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Taxonomy, pollution and the sludge worm

Limnodrilus species are annelid worms distributed worldwide in various freshwater sediments. The systematics of Limnodrilus has chiefly been based on morphology, but the genus has not been subject to any closer phylogenetic studies over the past two decades. To reconstruct the evolutionary history of Limnodrilus, and to assess the monophyly of this genus and its systematic position within the subfamily Tubificinae (Annelida: Clitellata: Naididae), 45 Limnodrilus specimens, representing 19 species, and 35 other naidid species (representing 24 genera) were sampled. The data consisted of sequences of three mitochondrial genes (COI, 12S and 16S rDNA) and four nuclear markers (18S and 28S rDNA, Histone 3, and ITS). The phylogeny was estimated, using Maximum Likelihood and Bayesian analyses of concatenated data of seven DNA loci, as well as a multi-locus coalescent-based approach. All analyses strongly suggest that Limnodrilus is monophyletic, but only if the morphospecies L. rubripenis is removed from it. Limnodrilus rubripenis and (at least) Baltidrilus, Lophochaeta and some species attributed to Varichaetadrilus comprise the sister group to the clade Limnodrilus sensu stricto, and the latter is further divided into three well-supported groups. One of them contains morphospecies characterized by short cuticular penis sheaths and enlarged chaetae in anterior segments (L. udekemianus, L. silvani and L. grandisetosus). The second is a small group of species with moderately long penis sheaths, i.e., L. sulphurensis and L. profundicola. The third, and largest group, includes not only the multitude of cryptic species in the L. hoffmeisteri complex, but also other, morphologically distinct, species nested within this complex. All studied species in this large group have long penis sheaths, which are exceptionally long in L. claparedianus, L. maumeensis, and a form morphologically intermediate between L. claparedianus and L. cervix. The identification and classification of these groups provide a framework for directed sampling in further phylogenetic studies, and for revisionary work on the L. hoffmeisteri complex and other unresolved Limnodrilus species.

Lipophilic chemicals released into the aquatic environment tend to adsorb to suspended material and sediments. These compounds are therefore not bioavailable to many pelagic organisms, and their overall bioaccumulation is often not adequately covered by the currently prescribed measuring of bioconcentration in fish. The assessment of bioaccumulation in sediment-dwelling organisms shall give more detailed information on the hazard of chemicals for aquatic ecosystems. Therefore a standardised test design is required. The freshwater tubificids Tubifex tubifex and Limnodrilus hoffmeisteri were chosen as test organisms, while ¹⁴C-Lindane (γ-Hexachlorocyclohexane, γ-HCH), and ¹⁴C-Hexachlorobenzene served as test substances. The bioaccumulation studies were carried out in a static system consisting of artificial sediment and reconstituted water. Uptake as well as deputation phases were examined. Bioaccumulation factors (BAF) were determined from the ratio of concentration in worm tissue to concentration in sediment, and from the ratio of uptake rate constant (k_s) to elimination rate constant (k_e), respectively. Lindane concentrations were found to be about 4 times higher in the worms compared to the sediment, HCB was accumulated by a factor of approximately 7. The tubificid bioaccumulation factors of Lindane and HCB are discussed in comparison to corresponding fish bioconcentration data from lieterature. The presented data show that the assessment of bioaccumulation based exclusively on the extrapolation of fish-BCF to other organisms and environmental compartments is not appropriate.

The distribution, abundance and numerical dominance of oligochaete species together with associated values of Shannon-Weaver diversity are presented, using the results of two intertidal surveys of the south bank of the Humber Estuary (UK). Data concerning oligochaete distribution and abundance must be interpreted with care, but Tubificoides benedeni appears to be a valid indicator of pollution in the Humber. Ecological justification of this finding is offered with reference to the relevant literature. The role of T. benedeni as a pollution indicator in the Humber is discussed and a polluted area in the lower Humber is identified.