Occultammina

''Occultammina'' is a genus of xenophyophorean foraminifera known from the Atlantic and Pacific oceans. It is notable for being the first known infaunal xenophyophore as well as for being a possible identity for the enigmatic trace fossil ''Paleodictyon''.

Distribution and habitat

Like all other known xenophyophores, ''Occultammina'' is found in the deep ocean; the first known specimen was first discovered in 1980 at a depth of in the Ogasawara Trench, off the coast of Japan and described in 1982 by a joint research team from the University of Copenhagen and the University of Tokyo. Further specimens referred to ''Occultammina'' sp. have been found at a depth of in the Porcupine Abyssal Plain, in the North Atlantic. Further studies have expanded its geographical and bathymetric range from in the Ogasawara Trench and from in the North Atlantic, and also recorded its presence at in the Japan trench. ''Occultammina'' sp. has also been recovered at a depth of about near the Clipperton Fracture Zone, off the coast of western Mexico.

''Occultammina'' is infaunal; it typically is found at a depth of or less below the sediment's surface. It was the first known infaunal xenophyophore.

''Occultammina'' is found at turbidite facies in today's oceans.

Description

''Occultammina'' is an exceptionally large single-celled organism; like other xenophyophores it constructs a complex, single-chambered shell or "test" from sediment particles. It grows in a net-like shape, the test consisting of hollow tubes that branch or form loose polygons. It is typically flattened compared to other xenophyophores. It also has vertically ascending tubular outlets. ''Occultammina'' tubes are similar to those of '' Tendalia'' in general shape, though the test structure differs.

The test wall typically ranges from 70-120 μm in thickness; it is poorly cemented and consists of two layers. The outermost is 15-30 μm in thickness and composed primarily from clay; the inner layer is 50-90 μm thick and composed of silt and radiolarian tests. The interior of the test lining has between one and four ridges separating the stercomares, or waste masses.

Individual ''Occultammina'' tubes are between in diameter. Networks of ''Occultammina'' sized across have been found at the Ogasawara trench.

A specimen of ''Occultammina profunda'' was found to have unusually high levels of the radioactive isotopes Lead-210 (half-life=22.3 years), Polonium-210 (half-life=138.376 days) and Radium-226 (half-life=1600 years), specifically in the grannelare and stercomare, among the highest levels of Lead-210 recorded in any living organism. The authors suggest that the relative distribution of radionuclides in the organism's body parts implies that it grows and excretes comparatively rapidly.

Unfortunately no DNA samples have been collected from ''Occultammina'', preventing further examination of its evolutionary relationships.

Relation to ''Paleodictyon''

The enigmatic "graphoglyptid" fossil ''Paleodictyon'' bears a passing resemblance to ''Occultammina'', and this has led to suggestions of relationship between the two. ''Paleodictyon'' fossils are known from sediments interpreted as representing abyssal paleoenvironments associated with turbidite deposits, potentially lending credence to the hypothesis. ''Paleodictyon'' also preserves vertically ascending tubules that have been likened to those of ''Occultammina''.

However, this relationship has been contested. The large size (up to ) of some ''Paleodictyon'' is unknown in modern ''Occultammina''; the regular hexagonal pattern of ''Paleodictyon'' is similarly not represented in ''Occultammina''. The apparent absence of collected sediment particles (known as xenophyae) in graphoglyptid fossils further casts doubt on the possibility.

Modern examples of ''Paleodictyon'' have been discovered; however, they have not been able to clear up the issue. These specimens come from near the mid-Atlantic Ridge at a depth of . The specimens ranged from in diameter. Dissection of one specimen did not reveal tubes, mucus, protoplasm, or evidence of a test. Staining of a second specimen did not reveal soft-bodied evidence for xenophyophores such as stercomares or granellae; some small agglutinated fragments were found that could have been from a psamminid, but this is not unexpected in deep-sea sediments. DNA analysis also did not reveal evidence of xenophyophores in the sample. The modern samples also lacked the barium concentration caused by the barite crystals in xenophyophore tests. This study suggested that ''Paleodictyon'' could represent a burrow system or a glass sponge.

References

{{Taxonbar, from=Q23773166
Foraminifera genera
Monothalamea
Xenophyophorea