Tetractines

Tetractines are a type of spicule found in certain marine sponges, specifically those belonging to the class Demospongiae. These spicules are characterized by having four rays or arms radiating from a central point. The rays are typically of equal length, though variations can occur. Tetractines are a common form of skeletal element that provides structural support to the sponge's body.

The formation of tetractines, like other sponge spicules, involves specialized cells within the sponge called sclerocytes. These cells secrete silica or calcium carbonate, which then crystallizes into the characteristic four-rayed structure. The precise shape and arrangement of tetractines, along with other spicule types, are important taxonomic features used by scientists to classify and identify different sponge species.

While tetractines are a defining feature of some sponges, their presence and morphology can vary significantly. Some species may possess abundant tetractines, while others may have few or none. Their distribution throughout the sponge body can also differ, contributing to the overall architecture and texture of the organism. The study of tetractines and other spicules is a crucial aspect of sponge biology and paleontology.