Porifera

Sponges, the intriguing creatures of the animal kingdom, belong to the phylum Porifera. With approximately 5,000 known species, they exhibit a wide array of fascinating characteristics. Commonly known as “pore bearers,” these remarkable organisms possess numerous tiny pores in their body walls.

Habitat and Sessile Lifestyle

Sponges are exclusively aquatic organisms, primarily found in marine environments. However, some species have adapted to freshwater habitats as well. One such example is Spongilla, a freshwater sponge frequently found in lakes and slow streams. Sponges lead a sessile life, permanently attached to underwater objects called substrata. Unlike most animals, they cannot move freely.

Feeding mechanism

Water enters the body of the sponge through minute pores called ostia. The water inside, is then filtered for microorganisms, microscopic plants, and organic particles. This process is carried out by specialized cells called choanocytes or collar cells, which create water currents to capture and ingest food. The filtering capability of sponges makes them efficient filter feeders.

Symmetry and level of organization

Sponges are regarded as primitive multicellular animals due to their simple body organization. They have a cellular level of organisation. Most of them lack a definite symmetry, and any plane passing through their body center cannot divide them into equal halves. At the cellular level of organization, the cells do not form tissues in sponges. Instead, individual cells function more independently, exhibiting limited coordination. However, some division of labor exists among these cells.

Distinctive body form

The most striking feature of sponges is their unique body form. Their body walls consist of three layers: the outer cellular layer called pinacoderm (dermal layer), the inner cellular layer called choanoderm (gastral layer), and the non-cellular layer between them known as the mesohyl (mesenchyme). The choanoderm is made up of highly specialised flagellated cells called choanocytes, responsible for food ingestion, mesohyl secretion, and differentiation of sex cells. Mesohyl also consists of totipotent cells- archaeocytes, nurse cells-trophocytes, collenocytes, thesocytes, myocytes, chromocytes, etc. Sponges have remarkable capacity of regeneration.

Canal system and water transport

The body of a sponge is organised to form a complex system of pores and canals, aptly named the canal system. There are three types of canal systems found in sponges: ascon type, sycon type, and leucon type. The canal system serves a crucial purpose in the life of sponges. Since they are sessile, a constant water flow through their bodies is vital for obtaining food, oxygen, and removing waste. The canal system ensures this water transport and facilitates nutrition, respiration, excretion, and reproduction.

Skeleton

Almost all sponges possess an internal skeleton, also known as the endoskeleton, providing support to their bodies. This skeleton is composed of spicules or spongin fibers, or sometimes both. Spicules are structural elements made of either calcium or silica.

Reproduction and development

Sponges are hermaphrodites, meaning they produce both male and female gametes. They reproduce both sexually and asexually. Fragmentation, budding, and gemmule formation are common asexual modes, while sexual reproduction involves the formation of haploid male and female gametes, leading to internal fertilization. The development is indirect and involves larval forms like parenchymula or amphiblastula.

Classification

The phylum Porifera is classified into three classes based on the type of skeleton they possess:

• Members of Class Calcarea or Calcispongiae are exclusively marine, commonly found in shallow waters. Skeleton is made of calcareous spicules (made of calcium). Examples: Leucosolenia, Sycon, Grantia.
• Members of Class Hexactinellida or Hyalospongiae are exclusively marine, mostly found in deep seas. Skeleton is made of siliceous spicules (made of silica) with six rays (hexactines). Examples: Euplectella (venus flowerbasket), Hyalonema (creeseoee sconce).
• Members of Class Demospongiae are found in both marine and freshwater environments. Skeleton may contain either spongin fibers or siliceous spicules, or both. Examples: Euspongia (bath sponge – spongin fiber skeleton), Spongilla (freshwater sponge), Cliona (boring sponge), Chalina (“deadman’s fingers”).

Summary

Sponges, belonging to the phylum Porifera, are aquatic, sessile organisms with a unique body form and a complex canal system. They feed by filtering water through pores and specialized cells called choanocytes. Sponges have an internal skeleton and reproduce both sexually and asexually. There are three classes of sponges based on their skeletal composition: Calcarea, Hexactinellida, and Demospongiae. These fascinating creatures play essential roles in their aquatic ecosystems.

FAQs on Porifera

What are sponges?

Sponges are fascinating aquatic creatures belonging to the phylum Porifera. They are commonly known as pore bearers due to the numerous tiny pores present in their body walls.

Where do sponges live?

Sponges are exclusively aquatic organisms, primarily found in marine and freshwater environments.

How do sponges obtain their food?

Sponges are filter-feeders. Water enters their bodies through small pores called ostia and is filtered for microorganisms, microscopic plants, and organic particles. Choanocytes, specialised cells, create water currents to capture and ingest the food.

What is the unique feature of sponge body organisation?

Sponges are considered primitive multicellular animals. They lack a definite symmetry, and any plane passing through their body center cannot divide them into equal halves. Their cells do not form tissues; instead, they function more independently.

What is the purpose of the canal system in sponges?

The canal system in sponges serves a crucial role in water transport. Since sponges are sessile, the canal system ensures a constant water flow through their bodies for obtaining food, oxygen, and removing waste.

How do sponges reproduce?

Sponges reproduce both sexually and asexually. Asexual reproduction occurs through fragmentation, budding, and gemmule formation. Sexual reproduction involves the formation of male and female gametes, with internal fertilisation taking place.

What types of skeletons do sponges have?

Sponges have internal skeletons known as endoskeletons. These skeletons are composed of either spicules (made of calcium or silica) or spongin fibers, or sometimes a combination of both.

How do sponges play a role in the ecosystem?

Sponges play essential roles in their aquatic ecosystems. They filter water and contribute to nutrient cycling by consuming microscopic organisms. Additionally, their skeletons provide habitats for other marine organisms.

Can sponges be harmful to humans?

While most sponges are harmless, a few species may contain toxic compounds that can cause skin irritation or other adverse reactions in humans. However, these instances are relatively rare.

Can sponges regenerate if damaged?

Yes, sponges have an impressive power of regeneration. They can often regrow and repair damaged parts of their bodies, contributing to their survival in their habitats.

Are sponges valuable in any industries?

Sponges, particularly those with natural sponge fibers, have been used historically in various industries, such as cleaning and personal care. However, today, synthetic sponges have become more commonly used due to sustainability concerns.