Sponges are ancient, simple multicellular animals that inhabit diverse aquatic environments across the globe. These sessile invertebrates, belonging to the phylum Porifera, lack true tissues and organs. Specialized skeletal elements known as spicules provide their structural integrity and survival.
Defining Spicules
Spicules are microscopic, rigid structures forming the internal skeleton of most sponges, providing form and stability. These skeletal elements are primarily composed of either calcium carbonate or silica (silicon dioxide). Calcareous spicules characterize sponges in the class Calcarea, while siliceous spicules are found in Demospongiae and Hexactinellida sponges. Their appearance varies, ranging from simple needle-like rods to intricate star-shaped or branched forms. The arrangement and composition of spicules are distinct among species and are used for classification.
Types of Spicules
Spicules are broadly categorized by size into megascleres and microscleres. Megascleres are larger spicules, ranging from 60 to 2000 micrometers (µm), forming the main supporting framework. Microscleres are smaller, measuring between 10 and 60 µm, and are scattered throughout the sponge’s tissue, contributing to protection and additional support.
These size categories encompass diverse shapes, classified by the number of axes or rays they possess. Monaxons grow along a single axis and can be straight or curved, with ends that may be pointed (oxeas), rounded (strongyles), or knobbed (tylotes). Triaxons have three intersecting axes, forming six rays, and characterize glass sponges (Hexactinellida). Tetraxons feature four rays, while polyaxons have multiple rays radiating from a central point, forming star-like shapes called asters. Other forms include C-shaped sigmas, bow-shaped toxas, and chelas with recurved hooks.
Roles of Spicules
Spicules provide structural support, forming a rigid skeletal framework that prevents the sponge’s soft body from collapsing. This internal scaffolding facilitates the efficient flow of water through their intricate canal systems, essential for filter feeding and gas exchange.
Spicules also serve as a defense mechanism against predators. Their sharp, pointed nature makes sponges unpalatable or difficult for many organisms to consume. Beyond structural support and defense, specialized spicules in deep-sea sponges, such as the Venus flower basket, exhibit light-channeling abilities. These siliceous spicules act like natural fiber optic cables, gathering and focusing ambient light deep within the sponge’s body, which may benefit symbiotic algae or attract other organisms.
How Spicules Are Formed
The formation of spicules, known as biomineralization, is carried out by specialized cells called sclerocytes. These cells are located within the mesohyl, the gelatinous matrix between the outer and inner cell layers of the sponge. Sclerocytes absorb dissolved minerals, such as calcium ions or silicic acid, directly from the surrounding seawater.
Once absorbed, these minerals are secreted and precisely shaped by the sclerocytes to form the spicule morphology characteristic of that sponge species. The process involves the formation of an organic filament or sheath that acts as a template, around which the mineral is deposited. Each sclerocyte can produce a spicule, and multiple sclerocytes work together to form larger or more complex spicules.