Abstract
Three new species of the order Monhysterida are described based on specimens obtained at depths of 8081 and 9177 m in the Kermadec Trench. Thelonema clarki sp. nov. is characterised by a large body size (3230–4461 µm), short cylindrical buccal cavity, gubernaculum without apophyses, and long conico-cylindrical tail. This is the first record of the genus since its original description over two decades ago from the Peru Basin. Metasphaerolaimus constrictus sp. nov. is characterised by a relatively long body (1232–1623 µm), slightly arcuate spicules without gubernaculum, and conico-cylindrical tail with inner cuticle conspicuously thickened immediately anterior to cylindrical portion. Monhystrella kermadecensis sp. nov. is characterised by a circle of papillose outer labial sensillae slightly anterior to the four short cephalic setae, gubernaculum with caudal apophyses, the presence of distinct cuticularised piece along anterior vaginal wall, and a relatively short conical (males) or conico-cylindrical tail (females) with conical, ventrally-curved spinneret. M. 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. The new species is classified within the Monhysteridae, and not the closely-related Xyalidae, based on the small body size, a smooth cuticle, and the presence of six outer labial papillae and only one testis. Further work is required to clarify the placement of M. kermadecensis sp. nov. relative to other monhysterid genera. A tabular key to all ten valid Metasphaerolaimus species is presented.
Highlights
The Order Monhysterida is well represented in hadal trench samples (> 6000 m depth), with the family Monhysteridae often dominant (Gambi et al 2003; Vanhove et al 2004)
The families Sphaerolaimidae and Linhomoeidae are common in hadal trench samples, but are typically much less abundant (e.g., Tietjen 1989)
Thelonema clarki sp. nov. and Metasphaerolaimus constrictus sp. nov. were relatively common at the 9177 m site, representing 3 and 5%, respectively, of total nematode abundance
Summary
The Order Monhysterida is well represented in hadal trench samples (> 6000 m depth), with the family Monhysteridae often dominant (Gambi et al 2003; Vanhove et al 2004). The families Sphaerolaimidae and Linhomoeidae are common in hadal trench samples, but are typically much less abundant (e.g., Tietjen 1989). Length) and often represented by many morphologically similar species in deep-sea samples Data on the distribution of Monhysteridae species in the deep sea are scarce despite their high abundance (Tietjen 1989; Miljutin et al 2010). Vanhove et al (2004) and Gambi et al (2003) recorded high densities of the genus Monhystera Bastian, 1865 in hadal trenches, despite a taxonomic revision by Jacobs (1987) almost two decades earlier resulting in all marine Monhystera species being transferred to either Thalassomonhystera or Monhystrella (Fonseca & Decraemer 2008). More work is clearly needed on the taxonomy of this family, as well as other Monhysterida, in order to identify patterns in species distribution within and among trenches
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