Phylum Porifera

The most primitive groups of animals include the phylum Porifera, generally known as sponges. These are multicellular, primarily aquatic animals, which indeed possess a simple organization of the body. The term "Porifera" is from the Latin words porous, meaning pore and fera, meaning to bear, indicating its pores. Poriferans are sessile animals, which means they spend their adult life fixed and immobile on some substrate.

1.0Features of the of Phylum Porifera

• Porifera has the following distinct characteristics from the other phyla:
• Multicellular, but primitive: Porifera is one of the first divergences in the animal's evolutionary tree. They lack true tissues and organs.
• Cellular-level organization: Their body is organized at the cellular level. There are no tissues or organs in their body.
• Sessile lifestyle: The adult sponges grow attached on the surfaces of rocks, corals, or ocean beds.
• Aquatic habitat: Nearly all sponges are marine, although certain species can be found in freshwater.
• Filter feeders: Poriferans feed through the process of filtering water across their porous body wall, trapping plankton and small particles.
• Asymmetrical body: Poriferans generally have an asymmetrical body, even though some species may exhibit radial symmetry.
• Asexual and sexual reproduction: Poriferans have the ability to reproduce either by asexual or by sexual methods, showing a range of different reproduction strategies.

2.0Classification of Porifera

Porifera is divided into three main classes depending on the type of skeletal composition:

1. Calcarea Class: This class includes those sponges that have calcium carbonate spicules. These sponges are primarily small marine.
2. Glass sponges, or Hexactinellida: These sponges have six-rayed pattern siliceous spicules and dwell in deep seas.
3. Class Demospongiae: This class comprises the largest sponges, covering 90% of all species. Its skeleton has only spongin fibres, silica spicules or both.

3.0Anatomy and Morphology

Body Structure and Organization:

1. Porous body wall: The body of a sponge contains many pores (ostia) through which water enters and large openings (oscula) through which water leaves.
2. Canal system: The sponges possess a very specialized canal system for the flow of water. Classified into asconoid, syconoid, or leuconoid according to the complexity of this system.

• Asconoid: the simplest type, with a tube-shaped body.
• Syconoid: with folded walls for increasing surface area.
• Leuconoid: It has a very complicated canal system that is profoundly branched.

Cellular Composition

Three different types of cells exist in sponges

• Pinacocytes: They are flat cells forming an outer cover, called the pinacoderm.
• Choanocytes (collar cells): These special cells line the inner canals of sponges. They have a flagellum and collar of microvilli and helps to create a water current with which to trap food particles.
• Amoebocytes: These are free living cells that move around within the sponge body to carry nutrients to different parts and also help in repairing and reproduction. They may change into any other type of cell.

Skeleton:

• Spicules: Some sponges have a skeleton made up of tiny spiny fragments called spicules. Most of the time, they are made of calcium carbonate (in Class Calcarea) or silica (in Class Hexactinellida and Demospongiae).

• Spongin: In Demospongiae, the skeleton is a soft flexible element made up of a protein called spongin.

4.0Reproduction

1. Asexual Reproduction

• Budding: Some sponges reproduce asexually by budding, where an offshoot of the parent sponge grows to become a new organism.

• Gemmules: Freshwater sponges and some marine ones can also form gemmules, internal buds which help the sponge survive poor periods. They could then remain in the sponges in suspended animation until things pick up.

2. Sexual Reproduction:

• Hermaphroditism: Most of the sponges are hermaphroditic having both the male and female reproductive cells.
• Fertilization: Sperm released into the water by one sponge is taken by another sponge inside it and fertilizes eggs. The fertilized eggs grow into free-swimming larvae, which finally settle on a surface and form a new sponge.

5.0Ecological and Economic Importance

1. Ecological Role

• Sponges are precious members of marine ecosystems. They filter the water, clean up, and recycle nutrients.
• They can also host other marine animals like bacteria, algae, and even fish.
• Some species of sponges are symbiotic with microbes, such as cyanobacteria, that perform photosynthesis, thus providing the sponge with food.

2. Economic Significance

• Wild: The demosponges' sponges, particularly those that consist of spongin have been harvested as wild bath sponges.
• Biotechnology: Sponges synthesize various bioactive compounds which, through a process of development, could be useful in pharmaceutical firms, including anticancer, antimicrobial, and anti-inflammatory.
• Biomonitoring: Since they are very effective at filtering large volumes of water, they are used in biomonitoring to monitor the health of aquatic environments as bioindicators.

6.0Evolutionary Significance

• Sponges belong to the most ancient animals, with fossil records dating back more than 600 million years ago. 
• They represent one of the earliest-branching animals and give a wonderful insight into the course of multi cellularization. 
• Simple body organization in sponges reflects a transition from unicellular to multicellular organisms and gives a model for studying the evolution of more complex animals.