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MICROSCOPES
Lesson 5 - Page 1

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Meroplankton refers to organisms that live only part of their life as plankton.  Usually meroplankton has either complete or incomplete metamorphism.  The early stages are usually planktonic and then they emerge either as a land or flying critter.  Eggs of fish or other animals (i.e., frogs) are included as meroplankton. Fish eggs are dependant on water currents and not independent swimmers (nekton) until they hatch. Other organisms include their larval or nymph stages.  Some groups, like the dragonflies can spend up to 2 years as plankton and then emerge only to live on land for a few months. 

A dragonfly changes from a nymph (A) found in fresh water.  In B it emerges from the water on an aquatic plant. 
In C it emerges from the nymph and in D the land dragonfly is born.

The common meroplankton are arthropods with incomplete metamorphism.  They usually have different stages as they grow.  We will look at a few groups and when you look for meroplankton use this guide to help identify a critter.

The Order Diptera includes many members that are in their larval and pupa stages and adapted to fresh water. The transition from larval and pupa may have very different forms.  The larval stages may last several weeks or up to two years depending on species, temperature, and food availability.  The skin usually molts 3 times.  Adults are never truly aquatic.  All aquatic representatives are legless although there may be pseudopods on the thorax or abdomen.  Diptera include the common mosquito, midges, gnats, crane fly, and the common fly.