Metasphaerolaimus constrictus, Leduc, 2015

Leduc, Daniel, 2015, New species of Thelonema, Metasphaerolaimus, and Monhystrella (Nematoda, Monhysterida) from Kermadec Trench, Southwest Pacific, European Journal of Taxonomy 158, pp. 1-19 : 9-13

Metasphaerolaimus constrictus sp. nov.

urn:lsid:zoobank.org:act:C2DD1CB2-2B9B-4C31-BF13-C0E1A143A275

Figs 4–5 , Table 1

Diagnosis

Metasphaerolaimus constrictus sp. nov. is characterised by a relatively long body (1232–1623 μm), amphids located 1.0–1.4 cbd from anterior extremity, slightly arcuate spicules 1.3 abd long without

gubernaculum, tail conico-cylindrical with inner cuticle conspicuously thickened immediately anterior to cylindrical portion and with three short terminal setae.

Etymology

The species name is derived from the latin constrictus (= drawn together or contracted), and refers to the distinctive shape of the tail with thickening of the inner cuticle.

Material examined

Holotype

KERMADEC TRENCH: ♂ ( NIWA 99763 ), collected 6 May 2014 (WHOI cruise TN309, Nereus dive N074).

Paratypes

KERMADEC TRENCH: 3 ♀♀ ( NIWA 99764-6), collected 7 May 2014 ( WHOI cruise TN309, Nereus dive N075).

Type habitat

KERMADEC TRENCH: water depth: 8081 m (178.17571º W, 34.34030º S), sediment depth: 2–3 cm (holotype); water depth: 9177 m (177.65414º W, 32.85037º S), sediment depth: 0–2 cm (paratype).

Description

Male

Body cylindrical, tapering slightly towards both extremities. Cuticle faintly striated along entire body. Eight rows of somatic setae, relatively long (4–7 μm) and numerous in pharyngeal region, short and sparse elsewhere. Head rounded, with well-developed lip region. Inner labial sensillae not observed; six outer labial setae, 1–2 μm long, and four cephalic setae, 2–3 μm long, in one circle. Eight groups of three to four sub-cephalic setae, 3–8 μm long. Large circular amphideal fovea with strongly cuticularised outline, 1.4 cbd from anterior body extremity. Buccal cavity large, 24 μm deep and 13 μm wide; six H-shaped mandibles hooked anteriorly and with wide base articulating onto cuticularised rim posteriorly. Posterior portion of buccal cavity surrounded by pharyngeal tissue. Pharynx muscular, cylindrical, widening very slightly towards posterior extremity, with strongly cuticularised lumen. Cardia extend into intestine lumen. Nerve ring situated at middle of pharynx length. Secretory-excretory system not observed.

Reproductive system diorchic with two outstretched testes; anterior testis to the left and posterior testis to the right of intestine. Mature sperm cells nucleated, spherical to globular, 9 × 9–13 μm. Spicules 1.3 abd long, slightly arcuate, with swollen proximal ends and pointed distal ends. Gubernaculum and pre-cloacal supplements absent. Tail conico-cylindrical with conspicuous thickening of inner cuticle (maximum thickness ~7 μm) immediately anterior to cylindrical portion ( Fig. 5D ); cylindrical portion shorter than conical portion. A few short, sparse caudal setae present sub-ventrally and sub-dorsally; three short terminal setae, 2 μm long. Caudal glands not observed.

Female

Similar to male, but with longer body and substantially smaller amphids (0.11–0.14 vs 0.48 cbd) situated 1.0–1.4 cbd from anterior body extremity. Reproductive system monodelphic with outstretched anterior branch to the left or right of intestine. Spermatheca present, simple and not cuticularised. Vulva transverse, situated at almost two thirds of body length from anterior. Vaginal glands not observed. Three caudal glands observed in one specimen. Cuticle immediately anterior to cylindrical portion of tail up to 9 μm thick.

Remarks

Metasphaerolaimus constrictus sp. nov. can be differentiated from all other species of the genus by the distinctive tail shape with conspicuous thickening of cuticle (up to 7–9 μm thick) immediately anterior to the cylindrical portion. M. constrictus sp. nov. is most similar to M. gerlachi Jensen, 1992 , M. hadalis (Freudenhammer, 1975) , and M. inglisi Gourbault & Boucher, 1981 . The new species can be differentiated from M. gerlachi based on larger body size (1232–1623 vs 893–971 μm), larger amphids in males (0.48 vs 0.30–0.40 cbd), and presence of short terminal setae (vs long terminal setae in M. gerlachi ); from M. hadalis by higher values of a (30–38 vs 26–27), larger amphids in males (0.48 vs 0.35 cbd) and shorter tail (2.9–3.1 vs 3.4–3.8 abd); from M. inglisi by the absence of setae immediately anterior to amphids in females (vs three long setae in M. inglisi ), and presence of terminal setae (absent in M. inglisi ).

Metasphaerolaimus was initially described by Gourbault & Boucher (1981) to accommodate species similar to Sphaerolaimus Bastian, 1865 , but with a buccal cavity with six mandibles instead of a solid, heavily cuticularised capsule. Gourbault & Boucher (1981) described three new species ( Metasphaerolaimus cancellatus Gourbault & Boucher, 1981 , M. hamatus Gourbault & Boucher, 1981 and M. inglisi Gourbault & Boucher, 1981 ), and transferred the species Sphaerolaimus campbelli Allgén, 1927 , Sphaerolaimus hadalis Freudenhammer, 1975 , and Sphaerolaimus crassicauda Freudenhammer, 1975 to Metasphaerolaimus . Fadeeva (1983) later described the identical genus Ceratosphaerolaimus Fadeeva, 1893 , including one new species ( Ceratosphaerolaimus japonicus Fadeeva, 1983 ); the latter author also transferred Sphaerolaimus horrendus Sergeeva, 1981 to Ceratosphaerolaimus . Ceratosphaerolaimus was synonymised with Metasphaerolaimus by Jensen (1992), who also described Metasphaerolaimus gerlachi Jensen, 1992 . There are currently ten valid Metasphaerolaimus species including Metasphaerolaimus constrictus sp. nov. ( Table2 ). Species of this genus are mainly differentiated based on amphideal fovea size, spicule length, and length and shape of the tail. The majority of species occur in the deep sea below 1000 m depth, except for Metasphaerolaimus campbelli , M. horrendus , and M. japonicus , which were described from coastal waters. These three shallow water species are also the only species of the genus that possess a gubernaculum with dorso-caudal apophyses.

Figures

Fig. 4. Metasphaerolaimus constrictus sp. nov. A. Holotype, ♂, anterior body region. B. Holotype, ♂, head. C. Female head. D. Female anterior body region. Scale bar: A = 50 μm; B–C = 25 μm; D = 55 μm.

Fig. 5. Metasphaerolaimus constrictus sp. nov. A. Entire ♀. B–C. Female posterior body region. D. Male posterior body region. E. Male gonads. Scale bar: A = 300 μm; B–C = 55 μm; D = 45 mm; E = 80 μm.