European green crabs are an invasive species which have

European green crabs are an invasive species which have recently colonized the Northwest coast of North America.
They disrupt ecosystems and prey heavily on native shellfish.
However, in Europe, the population of green crabs is kept relatively low by a parasitic barnacle called Sacculina carcini.
This barnacle could likely be introduced to areas where the European green crab threatens to ruin established ecosystems.

Which of the following, if true, most strongly supports the argument that it would NOT be advisable to attempt to eliminate European green crabs from the Northwest coast of North America by introducing to the area the parasitic barnacle discussed above?

A. In their native habitat, the parasites often threaten the elimination of local green crab populations.
(They keep crab population low in their native habitat but not eliminate them. Also native scenario does not affect the proposal in NA.)

B. European green crabs are also occasionally preyed upon by fishes, gulls and other species of crab in their native ecosystems.
(This is only occasional and even this is the info regarding their native ecosystems not NA. OFS)

C. Though the parasite may survive in North American waters, no large-scale facility for breeding and harvesting the parasites exists as of yet.
(we are not bothered about the survivial of parasite in NA waters. OFS)

D. Many other invasive species on the Northwest coast of North America are not suitable hosts for this parasite.
(we have info regarding any other invasive species other than European green crabs as of now. OFS)

E. Though the barnacle cannot reproduce inside of native shore crabs, it can still infect and kill native shore crabs.
(instead of green crabs we may lose shore crabs because of introducing these parasites. This indicates that the proposal may not be advisable.)

i chose E but doesn't native shore crabs mean native to barnacle's region it is inaccurate to assume the same species exists in north america?

You have to answer this question:

Which of the following, if true, most strongly supports the argument that it would NOT be advisable to attempt to eliminate European green crabs from the Northwest coast of North America by introducing to the area the parasitic barnacle discussed above?

We are talking about the North American area. So if we say that though the barnacle cannot reproduce inside of native shore crabs, it can still infect and kill native shore crabs, we are talking about the native shore crabs of America. We are saying that we should not introduce the parasitic barnacle because it can infect and kill native shore crabs (along with European green crabs).

Knewton Official Solution:


European green crabs are an invasive species which have recently colonized the Northwest coast of North America, among other places. They disrupt ecosystems and prey heavily on native shellfish. However, in Europe, the population of green crabs is kept relatively low by a parasitic barnacle called Sacculina carcini. This barnacle is hardy and could likely be introduced to areas where the European green crab threatens to ruin established ecosystems.

Which of the following, if true, most strongly supports the argument that it would NOT be advisable to attempt to eliminate European green crabs from the Northwest coast of North America by introducing to the area the parasitic barnacle discussed above?


A. In their native habitat, the parasites often threaten the elimination of local green crab populations.
B. European green crabs are also occasionally preyed upon by fishes, gulls and other species of crab in their native ecosystems.
C. Though the parasite may survive in North American waters, no large-scale facility for breeding and harvesting the parasites exists as of yet.
D. Many other invasive species on the Northwest coast of North America are not suitable hosts for this parasite.
E. Though the barnacle cannot reproduce inside of native shore crabs, it can still infect and kill native shore crabs.


What is INVASIVE species? as also hinted in the argument, a species that is not native to the location and spreads in a way to threaten the local ecosystem. This argument describes a European crab that is invading North American coastal waters. It notes that a parasite controls the populations of such crabs in Europe and suggests the parasite could be introduced to control invasive populations in North America.

We are asked to find a reason why it would not be a good idea to introduce this parasite. The correct answer should provide some reason why the parasite would be ineffective at controlling the crab population, or provide some undesirable, unconsidered effect of introducing the parasite.

Choice E notes that the parasite can "infect and kill" native shore crabs. The parasite is being introduced to protect native species; if, instead, the parasite kills them, the native ecosystem is not protected. Choice E is correct.

Choice A: Opposite. This choice states that the parasites often "threaten the elimination" of local crab populations in Europe. Such a statement suggests that the parasite would be highly effective in destroying the invasive green crabs in North America. This choice argues for, not against, introducing the parasite.

Choice B: Irrelevant. This choice states that other predators also control the population of green crabs in Europe. This might suggest that parasites alone may be less effective at controlling the green crabs than parasites AND other predators, but it does not demonstrate that parasites alone would not lower the crab population.

Choice C states a logistical obstacle to the plan; there is not currently a facility which can breed large numbers of the parasite. While this may make the plan difficult to execute, it does not mean the plan is not advisable.

Choice D: Irrelevant. This choice suggests that other invasive species would not be affected by the parasite. This has no bearing on whether the parasite would control the population of green crabs.


Answer: E