H, I = 100 µm; F = 25 µm; G = 200 µm.

FIGURE 4. Prosaetes rhinodontis (Wright, 1876), adult female. (A) left leg 3, anterior; (B) left leg 4, anterior; (C) right leg 5, ventral. Scale bars: A, B = 100 µm; C = 50 µm.