Aquatic Invertebrate Life History and Populations

Engage:

1. Where does a large insect such as a dragonfly, cranefly or mayfly begin its life? Review/discuss the life cycles of insects that begin their lives in aquatic environments using the mayfly as an example. Watch educational video, review diagram, and introduce/discuss related vocabulary (larvae, emergence, subimago, imago).
2. What could affect the life cycle or the population size of aquatic macroinvertebrates? Make a list of factors that might affect these organisms at their various life stages.

Explore:

1. Students will be divided into small groups (3-5 people, depending on class size) and given an envelope with a unique environmental scenario (below) and a data sheet.  Students will review their scenario, in which population sizes of larval and adult mayflies change between two sampling dates.
2. Using the data sheets, students should describe the physical and biological aspects of the environment in their scenario.
3. Count the number of larval and adult mayflies that they “sampled” in this environment on two different dates, and record on the data sheet.
4. Hypothesize why the populations sizes of larva and adults changed from one date to the next, based on environmental conditions and events that occurred at their sites.
5. Students should share their results with the class.
6. If time allows, students can review the class scenario together, or it can be used for a homework assignment.

Explain:

1. Many small animals, called macroinvertebrates (lacking a back bone), live in aquatic environments, including streams and rivers. Some macroinvertebrates live in streams continuously, such as snails, freshwater clams and mussels, and crayfish. Aquatic macroinvertebrates also include insects, such as dragonflies, damselflies and mayflies, which spend the early stages of their life in the water.
2. Mayflies, like many aquatic macroinvertebrates, go through several life stages that occur in different environments. Adult Mayflies lay their eggs in streams, and these eggs hatch into tiny larva in the stream environment. These larvae grow over time, molting their exoskeletons as they grow larger. Mayfly larvae can live in a stream for a year or more. They breathe oxygen through feathery gills, and eat algae and detritus as part of their diet. When they complete their larval stage, mayflies float to the surface of the water and break out of their larval exoskeleton. This process is called “emergence”. This is a very risky process for the mayfly, as it exposes them to fish and other predators. Mayflies emerge from their larval stage complete with wings, but still immature. This immature adult stage is called the “subimago”. The adult mayfly must molt one more time to become a sexually mature adult called an “imago”. In this stage, the mayfly has mature reproductive organs, but no mouth parts or digestive tract. Therefore, in the imago stage, the mayfly survives on its stored energy long enough to mate, and then it will die.
3. Various biotic and abiotic factors can affect the life cycles and population sizes of aquatic macroinvertebrates. Water temperature, depth, flow rate, dissolved oxygen levels, turbidity (cloudiness), and other physical characteristics of the environment (geology, stream substrate, etc) all influence what organisms can survive in a stream. Beyond the water channel, the terrestrial environment bordering the stream can affect stream diversity and population sizes by influencing water quality. For example, a heavily shaded stream in a forested environment will have cooler water than a stream that has much of its area exposed to sunlight, such as one running through a field or meadow. Warmer streams may have lower levels of dissolved oxygen, which some aquatic invertebrates needs for respiration. A stream in a more developed environment may have greater inputs of runoff from roads, parking lots, etc. This would influence water clarity and pollution levels. Some macroinvertebrate species, including mayflies, are very sensitive to water quality.

Extend:

1. Have students review the class scenario, or any group scenario that wasn’t covered, and answer the related questions.
2. Create a field guide with your students on aquatic macroinvertebrates.  Working individually or in pairs students can research a specific group of macroinvertebrates (mayflies, dragonflies, caddisflies, etc) and develop a one page summary on their group.  Students can easily find information on the internet. Remind students to always reference where they get their information.  Their summary could include: pictures of larvae and adults, food and habitat requirements and interesting facts (for example:  Did you know that jewelry is made from caddisfly cases?). Compile all student work together to create one book.

Evaluate:

1. Evaluations can be based on student performance when developing hypotheses, predictions, etc, for their scenarios and presenting their results.
2. Homework: Pass out Macroinvertebrate Research Cards and have students research and share information about their organism during the next lesson.
3. Evaluate hypotheses developed for the class scenario for homework or as an exit ticket.