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  3. The Mediterranean’s Killer Algae 1. Today there are many invasive species threatening ecosystems all over the world, but few

The Mediterranean’s Killer Algae 1. Today there are many invasive species threatening ecosystems all over the world, but few

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问题                  The Mediterranean’s Killer Algae
   1. Today there are many invasive species threatening ecosystems all over the world, but few are of as much concern as Caulerpa taxifolia.This species of green algae grows in long, thin blades similar in appearance to sea grass. Each individual alga is single-celled, with a length of up to a meter, they are some of the largest cells in the world. Algae contain chlorophyll and produce their own food through photosynthesis. Also they reproduce asexually, meaning that if an alga is cut in two each half wilt develop into a whole new organism.
   2. Caulerpa taxifolia’s native habitats are the tropical waters of the Caribbean, south Pacific, east Atlantic, and Indian Ocean. However, in the 1970s, a new strain was bred in Europe to decorate exhibits in saltwater aquariums. The strain proved useful for this purpose, for it was able to grow more quickly, form denser patches, and tolerate a greater temperature range--from 10 to 31 degree Celsius--than the original. Created in Germany, the improved algal strain was distributed in 1980 to, among other facilities, the Oceanographic Museum in Monaco, a tiny country on the French coast. Four years later, a sample of the algae escaped the museum and began to grow on the seafloor just outside.
   3. By 1989, this small sample had turned into a 2.2-acre field of Caulerpa taxifolia.A It continued to spread rapidly throughout the Mediterranean, appearing at locations up to 350 kilometers from Monaco in 1991, and by 1994 it had arrived in the waters of Italy, Croatia, and Spain.B Officials in the region, however, were slow to respond,C Their delays allowed the species to develop a strong foothold in the Mediterranean, and the invasion reached global proportions when the algae were spotted off the coasts of California in 1998 and Australia in 2000.D
   4. Dubbed "killer algae" by many scientists, the aquarium strain of Caulerpa taxifolia poses a whole host of threats to the areas it colonizes. Its rapid, dense growth patterns enable it to successfully out-compete all native varieties of algae and sea grasses. Due to its ability to thrive in both warm and cold waters at a variety of depths(anywhere from one to 100 meters), few coastal marine environments are safe. The disappearance of native vegetation affects all the animals that depend on it for food and shelter, while Caulerpa taxifolia itself is inedible to most species because it secretes many harmful toxins.
   5. The algae also cause negative repercussions for humans, in particular the commercial fishing industry. As a result of habitat loss, many species fishermen have relied on for centuries are no longer available, and the algae are notorious for clogging fishing nets and even snagging boat propellers. In addition, the decrease in the natural diversity of plant and animal life in the Mediterranean has led to a severe decline in marine tourism.
   6. In recent years, many methods have been employed in attempts to eliminate the invasive algae. The simplest is known as manual uprooting, in which individual divers use their hands to pull the algae out by the roots. Unfortunately, Caulerpa taxifolia has spread in the Mediterranean to the point that this method is no longer feasible there. While it holds promise for removing small, isolated patches, extreme care must be taken, as any small fragments of the algae left behind could grow and take root once again.
   7. Various physico-chemical procedures have been tried as well. These include exposure to copper and salt, air suction, ultrasound devices, and hot water jets, but each seems more effective at slowing the algae’s expansion than actually eliminating it. Studies of biological methods, such as the introduction of sea slugs* that feed on the algae, have also yielded discouraging results. Instead of consuming the algae blades, the slugs merely cut them into small pieces that could easily disperse and form new colonies elsewhere. Until an answer to the problem is found, scientists urge caution, encouraging marine organizations to be on the lookout for new algae growth and to help educate the public about the serious risks this invasive species represents.
   slug* a small slow-moving creature with a long soft body and no legs, like a snail without a shell.
The word them in the passage refers to
选项 A、sea slugs.
B、discouraging results.
C、algae blades.
D、new colonies.
答案C
解析
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