Stilbonematinae

Stilbonematinae is a subfamily of the nematode worm family Desmodoridae that is notable for its symbiosis with sulfur-oxidizing bacteria.

Stilbonematinae
Scientific classification
Kingdom:
Animalia
Phylum:
Nematoda
Class:
Chromadorea
Subclass:
Chromadoria
Order:
Desmodorida
Family:
Desmodoridae
Subfamily:
Stilbonematinae

Chitwood, 1936
Genera

(See article text)

Systematics

Stilbonematinae Chitwood, 1936 belongs to the family Desmodoridae in the order Desmodorida. Nine genera have been described.[1]

  • Adelphos Ott, 1997
  • Catanema Cobb, 1920
  • Centonema Leduc, 2013
  • Eubostrichus Greeff, 1869
  • Laxus Cobb, 1894
  • Leptonemella Cobb, 1920
  • Parabostrichus Tchesunov et al. 2012
  • Robbea Gerlach, 1956
  • Squanema Gerlach, 1963
  • Stilbonema Cobb, 1920

Description

Stilbonematines can be up to 10 mm long, with a club-like head. The worms are completely covered in a coat of ectosymbiotic sulfur-oxidizing bacteria except for the anterior region. The presence of the bacteria, which often contain intracellular inclusions of elemental sulfur, gives the worms a bright white appearance under incident light. They have small mouths and buccal cavities, and short pharynges. Many species have multicellular sensory-glandular organs in longitudinal rows along the length of the body, which secrete mucus that the bacterial symbionts are embedded in.[1]

Stilbonematines are found in the meiofaunal habitat in marine environments.[2] Another group of meiofaunal nematodes with sulfur-oxidizing symbionts is the genus Astomonema, although in Astomonema the bacteria are endo- rather than ectosymbionts.

Symbiosis with sulfur-oxidizing bacteria

The bacterial symbionts of stilbonematines are of different shapes and sizes, ranging from small coccoid cells to elongate crescent-like cells, but each host species has only a single morphological type associated with it.[3] The bacterial symbionts of stilbonematines are closely related to the sulfur-oxidizing symbionts of gutless phallodriline oligochaete worms: these bacteria were all descended from a single ancestor, and each host species has its own specific bacterial species.[4]

The bacterial symbionts are chemosynthetic, gaining energy by oxidizing sulfide from the environment, and producing biomass by fixing carbon dioxide through the Calvin-Benson-Bassham cycle.[3] The bacteria benefit from the symbiosis because the host animal can migrate between sulfide- and oxygen-rich regions of the sediment habitat, and the bacteria require both these chemical substances to produce energy. The hosts are believed to consume the bacteria as a food source, based on evidence from their stable carbon isotope ratios.[5]

The specificity of the bacterial symbionts to their respective host species is controlled by a lectin called Mermaid that is produced by the worms. Mermaid occurs in different isoforms, which have differing affinities for the sugar compositions of the lipopolysaccharide coat in different bacterial species.[6]

See also
  • Olavius algarvensis - A species of gutless phallodriline oligochaete worms whose sulfur-oxidizing bacterial symbionts are related to those of the stilbonematine nematodes.
  • Astomonema - A genus of nematodes (from a different family) that also has symbiotic sulfur-oxidizing bacteria.

References
  1. Tchesunov, Alexei V. (February 2013). "Marine free-living nematodes of the subfamily Stilbonematinae (Nematoda, Desmodoridae): taxonomic review with descriptions of a few species from the Nha Trang Bay, Central Vietnam". Meiofauna Marina. 20: 71–94.
  2. Ott, Jörg; Bright, Monika; Bulgheresi, Silvia (2004). "Symbioses between marine nematodes and sulfur-oxidizing chemoautotrophic bacteria". Symbiosis. 36: 103–126.
  3. Polz, Martin F.; Felbeck, Horst; Novak, Rudolf; Nebelsick, Monika; Ott, Jörg A. (1992-11-01). "Chemoautotrophic, sulfur-oxidizing symbiotic bacteria on marine nematodes: Morphological and biochemical characterization". Microbial Ecology. 24 (3): 313–329. doi:10.1007/bf00167789. ISSN 0095-3628. PMID 24193210.
  4. Zimmermann, Judith; Wentrup, Cecilia; Sadowski, Miriam; Blazejak, Anna; Gruber-Vodicka, Harald R.; Kleiner, Manuel; Ott, Jörg A.; Cronholm, Bodil; De Wit, Pierre (2016-07-01). "Closely coupled evolutionary history of ecto- and endosymbionts from two distantly related animal phyla". Molecular Ecology. 25 (13): 3203–3223. doi:10.1111/mec.13554. ISSN 1365-294X. PMID 26826340.
  5. Ott, J. A.; Novak, R.; Schiemer, F.; .Hentschel, U; Nebelsick, M.; Polz, M. (1991-09-01). "Tackling the Sulfide Gradient: A Novel Strategy Involving Marine Nematodes and Chemoautotrophic Ectosymbionts". Marine Ecology. 12 (3): 261–279. doi:10.1111/j.1439-0485.1991.tb00258.x. ISSN 1439-0485.
  6. Bulgheresi, Silvia; Gruber-Vodicka, Harald R.; Heindl, Niels R.; Dirks, Ulrich; Kostadinova, Maria; Breiteneder, Heimo; Ott, Joerg A. (June 2011). "Sequence variability of the pattern recognition receptor Mermaid mediates specificity of marine nematode symbioses". The ISME Journal. 5 (6): 986–998. doi:10.1038/ismej.2010.198. ISSN 1751-7362. PMC 3131856. PMID 21228893.
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