Nematosomes are multicellular motile bodies found in the
gastrovascular cavity
The gastrovascular cavity is the primary organ of digestion and circulation in two major animal phyla: the Coelenterates or cnidarians (including jellyfish and corals) and Platyhelminthes (flatworms). The cavity may be extensively branched into ...
of the model sea anemone ''Nematostella vectensis''
starlet sea anemone
The starlet sea anemone (''Nematostella vectensis'') is a species of small sea anemone in the family Edwardsiidae native to the east coast of the United States, with introduced populations along the coast of southeast England and the west coast ...
. First described by
Stephenson in 1935, nematosomes are the defining apomorphy (
synapomorphy
In phylogenetics, an apomorphy (or derived trait) is a novel Phenotypic trait, character or character state that has evolution, evolved from its ancestral form (or Plesiomorphy and symplesiomorphy, plesiomorphy). A synapomorphy is an apomorphy sh ...
) of the genus ''Nematostella'' but have received relatively little study. Nematosomes can be observed circulating through the body cavity and tentacle lumen of adult anemones, occasionally coming to rest on the
gastrodermis
Gastrodermis (from Ancient Greek: , , "stomach"; , , "skin") is the inner layer of Cell (biology), cells that serves as a lining membrane of the gastrovascular cavity in cnidarians. It is distinct from the outer epidermis and the inner dermis and ...
.
Nematosomes that are dislodged from rest return to circulation. The lifespan of a single nematosome has not been studied.
Development
Nematosomes arise by budding from the cnidoglandular tract of the mesenteries in ''N. vectensis''; thus, they are of
ectoderm
The ectoderm is one of the three primary germ layers formed in early embryonic development. It is the outermost layer, and is superficial to the mesoderm (the middle layer) and endoderm (the innermost layer). It emerges and originates from the o ...
al origin. Although material can be seen circulating in the gastrovascular cavity from the onset of body cavity development, true nematosomes do not appear until approximately the same time that the polyp reaches sexual maturity. Inspection of the jelly matrix surrounding a spawned egg mass will reveal abundant nematosomes; male anemones do not appear to release nematosomes during spawning. Once outside the body cavity, nematosomes can live from one to several days in dilute seawater, dependent upon the temperature at which they are housed.
Cell Biology
Since the time of their description, nematosomes have been known to be populated by abundant
cnidocyte
A cnidocyte (also known as a cnidoblast) is a type of cell containing a large secretory organelle called a ''cnidocyst'', that can deliver a sting to other organisms as a way to capture prey and defend against predators. A cnidocyte explosively ...
s (also known as nematocytes) and "flagellated cells". Two of the three types of cnidocytes found in ''N. vectensis'' are commonly found in nematosomes: basitrichous isorhizas and microbasic p-mastigophores. The flagellated cells are known only to have a long motile cilium. Although they arise from a portion of the mesenteries that also contains abundant gland cells, no gland cells have ever been found in nematosomes.
A recent study found cells with the capacity to undergo phagocytosis in the nematosomes.
These phagocytotic cells appear to be the same as the "flagellated cells" described previously.
Putative Function
The abundance of cnidocytes in nematosomes led to the early hypothesis that this tissue plays a role in immobilization of prey.
Although the veracity of this hypothesis was challenged, recent evidence has confirmed that cniodcytes in nematosomes are capable of firing and immobilizing juvenile brine shrimp.
Because nematosomes are embedded in the spawned egg mass, it is likely they also play a role in deterring potential egg predators. This hypothesis has not been tested but anecdotal observations suggest killifish may indeed be deterred by the nematosomes surrounding eggs. The presence of phagocytes in this tissue suggests nematosomes may play a role in clearing the gastrovascular cavity of foreign material and/or pathogens.
Explicit testing of this hypothesis is needed.
References
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Cnidarian anatomy