Leduc, D.; Zhao, Z. Q. (2018). Phylogenetic relationships within the Cyatholaimidae (Nematoda: Chromadorida), the taxonomic significance of cuticle pore and pore-like structures, and a description of two new species. Marine Biodiversity. 48(1): 217-230.
The family Cyatholaimidae Filipjev, 1918 is a relatively diverse group of mainly marine nematodes which has been shown to be monophyletic by morphological and molecular phylogenetic analyses. There are, however, no morphological synapomorphies for any of the four subfamilies that currently comprise the Cyatholaimidae. The two types of cuticle pore and pore-like structures often observed in cyatholaimids may be of taxonomic significance, but the terminology used is inconsistent, and their description is often too limited to provide sufficient information for comparisons. Here, we describe two new cyatholaimid species, Paracanthonchus miltommatus sp. n. and Metacyatholaimus delicatus sp. n., from intertidal and upper continental slope sediments of New Zealand, and investigate the distribution and morphology of cuticle pore and pore-like structures in these two new species using light and scanning electron microscopy. We also investigate phylogenetic relationships within the Cyatholaimidae using SSU and D2-D3 of LSU rDNA sequences. The first type of cuticle structure, first termed pore complex by Wright & Hope (Can J Zool 46:1005–1011, 1968), consists of circular structures usually arranged in sublateral, subventral and subdorsal longitudinal rows, with a slit-like pore and ring-like development of dense material in the middle cuticle layer and typically associated with an underlying cell. The second, less studied type, for which we propose the term lateral pore-like structure, consists of structures arranged along the mediolateral lines in one or more longitudinal rows or sometimes irregularly, usually with a central, non-cuticularised dome, and a round or elliptical cuticularised opening supported by unmodified or modified punctations at anterior and posterior extremities. The function of the lateral pore-like structures remains unclear but their morphology is inconsistent with their descriptions as pores or modified punctations by some authors. The limited information available suggests that the number, distribution, and morphology of pore complexes and lateral pore-like structures could provide taxonomically useful information for defining cyatholaimid genera such as Longicyatholaimus Micoletzky, 1924 and Marylynnia (Hopper, 1972). Our SSU-based molecular phylogenetic analysis retrieved two monophyletic clades with low support, which approximately correspond to the subfamilies Paracanthonchinae and Cyatholaiminae; however, we found that Praeacanthonchus, which is currently classified with the Cyatholaiminae based on the structure of the gubernaculum, may be better placed in the subfamily Paracanthonchinae. This finding suggests that the gubernaculum structure may not be a meaningful character for defining cyatholaimid subfamilies or genera. Features of the cuticle, some of which are already used for defining genera such as Metacyatholaimus Schuurmans Stekhoven, 1942, may better represent phylogenetic relationships within the family, and should, therefore, be described more comprehensively in the future.