Why are sponges considered colonial organisms?

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Sponges are simple animals in the Phylum Porifera. They have two layers of cells with a jelly-like material sandwiched between. They do not have highly specialized systems in the body. Rather, they rely on water flow throughout the sponge transporting food, oxygen and wastes. When they reproduce, they can carry out both sexual and asexual reproduction. During asexual reproduction, if a piece of a sponge breaks off, it can reattach someplace else and form a new colony. When well-fed, a sponge will asexually reproduce by forming identical buds. These may remain attached to the colony, or break off and form new colonies. Sometimes, sponges send out gemmules, or survival pods, when conditions are not quite perfect for their survival. Sometimes, these may merge with cells of other sponges from the same species to form a new colony.

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Sponges are considered colonial, what is the evolutionary significance of this?

In classical Binomial Nomenclature, all life was categorized into 5 different Kingdoms (Protista, Monera, Fungi, Plants and Animals.)  However, as more has been understood about cellular structure, tissue organization, and even DNA analysis, these categories, which were once considered definitive, have become only one method among many in the science of classification.

In the case of sponges, although traditionally classified in the Animal Kingdom, they have many characteristics that argue for classification in the Protist Kingdom.  The fact that they have characteristics across Kingdoms suggests that they are an ancient life form; studies indicate they originated about 700 million years ago. Excepting sponges, all multicellular animal life incorporates various tissues into their structure.  In fact, the lack of tissues in sponges, yet their multicellularity, suggests they have characteristics like a colony of protists.  

Sponges can thus be categorized as the only colonial animals, a physical structure from which all animals and protists diverged millions of years ago, and may be considered as the source of all animal life, according to recent research.  See more at the links: 

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Sponges are considered colonial, what is the evolutionary significance of this?

Sponges (Phylum Porifera) are invertebrate animals that live in water. They have pores all over their bodies that they use to allow the passage of water in their system for growth and metabolism. Unlike other animals, sponges do not have organ systems, tissues, complex cellular communications and lack body symmetry. Sponges form colony in a particular area for mutual benefits. Species of sponges should be within reach for sexual reproduction to happen. The sperm cells are often released on water and are caught by the receiving sponge in order to reproduce. However, they can also reproduce asexually when a part of the sponge is broken and attached somewhere else to form a new colony.

Evolutionary patterns of sponges' physiology can be traced back to their ancestors, the protozoan. Scientist believed that sponges evolved from colonial protists because of genetic sequences and profiles. Sponges are related to choanoflagellates, a colonial protist, who happens to be their closest protist ancestors. These early protists might have formed colony in shallow waters and have symbiotically lived with each other up to the point that they can no longer survive as single-celled organisms.

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