Sponges – Morphology, Physiology and Reproduction

This post vividly describes the physiology, morphology and reproduction in sponges. Here we shall explain and see the characteristic behavior of sponges in their habitat and their significance to the overall ecosystem.

Physiology of Sponges

All sponges feed holozoically. The flagella of the choanocytes beat inwards, creating inhalant currents through the ostioles. Small food particles of many types are ingested by the choanocytes at the base of the protoplasmic collar. The exact manner in which the ingestion takes place is not known. The food is digested intracellularly and it is presumed that the wandering amoeboid cells collect digested food from the choanocytes and distribute it to other types of cells. Water containing defecated particles and excretory materials is swept out through the osculum by the exhalant current.

Respiration is aerobic, diffusion of oxygen being fully effective since no cell is far removed from the water. Growth is interstitial; every type of cell is capable of division. Most sponges are colonial and extension takes place by means of horizontal outgrowths, giving a basal portion which secretes a cementing substance fastening the colony to the substratum. Then, at irregular intervals, vertical outgrowths give rise to upright branches.

Excretion of nitrogenous material is probably largely performed by the amoeboid cells. They are said to convey such materials from other cells and discharge them into the paragastral cavity. With such ready access to water, diffusion would suffice for all excretion of soluble and diffusible products.

Sponges have no nervous system or special receptor cells. Responses to stimuli, even if the latter are violent, are slow and local. In many species, there is an ability to effect slow closure of the inhalant pores by means of special contractile cells around their edges. These cell are stimulated to action by the cessation of water movement, and in still water, they contract slowly and effect closure. Thus they act both as receptors and effectors.

Detailed Description of the Physiology of Sponges

In describing the physiology of sponges, we shall first take a look at its feeding mechanism. All sponges feed holozoically. The flagella of the choanocytes beat inwards, creating inhalant currents through the ostioles. Small food particles of many types are ingested by the choanocytes at the base of the protoplasmic collar. The exact manner in which ingestion takes place is not known. The food is ingested intracellularly and it is presumed that the wandering amoeboid cells collect digested food from the choanocytes and distribute it to other types of cells. Water containing defaecated particles and excretory materials is swept out through the osculum by the exhalant current.

Respiration is aerobic, diffusion of oxygen being fully effective since no cell is far removed from water, growth is interstitial; every type of cell is capable of division. Most sponges are colonial and extension takes place by means of horizontal outgrowths, giving a basal portion which secretes a cementing substance fastening the colony to the substratum. Then, at irregular intervals, vertical outgrowths give rise to upright branches.

Excretion of nitrogenous material is probably largely performed by the amoeboid cells. They are said to convey such materials from other cells and discharge them into the paragastral cavity. With such ready access to water, diffusion would suffice for all excretion of soluble and diffusible products.

Sponges have no nervous system or special receptor cells. Responses to stimuli, even if the latter are violent, are slow and local. In many species, there is ability to effect slow closure of the inhalant pores by means of special contractile cells around their edges. These cells are stimulated to action by the cessation of water movement, and in still water, they contract slowly and effect closure. Thus they act both as receptors and effectors.

Reproduction occurs by asexual and sexual methods, though with many species, sexually-produced larvae have not been demonstrated. In asexual reproduction, external buds may become separated, or internal buds called gemmules, may develop. A gemmule is formed from a cluster of amoeboid cells; it becomes detached from the parent body and is drifted away to found another individual or colony.

Sperm and ova are produced from wandering amoeboid cells. All sponges are hermaphrodite and most are protandrous, so that cross-fertilization is probably the rule. One amoeboid cell divide many times to form a mass of small, tailed sperm. Each ovum is formed from a single cell which enlarges and becomes spherical. After fertilization, the zygote becomes invested with a layer of the parent’s cells. Within this, the early stages of development take place.

After cleavage, an ovoid blastula is formed, wholly or partly covered with flagellated cells; this is released into the water as a free-swimming larva and thus dispersal of the species is effected. Eventually, the larva settle down and by a remarkable process, the flagellated cells migrate through the inner cell layer and become the choanocytes, the internal cells passing out to form the coherent dermal layer. An aperture develops at the free end and becomes the osculum; special amoeboid cells become the porocytes; the jelly is secreted and growth begins.

Having developed a sedentary mode of life, sponges have developed suitable adaptations. Feeding, respiratory, and defaecating currents are set up by the constant inward beating of the flagella. The minute ostia prevent entry of particles too large for ingestion into the choanocytes, and the slow closure of the ostia may be a protective mechanism under certain conditions. There is effective dispersal by the swimming larvae. The sharp pointed spicules, which frequently project from the surface, render them immune from predation; no aquatic animals are known to eat sponges.

Special Features of Sponges that are  of Biological Importance

In describing further the physiology of sponges, it is important to note that sponges have reached the cellular level of organization. Although there are few types of cells, two types cover a considerable proportion of the surface; they are the pinacocytes and the choanocytes. The lack of any co-ordinating mechanisms precludes consideration of them as tissues. The presence of the highly-characteristic and peculiar chaonocytesindicates strongly their probable line of descent from chaonoflagellate protozoa.

In studying the physiology of sponges, it is also imperative to note that sponges possess remarkable powers of regeneration. Any small piece of the body can grow into a complete animal. If a sponge is forced through silk bolting-cloth into a dish of water, the cells soon aggregate into small clumps and each clump will develop slowly into the mature form.

Certain features distinguish the Porifera very clearly from the Metazoa. The principal and largest opening is exhalant. In all metazoan animals where there is a single large opening. It is inhalant primarily and exhalant secondarily. The minute pores of the sponges limit the food material to microscopic particles. Digestion is not a common function shared by a layer of cells all working in co-ordination, but each choanocyte acts as an independent unit. The complete absence of nervous system and sensory cells is another peculiarity. Finally, the migration of the flagellated cells from exterior to interior during development is found nowhere else.

Reproduction in Sponges

Reproduction occurs by asexual and sexual methods, though with many species, sexually-produced larvae have not been demonstrated. In asexual reproduction, external buds may become separated, or from a cluster of amoeboid cells; it becomes detached from the parent body and is drifted away to found another individual or colony. Sperm and ova are produced from wandering amoeboid cells. All sponges are hermaphrodite and most are protandrous, so that cross-fertilization is probably the rule. One amoeboid cell divides many times to form a mass of small, tailed perm. Each ovum is formed from a single cell which enlarges and becomes spherical. After fertilization, the zygote becomes invested with a layer of the parent’s cells. Within this, the early stages of development take place. After cleavage, an ovoid blastula is formed, wholly or partly covered with flagellated cell’ this is released into the water as a free-swimming larva and thus dispersal of the species is effected. Eventually the larvae settle down and by a remarkable process, the flagellated cells migrate throygh the inner cell layer and become to form the coherent dermal layer. An aperture develops at the free end and becomes the osculum; special amoeboid cells become the porocytes; the jelly is secreted and growth begins.

Having evolved a sedentary mode of life, sponges have developed suitable adaptations. Feeding, respiratory, and defaecation currents are set up by the constant inward beating of the flagella. The minute ostia prevent entry of particles too large for be a protective mechanism under certain conditions. There is effective dispersal by the swimming larvae. The sharp- pointed spicules, which frequently project from the surface, render them immune from predation; no acquatic animals are known to eat sponges.

Morphology of Sponges

All sponges are aquatic, the majority living in the sea where they are found at all depths from the intertidal zone to the deepest abysses. One family, the Spongillidae, contains only fresh water members; they inhabit rivers and lakes, practically all over the world.

In the life history of some sponges, a simple stage known as the Olynthus appears. It is probable that all sponges are modifications of this simple type. The Olynthus is a vase shaped structure attached to the substratum at the narrower end. The lateral surfaces are perforated by microscopic pores called ostia, which lead into a large cavity, the paragaster. At the free end is a single exhalant aperture, the osculum.

The outer surfaces is covered by flattened cells called pinacocytes, closely cemented together. The inhalant pores are perforated through single cells, the porocytes. Lining the internal surfaces are collared-flagellate cells, the choanocytes. The outer dermal and inner gastral layers are separated by a stiff jelly in which are two types of cells. Scleroblasts secrete the tir-radiate spicules which support the body, and frequent amoeboid cells wander through the jelly.

No adult sponge retains the Olynthus type of structure, but all show various modifications which are essentially concerned with increasing the area of surface occupied by choanocytes. The three principal grades of structure are the ascon, sycon and leucon. They are illustrated diagrammatically above, with arrows showing the direction of flow of the water currents.

Skeletal Elements In Sponges

The jelly which gives coherence to the body is strengthened by deposition of various elements by the scleroblasts. In the class Calcarea, the spicules are always made of calcium carbonate and are commonly tri-radiate. The Hexactinellida are characterized by siliceous spicules, of which there is a wide array. The shapes of these spicules are useful in classifying this group. Some members of the Demospongiae have siliceous spicules, but the more common skeletal material is a network of sponging fibres secreted by spongoblasts; this sponging in some genera is impregnated with spicules or granules of silica.