The trochophore metamorphoses into a veliger with the development of the velum - a ciliated feeding and swimming organelle.
D. polymorpha umbonal veliger actively feeding using its velum.
Although the term “veliger” is often used popularly to refer to all planktonic (i.e., floating in water column) stages in the life cycle, strictly speaking this term does not include the trochophore stage, but rather refers only to the stages that possess a velum, i.e., the straight-hinged stage (»97-112 mm), the umbonal stage (»112-347 mm), and the pediveliger stage (»231-462 mm).
Veliger densities typically peak in midsummer in North America, with lower densities present in spring and autumn. Rarely are the planktonic life stages of zebra mussels found in winter, but in the Ohio River, for example, Reed et al. (1998) recorded the presence of larvae at low temperatures in March and suggested that these veligers may have delayed settlement the preceding autumn and overwintered in the river.
The velum of this straight-hinged larva aids in swimming and feeding.
Typically within 2-9 days after fertilization, veligers secrete an unornamented D-shaped shell from their shell glands. The larvae are then referred to as D-shaped or straight-hinged veligers, since the body side where the hinge is forming is straight while the open valve side is rounded. A rudimentary shell may be forming in this stage, but the larval mussel is still quite transparent. Straight-hinged veligers feed on small (1- to 4-µm diameter) algae (Nichols 1993) which they filter out of the water column using the bands of cilia located on the velum.
The next stage of development generally occurs 7-9 days after fertilization, when an ornamented larval shell is secreted by the mantle tissue. The shell has a more pronounced umbonal region near the hinges and is round in profile. The “umbone” is the extended bump of the mussel’s shell that covers the hinge. This umbonal stage resembles a very small native clam and represents the last veliger stage that is completely planktonic.
The final veliger stage, the pediveliger, can either swim using its velum or crawl using its foot.
A pediveliger crawls using its foot.
Primary settlement generally occurs between 18 and 90 days after fertilization, when a pediveliger will attach onto a substrate. The pediveliger crawls on the surface of a substrate until it receives the appropriate cues (probably physiochemical) to attach a byssal thread.
This thread provides the anchor that enables the pediveliger to stay securely attached during its transformation into the plantigrade stage. The pediveliger is considered by some to be the final larval form, with the plantigrade as a stage between larval and juvenile stages (Ackerman et al. 1994).