Sponges Cnidarians 2BIbDF09 10/11
What are Sponges?
Sponges, from the Latin pore bearers, are very primitive animals of the phylum Porifera, more precisely they are the first multicellular animals. They evolved very early from colonial protists. Their ancestors are the choanoflagellates (group of free-living unicellular and colonial flagellate eukaryotes considered to be the closest living relatives of the animals). There are between 8'000 and 9'000 species of sponges, depending of the textbook.
They are invertebrates, which are animals without backbones. There exist different sponges of different forms and different shapes, including tubes, fans, cups, cones, blobs, barrels, and crusts.
- see Appendix Sponges 1
What are the main characteristics of Sponges?
Sponges are loosely organized and lack true tissues. The body of a sponge has two outer layers separated by an acellular gel layer called the mesophyl (also called the mesenchyme). In the gel layer are either spicules (supportive needles made of calcium carbonate) or spongin fibers (a flexible skeletal material made from protein).
Their sizes are from a few millimeters to more than 2 meters in diameter. They don't have any symmetry.
The body of a sponge looks similar to a vase pierced with small holes and pores. It has thousands of pores. Sponges pump remarkable amounts of water. Water is drawn through the little pores into a central cavity, the spongocoel, then flows out the sponge through a larger opening, the osculum. In this process, they collect small particulate matter and tiny planktons, by the trapping action of the choanocytes, which are collar cells. That's why we call them filter feeders. Sponges do not have nervous, digestive or circulatory systems.
As adults, sponges are sessile. That means that they are permanently attached to a substrate and are unable to move on their own.
Most sponges are hermaphrodites (each adult can act as either the female or the male in reproduction). They produce sperm and eggs which are formed by aggregations of cells called amoebocytes in the body wall. During the proper time of the year, they release sperm cells into the sea. These cells are then captured using the same process the sponges use to capture food. The difference is that the sperm cells are moved to the "ova" of the sponge parent. And now the fertilization can take place. The eggs develop within the parent, the amount of time this takes depends on the species. When the eggs reach the "larvae" stage they are released back into the sea to seek out a place to settle and colonize.
Sponges can be either white, red, orange, green, yellow, brown, purple or black. They are known for regenerating from fragments that are broken off..
- see Appendix Sponges 2
Where do Sponges live?
Of the 9'000 or so species of sponges, only about 100 (made of spongin fiber) live in fresh water, the rest are marines. They are able to grow in most environments. Sponges can be attached to surfaces anywhere as deep as 8km in the ocean on the bottom of the ocean floor. There are a higher number of sponges in the tropics because the water is warmer. They like to live in clearer waters over murky waters formed by currents because the murky waters can clog the pores of the sponges so the sponge cannot get its nutrition and oxygen to survive.
Why are sponges considered as animals?
For years sponges were considered as plants, but after looking at them better, biologists decided that they have more characteristics of animals. Even sponges don’t have a definite form of symmetry, lack organs or true tissues, and are poorly organized, they are still considered as animal because of the following points. They don’t create their own food. There are considered as heterotrophes whereas plants are autotrophic orgnaisms. They have a skeleton like structure made of calcium carbonate and colalgen. They exhibit growth (basically they are born, they grow, they reproduce and then die), which is a characteristic of all animals. Their cells lack cell walls which is again a main characteristic of animal cells (plants have cellulose in their cell wall).
What are Cnidarians?
The name Cnidaria comes from the Greek word "cnidos" which means stinging nettle. Cnidarians (phylum Cnidaria) are mostly marine species (more than 10'000) and are the oldest of the true metazoan phyla . All the Cnidaria are aquatic and nearly all are marine and are characterized by a radially symmetrical body. They don't have a head but a mouth surrounded by a crown of tentacles. All Cnidarians have stinging cells known as nematocysts, or cnidae, at the end of tentacles surrounding the creature's mouth.
- see Appendix Cnidarians 1
What are the main characteristics of Cnidarians?
The body wall of all Cnidarians consists of three layers: a out layer called the epidermis which contains a lot of different cells like the epitheliomuscular cells which contract and can move, interstitial cells which give rise to other cells such as sperm and egg, cnidocytes which are specialized cells unique to Cnidarians that in fact contain stinging structures, cells which are glandular cells that secrete mucus called mucus-secreting and receptor and nerve cells which collect and transmit sensory informations. A middle layer known as the mesoglea and an in layer called the gastrodermis.
They are two different forms of Cnidarians body: Polypoid Shape or Medusoid Shape.
The polypoid is the shape of corals and anemones. Its characteristics are the mouth and the tentacles facing up and the other side is anchored to a colony of the same aspect: for example, a coral reef.
Medusoids are jellyfish. They are free swimmers and their tentacles and mouth are pointed down (the opposite of polypoids).
Cnidarians have a mouth and a simple digestive system at the center of their tentacles, that is the reason why they are considered as animals and not plants. They have a radial symmetry. All cnidarians use tentacles which have stinging cells to subdue and capture food. They generally eat small planktonic animals.
The digestive system of Cnidarians is an internal sac for digestion which is called the gastrovascular cavity. It has only one opening, the mouth through which Cnidaria takes food in and releases waste and their tentacles radiate outward from the rim of the mouth.
- see Appendix Cnidarians 2
What are the four main groups of Cnidarians?
There are four groups of Cnidarians: Hydrozoa, Scyphozoan, Cubozoa, and Anthozoa.
Hydrozoans are in general small and colonial animals that usually exist as polyps. They live attached to rocks, sea grasses, and other solid objects, where they feed on small crustaceans and other zooplankton. There are around 2,700 described species of hydrozoans: this species have a life cycle with both polyp and medusa stages.
Scyphozoans, or jellyfish, are found in the oceans of the world and eat small planktonic animals (zooplankton) and sometimes fish. There are about 200 described species of scyphozoans, and, like hydrozoans, they have three separate life stages: a sexually produced planula stage, a strictly asexual polyp stage (usually not colonial), and a dominant, sexual, medusa stage. They can measure between a few centimeters and a meter.
Cubozoans, or box jellyfish, are a class in which the medusa is the dominant life stage. They are small, rectangular boxes (1-10 cm in length) with four tentacles (nematocyst coverded tentacles or bundless of tentacles). There are about twenty species of Cubozoans. They are fast and agile swimmers and fish on fish and crustaceans. Unlike the other Cnidarians, Cubozoans have numerous well-developed eyes along the margin of the bell which are remarkably complex because they have a cellular cornea, a lens, pigment cells and a photoreceptive, enervated retina. This eyes are used for feeding and during reproduction. Their life cycle is similar to Scyphozoans with one exception: box jellies mate. Cubozoans are highly toxic and can kill humans.
Anthozoans have te greatest species diversity, with over 6'000 species. They have too the greatest morphological diversity, encompassing sea anemones, sea pansies, sea whips and fans, corallimorphs and soft and hard corals. The dominant life stage og Anthozoa is the polyp. It shows a greater complexity than Hydrozoans and Scyphozoans polyps. Unlike the other Cnidarians, its gut is partially subdivided by thin, folded layers of tissue taht are vertically arranged around the inside og the gut. So its medusa is completely absent. Planula develop into a reproductively capable polyp and the development can occur within the gastrovascular cavity or in the external milieu.
- see Appendix Cnidarians 3
How do Cnidarians reproduce?
Cnidarians reproduce both asexually and sexually.
The reproduction is made by budding. That is a process of asexual reproduction in which a new individual develops from some generative anatomical point of the parent organism. Cnidarians have separate sexes, which specialized cells in each producing the gametes, which are excreted with the aim of meeting gametes produced by the opposite sex. When this happens a microscopic free-swimming, a larva known as a planula forms moves through the water by moving cilia that cover its body. If it survives against the predators, it eventually settles down on the sea floor and develop into polyp. After the polyp is etablished, it will reproduce by budding. These will break off and form medusoid floaters, which reproduce sexually, and begin the cycle again. This alternation of generations is unique to Cnidarians, although not all follow this pattern.
The way of reproducing sexually is different according to the species of Cnidarians. For example for Scyphozoa (jellyfish) and Cubozoa (box jellies) a larva swims until it finds a site and then becomes a polyp. But the pricinple is the same for all. In fact, male and female produce gametes that join through external fertilization to produce polyps.
Asexual reproduction spreads healty genotypes in huge numbers, while sexual reproduction secure genetic variation.
- see Appendix Cnidarians 4
Where do Cnidarians live?
They live under water. Some Cnidarians live in the oceans and seas, and a few live in freshwater. But they mostly live in saltwater or oceans.
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