Phaeodarea or Phaeodaria is a group of amoeboid organisms. They are traditionally considered ,
The term "Radiozoa" has been used to refer to radiolaria when Phaeodarea is explicitly excluded.
Phaeodarea produce hollow skeletons composed of amorphous silica and organic material, which rarely fossilize. The endoplasm is divided by a cape with three openings, of which one gives rise to feeding , and the others let through bundles of that support the . Unlike true radiolarians, there are no cross-bridges between them. They also lack symbiotic , generally living below the photic zone, and do not produce any strontium sulfate.
Characteristics
Cell structure
Phaeodarea are
unicellular protists that grow a capsule with a thick, double-layered
cell wall containing two kinds of pores or openings: the large type, known as "
astropylum" or oral pore, from which a massive strand of
cytoplasm protrudes; and the smaller type, known as "
parapylae" or lateral pores, from which thinner strands of cytoplasm protrude. External to the capsule there is a large, often darkly pigmented, mass of granular cytoplasm called "
phaeodium" which contains undigested or partially digested food or debris. Their
crista are tubular.
Most Phaeodaria have a siliceous skeleton composed of amorphous silica joined by organic matter. They may contain more organic matter than Polycystinea (Radiolaria). Skeletons of some species are composed of hollow bars, instead of solid rods as in Polycystinea. Others have ornate spicules scattered through the external cytoplasm in a variety of forms, including geodesic frameworks, spheres or polyhedra. Other species have porous protist shell that are either bivalved, resembling clams, or vase-like with ornamentations around the opening. Other species have highly branched antler-llike spines stemming from a central shell. Unlike Polycystinea, the capsular wall surrounding the denser endoplasm of Phaeodarea lacks fusules.
Their cell size ranges from hundreds of to a few , roughly depending on the family. For example, species of Tuscaroridae exceed 3 mm, while Challengeriidae are generally smaller than a few hundred μm. Some phaeodarians with spherical skeletons are similar to Polycystinea and Acantharea (Radiolaria), although they are more porous and fragile. Some species of the family Challengeriidae resemble marine , but can be correctly distinguished by the presence of a phaeodium and absence of grooves.
Nutrition
The continuous, massive strand of cytoplasm in the astropyle of Phaeodarea provides a pathway to carry digested prey matter into the
endoplasm, similarly to some
testate amoebae and
foraminifera. Among the limited evidence of phaeodarian
predation, it was reported in 1986 that a
mesopelagic phaeodarian had absorbed micro
flagellate and
metazoan prey.
When introduced in the laboratory cultures,
copepods and
salps also were snared by phaeodaria.
Reproduction
Sexual reproduction of Phaeodaria has not been confirmed, but the release of motile swarmers that are likely
is widely documented. In the species
Coelodendrum ramosissimum, dispersal starts with the disappearance of the phaeodium, followed by the dissolution of the capsule and the creation of small plasmodial spheres in the ectoplasm; each of the spheres produces hundreds of
amoebae that eventually form swarmers with two
undulipodia (
flagella).
Ecology
Phaeodaria are exclusively
marine life,
holoplanktonic unicellular
protists that play an important role in
. They are
heterotrophic plankton (
zooplankton) that chiefly live in
pelagic open
, from the
surface water to the
deep sea. They have not been reported from brackish and high-salinity environments, but they are present in all oceans.
Very little is known about their role in the trophic web and about their predators. Dinoflagellate are reported to infect species of Phaeodaria, such as Syndinium nucleophaga.
Systematics
History
As a
protist group often broken by normal sampling methods and not very abundant in comparison with other organisms in the
euphotic zone, Phaeodaria have attracted little attention from
plankton researchers. The majority of taxonomic and ecological information regarding Phaeodaria, studied by German scientists, ended after World War I, and it has been little updated until today. Because of the presence of an organic central capsule and "ray-like pseudopodia", Phaeodaria were historically regarded as
Radiolaria, along with
Polycystinea and
Acantharea.
The marine Radiolaria were, along with the freshwater
Heliozoa, assigned to the phylum Actinopoda due to their elaborate siliceous skeletons surrounding the central capsule with pores from which
axopodia emerge.
Through molecular phylogenetics, Actinopoda was rendered invalid due to being polyphyletic, not a clade. Phaeodaria are now classified as a subclass of the phylum Cercozoa, while the ecologically different "true" Radiolaria belong to the related phylum Retaria, both within the higher Rhizaria supergroup.
Phylogeny
Through phylogenetic analyses it has been discovered that Phaeodarea is a
monophyletic clade, but the historical orders and families comprising it aren't. Instead, the clade consists of 11 subclades defined by morphological and phylogenetic values that do not correspond with the traditional orders and families:
Despite this, the current taxonomy by Cavalier-Smith maintains the original classification of suborders
Modern classification
The modern classification is the following, with the subclass containing a total of 2 orders, 7 suborders,
16 families and 39 genera.
-
Order Eodarida
-
Suborder Phaeogymnocellina (=Phaeogymnocellida)
-
Suborder Phaeocystina (=Phaeocystida)
-
Order Opaloconchida
