Tuesday, January 26, 2010
Manta Ray (Manta birostris)
Mantas are found world-wide in tropical to warm temperate seas.
Like sharks and other rays, Mantas are fertilized internally. During courtship, one or more male Mantas chase a female for prolonged periods. Eventually a successful male grasps the tip of one of her pectoral wings between his teeth and presses his belly against hers. Then, the male flexes one of his claspers and inserts it into her vent. The fertilized eggs develop inside a mother Manta's body for a lengthy but unknown period that may be 9 to 12 months or more. One to two pups are born per litter, but no one knows where or when Mantas give birth.
Based on the smallest free-swimming members of this species, newborn Mantas are about 1.2 metres across.
Records of giant Mantas are notoriously difficult to verify. The largest reported in the scientific literature measured 6.7 metres across and there is one report of an individual 9.1 metres across. But most Mantas encountered by people are about 4 metres across.
No one knows how long Mantas live. Based on known lifespans of closely related (but much smaller) warm-water rays, Mantas may live up to 25 years or so.
There are numerous reports, mostly anecdotal, of harpooned Mantas leaping on small vessels and smashing them or their occupants. Some of these cases may have resulted in the death of one or more people due to crushing or drowning, but it must be born in mind that the animal was simply trying to defend itself. Divers have sometimes been injured accidentally while trying to ride or photograph Mantas from too close. But there is no record of an unprovoked Manta attacking or injuring a person.
Only large warm-water sharks, such as the Tiger Shark , are known to prey upon Mantas.
This flattened body form is advantageous for hiding in the bottom sediment, making it difficult for predators to see partially buried stingrays. During the course of their evolution, Mantas lost the stinging barb and their pectoral fins developed into graceful, flapping wings, but their flattened body shape remained .Given the scattered nature of their planktonic food, it is very likely that Mantas migrate. But little is known about where they travel and when.
In vertebrates (backboned animals), sleep is characterized by a profound change in brainwaves. This has never been demonstrated experimentally in Mantas or any other elasmobranch (shark or ray). Actively swimming sharks and rays, such as the Manta, are believed to swim constantly, never stopping from birth to death. Although it is feasible that some parts of the brain shut-down while the parts of the brain responsible for coordinating swimming movements stays awake, this has not been demonstrated experimentally.
Apart from courtship and mating (which is quite elaborate, see above), Mantas do not appear to be particularly social. Mantas frequently aggregate at rich feeding sites and cleaning stations, but there is little evidence of social interaction among them.
Mantas are most commonly seen during daylight hours because that's when most observers are most active. Mantas may feed most actively at night, when many planktonic creatures rise surfaceward, providing a rich bounty on which Mantas may feed.
Divers have observed Mantas as deep as 30 metres, but no one knows how deep they can swim.
The forward-pointing, paddle-like organs at each corner of a Manta's mouth are termed "cephalic lobes". Mantas have been observed using their cephalic lobes like scoops to help push plankton-bearing water into their mouths.
In the 1990's, targeted harpoon fisheries in the Philippines and off the Pacific coast of Mexico decimated resident populations. Due to the scarcity of catches, Mantas are rarely targeted in these locations today.
Mantas are valued commercially for their tasty meat, sandpapery hide, and oil-rich liver. A Manta in captivity is worth a small fortune to any public aquarium with the facilities to display it, a reality that drives a small but recurrent harvest in Japan and possibly other areas.
Mantas seem to be fairly abundant in some areas, rare or absent in others. Until we understand the extent and dynamics of Manta stocks, there is no way to assess their conservation status. Based on their low birth rate, Mantas are probably highly vulnerable to sustained fishing pressure and habitat degradation. This likelihood would seem to favor a cautionary approach to Manta exploitation and management until such time as we have the sound scientific data to make a more informed assessment of this species' risk of extinction.
So little is known about the basic biology and life history of Mantas, little can be said about their importance from a scientific or ecological standpoint. It can be argued that Mantas are important because they add to the beauty, diversity, and mystery of our world. Without Mantas, our planet would seem a significantly poorer place.
The class took place in the laboratory downstairs.
Homework: The photocopy about questions was corrected. Look, many of you don´t do your homework.
I must say that it is up to you to pass your exams or to fail them. My duty is to provide you with the necessary knowledge in the four sills (listening, reading, writing and speaking) , However, if you don´t study enough , you´ll have to be responsible of your exam results, not me.
St.B. p. 38,39 and Wk.p. 28,29
Listening: Three videos of units 1,2,3. With questions, useful phrases and interaction activities.
Homework: Wk.p. 30 and ST.B.p. 40 The reading about Zara and exercises a,b,c.
Homework: St.B.p.46 and exercises c,d,e .About Henry VII and his wife Katherine Howard.
Listening: St.B.p. 47 .Aboyt Henry VIII´s other wives. a chart to be filled in.
Listening: video of unit 1 ,about a radio actor. I´m afraid I can´t post this activity because of license problems. The exercises were of two kings: questions and true-false statements.
Homework: St.B.p.50. The writing activity. Read the story, understand it and fill in the the correct expressions given. a compositions has to come out of this activity.