Spring 2001

By Mary S. Ochsenschlager

Pop! Something in the net jumped. The child looked carefully, but could see nothing more than the usual collection of pieces of grass and small pebbles. She brought her catch to the naturalist who was leading her stream exploration class and pop! — it jumped again. Looking very closely, they noticed something that looked like clear jello in the bottom of the net. After carefully removing the object into a pan of water, they saw that they had a small, transparent, shrimp-like creature. With a little detective work in Golden Guide: Pond Life, they were able to identify it as a fresh-water shrimp.

Palaemonetes kadiakensis, in the family Palaemonidae and order Decapoda, is found in the central part of the United States. It is often encountered in lakes, ponds, and backwaters in the southern portion of its range; it is less common in the north. There are reports of it from all along the Illinois River, from Wolf Lake in Cook County, and from the Kankakee River (both a pre-1898 and a 2000 record). In 2000, it was found consistently in Otter Creek in Kane County, throughout the late summer and fall.

This shrimp, variously called glass shrimp, grass shrimp, popcorn shrimp, glass pawns, hardbacks, and jumpers, can reach a length of 53 millimeters, but is smaller in the north. Females are larger than males, and generally, longer females produce more eggs than shorter females. Most individuals live only one year; however, in southern areas some may live into the second year.

The glass shrimp is aptly named, as it is completely transparent except for its eyes. At times its abdomen may have a green tinge. This coloration is from recently ingested plants still in its intestines. The freshwater shrimp closely resembles the marine shrimp we eat, but is much smaller. It has very long antennae, and its body is flattened laterally. Glass shrimp move in several ways. While swimming, they use the five sets of swimmerets attached to the abdomen, called pleopods. To walk, they use the long, leglike appendages attached to the thorax, called periopods. And they dart and jump using the tail just as crayfish do.

The glass shrimp eats algae, plants, and both live and dead animals. It is consistently found in aquatic vegetation. When present, it is an important food for fish, especially small fish that also hide in vegetated areas. Interest-ingly, its annual cycles of abundance and reduction precede similar cycles of aquatic insects by one or two months. Obviously this would be advantageous for fish and other aquatic predators, because it helps to provide a fairly continuous abundance of food throughout the year.

Female glass shrimp carrying eggs in their swimmerets, appear around May at our latitude or when the temperature reaches between 15 and 20°C. Egg production continues into August, and females can have more than one brood. Egg number per female varies considerably — from as low as 11 to occasionally over 100. When the eggs hatch, in three to four weeks, the female releases them into the water. The larvae develop in six distinct phases over a period of approximately three weeks. The juvenile shrimp grow rapidly in the fall and spring, but slow down in the winter. There is a die-off in the adult shrimp population after August each year.

This summer look for freshwater shrimp when you are dipping in vegetated water or along the shores where vegetation hangs into the water. It will be fun to add this interesting creature to your list of inhabitants in your favorite Chicago Wilderness stream or pond.