Monhystrella kermadecensis, 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 : 15-17

Monhystrella kermadecensis sp. nov.

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Fig. 6 , Table 1

Diagnosis

Monhystrella kermadecensis sp. nov. is characterised by circle of papillose outer labial sensillae slightly anterior to circle of four short cephalic setae, amphideal fovea 1.9 cbd from anterior extremity and 0.33– 0.43 cbd (females) or 0.43–0.50 cbd (males), gubernaculum with caudal apophyses, presence of distinct cuticularised piece along anterior vaginal wall, relatively short conical (males) or conico-cylindrical tail (females) with conical, ventrally-curved spinneret.

Etymology

This species is named after the type locality.

Material examined

Holotype

KERMADEC TRENCH: ♂ ( NIWA 99767 ), collected 6 May 2014 (WHOI cruise TN309, Nereus dive N074).

Paratypes

KERMADEC TRENCH: 2 ♂♂ ( NIWA 99768) and 5 ♀♀ ( NIWA 99769) same data as holotype.

Type habitat

KERMADEC TRENCH: water depth: 8081 m (178.17571º W, 34.34030º S), sediment depth: 0–1 cm.

Description

Male

Body cylindrical, tapering slightly towards both extremities. Cuticle smooth. Anterior end rounded or slightly truncated. Six minute inner labial papillae, difficult to observe; circle of six small outer labial papillae slightly anterior to circle of four short cephalic setae, 0.23 cbd long. Amphideal fovea mediumsized, circular, situated 1.9 cbd from anterior end, outline not cuticularised. Two short setae situated in sublateral row posterior to amphideal fovea. Buccal cavity small, funnel-shaped, surrounded by pharyngeal tissue. Pharynx gradually widening posteriorly, apparently forming a weak posterior bulb in some cases. Nerve ring situated slightly posterior to middle of pharynx. Secretory-excretory system not observed. Cardia small, partly surrounded by intestinal tissue. Progaster not observed. Distinct layer of glycocalyx present in intestinal lumen.

Reproductive system monorchic with anterior outstretched testis situated to left or right of intestine. Mature sperm nucleated, globular, 1.3–2.0 × 1.5–2.4 μm. Spicules paired, arcuate, without capitulum, widest at proximal end and with pointed distal tip. Gubernaculum with caudal apophyses and wide lateral crurae. Pre-cloacal supplements or setae not observed. Tail conical, with two pairs of sub-ventral setae and one pair of sub-dorsal setae. Well-developed, ventrally curved, conical spinneret; caudal glands not observed.

Female

Similar to males but body widest immediately anterior to vulva, then markedly smaller immediately posterior to vulva; body width decreases gradually towards both extremities. Amphideal fovea slightly smaller than in males (0.33–0.43 vs 0.43–0.50 cbd), only one seta present posterior to amphideal fovea; tail longer than in males (7.8–11.0 vs 6.2–6.7 abd), conico-cylindrical, without setae. Reproductive system monodelphic with outstretched anterior branch to the left (two specimens) or right of intestine (three specimens). Mature egg dimensions up to 12 × 39 μm. Vulva located at almost two thirds of body length from anterior. Vagina transverse or oblique, with distinct cuticularised piece situated along anterior wall ( Fig. 6D ). No vaginal glands visible. Post-vulvar sac not observed.

Remarks

Monhystrella kermadecensis sp. nov. differs from most other Monhystrella species (except M. marina Timm, 1964 ) by the relatively short tail lacking a cylindrical filiform posterior portion. This trait may suggest affinities with Thalassomonhystera Jacobs, 1987 , but species of this genus always possess a well-developed secretory-excretory system and lack a posterior pharyngeal bulb ( Fonseca & Decraemer 2008). M. kermadecensis sp. nov. is also unusual in the position of the vulva at almost two thirds of body length from the anterior extremity instead of near mid-body, a trait which suggests affinities with Halomonhystera Andrássy, 2006 . The vulva in M. kermadecensis sp. nov. is, however, situated more anteriorly than in Halomonhystera (62–64 vs 76–92% of body length from anterior extremity); the new species also differs from Halomonhystera in the absence of secretory-excretory system and shape of the buccal cavity ( Fonseca & Decraemer 2008). Elucidating the taxonomic relationships between M. kermadecensis sp. nov. and other Monhysteridae will require detailed molecular analyses which are not possible at present based on available specimens.

The holophyly of the Monhysteridae was established based on the anterior gonad always positioned to the right of the intestine ( Lorenzen 1981). Monhystrella kermadecensis sp. nov. can be differentiated from all other species of the genus, and indeed the entire family, based on the variable position of the anterior gonad relative to the intestine (to the right or left of intestine in both sexes). The holophyly of the closely related family Xyalidae was established based on the anterior gonad always positioned to the left of the intestine and the posterior gonad (when present) always to the right of it ( Lorenzen 1981). The new species therefore shows affinities with both the Mohysteridae and Xyalidae ; within the Xyalidae it resembles the genus Theristus Bastian, 1965 most due to the conical shape of the tail and lack of terminal setae. The following traits, however, suggest closer affinities with the Monhysteridae : small body size (<500 μm in length), smooth cuticle (always striated in Xyalidae ; Lorenzen 1981), presence of only four setae in second circle (always ten setae in Theristus ), and presence of only one testis (two testes often present in Xyalidae ).

Figures

Fig. 6. Monhystrella kermadecensis sp. nov. A. Male anterior body region. B. Male head region. C. Female anterior body region. D. Male posterior body region. E. Female reproductive system. F. Entire ♀. G. Entire ♂, holotype. H. Male copulatory apparatus. I. Female posterior body region. Arrow shows position of cuticularised piece anterior to vulva. Scale bar: A, C–E, I = 25 μm; B = 7 μm; F–G = 70 μm; H = 12 μm.