Acantharia

Acantharia, commonly called acantharians, are a group of marine single-celled protists within the radiolarians. They are best known for their hard skeletons made of strontium sulfate, rather than the silica characteristic of many other radiolarians, and for their spiny, branching exoskeletons that form intricate lattices.

Anatomy and growth: Like other radiolarians, acantharians have a central capsule that divides the cell into an inner endoplasm and an outer ectoplasm. From the capsule extend numerous slender pseudopodia called axopods, which protrude through pores in the skeleton. The axopods serve for capturing prey, sensing the environment, and sometimes for maintaining buoyancy.

Feeding and symbiosis: Acantharians are primarily heterotrophic and feed by ensnaring and phagocytosing small planktonic organisms along their axopods. Some species host endosymbiotic algae or dinoflagellates, which can provide photosynthetic nutrients under suitable conditions and contribute to the host’s energy budget.

Distribution and ecology: They are widespread in oceans worldwide, occupying a range of habitats from surface waters to deeper layers. Individual organisms are typically several tens to a few hundred micrometers in size. Their siliceous to strontianite skeletons contribute to marine sediments when organisms die, forming part of radiolarian ooze.

Reproduction and life cycle: Reproduction is largely by asexual cell division, producing daughter cells that inherit the skeleton. In many species, details of sexual reproduction are not well documented and may be rare or episodic.

Taxonomic notes and significance: Acantharia represent a major lineage within Radiolaria, distinguished by their strontianite skeletons and axopod-bearing cytoplasm. Their fossilizable skeletons provide tools for paleoceanographic studies and help illuminate past marine conditions.