Biota of Rivers and Streams - Hydro-Environmental Analysis: Freshwater Environments - page 173

Spire of shell Body whorl

Te ntacle

Apex

Pneumosome Foot

Eye

Operculum

FIGURE 6.9 A pulmonate snail ( upper ) and a prosobranch snail with an operculum ( lower ). (From Clifford,
H., Aquatic Invertebrates of Alberta , pp. 638, University of Alberta Press, Edmonton, Alberta, Canada, 1991.
With permission.)

FIGURE 6.10 Radula of Helisoma trivolvis (0.03 mm). (From Clifford, H., Aquatic Invertebrates of Alberta ,
University of Alberta Press, Edmonton, Alberta, Canada, 1991.)

200,000-17,000,000 glochidia released per mussel per year (McMahon and Bogan 2001). The large
number aids in survival since the glochidia can only survive for a few days without inding a host.
Also, there are only certain ish that the glochidia can parasitize, and these and other factors result
in high glochidia mortality. However, while their mortality rates are high, the glochidia life stage
and the parasitism on ish provide a means of transporting the mussels to new habitats.
In addition to drifting, some species use structures or shapes to attract ish hosts to the female,
such as a lap lure (Figure 6.12). Once the mantle lure is struck, the female expels the glochidia to
infect the ish.
Once attached to the ish, the glochidia remain for a period of several weeks to a month and
then drop off to the riverbed as juveniles. If dropped to an unfavorable habitat, they do have some