Microlaimus korari, Leduc, 2016

Leduc, Daniel, 2016, One new genus and three new species of deep-sea nematodes (Nematoda: Microlaimidae) from the Southwest Pacific Ocean and Ross Sea, Zootaxa 4079 (2), pp. 255-271 : 259-263

Microlaimus korari n. sp.

( Figs. 1 & 2 ; Table 1)

Diagnosis. Microlaimus korari n. sp. is characterised by having annulated cuticle with faint longitudinal bars, round amphideal aperture slightly smaller than the cryptospiral amphideal fovea, spacious and heavily cuticularised buccal cavity with large dorsal tooth and smaller right subventral tooth situated anteriorly relative to left subventral tooth, pharynx with large oval posterior bulb, excretory pore situated anterior to nerve ring, slender spicules 4.4 cloacal body diameters long, gubernaculum 1.2 cloacal body diameters long with laterally curved distal end and swollen proximal end, precloacal supplements absent, vulva situated slightly posterior to mid-body, and vagina directed posteriorly.

Etymology. The species name is a noun in apposition derived from the Māori term kōrari (= flower stems of New Zealand flax Phormium tenax , which are long and stiff) and refers to the long, slender shape of the spicules.

Material examined. Holotype male ( NIWA 99774 ), collected 28 May 2007 ( NIWA cruise TAN0707 , station 23), central Challenger Plateau , Southwest Pacific (168.9450º E, 38.6200º S); water depth : 482 m, sediment depth: 0–5 cm, silt/clay content: 35.5%, CaCO3 content: 89.5%. One paratype female ( NIWA 99775 ), same data as holotype. One paratype female ( NIWA 99776 ), collected 6 March 2008 ( NIWA cruise TAN0802 , station 233), abyssal plain near Scott Seamount , Ross Sea (178.8285º W, 67.6198º S); water depth : 3543 m.

Description. Male Body colourless, cylindrical, tapering towards anterior body extremity. Cuticle annulated from immediately posterior to cephalic setae to near tail tip; very faint longitudinal bars visible at level of pharyngeal bulb and posteriorly. Two subventral rows of short, stout somatic setae observed in pre-cloacal region, one subdorsal pair also present at level of spicules; somatic setae apparently absent elsewhere. Porids and epidermal glands not observed. Head rounded, slightly set off by constriction immediately posterior to cephalic setae. Six inner labial papillae and six outer labial papillae in two separate circles; four short cephalic setae situated further posteriorly, 0.15 corresponding body diameter (cbd) long. Cryptocircular amphideal fovea with cuticularised outline, situated about 1.0 cbd from anterior extremity; amphideal aperture circular, smaller than amphideal fovea ( Fig. 2A ). Buccal cavity large, 14 µm deep, funnel-shaped, heavily cuticularised, with large dorsal tooth and two smaller subventral teeth; right subventral tooth situated approximately 4 µm anteriorly relative to left subventral tooth. Cheilostoma with twelve longitudinal folds. Pharynx surrounding buccal cavity slightly swollen and forming a small bulb; large, muscular posterior pharyngeal bulb present, oval-shaped, 24 µm long. Secretoryexcretory system present; renette cell situated slightly posterior to cardia and excretory pore situated slightly anterior to nerve ring. Nerve ring situated at almost two thirds of pharynx length from anterior. Cardia 7 µm long, not surrounded by intestinal tissue.

Reproductive system diorchic, testes opposed and outstretched. Sperm cells globular, 6 × 7–8 µm. Two equal spicules, very long (4.4 cloacal body diameters) and slender, with swollen proximal end and pointed distal end. Gubernaculum 1.2 cloacal body diameters long, with distal end curving laterally and swollen proximal end, situated dorsally relative to spicules and without crurae. Precloacal supplements not observed. Tail conical, with five pairs of short and stout sub-ventral setae and three pairs of sub-dorsal setae. Three caudal glands and spinneret present.

Females Similar to male in most respects, but with slightly lower values of a, and without caudal setae. Ross sea specimen with more conspicuous longitudinal bars on cuticle, slightly smaller amphideal fovea, and with somatic setae visible in pharyngeal region. Renette cell up to 18 × 12 µm. Reproductive system didelphic, with opposed and outstretched genital branches both situated ventrally relative to intestine. Vulva situated slightly posterior to mid-body, directed posteriorly. Mature egg 44–45 µm long and 15–17 µm wide. Proximal portion of vagina surrounded by constrictor muscle.

Differential diagnosis. Microlaimus korari n. sp. differs from most other species of the genus in having long and slender spicules. Only three other species have similar spicules: M. acicularis , M. karachiensis , and M. capillaris . Microlaimus korari n. sp. can most easily be differentiated from M. acicularis by the structure of the buccal cavity, which is conspicuously larger, more heavily cuticularised, and with larger teeth in M. korari n. sp. than in M. acicularis . In addition, the gubernaculum of M. acicularis is slender along its entire length and is not curved distally or swollen proximally. Microlaimus korari is also characterised by shorter spicules than M. acicularis (84 vs 105 µm, respectively). Microlaimus . korari n. sp. can be differentiated from M. karachiensis by the structure of the buccal cavity, which is smaller and only weakly cuticularised in M. karachiensis . In addition, the excretory pore is situated posteriorly to nerve ring in M. karichiensis (vs anterior to nerve ring in M. korari n. sp.), and the gubernaculum is relatively short (<1.0 cloacal body diameter) and lacks a swollen proximal end (vs gubernaculum 1.2 cloacal body diameters long and with swollen proximal end in M. korari n. sp.). Microlaimus korari n. sp. is most similar to M. capillaris , which was originally described from a mangrove mud flat in Brazil.

The structure of their buccal cavity and pharynx is similar and the copulatory apparatus virtually identical. Microlaimus korari , however, can be differentiated from M. capillaris based on the presence of only one reflexed testis in M. capillaris (vs two outstretched testes in M. korari n. sp.) and the excretory pore situated posterior to the nerve ring in M. capillaris (vs anterior to nerve ring in M. korari n. sp.).

Species Microlaimus korari n. sp. Bolbolaimus tongaensis n. sp. Maragnopsia hadalis n. gen. n. sp.

Male Females Male Females Males Female Remarks. Jensen (1989) provided a re-description of M. capillaris based on specimens from a mangrove mud flat in Australia. These specimens, however, differ from the original description in the shape of the gubernaculum, which is slender and lacks the proximal swelling described by Gerlach (1957). Jensen (1989) also described unequal spicules, which were not observed in the Brazilian specimens, and argued that this feature was overlooked in the original description. It is likely that the two descriptions in fact represent distinct species; clarification on this matter will require observations on the Brazilian type material.

Figures

FIGURE 1. Microlaimus korari n. sp. Holotype. A: Anterior body region; B: lateral view of head; C: posterior body region; D: entire male. Scale bar: A = 20 µm; B = 45 µm; C = 26 µm; D = 65 µm.

FIGURE 2. Microlaimus korari n. sp. Female. A: Lateral view of head (Ross Sea specimen); B: lateral view of head; C: anterior body region; D: anterior body region (Ross Sea specimen); E: entire female; F: posterior body region; G: posterior body region (Ross Sea specimen). Scale bar: A & B = 20 µm; C, D & G = 50 µm; E = 65 µm; F = 38 µm.