|Stilbonematid nematodes are highly abundant members of the meiofauna in coastal
sediments. These nematodes are covered by sulfur-oxidizing ectosymbiotic
bacteria. Co-occuring 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. My aim is to characterize
the nematodes and their ectosymbiotic bacteria with molecular methods.
Phylogenetic analysis of the host, based on the 18S rRNA gene showed that at
least four different genera co-occur in the same habitat. Each of the nematodes
hosts was covered by a particular arranged bacterial coat consisting of a
certain morphotype. Nematodes of the genus Catanema, for example, are covered by
a monolayer of rods, whereby Eubostrichus sp. has long filamentous bacteria on
the cuticle. Phylogenetic analyses, based on the 16S-23S-ITS region, showed that
nematodes of different genera are associated with distinct ectosymbiotic
bacteria belonging to a gammaproteobacterial clade of sulfur-oxidizing symbionts
from marine nematode and oligochaete worms. Each of the stilbonematid nematode
genera was associated with its own particular 16S-23S-ITS type. Moreover
individuals of two different Leptonemella sp. hosted two closely related but
distinct ectosymbionts, indicating that the symbiotic associations are host
species-specific. While highly monospecific associations were shown for the
genera Leptonemella and Catanema, Laxus sp., Eubostrichus sp. and Stilbonema sp.
had a higher diversity in their bacterial community. To exclude that these other
bacteria are contaminants and show that the gammaproteobacterial bacteria are
the major ectosymbionts, fluorescence in-situ hybridization (FISH) with specific
probes will be performed.