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Abstract:
Stilbonematid nematodes can be highly abundant members of the meiofauna in coastal
sediments. These nematodes are covered by sulfur-oxidizing ectosymbiotic bacteria. Cooccurring
stilbonematid nematodes of various genera have been described from sandy
sediments of the Mediterranean island of Elba (Italy), but nothing is known about their
ectosymbiotic bacteria. The aim of this study was to characterize the nematodes and their
ectosymbiotic bacteria with molecular methods. Both morphological and 18S rRNA gene
phylogenetic analysis of the hosts revealed that at least four different genera co-occur in the
same habitat. Each of the nematode hosts is covered by a particularly arranged bacterial coat
consisting of a specific morphotype. Nematodes of the genus Catanema and Laxus for
example, are covered by a monolayer of rods, Leptonemella and Stilbonema by a multilayer of
coccoid bacteria, whereby Eubostrichus sp. has long filamentous bacteria on the cuticle.
Phylogenetic analyses based on 3 genes (16S rRNA, ITS and partial-23S rRNA) showed that
nematodes of different genera are associated with distinct ectosymbiotic bacteria that belong to
the gammaproteobacterial clade of sulfur-oxidizing symbionts, typically associated with
marine nematode and oligochaete worms (MONTS clade). Each of the stilbonematid nematode
genera was associated with its own particular 16S-ITS-partial-23S MONTS phylotype. The
individuals of two different species of Leptonemella hosted two closely related but distinct
ectosymbionts, indicating that the symbiotic associations are host species-specific. While
monospecific associations were shown for the genera Leptonemella and Catanema, the genera
Laxus sp., Eubostrichus sp. and Stilbonema sp. had a higher diversity in their bacterial
community as detected with our PCR assay. However, the symbiotic nature of these additional
bacterial phylotypes remains unclear. Furthermore, we investigated the possible mode of
transmission by comparing host and symbiont phylogenies. The incongruence of host and
symbiont tree topologies both on the genus and species level suggests that hosts and symbionts
did not cospeciate and that the hosts acquire their symbionts from their environment. Due to
the highly specific association of host and symbionts coevolved recognition mechanisms to
ensure a stable symbiosis over time are postulated.
