Porifera: Skeleton | Zoology Optional Notes for UPSC PDF Download

Unlike many other animals, sponges possess a form of skeleton that provides their bodies with shape and structural support. In all sponge species, a skeleton is found within the mesenchyme, and this skeleton is composed of distinct spicules, interwoven sponge fibers, or a combination of both. The primary functions of this skeleton are to uphold and safeguard the delicate body components of sponges. Furthermore, the type of skeleton present serves as a fundamental criterion for classifying sponges into different classes, including Calcarea, Hexactinellida, and Desmospongia.

Spongin Fibers

Structure of spongin

Spongin is an organic, elastic material with a chemical composition resembling silk. It is a scleroprotein containing sulfur and shares a chemical relationship with collagen. 

• It is both water-insoluble and resistant to protein-digesting enzymes. Spongin fibers are fine threads composed of a soft, granular core surrounded by concentric layers of spongin.
• Spongin contains substantial amounts of iodine, which historically led to the use of bath sponges as a remedy for croup, a throat condition in children resulting from inflammation and partial larynx obstruction.
• Within the Desmospongia class, spongin fibers can take on various forms. They may act as a cement, connecting siliceous spicules, or be present in the form of branching fibers with embedded siliceous spicules.
• In the Keratosa subclass, spicules are entirely absent, and spongin is the sole structural component.

Development of spongin

Spongin fibers are secreted by flask-shaped mesenchyme cells called as spongioblast cells. During the development the spongioblast cells are arranged in the rows and the spongin rods secreted by them are fused with the neighboring cells to form a long fiber. Later the spongioblasts vacuolated and finally get degenerated after secreting certain amount of spongin.

Spicules

Structure and types

Spicules are microscopic crystalline structures which gives the sponges their rigidity and form. Spicule consists of spines or rays that radiate from a point. These are secreted by special mesenchymal amoebocytes called scleroblast cells. The following are various types of spicules:

On basis of type of deposit on core organic matter: All kinds of spicules have a core of organic material around which either calcium carbonate or colloidal silica is deposited. Accordingly spicules are of two types:

• Calcareous spicules: The organic material in this type of spicules is calcium carbonate or calcite. This is the characteristic of the sponges of class Calcarea.
• Siliceous spicules: The organics material in this type of spicules is Colloidal silica or Silicon. These types of spicules are the characteristic of the sponges of class Hexactanellida.

On the basis of size and function: Spicules can be of large size or small size. Accordingly spicules can be of two types:

• Megascleres: These are larger spicules constituting main skeleton of sponge body.
• Microscleres: These are the small spicules occurring interstitially.

On the basis of number of axes and rays: Spicules may occur in several forms like the simple rod form or in the form of forks, anchors, shovels, stars, plumes etc. The spicule forms depend on the presence of number of axes and rays. Accordingly, they can be divided into the following forms:

Monaxon: These kinds of spicules are formed by the growth along one axis. They may be straight needle-like or rod like or may be curved. Their ends may be pointed or hooked or knobbed. Monaxons can be both calcareous and siliceous types.

These monaxon spicules are further divided into two kinds,

• Monactinal: the growth of the spicule takes place only in one direction
• Diactinal: The growth of the spicule takes place in both the directions.

Tetraxon

These spicules have four rays each pointing in different direction. Usually one of the four rays is elongated giving the appearance of a crown of 3 rays. Such spicules are called as triaenes.

• Sometimes all the rays are equal, when all the rays are equal it is termed as calthrops.
• When all the four rays persist it is called as tetraradiate or quadriradiate.
• Sometimes one of the rays is lost and then it is known as triradiate. These triradiate rays are characteristic of calcareous sponges.
• If the elongated ray bears a disc at both ends, it is called as amphidisc.

Triaxon

These spicules have three axes that cross one another at right angles to produce six rays. Thus it is also called hexactinal spicule. These triaxon spicules are characteristic of glass sponges of the class Hexactanellida.

Polyaxon 

These are the spicules with several equal rays radiating from a central point. They may be grouped to give star-like appearance. Polyaxon spicules are found along with microscleres.

Development of Spicules

The calcareous spicules are secreted by special type of cells called as sclerocytes. These sclerocytes are derived from binucleated mesenchymal scleroblasts. A monaxon spicule or each ray of the triradiate spicule is secreted by a group of two sclerocytes. Among these two sclerocytes one acts as thickener cell and the other acts as the founder cell.

The initiation of the formation of the spicule starts with the deposition of a particle of calcium carbonate between the two nuclei of the binucleated mesenchymal cells. This particle grows drawing apart the two nuclei and then two sclerocytes are formed. Now the thickener cell lays down additional layer of calcium carbonate adding to the thickness of the spicule. When the spicule is fully formed, both the cells i.e. thickener cell and the founder cell wander into the mesenchyme. The scleroblast secreting a calcareous spicule is called as calcoblast, while the scleroblast secreting a siliceous spicule is called silicoblast.