Stilbonematinae is a subfamily of the
nematode worm family
Desmodoridae
Desmodoridae is a family of nematodes belonging to the order Desmodorida
Desmodorida is an order of nematodes
The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelm ...
that is notable for its
symbiosis
Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...
with
sulfur-oxidizing bacteria.
Systematics
Stilbonematinae Chitwood, 1936 belongs to the family Desmodoridae in the order Desmodorida. Nine genera have been described.
* ''
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
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
, 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
Mucus ( ) is a slippery aqueous secretion produced by, and covering, mucous membranes. It is typically produced from cells found in mucous glands, although it may also originate from mixed glands, which contain both serous and mucous cells. It is ...
that the bacterial symbionts are embedded in.
Stilbonematines are found in the
meiofaunal habitat in marine environments. 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.
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.
The bacterial symbionts are
chemosynthetic
In biochemistry, chemosynthesis is the biological conversion of one or more carbon-containing molecules (usually carbon dioxide or methane) and nutrients into organic matter using the oxidation of inorganic compounds (e.g., hydrogen gas, hydro ...
, gaining energy by oxidizing sulfide from the environment, and producing biomass by fixing carbon dioxide through the
Calvin-Benson-Bassham cycle.
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.
The specificity of the bacterial symbionts to their respective host species is controlled by a
lectin
Lectins are carbohydrate-binding proteins that are highly specific for sugar groups that are part of other molecules, so cause agglutination of particular cells or precipitation of glycoconjugates and polysaccharides. Lectins have a role in rec ...
called Mermaid that is produced by the worms. Mermaid occurs in different
isoforms
A protein isoform, or "protein variant", is a member of a set of highly similar proteins that originate from a single gene or gene family and are the result of genetic differences. While many perform the same or similar biological roles, some isof ...
, which have differing affinities for the sugar compositions of the
lipopolysaccharide
Lipopolysaccharides (LPS) are large molecules consisting of a lipid and a polysaccharide that are bacterial toxins. They are composed of an O-antigen, an outer core, and an inner core all joined by a covalent bond, and are found in the outer m ...
coat in different bacterial species.
See also
* ''
Olavius algarvensis
''Olavius algarvensis'' is a species of gutless oligochaete worm in the family Tubificidae which depends on symbiotic bacteria for its nutrition.
Habitats and research
''Olavius algarvensis'' lives in coastal sediments in the Mediterranean. It w ...
'' - 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
{{Taxonbar, from=Q21221178
Chromadorea
Chemosynthetic symbiosis