Why Are Sponges Considered Animals? (7+ Reasons To Consider)

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People often think and consider sponges as plants, rather considering them as animals.

This misconception is due to some of the characteristics of the sponges that make them seem like a plant.

Like plants they do not move, i.e. they are sessile. Like plants, they remain adhered to one place at the bottom of the seawater or at the coral reefs.

But wait, there are some reasons to consider them as animals and not plants. We’ll discuss them all in this article so just keep reading.

Why Are Sponges Considered Animals? A sponge is an animal and not a plant because it is classified under Phylum Porifera of the Animal Kingdom, they are holozoic in nature and so capture food to eat, they don’t have cell walls, their life cycle includes larval stages. Moreover, they are diploblastic and have glycogen as the reserved food materials thus, they are called animals.

There’s is no such strong evidence to consider it under the plants and so, it is not classified in any way under the Plant Kingdom. But, there are various strong pieces of evidence to consider them under the animals.

Sponges have managed to hide their true animal nature for several centuries. They are sessile, variously branched, have no apparent way of capturing food or eliminating waste, and show little response to stimuli.

Some species of the sponges are even green due to the presence of unicellular symbiotic algae.

Thus, earlier workers like Aristotle, Pliny, Gerade, Grew, etc. had considered sponges as plants and some had even termed them as non-living organisms.

But it was Robert Grant in the year 1857, who was the first to recognize and prove the true animal nature of sponges and consider it as living beings.

He had properly explained why sponges must be considered animals and not plants. The modern-day classification system of the sponges under the Animal Kingdom is followed as per his findings.

So, why is a sea sponge more closely related to an animal than being a plant or non-living thing? Read on to find out!

Sponges in Caribbean Sea, Cayman Islands
Sponges in Caribbean Sea, Cayman Islands

Why Are Sponges Considered Animals? Here are the prominent reasons you should consider…

#1. Sponges belong to the phylum Porifera

Yes, sponges belong to phylum Porifera and Infrakingdom Parazoa of the Animal Kingdom. You won’t find any such phylum named “Porifera” under the plant kingdom classification.

Sponges are all multicellular organisms with only the cellular level of body organization, with no distinct tissues or organs.

Moreover, the majority of sponges are marine in nature. They are 99% marine, a few freshwater species, and all aquatic in nature. All of them are sessile and fixed living in nature.

Taking about histology, their body wall is differentiated into outer pinacoderm (dermal epithelium), inner choanoderm (gastral epithelium), and gelatinous non-cellular mesenchyme in between.

Their body wall contains pores (ostia) that leads to canals and chambers that serve for the flow of water through it. One or more water exits called oscula are present.

Their digestion is all intracellular and, no respiratory or excretory organs are present.

All sponges are hermaphrodite (having both male and female sex organs in one body) but, cross-fertilization is the only rule of sexual reproduction.

These above-mentioned characters show that they are all living organisms that belong to Phylum Porifera.

I highly recommend you to read any of these books Biology of the Invertebrates by Pechenik J.A or Invertebrates by Richard C. Brusca if you need better knowledge of invertebrates including the sponges. There are no color pictures in the book, but the text is clear and readable and the diagrams are clear and helpful. Everything is well organized, and I particularly like the updated phylogenetic information. This may be what you will need for your next invertebrates class.

#2. Sponges are Sessile, Pore bearing, Monoblastic animals

Yes, all sponges wherever they may be, are all sessile, have pores in their body, and all are monoblastic in nature.

Adult sponges are called sessile because they are unable to move from place to place. They remain fixed by adhering itself to its substratum which may be a rock, coral reef, under the water surface.

During embryonic development, only the larvae are the free-moving organisms which later shows metamorphosis and becomes an adult by fixing itself to a substratum. Thus, they become sessile.

They have various tiny pores in the body. These pores lead to the passage of the canal system where water current enters and bring in food and oxygen to the body, and excreta and reproductive bodies are carried out through Oscula pore.

They are all monoblastic. Monoblastic is a condition that is only shown during the embryonic stage development of the animals in which the cells are arranged having or derived from a single embryonic germ layer.

Sponges are neither Diploblastic nor Triploblastic because they have Cellular Level of Organization.

#3. Sponges capture their food to eat

Yes, sponges can capture their food to eat. They follow the filter-feeding mechanism to eat.

They can eat, excrete, and can do a lot more things. It’s possible because of their Canal system which is also known as the aquiferous system.

Their mechanism of feeding and then digestion of the captured food is actually done in a filter-feeding way.

This all happens in the canal system. In order to obtain food, sponges pass water through their bodies that is via. the canal system pathway.

The pinacocytes are very selective in the entry of food particles. On the other hand, choanocytes only engulf the food particles that meets the feeding and digestive criteria of the sponges.

This selective nature of capturing and digesting food is called the filter-feeding process in sponges.

The choanocytes and the amoebocytes that are present in the canal system of the sponge helps in intracellular digestion where food particles are digested inside the cellular food vacuoles by means of various acidic and alkaline enzymes.

That’s the holozoic type of nutrition which is seen in animals and not plants. Plants prepare their own food using inorganic compounds.

#4. They absorb organic carbon

They absorb organic carbon rather than fixing it from inorganic sources such as carbon dioxide.

Plants can fix carbon dioxide into carbon thus they themselves become a carbon sink. An animal cannot fix carbon but in fact, it takes carbon for its various needs from the carbon sink like plants.

The phytoplanktons use the dissolved carbon dioxide present in the water and fix it in its body during photosynthesis using sunlight. When these phytoplanktons are eaten by the sponges, carbon gets passed to the sponge body.

Thus, the phytoplankton taking the role of a plant acts as a carbon sink whereas, the sponge taking the role of an animal gets the carbon from the phytoplankton.

Sponges receive the largest quantities of carbon from the algae, phytoplanktons, and corals respectively. That’s by heterotrophic feeding, sponges can quickly deplete carbon in the surrounding water.

Therefore, that is usually how they act like animals by absorbing organic carbon.

#5. They produce sperm cells, not pollen grains

Sponges reproduce both sexually and asexually. When they reproduce in a sexual way, they produce sperms.

They don’t have any definite reproductive organs as their body possesses a cellular level of organization.

Most sponges are monoecious (hermaphrodite i.e. they can produce both sperms and eggs from the same body), only a few are dioecious (unisexual) in nature.

If monoecious, then the same organism can release both sperm and ovum but not at the same time.

The sperms are derived from the amoebocytes or choanocytes depending on the species by gametogenesis.

When the sponges produce their sperms, its concentration in the body increases and thus get released into the aquatic environment through the oscula.

In plants, sperms are not produced except for some plants such as ferns and mosses that produce sperm that move. Plants mostly produce pollen grains which is the male reproductive gamete.

#6. Their cells do not have cell walls

Sponges don’t possess a cell wall as plant cells have. In fact, they only have eukaryotic cells with the plasma membrane.

The plasma membrane supports the cell forming the cell boundary. In some cells like the choanocytes, small microvilli and flagella are present that bulges out of the cell membrane.

These microvilli help in the engulfment of the food particles and in digesting it by sending it to the food vacuoles. The flagella help in the creation of water current that passes through the canal system.

The cell membrane of the pinacoderm (outermost epithelial layer) is composed of pinacocytes that is harder and often with spicules in some species but, don’t contain a cell wall.

The plasma membrane in pinacocytes itself represents the same or even more hardness as compared to the plant cell wall.

Other cells like the scleroblasts, calcoblasts, chromocytes, gland cells, amoebocytes, etc. are all animal types in origin.

#7. The reserve food material of sponges is glycogen

The reserve food material of sponges is glycogen, which is the reserve food of all animals. In plants the reserve food is starch.

Glycogen is the storage form of glucose in animals and humans which is analogous to the starch in plants. In case of higher animals, glycogen is synthesized and stored mainly in the liver and the muscles.

But here, Thesocyte is the reserve cell, which is present in sponges and it stores glycogen.

Thesocytes contain yolk platelets as stored energy and high concentrations of polyols to maintain high osmotic concentration in the cells of the gemmule.

Thesocyte (in some sponges its called gray cells) are capable of synthesis and accumulation of glycogen and responsible for its transfer to sites of more intense metabolism (growth, bud, blastema).

#8. Other reasons to consider sponges as animals

  • They show changes in the diameter of their central body cavity and are rapidly involved in the creation of distinct water currents with the help of choanocytes.
  • A molecular biological study has shown that sponges are actually more complex animals (like humans) developed from a common ancestor. Their phylogenetic relationships with animals are well-established.
  • Sponges possess many of the qualities biologists use to distinguish animals from plants. Just like the plant cell contains a cell wall, chloroplasts, plastids, which sponge cells don’t.
  • The skeleton (spicules with spongin fibres) of sponges are made from collagen protein giving it the property of elasticity. The same type of protein is found in higher animal tendons and skin.
  • Transportation of food materials within the sponge body occurs by choanocytes and amoebocytes.
  • Sponges are holozoic in nutrition and feeding.
  • The life cycle of sponges includes free-swimming ciliated larval stages resembling those of other marine animals. Such larval stages do not occur in plants.

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