Marine zoology: where the fantasy of nature gives its best!
Zoology is the branch of Biology that focuses on the structure, the function, the behaviour, and the evolution of animals. When the studied animals are living in the sea or in lagoons we add the adjective “marine”.
This discipline studies many groups of animals, from the simplest ones to the most evolutes.
Let’s start a journey into the marine zoology taxonomy!
Phylum PORIFERA (Sponge)
The sponges are the most primitive animals living in the sea. Their structure is a sort of assemblage of cells with the same function, forming channels (or pores, from which “Porifera”) where flues water. Neither proper tissues, nor organs, nor nervous system are present.
The most characteristic cell of a sponge is called choanocyte; it presents a flagellum with whom it creates a water stream.
Water, carrying food and oxygen, enters through numerous microscopic pores called ostia. The food particles present in the water are captured by the choanocytes. The filtered water flues out of the sponge body through some bigger holes called oscula. Digestion take place either in the choanocytes or in other cells called amoebocyte, able to move inside the sponge body.
In base to their internal structure sponges are divided in 3 categories:
Ascon: very simple and inefficient. Their shape is like a tube with only one big water chamber. They are very small and few.
Sycon: they present a central water chamber and numerous small chambers where the filtration takes place. This structure enhances significantly the surface of the sponge which comes in contact with water.
Leucon: The water chamber does not exist anymore but the entire internal surface is divided in very numerous small chambers interconnected between them. More oscula are present.
There are about 4300 species of sponges; the majority of them are living in the sea. Only 150 species are living in sweet water.
A characteristic of the sponge is to have an internal skeleton formed by small calcareous, siliceous or protein (spongin) structures called spicules.
In taxonomy 4 Classes of Porifera, divided in base of their spicules, are identified:
• Calcispongiae: (calcareous sponges) normally very small and simple. Ascon and Sycon forms. They are all living in the sea and in the first 100 m.
• Hyalospongiae: (siliceous sponges) sponges of the big depth, they normally live between 500 and 5000 m. Their shape is normally cylindrical and their dimension between 1 cm up to 1 m. The spicula has always 6 rays.
• Sclerospongiae: (coral sponges) small group of tropical sponge with a complicated calcareous structure with siliceous and spongin spicules included.
• Demospongiae: (corneous sponges) 95% of all the living sponges, they are living in all the seas and lagoons. Their skeleton is spongin or mixed siliceous and spongin. Their shapes and dimensions are very various. They are all of Leucon type.
The sponges have both asexual and sexual reproduction. The asexual is obtained through gemmation, where a part of the sponge leaves the colony to build a new one.
Sexual reproduction takes place when conditions are good. Sponges are hermaphrodites and produce alternatively eggs and spermatozoa. These last ones are released in the water and are collected by the other sponges. From the choanocytes they pass in the mesolia. From the union of an egg and spermatozoon origins a zygote, from which it develops a flagellated larva.
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