501 Extraordinary Journey of the Opossum Shrimp - webapps8

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Carapace (bony outer covering) Antenna Uropod (last abdominal leg) More Facts About Mysis Body Parts Like most arthropods, a mysid has three body regions — head, thorax, and abdomen. An extension of the head, the carapace, covers the thorax. The head has five pairs of "legs" or appendages — first antennae, second antennae, and three pairs of mouth parts: mandibles, first maxillae, and second maxillae. The thorax has eight pairs of legs. The first two pair assist with feeding, and are called "maxillipeds." The remaining six pairs are used for swimming and are called "periopods." The abdomen has six pairs of legs. The first five abdominal pairs are very small. The last abdominal appendages are very large and are called the "uropods." The posterior of a mysid is called the "telson." Speed of Migration In a Lake Michigan study, samples taken at different times and at different depths showed that they swim 2.62 feet per minute. Light Sensitivity Mysis detect differences in light intensity with their eyes. They are so sensitive to low light intensity that, on a moonlit night, they do not travel as far up in the water column to feed. Defense Against Predators A mysid does not really have any defense mechanisms. One theory for mysid night-feeding behavior is that it avoids predation by fish. However, many fish are also night feeders. Adaptation to High Water Pressures The exact mechanism that enables mysis and other deep-water crustaceans to survive in deep water is unknown. Experimentally, mysis have been exposed to much greater pressure than those they have been subjected to in deep lakes. At these high pressures, the tiny shrimp ceased activity, but resumed normal activity when pressure was removed. Which Way Is Up? Mysis detect the force of gravity with a structure called the "statocyst" — an inward folding of the body wall, filled with fluid, located in the uropod. A tiny grain, called a "statolith," is secreted by the cells in the statocyst. The statolith floats in the fluid of the statocyst. When it touches certain sensory hairs on the statocyst wall, the statolith detects the force of gravity and instructs the mysid about its equilibrium in the water column. The statocyst of a mysid functions much the same way our inner ear functions to help us maintain equilibrium. JULY-AUGUST 1985 39
Opossum Shrimp Shrimp Traveler. An interesting problem that confronted biologists called "zoogeographers is how the opossum shrimp came to occupy its present habitats. Zoogeographers are scientists who have a background in zoology, geology, and geography. Two interesting facts emerged early in the study of the opossum shrimp. First, it has a wide distribution: northern U.S., Canada, Russia, northern Europe, and Scandinavia. In short, its southernmost reach coincides well with the southernmost extent of the last glacial period in earth's history. The second interesting fact is that a closely related, almost identical species, Mysis oculata, is found in the Arctic Ocean. The summation of these two facts resulted in different conclusions among zoogeographers. The most recent theory is that, during the early part of the last glacial period, sea levels were much lower than they are today because the water was tied up in snow on land. The Arctic Ocean was cut off from the rest ol the oceans except for a narrow strait. It also received inflows of freshwater from rivers. Because of its greater isolation, the Arctic Ocean developed a gradation of brackish to salty water. This set the stage for some saltwater animals to adapt to a different habitat, brackish water. Subsequently, a new species of Mysis evolved from these conditions. But how did the new species arrive in Minnesota? The advancing glaciers provided the opportunity for travel, although the trip took several thousand years. Glaciers create lakes as they advance. The lakes are formed by the enormous weight of the ice depressing the earth's crust. Melting water pools at the front of the glacier, forming a lake. This is called a "proglacial lake. Opossum shrimp traveled south in these advancing proglacial lakes. When the earth's climate warmed again, glacial melt waters formed huge lakes. Opossum shrimp were redistributed northward again via these vast meltwater lakes. As these lakes eventually drained, opossum shrimp were confined to isolated lakes throughout the former range of the last glacial ice sheet. THIS ACCOUNT of opossum shrimp history may one day be replaced by a newer theory. However, there are many interesting aspects of a creature's distribution that may be related to geological history. For example, where did fish go when the glaciers arrived? But that's another story. Q Nancy Kirsch is an aquatic biologist with the DNR Ecological Services Section, St. Paul. She wrote "Trout Chow: Insect Life in a Trout Stream," in the March-April 1984 issue of the Volunteer. 40 THE MINNESOTA VOLUNTEER
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