P8 - Structure, Function and Diversity of Terrestrial Vertebrates 2 (Frog)
Amphibians evolved during the Devonian from fish and further evolved to form organisms that were totally terrestrial, the reptiles. The key characteristics of Amphibians are they are ectothermic vertebrates, with three-chambered hearts, claws on their toes are absent and their skin lacks scales, feathers and hair and is generally smooth as well as moist for efficient breathing. The main of the experiment is to study the habitat, lifecycle, external features and internal anatomy of a typical frog species: Rana temporaria.
* Dissection kit
* Dissecting awls
* Drawing utensils and A4 drawing paper
The external features of the frog were observed. Then the skin of the frog was peeled back during ventral dissection to reveal the internal organs. The pectoral girdle was removed to see the heart and major arterial and venous blood vessels. Next the kidneys and reproductive organs were looked at before any remaining viscera were removed to allow the vertebral column and the spinal nerves to be visible. Students were then taken to a Zoological Museum in order to observe some specimens, including the evolution of Amphibians, their life stages. The various habitats of Amphibians were looked at as well as the possible effects of climate change and anthropogenic activities. Amphibians were then compared to the fishes, particularly the lungfishes.
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*See the diagrams made during the practical which follow the conclusion
Common Frog identification featuresDiscussion
Species: R. temporaria
Frogs belong to the Anura order of amphibians. They have powerful legs for hopping and are recognised as the greatest jumpers of all vertebrates. Their skin is permeable. Convergent evolution has caused a distinct separation between the frogs and toads. Most frogs are found in tropical rainforests; however a few are present in the subarctic regions of the world. Rana temporaria is also known as the European Common frog. Rana temporaria hind-limbs are much larger and longer than the fore-limbs and their digits are webbed to enable swimming. The soft and fleshy tongue is fixed to the symphysis of the lower jaw front. The upper jaw bears teeth. Their smooth skin is variably spotted or stripped and is variable in colour. These dark bands are the result from an adaptation towards camouflage. Sexual dimorphism is common in frogs, in this species of study; the males are usually smaller than females. Females lay spawn in clumps, typically 1000-2000 gelatinous eggs containing black embryos with a white spot. Rana temporaria can be mistaken for the common toad, Bufo bufo. However, this toad is larger and usually walks unlike the hopping frog. Toads also lack the distinctive dark patch behind the eye and usually have a rounder snout than frogs. Common frogs hibernate throughout the winter months and the rest of the year they hunt for food. Typical food includes snails, slugs, worms and insects.
It has been found by scientific evidence that male frogs can be changed into females by the introduction of pesticides containing the chemical atrazine. They were found to be able to successfully mate with males even though they were genetically male.
University of California - Berkeley (2010, March 1). Pesticide atrazine can turn male frogs into females. ScienceDaily. Retrieved March 13, 2010, from http://www.sciencedaily.com/releases/2010/03/100301151927.htm
In spring, males arrive at breeding areas before females, and it is thought that individuals return to their natal ponds by following scents (5). There is typically heavy competition amongst males for females, involving much croaking and wrestling. Males grab a female and remain clasped to her body for days or weeks before spawning takes place. All of the frogs in a pond tend to spawn roughly within a few days of each other. The female releases 1000 to 2000 eggs, the male then releases sperm. The eggs are coated in jelly, and are popularly known as 'frogspawn'. After 10-14 days, the tadpoles hatch, becoming free-swimming a few days later, and undergoing metamorphosis into adults 10-15 weeks after hatching. Tadpoles are vulnerable to predation by a range of aquatic creatures, including water beetles, newts and fish (5).
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Some frogs are known to contain poisons in order to make them unpalatable to predators. These types of frog usually display bright colours to advertise their toxicity.
In adult frogs, respiration is carried out by lungs and through the skin to the blood system. The frog closes its mouth, and air is taken in via the nostrils, filling the buccal cavity. The posterior nares close by the contraction of the muscles of the cheeks and pharynx and the glottis opens to force the inspired area into the windpipe leading to the lungs.
Amphibians include the frogs, toads, salamanders, newts and caecilians. They are ectotherms that lay non-shelled eggs in water. Most metamorphose as a juvenile to an adult from water breathing to air breathing.
Frogs have a three chambered heart by which oxygenated blood from the lungs and deoxygenated blood from the respiring tissues enter via separate atria and directed through a valve to through the aorta for oxygenated blood and pulmonary artery for deoxygenated blood. This structure was adapted from the two chambered heart, usually found in most fish, in order to avoid the mixing of oxygenated and deoxygenated blood.
Amphibians arose during the Devonian Period from the adaptation of fish similar to the modern lungfish, which has evolved multi-jointed leg- like fins that enabled them to crawl along the sea bottom. Amphibians are terrestrial creatures apart from having to return to water to lay their eggs. In moving to land, there were many resources available. The earliest known fossil of an amphibian origin is the aquatic or semi-aquatic organisms, Ichthyostega. Their fore-legs were well supported by shoulder bones and the hind legs were flipper-shaped. Broad ribs that overlap each other formed a solid cage for the lungs and heart. It was discovered that the organism that developed between fish and Ichthyostega was found to be a Tiktaalik, which had gills and scales like a fish and a neck similar to an amphibian. Its forelimbs and lobe fins were identical to those found in an amphibian. Modern amphibians belong to three groups: Anura, Caudata, and Apoda.
Reproduction can be external or internal in amphibians. Salamanders and caecilians usually reproduce internally, whereas anurans usually reproduce externally. Reproduction generally occurs in fresh water, apart from a few exceptions that can tolerate brackish water. However, there are no true marine amphibians. Mating usually occurs around springtime. During the larval stage of development, the tadpoles breathe with exterior gills. After hatching, they begin to metaphorphose and usually leave the water to become terrestrial adults. During metamorphosis, four legs are formed to support their body on land, the gills are replaced by lungs, an eardrum is developed to lock the middle ear, the skin changes and develops glands, the eyes develop eyelids and adapt to vision outside the water. Also, in frogs and toads, the tail disappears.
After fertilization, the nucleus of a single celled egg divides, producing a 2- cell stage via the division of the cytoplasm. This process continues to produce many cells. The tadpole breathes through its external gills at just three days old. It attaches itself to jelly or weed with its mucous gland and remains relatively sessile. At four days old the tadpole begins to have vigorous bursts of swimming by using its tail muscles. The operculum grows over the gills after six days and these internal gills take over. A spiracle in the operculum allows the breathing current to exit. The intestine is visible through the skin and at this point the tadpole takes part in continuous active swimming. At three weeks, the external gills and mucous gland have disappeared. Very vigorous swimming occurs as the tadpole increases in size for the next seven weeks.
http://www.arkive.org/common-frog/rana-temporaria/info.html ...2003-2009 .... accessed 13/03/10