REVIEW OF THE EARLY-LIFE HISTORY OF ZEBRA MUSSELS (DREISSENA-POLYMORPHA) - COMPARISONS WITH MARINE BIVALVES

The ecological and economic impacts of the introduced zebra mussel (Dreissena polymorpha (Pallas)) have been due in part to a life history that is conserved with marine bivalves but unique among the indigenous freshwater fauna. There are a number of life history events in D. polymorpha that follow external fertilization and embryology. The first is a brief trochophore stage. The development of a velum and secretion of a larval shell lead to a D-shaped veliger, which is the first recognizable planktonic larva. Later a second larval shell is secreted and this veliconcha is the last obligate free-swimming veliger. Conversely, the last larval stage, the pediveliger, can either swim using its velum or crawl using its foot. Pediveligers select substrates on which they ''settle'' by secreting byssal threads and undergo metamorphosis to become plantigrade mussels. The secretion of the adult shell and change in growth axis lead to the convergent heteromyarian shape. Zebra mussels produce byssal threads as adults, but these attachments may be broken, enabling the mussels to translocate to new areas. The recognition of these life history features will lead to a better understanding of zebra mussel biology. In summary, life history stages of zebra mussels are similar to those of marine bivalves and should be identified morphologically rather than on the basis of size.