Sponges or Poriferans resemble most animals in the context of cell wall absence, multicellular organisation of cells and heterotrophic mode of nutrition.
Phylum Porifera: An overview
Poriferans come under the kingdom Animalia and share basic characteristics with other animals. Poriferans, however, do not possess actual tissues or organs, they have a very basic cellular organisation.
Common morphological characteristics of all sponges
{Poriferans can be either radially symmetrical or unsymmetrical. They do not exhibit bilateral symmetry (this type of symmetry is, most often, observed in higher-order animals. All species of Porifera share the following common characteristics-
- Body organisation is such that water flow through the body is maximised.
- Sponges are sessile, meaning they are attached to the floor of the ocean or water body.
- Water enters through small holes called Ostia and exits through larger pores called the osculum.
- Poriferans usually have cylindrically shaped bodies.
- Most sponges possess an internal skeleton that is made up of either spicules (needle-like structures) or spongin. Spicules are secreted by scleroblasts while spongin is secreted by spongioblasts.
- They possess endoskeletons made up of mesophyll which is a gelatinous matrix.
- The inner lining of the body is composed of choanocyte cells which are cells with whip-like flagella that direct the flow of water inside the body of the sponge.
It is important to note that sponges do not possess specialized cells like higher-order animals but instead possess unspecialized cells that can transform into other cell types. These cells often migrate between the mesophyll layer and the main cell layers during the process of cell transformation.
Sponges exchange gas and excrete nitrogenous wastes via the process of diffusion. All sponges have a main central cavity called the atrium which opens to the external environment via the osculum. In some exceptional cases, sponges may possess an external calcium carbonate skeleton in addition to their internal skeleton.
Feeding habits
- Nutrition in sponges is holozoic, meaning food is ingested and then processed using digestive enzymes.
- Sponges are heterotrophic. However, some sponges host autotrophic microorganisms and form symbiotic relationships with these organisms. These endosymbionts provide the sponge with food and oxygen.
- Sponges usually feed on bacteria and microscopic food particles present in the water.
- Some sponges have evolved to become carnivores and feed on small crustaceans.
Reproductive habits
- Sponges reproduce via the release of sperms and ova into the water and then rely on external fertilisation to take place.
- Some sponges retain the ova in the “mother’s body”.
- The fertilised eggs morph into motile larvae.
- Motile larvae swim to suitable habitats before settling down and becoming fully sessile.
- Some sponge species reproduce via the process of budding.
- In case of hostile environmental conditions, sponges form small pods called “gemmules” which in essence, is a protected mass of cells that remain dormant till the environmental conditions become hospitable again.
Sub-classification of sponges
Based on body organisation, skeletal structure and habitat, sponges can be classified into the following three classes-
Calcarea
- These sponges are found in shallow, marine waters.
- They have an endoskeleton made up of calcium carbonate spicules.
- They are radially symmetrical.
- Body organisation can be leuconoid (lack of a spongocoel), asconoid or syconoid.
Hexactinellids
- Usually found in deep-sea waters.
- Body canal is sycon or leucon.
- Endoskeleton is composed of six-rayed spicules which are made up of silica.
- The body is cylindrical and they exhibit radial symmetry.
Desmospongiae
- These sponges are found in both marine and freshwater water bodies.
- The body canal is leuconoid type.
- The endoskeleton is made up of spongin fibres and siliceous spicules.
- They are cylindrical in shape and unsymmetrical.
Commercial uses of sponges
In the early 18th century, sponges belonging to the class Desmospongiae were used as cleaning tools due to the soft and fibrous skeleton of the organism. Sponges also possess medicinal significance as
antibiotics, this can be attributed to the endosymbionts present in sponges that produce various biochemical compounds that could be used as potential antiviral or antibiotic drugs.
Conclusion
Poriferans, commonly known as sponges, are multicellular, heterotrophic organisms found in aquatic habitats. These organisms show great diversity in their skeletal structures and are one of nature’s marvels. Sponges used to be over-exploited for commercial use, so much so that two species of sponges came very close to extinction.
Kashmiri Biryani -Erakina
