Early Warning Detection of Zebra Mussels
* Is it feasible to use a molecular genetic assay technique based on the polymerase chain reaction for the early and accurate detection of zebra mussel at threatened Reclamation reservoir facilities?
Need and Benefit
The zebra mussel (_Dreissena polymorpha_) has the potential of becoming the worst biofouling aquatic pest ever introduced to North America. The larval veliger is the primary means of zebra mussel colonization in new freshwater environments. As a broadcast spawner, a single female zebra mussel can produce as many as a million eggs each year. After external fertilization, planktonic veligers are easily dispersed by flowing water or transported overland by trailered boats. Reclamation reservoirs that are at high risk of initial infestation may be monitored by detecting for the presence of veligers.
Reclamation has a need for an early warning monitoring program that will allow managers to implement in a timely manner accomodating and/or preventative measures at Reclamation facilities. Among the impacts zebra mussel posed for Reclamation structures and reservoirs are:
* Blockage of water deliveries
* Accelerated metal corrosion and concrete spalling
* Disruption of reservoir ecology caused by the collapse of the natural reservoir food chain.
There are currently only two monitoring methods available to Reclamation:
* Cross-Polarized Light Microscopy: In tow plankton samples, zebra mussel veligers are one of the few objects that are birefringent. However, due to their small sizes (< 200 micrometer [um]) and relatively nonunique morphological features, identification is a tedious and labor-intensive process. This method also is relatively insensitive and inaccurate due to the small volume of water sample viewed under the microscope and its inability to distinguish similar naturally occurring birefringent objects (ostracods), which has caused false positive results.
* Substrate Sampling: Zebra mussel veligers mature into a settling stage when the growth of their shells becomes too dense to float and they seek a substrate upon which to attach. Passive samplers with filamentous substrates ("bridal veil" or scouring pads) are used. Samplers are then retrieved and viewed under a dissecting microscope, which is a tedious and labor-intensive process. This type of sampling only collects zebra mussels that are entering the settling stage. By then, the infestation would be well developed and past the initial stages. Substrate sampling must be deployed over a longer period of time--during which the samplers may be prone to clogging, disturbance, and vandalism. These samplers may be better suited to flowing waters situated elsewhere upstream or downstream of Reclamation reservoirs.
Neither technique is able to sensitively and accurately detect zebra mussels in the early larval stage of the life cycle. A new approach, the polymerase chain reaction (PCR), utilizes a zebra mussel-specific oligonucleotide (RNA) probe to accurately detect zebra mussel. With its process of exponential amplification, a single veliger in a plankton sample can be detected, which is over 300 times more sensitive than cross-polarized light microscopy based techniques. This technique holds promise as an early and accurate method of detection of microscopic veligers when the zebra mussel population is very low during the initial infestation.
Also, most current and developing control strategies target the larval life stage. Utilities use hot water or chemical treatments only when spawning occurs and veligers are detected. This prevents zebra mussel from reaching a critical density of colonization that will impact operations. New promising research also focuses on the early larval life stage by disrupting the synchronization of spawning by males and females. Early and accurate detection of veligers using this new PCR method before settlement is also critical for implementing and measuring the success of these control strategies.