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501 Extraordinary Journey of the Opossum Shrimp - webapps8

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<strong>Opossum</strong> <strong>Shrimp</strong><br />

more than three feet from <strong>the</strong> bottom.<br />

At dusk, however, <strong>the</strong>y rise<br />

close to <strong>the</strong> surface to feed on algae,<br />

debris, and o<strong>the</strong>r small crustaceans.<br />

At dawn, <strong>the</strong>y move back to deep<br />

water.<br />

These vertical migrations occur<br />

even in large, deep lakes like <strong>the</strong><br />

Great Lakes. In Lake Huron, a migration<br />

<strong>of</strong> 315 feet was recorded. In<br />

Lake Superior and Lake Michigan,<br />

<strong>the</strong>y have been found as deep as 900<br />

feet.<br />

To swim, <strong>the</strong>y use six pairs <strong>of</strong> legs.<br />

In addition, a pair <strong>of</strong> "legs' called<br />

"maxillipeds" is located near <strong>the</strong> animal's<br />

mouth. The maxillipeds strain<br />

food from water.<br />

Stomach Samples. Fisheries biologists<br />

studying feeding habits <strong>of</strong> lake<br />

trout have found opossum shrimp in<br />

<strong>the</strong>ir stomachs as well as in <strong>the</strong><br />

stomachs <strong>of</strong> burbot and members <strong>of</strong><br />

<strong>the</strong> whitefish family. The shrimp are<br />

important in <strong>the</strong> diet <strong>of</strong> young lake<br />

trout; older trout favor a fish diet.<br />

In Minnesota, opossum shrimp<br />

have been reported in Gunflint,<br />

Clearwater, Trout, and Kekekabic<br />

lakes in nor<strong>the</strong>astern Minnesota and<br />

in Lake Superior.<br />

Fisheries biologists can also deduce<br />

<strong>the</strong> presence <strong>of</strong> opossum<br />

shrimps if <strong>the</strong>y find a parasite called<br />

a "swim bladder nematode" in lake<br />

trout. Why? Because <strong>the</strong> swim<br />

bladder nematode must live in <strong>the</strong><br />

opossum shrimp during an early part <strong>of</strong><br />

its life cycle before it infects lake<br />

trout.<br />

Because opossum shrimp are desirable<br />

as a food source for trout,<br />

biologists have attempted to introduce<br />

<strong>the</strong>m as food for trout and related<br />

fish species in lakes where<br />

<strong>the</strong>y were not native. Specimens<br />

from a Canadian lake were successfully<br />

transplanted to Lake Tahoe in<br />

California. They were also introduced<br />

to o<strong>the</strong>r lakes in <strong>the</strong> western<br />

U.S. and Canada. In Minnesota,<br />

opossum shrimp were taken from<br />

Trout Lake near Grand Marais and<br />

introduced to Grindstone Lake in<br />

Pine County.<br />

On <strong>the</strong> negative side, however,<br />

biologists have since found that<br />

<strong>the</strong>se introductions are not always<br />

beneficial. <strong>Opossum</strong> shrimp may<br />

actually compete for food with<br />

plankton-eating fish. The opossum<br />

shrimp feeds on smaller crustaceans<br />

that some fish rely on as a food<br />

source, especially in <strong>the</strong>ir younger<br />

years.<br />

But biologists have found an unusual<br />

benefit to having opossum<br />

shrimp in a lake; <strong>the</strong> creature is an<br />

indicator <strong>of</strong> acid in a body <strong>of</strong> water.<br />

Research by Canadian scientists has<br />

shown that increases in a lake's<br />

acidity cause opossum shrimp to<br />

decline drastically. The decline<br />

occurs before <strong>the</strong> water becomes<br />

acidic enough to noticeably affect<br />

many o<strong>the</strong>r species.<br />

Because <strong>the</strong> opossum shrimp is<br />

present in nor<strong>the</strong>astern Minnesota,<br />

an area sensitive to acid rain damage,<br />

it may be a useful monitor <strong>of</strong> a<br />

lake's acidic level.<br />

38 THE MINNESOTA VOLUNTEER

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