Some organisms in nature have developed the ability to enter a state of suspended metabolism called cryptobiosis when environmental conditions are unfavorable. This state-transition requires execution of a combination of genetic and biochemical pathways that enable the organism to survive for prolonged periods. Recently, nematode individuals have been reanimated from Siberian permafrost after remaining in cryptobiosis. Preliminary analysis indicates that these nematodes belong to the genera Panagrolaimus and Plectus. Here, we present precise radiocarbon dating indicating that the Panagrolaimus individuals have remained in cryptobiosis since the late Pleistocene (~46,000 years). Phylogenetic inference based on our genome assembly and a detailed morphological analysis demonstrate that they belong to an undescribed species, which we named Panagrolaimus kolymaensis. Comparative genome analysis revealed that the molecular toolkit for cryptobiosis in P. kolymaensis and in C. elegans is partly orthologous. We show that biochemical mechanisms employed by these two species to survive desiccation and freezing under laboratory conditions are similar. Our experimental evidence also reveals that C. elegans dauer larvae can remain viable for longer periods in suspended animation than previously reported. Altogether, our findings demonstrate that nematodes evolved mechanisms potentially allowing them to suspend life over geological time scales.

Two nematode species living in association with chemoautotrophic prokaryotes were found in two deep-sea canyon/channel systems, the Whittard Canyon and Gollum Channels, north-east Atlantic. Parabostrichus bathyalis gen. nov. sp. nov. (Desmodorida: Desmodoridae: Stilbonematinae) relates to Eubostrichus Greeff 1869 but differs in having well-developed paired dorso-caudal apophyses of the gubernaculum, small pre- and postcloacal latero-ventral papillae with short apical setae, elongate tail with slender posterior portion, and the absence of thorn-like setae (porids) in males. Body of P. bathyalis is loosely covered with elongate cells of prokaryote ectosymbionts. Astomonema southwardorum Austen et al. 1993, originally found at a methane seep pockmark in the North Sea, constitutes a significant portion of nematode communities in certain areas of the deep-sea canyon/channel systems. Taxonomic difficulties within Astomonematinae are discussed in light of the character state of paired male gonads discovered in A. southwardorum. Canyon populations of A. southwardorum are characterized by frequent loss of part of the hind body and wound healing posterior to the vulva in females. Both species tend to occur in deeper subsurface layers of the bottom sediment. Abundance of the nematode species associated with aggregations of ectosymbiotic (Parabostrichus) and endosymbiotic (Astomonema) chemoautotrophic bacteria may indicate reduced conditions at sites in these deep-sea canyons/channels and suggests a potentially substantial ecological role for chemolitotrophic fauna there.

Parasitic life-strategies in the phylum Nematoda (roundworms) are remarkably diverse and intricate in terms of evolution and taxonomy. By analysing novel rDNA data obtained on rare host-associated groups with unusual biology, we reveal paraphyly of the last major taxon with uncertain higher-rank classification that united solely parasitic nematodes (Marimermithida) to show that primarily marine parasitism only emerged independently and repeatedly in a few free-living lineages. We report secondary seaward ingression of land-based parasites (Mermithida) via invading hosts in the subtidal zone to illustrate the host-borne scenario of oceanic fish and mammal colonization by primarily terrestrial parasites (Spiruria). We also present the first molecular data on marine nematodes from unicellular hosts (foraminiferan protozoans) to demonstrate the independent origins of exploitative nematode associations at a microscopic scale. We argue that, in contrast with primarily intestinal associations arising from saprotrophy and commensalism, non-intestinal host capture (colonization of host body cavity or internal organs) is likely to be a primary route of transition to truly exploitative parasitism in roundworms. Predispositions to host capture in nematode morphology, ecology and life cycles imply its evolution as part of innate pre-adaptations to crossing environmental boundaries to enable multiple successful transitions to parasitism in the phylum history.

A new species of the genusMesacanthionFilipjev, 1927 was discovered during a survey of natural beaches of Jeju Island in South Korea. The new speciesMesacanthion jejuensissp. nov.shares general morphology of the genus such as the outer labial and cephalic setae being situated at the middle of cephalic capsule, well-developed mandibles with two columns united by a curved bar, and three equally sized and shaped teeth shorter than the mandibles. The new species belongs to a group ofMesacanthionspecies in which spicules are shorter than two anal body diameters. The new species is most closely related toM. pannosum, first discovered in Puget Sound, Washington, in terms of having enlarged cervical setae flap at the end of cephalic capsule, spicules which are shorter than 2 anal body diameter, both supplementary organ and gubernaculum. It can be distinguished fromM. pannosumby its stronger inner labial setae, longer outer labial setae, and difference in the index value of b and c’. Along with the description ofMesacanthion jejuensissp. nov.,the genusMesacanthionFilipjev, 1927 is bibliographically reviewed and revised. Including the new species, a total of 48 species are described within the genus; 39 which are valid; eight which are considered to be species inquirenda due to misplacement of genus and poor description; one which is considered nomen nudum. An updated diagnosis of the genus is provided along with a compiled tabular key comparing different diagnostic morphological characters of all valid species, as well as a pictorial key consisting of 21 species with spicules shorter than two anal body diameters.