The resilience of ecosystems to negative impacts is generally higher when high gene flow, species diversity and genetic diversity are present. Population genetic studies are suitable to investigate genetic diversity and estimate gene flow between populations. Seaweed beds form a dynamic shallow water ecosystem influenced by climate change and human exploitation, as such, seaweed beds are a particularly powerful model to investigate ecosystem resilience in coastal areas. We studied the population genetic structure of the new nematode species Paracanthonchus gynodiporata associated with seaweeds in northeastern Brazil. Nematodes are generally believed to have a limited dispersal capacity because of the lack of planktonic larvae. Yet, they can drift on seaweeds, and water currents might be a natural barrier for their dispersal. Populations of P. gynodiporata were sampled over more than 1000 km coastline in regions across major oceanic currents with and without historical exploitation of seaweed. P. gynodiporata is described in an integrative way using mitochondrial and nuclear sequences and morphological data. The 3D model of the head region shows for the first time a detailed view of the ventrosublateral teeth, a character often overlooked in older taxonomic studies of the genus. A total of 17 mitochondrial COI haplotypes were found with one haplotype representing 63 to 83% of the frequencies in each population. AMOVA showed overall little population genetic structure (F ST = 0.05204), and no genetic subdivision between the populations under the influence of the two different water currents were found. Effects of historical seaweed exploitation on population genetic diversity were not detected. In contrast, significant differences between populations were found in morphometric characters. This discrepancy in genetic and morphological differentiation between populations across 1000 km of coastline is surprising in view of the frequently observed presence of several cryptic species at small geographical scale in other macroalgal associated nematodes. Our results show that cryptic species are not omnipresent in marine nematode species, suggesting that nematodes associated with seaweeds have been able to disperse over large distances across well-known biogeographic barriers.

Glochinema bathyperuvensis sp. n., the first record of Epsilonematidae from bathyal muddy sediments of the oxygen minimum zone (OMZ) off Callao, Peru (eastern Pacific Ocean) is described. The new species belongs to the subfamily Glochinematinae and is characterized by the possession of more than 8 pairs of subdorsal thorns asymmetrically arranged and a single dorsal one in the pharyngeal region, as well as by the presence of ten to thirteen blade-like cuticular protrusions forming a latero-dorsal palisade around the posterior head region and anterior cervical region. In addition, G. bathyperuvensis is characterized by its large size, more than 240 cuticular annules, provided with an extremely dense „hairy“ body ornamentation and, four rows of ambulatory setae: two inner subventral ones with 8 to 9 setae in males, and 5 to 6 setae in females, and two outer subventral rows composed each of 9-12 ambulatory setae followed by a large number of modified somatic setae. The dense hairy body spines, the large number of modified somatic setae, its dominant occurrence exclusive in oxygen-depleted, organic-rich soupy surface sediments indicate an adaptation to the stressed OMZ habitat conditions.

A study of type material of several longidorid species revealed misinterpretations of the amphid structure with respect to shape of the amphidial fovea and amphidial aperture. A standardised terminology of the amphid structure within the Longidoridae is proposed. Former and recent hypotheses on phylogenetic relationships within the family based, respectively, on morphological and on molecular data are discussed, as well as possible implications for the systematics of the family Longidoridae. As a result of this study the following new combinations are proposed: Longidorus boshi n. comb. (syn. Paralongidorus boshi); Longidorus monegrensis n. comb. (syn. P. monegrensis); Longidorus spiralis n. comb. (syn. P. spiralis) and Longidorus cedari n. comb. (syn. Longidoroides cedari). Additional information is provided for Longidoroides bikanerensis, Paralongidorus buchae and P. zenobiae.

Two burrowing nematode populations from Colombia were characterised using morphological, morphometric and molecular criteria. The morphological and morphometric characters of the two populations did not differ from those of Radopholus similis. The phylogenetic analysis based on sequence comparison of 18S, D2D3 regions of these populations with those of Radopholus sp., R. similis and other Tylenchomorpha from GenBank confirmed the close molecular relationship between the genus Radopholus and the Hoplolaimidae. The consensus tree based on the comparison of ITS1-5.8-ITS2 rDNA sequences of the Colombian populations with R. similis (many sources), R. arabocoffeae and R. duriophilus from GenBank grouped the Colombian populations in a clade which also included R. similis from different localities. Other R. similis populations grouped in separated clades, which were closely related to the clade containing R. arabocoffeae and R. duriophilus. The high genetic variability among the R. similis populations was observed and confirmed by the high ITS1-5.8-ITS2 sequence divergence within the R. similis populations studied (5.4%) which was comparable to the divergence (4.2-6.9%) between the Radopholus species included in the analysis. These findings and the unresolved phylogenetic placement of R. arabocoffeae and R. duriophilus among R. similis suggests that the burrowing nematode could be a species complex. Limited information on the variability of the 18S and D2D3 region gives an indication of their possible value, besides ITS1-5.8-ITS, as molecular markers within Radopholus. No complete and comprehensible relation was found between the phylogenetic position of the Radopholus populations and their geographical origin or host.