Dreissena polymorpha - Zebra Mussel

Developing Embryos or Pre-Shell Larvae

• Similar in morphology to Asian clams, quagga mussels, false dark mussels, and wedge clams.
• Can be differentiated from Asian clams by their location in water column.
• Not found internally in female; as with zebra mussels, false dark mussels, and wedge clams, eggs and sperm are released in the water column, and fertilization and embryo development occur outside of the female.

The ripe eggs of D. polymorpha.

The ripe sperm of D. polymorpha.

A female D. polymorpha releases eggs, visible as tiny white dots, into the water column via the exhalant siphon.

A D. polymorpha sperm enters an egg released by a female, and fertilization occurs.

An early stage of a straight-hinged larva.

A trochophore – the earliest larval stage of D. polymorpha.

Straight-Hinged Larvae

The velum of this D. polymorpha is visible internally in this D-shaped veliger.

The velum of this straight-hinged larva aids in swimming and feeding.

The velum of D. polymorpha is a ciliated structure present in early larval stages, which is then reabsorbed later in its development.

Although the shell of this early umbonal veliger still appears transparent, the veliger now begins to show distinct growth lines on the shell halves.

The shell of this pediveliger is thicker and more opaque than in earlier stages and the umbo is well-developed and more prominent.

• Overall shell length is about 97-112 µm.
• Viewed on the shell face, the shell is longer than tall (shell height = 80 percent of shell length).
• The hinge length is over one-half of the total shell length, as compared to the quagga mussel where the hinge length is usually less than one-half of the shell length.
• The veliger swims in a circular motion.
• The velum is readily visible in live specimens, and the base of the velum has melanophores and appears as a dark crescent; one way to distinguish veligers of false dark mussels and wedge clams from dreissenids is that they lack this dark crescent at the base of the velum.

Umbonal Veligers

D. polymorpha umbonal veliger.

D. polymorpha umbonal veliger actively feeding using its velum.

The velum is covered with fine cilia and is located opposite the umbo.

The early umbonal larva.

The umbonal veligers of both species of dreissenids are difficult to separate. This difficulty arises because the characteristics used to distinguish early umbonal larvae are not consistent throughout umbonal larval development.

Note that as the veligers mature the shell proportions approach those of the quagga mussel. Apply both early and older characteristics to each veliger to assure an accurate identification.

Early Umbonal Larvae

• Overall shell length is about 112-228 µm, with umbos of equal height.
• Viewed on the shell face, the shell still appears longer than tall.
• The hinge length is still over one-half of the total shell length, as compared to the quagga mussel, where the hinge length is usually less than one-half of the shell length.
• The veliger swims in a circular motion.
• The velum is readily visible in live specimens, and the base of the velum has melanophores and appears as a dark crescent.

A veliger beginning to develop an umbone.

Older Umbonal Larvae

• Umbos are of equal height unlike quaggas, which are not equal.
• Unlike quaggas, where one valve does overlap the margin of the second, the valve margins do not overlap.
• Overall shell length is about 140-347 µm.
• Viewed on the shell face, the shell appears round.
• The velum is still present and the veliger still swims in a circular motion.

An older umbonal veliger uses its velum to swim and feed.

Pediveligers

A pediveliger crawls with the use of the foot.

A ‘clam-shaped’ pediveliger.

A pediveliger uses its foot to maneuver.

·      Shell size is about 231-462 µm and the shell appears to be ‘clam-shaped.'

The shell of this pediveliger is thicker and more opaque than in earlier stages and the umbo is well-developed and more prominent.

A pediveliger uses its velum to move.

·      Foot present, veliger crawls as well as swims.

The foot is an extendible muscular organ located in the mid-ventral region of this plantigrade.

·      Velum present.

In the pediveliger stage, the veliger crawls using the foot and swims using the velum.

·      Umbos of equal height, unlike quaggas.

·      Valve margins do not overlap, unlike the overlap seen in quagga mussels.

The D. polymorpha pediveliger appears ‘clam-shaped’; the umbos are of equal height, and the valve margins are of equal size and do not overlap.

Plantigrade Veligers

• Shell beginning to switch from clam-shaped to elongated adult zebra mussel shape.
• Shell length greater than 340 µm.
• Valve margins do not overlap.
• Umbos pronounced and equal in height.
• Foot is present; velum has disappeared; the plantigrade cannot swim, and is searching for a suitable substrate to attach to.

Species Descriptions

Zebra Mussel Identification