Odontomolgus decens Humes 1978

Odontomolgus decens

TYPE MATERIAL.—55 , 98 , and 131 copepodids from two Fungia (Heliofungia) actiniformis (Quoy and Gaimard), in 2 m, southern end of Poelau Naira, Banda Islands, 4°31′45″S, 129°53′35″E, 2 May 1955. Holotype , allotype, and 144 paratypes (50 , 94 ) deposited in the National Museum of Natural History, Smithsonian Institution, Washington, D.C.; the remaining paratypes and the copepodids in the collection of the author.

OTHER SPECIMENS.—From Fungia actiniformis: 14 , 19 , and 16 copepodids from eight hosts, in 3 m, Natsepa, Ambon, 3°37′05″S, 128°17′00″E, 24 April 1975.

FEMALE.—Body (Figure 21a, b) slender. Prosome narrowed anteriorly in dorsal view and thickened dorsoventrally. Length 1.85 mm (1.77–1.93 mm) and greatest width 0.59 mm (0.54–0.62 mm), based on 10 specimens. Ratio of length to width of prosome 2.03: 1. Ratio of length of prosome to that of urosome 1.85: 1.

Segment of leg 5 (Figure 21c) 104 × 297. Genital segment 248 × 325, broadest in its anterior half where dorsolaterally it is slightly flattened and winglike (Figure 21b, c). Genital areas located dorsally in middle of segment. Each area (Figure 21d) with two small naked setae 6.5 and 11. Three postgenital segments from anterior to posterior 91 × 148, 66 × 135, and 95 × 135. Posteroventral margin of anal segment with row of small spinules on both sides.

Caudal ramus (Figure 21c) moderately elongated, 101 × 52, with ratio of 2: 1. Outer lateral seta (here inserted distally and a little dorsally) 440, dorsal seta 138, outermost terminal seta 300, and innermost terminal seta 528, all naked. Two long median terminal setae 847 (outer) and 880 (inner), both inserted between smooth dorsal flange and ventral flange with minute marginal spinules and both with lateral hairs in their midregions. Small spinules at insertions of outer lateral seta and outermost terminal seta. Sensilla on distal dorsal surface of ramus.

Body surface with very few hairs (sensilla).

Egg sac (Figure 21a) approximately 517 × 220, containing 25–30 eggs about 103 in average diameter.

Rostrum (Figure 21f) small and rounded, placed subterminally. First antenna (Figure 21g) 389 long. Lengths of seven segments: 35 (70 along anterior margin), 78, 49, 70, 62, 36, and 24 respectively. Formula for armature: 4, 13, 6, 3, 4 + 1 aesthete, 2 + 1 aesthete, and 7 + 1 aesthete (as in other species of the genus). All setae naked.

Second antenna (Figure 22a) 340 long with formula 1, 1, 3, and one terminal claw. All setae small and naked. Fourth segment 46 along outer side, 27 along inner side, and 22 wide. Claw 34 and distally bent slightly angularly (Figure 22b). Near base of claw a small inconspicuous setule (Figure 22a).

Labrum (Figure 22c) with two posteroventral lobes having very hyaline edges. Mandible (Figure 22d) in general form resembling that of Odontomolgus scitulus Humes, 1973, and Odontomolgus mundulus Humes, 1974, with characteristic large hyaline proximally directed leaflike process. Paragnath (Figure 22c) a small inconspicuous smooth lobe. First maxilla (Figure 22e) with four smooth setae. Second maxilla (Figure 22f) with second segment bearing inner hyaline seta with lateral spinules and posterior surficial seta minutely barbed; lash with row of slender spines. Maxilliped (Figure 22g) with two naked setae on second segment; third segment with two small naked spinelike setae and small clawlike tip.

Ventral area between maxillipeds and first pair of legs (Figure 22h) not protuberant.

Legs 1–4 (Figures 22i, j; 23a, b) with 3-segmented rami except for endopod of leg 4, which is 2-segmented. Formula for the armature as in Anchimolgus notatus above, except that the third segment of the exopod of leg 4 has III, I, 5 instead of II, I, 5.

Inner coxal seta of legs 1–3 plumose, but in leg 4 this seta with short spinules. Inner margin of basis in legs 1–3 with row of hairs, but this margin smooth in leg 4. First segment of endopod of leg 1 lacking distal outer spiniform process. Outer spine on first segment of exopod of leg 1 stronger and more sclerotized than more distal spines. Leg 4 with exopod 164 long. First segment of endopod 26 × 26, its inner distal seta 108. Second segment 42 ×, 23, its two barbed terminal spines 21 (outer) and 100 (inner). Outer margins of both segments haired.

Leg 5 (Figure 23c) with unornamented free segment 44 × 20. Two terminal naked setae 49 and 52. Seta on body near free segment 69 and naked.

Leg 6 represented by two setae on genital area (Figure 21d).

Living specimens in transmitted light opaque to gray, eye red, egg sacs dark gray.

MALE.—Body (Figure 23d) slender. Length 1.61 mm (1.54–1.76 mm) and greatest width 0.49 mm (0.47–0.51 mm), based on 10 specimens. Ratio of length to width of prosome 1.61: 1. Ratio of length of prosome to that of urosome 1.39: 1.

Segment of leg 5 (Figure 23e) 73 × 237. Genital segment 255 × 286, subquadrate in dorsal view and slightly wider in the anterior half than posteriorly. Four postgenital segments from anterior to posterior 65 × 120, 70 × 120, 55 × 109, and 78 × 117.

Caudal ramus resembling that of female but smaller, 81 × 43, with ratio of 1.88: 1.

Rostrum like that of female. First antenna similar to that of female but three aesthetes added (at points indicated by round dots in Figure 21g), so that formula is: 4, 13 + 2 aesthetes, 6, 3 + 1 aesthete, 4 + 1 aesthete, 2 + 1 aesthete, and 7 + 1 aesthete.

Second antenna (Figure 23f) resembling that of female but sexually dimorphic in having small spines along inner edge of second segment.

Labrum, mandible, paragnath, first maxilla, and second maxilla like those of female. Maxilliped (Figure 23g) 4-segmented (assuming that proximal part of claw represents fourth segment). First segment with small inner distal knob. Second segment with two naked setae and two groups of small spines. Small third segment unarmed. Claw 275 along axis (including terminal lamella), divided incompletely at its midregion, and bearing two unequal proximal setae, longer seta bent and having row of slender spinules along one side.

Ventral area between maxillipeds and first pair of legs similar to that in female.

Leg 1 with sexual dimorphism in endopod (Figure 23h) where third segment has formula I, I, 4; otherwise leg 1 as in female. Legs 2–5 resembling those of female.

Leg 6 (Figure 23i) a posteroventral flap on genital segment bearing two naked setae approximately 53 long.

Spermatophore not observed.

Living specimens with color similar to that of female.

ETYMOLOGY.—The specific name decens (Latin, = handsome or becoming) alludes to the well-proportioned body form.

COMPARISON WITH OTHER SPECIES OF Odontomolgus.—Among the five species of Odontomolgus known until now only Odontomologus scitulus Humes, 1973, from Fungia (Fungia) fungites (Linnaeus) in New Caledonia has the formula III, I, 5 on the third exopod segment of leg 4, as in Odontomolgus decens. The other four species (see Humes and Stock, 1973 and Humes, 1974) have II, I, 5 on this segment. While O. scitulus and O. decens have many similar features, the two species differ in several ways, the most obvious being the longer caudal rami in O. scitulus. Several points of distinction are mentioned in the key to the species of Odontomolgus given below.