501 Extraordinary Journey of the Opossum Shrimp - webapps8
501 Extraordinary Journey of the Opossum Shrimp - webapps8
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