Download Some Characteristics of the Phylum Cnidaria

World Applied Programming, Vol (3), Issue (9), September 2013. 391-395
ISSN: 2222-2510
©2013 WAP journal. www.tijournals.com
Some Characteristics of the Phylum Cnidaria
Hamed Nosrati *
Masoud Nosrati
Ronak Karimi
Kamran Makekian
Eslamabad-E-Gharb Branch,
Islamic Azad University,
Eslamabad-E-Gharb, Iran.
[email protected]
Eslamabad-E-Gharb Branch,
Islamic Azad University,
Eslamabad-E-Gharb, Iran.
[email protected]
Eslamabad-E-Gharb Branch,
Islamic Azad University,
Eslamabad-E-Gharb, Iran.
[email protected]
Eslamabad-E-Gharb Branch,
Islamic Azad University,
Eslamabad-E-Gharb, Iran.
Abstract: In this paper we have investigated some characteristics of the phylum Cnidaria. Cnidarians form an
animal phylum that is more complex than sponges. Cnidarians feed in several ways: predation, absorbing
dissolved organic chemicals, filtering food particles out of the water, and obtaining nutrients from symbiotic
algae within their cells. Cnidarians live in different ecosystems and have ecological differences. Some
cnidarians are parasites.
Keywords: Cnidarians as an animal phylum, Cnidarians' feeding and excretion, ecology of cnidarians, some
characteristics of the phylum Cnidaria
I. INTRODUCTION
In this paper we will investigate feeding, excretion, ecology and some characteristics of the phylum Cnidaria.
Section 2 is titled "about cnidarians" and we will introduce cnidarians briefly in this section.
Section 3 is about cnidarians' feeding ways. In this section we will also discuss about excretion in cnidarians.
Section 4 is dedicated to ecology of cnidarians.
At the end of paper conclusion is placed.
II.
ABOUT CNIDARIANS
Cnidarians form an animal phylum that is more complex than sponges, about as complex as ctenophores (comb jellies),
and less complex than bilaterians, which include almost all other animals. However, both cnidarians and ctenophores are
more complex than sponges as they have: cells bound by inter-cell connections and carpet-like basement membranes;
muscles; nervous systems; and some have sensory organs. Cnidarians are distinguished from all other animals by having
cnidocytes that fire like harpoons and are used mainly to capture prey but also as anchors in some species. [1]
Cnidaria is a large phylum composed of some of the most beautiful of all the salt and freshwater organisms: the true
jellyfish, box jellyfish, coral and sea anemones, and hydra. Although Cnidaria is an incredibly diverse group of animals,
there are several traits that link them together.
Most cnidarians are dipoblastic, which means that they are composed of only two layers of cells. The outer layer is known
as the ectoderm or epidermis, and the inner layer is known as the endoderm or gastrodermis. These layers contain the
nerve nets that control the muscular and sensory functions of the animal. Between these layers is a jelly-like noncellular
substance known as mesoglea, which in true jellyfish constitute the vast bulk of the animal (hence their common name). In
other species, the mesoglea may be nearly absent. All cnidarians have a single opening into the body which acts as both
the mouth and anus, taking in food and expelling waste. In most species the mouth is lined with tentacles which act to
capture food. The mouth leads to a body cavity known as the coelenteron, where the food is digested. This body cavity has
given this phylum its other, less commonly used, name of Coelenterata.
391
Hamed Nosrati, et al. World Applied Programming, Vol (3), No (9), September 2013.
Cnidarians have a complex life cycle that, depending on the species, may alternate between two forms. The first form is
known as a polyp, which is sessile (anchored to one spot). The polyps are tubular in shape, with the mouth, often lined
with tentacles, facing upwards. The bodies often contain a type of skeleton that may surround the tissues (exoskeleton) or
be surrounded by the tissues (endoskeleton). These skeletons may be composed of minerals like calcium carbonate, and/or
may consist of organic material such as chitin. Polyps also have a hydrostatic skeleton, where the muscles in the endoderm
work against the fluid contained in the coelenteron, thus extending the polyps. Hydrostatic skeletons are also present in the
tentacles, allowing them to be extended to capture food. Polyps often form large colonies, where a trait known as
polymorphism may occur: various polyps in the colony may take on specialized roles. For example, one polyp may only
be used for defence, while another is used for reproduction and another for capturing food. Not all polyps do this,
however, and may live solitary lives. Some cnidarians, such as true coral and sea anemones, live their entire lives in the
polyp stage and do not metamorphose into the second form, which is known as the medusa.
Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like
material, which is called the mesoglea in cnidarians; more complex animals have three main cell layers and no
intermediate jelly-like layer. Hence, cnidarians and ctenophores have traditionally been labelled diploblastic, along with
sponges. [1][2] However, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises
from the middle cell layer. [3] As a result some recent text books classify ctenophores as triploblastic, [4] and it has been
suggested that cnidarians evolved from triploblastic ancestors. [3]
In true jellyfish and in box jellyfish, the medusa is the most prominent form. They are free-floating or free-swimming,
with the mesoglea giving them buoyancy. Medusae generally have only hydrostatic skeletons, which allow the muscles
to work against the fluids in the coelenteron to enable the medusae to swim. The life cycle of cnidarians that contain both
the polyp and medusa forms goes generally as follows: adult medusae reproduce sexually, creating a small, ciliated (cilia
are small hairs that beat back and forth, allowing for locomotion) larva known as a planula. The planula eventually
settles on the sea floor and changes into a polyp. The polyp can then reproduce asexually, commonly using one of two
ways: it can split in two, creating two clones of the original, or it can form a colony, where the new polyps do not split
from the original but rather seem to grow off its sides. Then, depending on the species, medusae can be formed asexually
from the polyps, or, as occurs with the box jellyfish, the polyp itself can metamorphose into a medusa, and the cycle
begins again. As mentioned above, this cycle is common for the jellyfish and box jellyfish. The coral and sea anemones
remain as polyps. The hydrozoans are the most diverse when it comes to life cycles: some species may consist of both
polyps and medusae, while others are polyp-free, being only medusae, and yet others are medusa-free, being only polyps.
Cnidarians are generally carnivorous in nature, but some species, such as coral, get some of their food from special
symbionts (organisms that are benefited from and benefit the organisms they are with) living within them. There are two
main types of symbionts: zooxanthellae, which are photosynthetic protists (single-celled organisms) known as
dinoflagellates, and zoochlorellae, which are photosynthetic algae. These symbionts capture the energy from the sun to
produce sugars which are then passed on to their host as a source of food. Not all cnidarians possess these creatures
however, and thus must capture their own food. Because most cnidarians lack such sensory organs as eyes, it is thought
they hunt passively: namely, they just wave their tentacles and hope something brushes near. When prey does come in
contact with the tentacles, special structures known as nematocysts fire like harpoon guns into the flesh of the organism,
either injecting a toxin that paralyses and/or kills the prey, or entangling the prey.
Nematocysts are common in all cnidarians, and are the one major trait that separates this phylum from the others. There
are approximately 20-30 types of nematocysts know to date that help distinguish between the various classes.
Nematocysts are just one of three types of structures located within the cells of the tentacles and/or mouth lining that are
known as cnidae (which is how this phylum is named). The other two types are known as spirocysts and ptychocysts.
They all work in basically the same way: when the cnida receives the appropriate physical or chemical signal, a cover
known as the operculum is moved back, allowing a hollow structure known as a tubule to fire. This tubule may contain
toxins (as is the case with nematocysts), adhesives that stick to the prey (spirocysts), or may just entangle bits of mud
that then form a type of mud “home” around the organism (ptychocysts - found only in one order of the anemone and
coral class). Once a cnida has fired, it can no longer be used [14].
392
Hamed Nosrati, et al. World Applied Programming, Vol (3), No (9), September 2013.
All cnidarians can regenerate, allowing them to recover from injury and to reproduce asexually. Medusae have limited
ability to regenerate, but polyps can do so from small pieces or even collections of separated cells. This enables corals to
recover even after apparently being destroyed by predators.
In the Cnidaria sexual reproduction often involves a complex life cycle with both polyp and medusa stages. For example
in Scyphozoa (jellyfish) and Cubozoa (box jellies) a larva swims until it finds a good site, and then becomes a polyp.
This grows normally but then absorbs its tentacles and splits horizontally into a series of disks that become juvenile
medusae, a process called strobilation. The juveniles swim off and slowly grow to maturity, while the polyp re-grows
and may continue strobilating periodically. The adults have gonads in the gastroderm, and these release ova and sperm
into the water in the breeding season [13].
III. FEEDING AND EXCRETION
Cnidarians feed in several ways: predation, absorbing dissolved organic chemicals, filtering food particles out of the
water, and obtaining nutrients from symbiotic algae within their cells. Most obtain the majority of their food from
predation but some, including the corals Hetroxenia and Leptogorgia, depend almost completely on their endosymbionts
and on absorbing dissolved nutrients. [1] Cnidaria gives their symbiotic algae carbon dioxide, some nutrients and a place
in the sun. [2]
Predatory species use their cnidocytes to poison or entangle prey, and those with venomous nematocysts may start
digestion by injecting digestive enzymes. The "smell" of fluids from wounded prey makes the tentacles fold inwards and
wipe the prey off into the mouth. In medusae the tentacles round the edge of the bell are often short and most of the prey
capture is done by "oral arms", which are extensions of the edge of the mouth and are often frilled and sometimes
branched to increase their surface area. Medusae often trap prey or suspended food particles by swimming upwards,
spreading their tentacles and oral arms and then sinking. In species for which suspended food particles are important, the
tentacles and oral arms often have rows of cilia whose beating creates currents that flow towards the mouth, and some
produce nets of mucus to trap particles. [1]
The food is in the digestive cavity, gland cells in the gastroderm release enzymes that reduce the prey to slurry, usually
within a few hours. This circulates through the digestive cavity and, in colonial cnidarians, through the connecting
tunnels, so that gastroderm cells can absorb the nutrients. Absorption may take a few hours, and digestion within the
cells may take a few days. The circulation of nutrients is driven by water currents produced by cilia in the gastroderm or
by muscular movements or both, so that nutrients reach all parts of the digestive cavity. [2] Nutrients reach the outer cell
layer by diffusion or, for animals or zooids such as medusae which have thick mesogleas, are transported by mobile cells
in the mesoglea. [1]
Indigestible remains of prey are expelled through the mouth. The main waste product of cells' internal processes is
ammonia, which is removed by the external and internal water currents. [2]
IV. ECOLOGY
Many cnidarians are limited to shallow waters because they depend on endosymbiotic algae for much of their nutrients.
The life cycles of most have polyp stages, which are limited to locations that offer stable substrates. Nevertheless major
cnidarian groups contain species that have escaped these limitations. Hydrozoans have a worldwide range: some, such as
Hydra, live in freshwater; Obelia appears in the coastal waters of all the oceans; and Liriope can form large shoals near
the surface in mid-ocean. Among anthozoans, a few scleractinian corals, sea pens and sea fans live in deep, cold waters,
and some sea anemones inhabit polar seabeds while others live near hydrothermal vents over 10 kilometres (6.2 mi)
below sea-level. Reef-building corals are limited to tropical seas between 30°N and 30°S with a maximum depth of 46
metres (151 ft), temperatures between 20°C and 28°C, high salinity and low carbon dioxide levels. Stauromedusae,
393
Hamed Nosrati, et al. World Applied Programming, Vol (3), No (9), September 2013.
although usually classified as jellyfish, are stalked, sessile animals that live in cool to Arctic waters. [5] Cnidarians range
in size from Hydra, 5–20 millimetres (0.20–0.79 in) long, [6] to the Lion's mane jellyfish, which may exceed 2 metres
(6.6 ft) in diameter and 75 metres (246 ft) in length. [7]
Prey of cnidarians ranges from plankton to animals several times larger than themselves. [5][8] Some cnidarians are
parasites, mainly on jellyfish but a few are major pests of fish. [5] Others obtain most of their nourishment from
endosymbiotic algae or dissolved nutrients. [1] Predators of cnidarians include: sea slugs, which can incorporate
nematocysts into their own bodies for self-defense; [9] starfish, notably the crown of thorns starfish, which can devastate
corals; [5] butterfly fish and parrot fish, which eat corals; [10] and marine turtles, which eat jellyfish. [7] Some sea
anemones and jellyfish have a symbiotic relationship with some fish; for example clown fish live among the tentacles of
sea anemones, and each partner protects the other against predators. [5]
Coral reefs form some of the world's most productive ecosystems. Common coral reef cnidarians include both
Anthozoans (hard corals, octocorals, anemones) and Hydrozoans (fire corals, lace corals) The endosymbiotic algae of
many cnidarian species are very effective primary producers, in other words converters of inorganic chemicals into
organic ones that other organisms can use, and their coral hosts use these organic chemicals very efficiently. In addition
reefs provide complex and varied habitats that support a wide range of other organisms. [11] Fringing reefs just below
low-tide level also have a mutually beneficial relationship with mangrove forests at high-tide level and sea grass
meadows in between: the reefs protect the mangroves and seagrass from strong currents and waves that would damage
them or erode the sediments in which they are rooted, while the mangroves and seagrass protect the coral from large
influxes of silt, fresh water and pollutants. This additional level of variety in the environment is beneficial to many types
of coral reef animals, which for example may feed in the sea grass and use the reefs for protection or breeding. [12]
V. CONCLUSION
In this paper we investigated some characteristics of the phylum Cnidaria. We discussed about cnidarians as an animal
phylum that is more complex than sponges. Then we got into different ways of feeding in cnidarians. We also studied
about cnidarians' excretion. Finally we investigated ecology of cnidarians.
REFERENCE
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
Hinde, R.T., (1998). "The Cnidaria and Ctenophora". In Anderson, D.T.,. Invertebrate Zoology. Oxford University Press. pp.
28–57. ISBN 0-19-551368-1.
Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 111–124. ISBN 0-03025982-7.
Seipel, K., and Schmid, V. (June 2005). "Evolution of striated muscle: Jellyfish and the origin of triploblasty". Developmental
Biology 282 (1): 14–26. doi:10.1016/j.ydbio.2005.03.032. PMID 15936326.
Ruppert, E.E., Fox, R.S., and Barnes, R.D. (2004). Invertebrate Zoology (7 ed.). Brooks / Cole. pp. 182–195. ISBN 0-03025982-7.
Shostak, S. (2006). "Cnidaria (Coelenterates)". Encyclopedia of Life Sciences. John Wiley & Sons.
doi:10.1038/npg.els.0004117.
Blaise, C., and Férard, J-F. (2005). Small-scale Freshwater Toxicity Investigations: Toxicity Test Methods. Springer. p. 398.
ISBN 1-4020-3119-X. Retrieved 2008-11-21.
Safina, C. (2007). Voyage of the Turtle: In Pursuit of the Earth's Last Dinosaur. Macmillan. p. 154. ISBN 0-8050-8318-9.
Retrieved 2008-11-21.
Cowen, R. (2000). History of Life (3 ed.). Blackwell. p. 54. ISBN 0-632-04444-6. Retrieved 2008-11-21.
Frick, K (2003). "Predator Suites and Flabellinid Nudibranch Nematocyst Complements in the Gulf of Maine.". In: SF Norton
(ed). Diving for Science...2003. Proceedings of the American Academy of Underwater Sciences (22nd Annual Scientific
Diving Symposium). Retrieved 2008-07-03.
Choat, J.H. and Bellwood, D.R. (1998). Paxton, J.R. and Eschmeyer, W.N.. ed. Encyclopedia of Fishes. San Diego: Academic
Press. pp. 209–211. ISBN 0-12-547665-5.
394
Hamed Nosrati, et al. World Applied Programming, Vol (3), No (9), September 2013.
[11]
[12]
[13]
[14]
Barnes, R.S.K., and Mann, K.H. (1991). Fundamentals of Aquatic Ecology. Blackwell Publishing. pp. 217–227. ISBN 0-63202983-8. Retrieved 2008-11-26.
Hatcher, B.G. Johannes, R.E., and Robertson, A.J. (1989). "Conservation of Shallow-water Marine Ecosystems".
Oceanography and Marine Biology: An Annual Review: Volume 27. Routledge. p. 320. ISBN 0-08-037718-1. Retrieved
2008-11-21.
http://answers.yahoo.com/question/index?qid=20110327122053AAKcMTP
http://www.angelfire.com/mo2/animals1/phylum/jellyfish.html
395