Issue #36 August 2007
CFI Contributes To Rare Mussel Propagation
Many of you that read this newsletter are aware of the plight of our native mussels. Many are threatened with extinction…..some are already gone and others are probably functionally extinct because they aren’t able to reproduce! The result is that many are considered as threatened or endangered according to the Federal Endangered Species Act.
Those of you aware of this are probably also aware that these mussels have a very complicated life history involving fish as an intermediate host for the larval mussel. For those not familiar with this remarkable relationship, a brief summary:
When mussels spawn, males release their gametes into the water. Female mussels take up the male gametes via the incurrent siphon. Fertilization occurs within the female’s body, and develop into larvae called glochidia. Each larva resembles a tiny Pac Man. Here’s where the fun begins! In order to develop further, glochidia must spend part of their life attached to the gills or fins of a fish. The relationship between a given species of mussel and the fish species that host its larvae is frequently quite specific.
After a period of time spent parasitizing the host fish, the glochidia drop off to begin a sedentary existence as juvenile mussels, anchored in the substrate at the bottom of a river or stream.
In many mussel species, females possess elaborate lures to attract a suitable host fish. Some lures mimic fish, others mimic aquatic insects upon which a host fish might feed. Some of the lures are so effective the host fish may attack and actually swallow the lure! As the fish approaches the lure, the female releases her glochidia. Thus, the host fish becomes “infected” with mussel larvae.
Nope, that's not a fish, it's a mussel lure!
OK, now you’re experts on mussel reproductive life history.
While CFI has never previously been directly involved with propagating any mussels, we have recently begun to contribute to that effort. The past few years have seen the establishment of several federal, state and even private mussel propagation facilities. For some of the mussel species, the host fish is known. Some fish species serve numerous mussel species as hosts. Others may be a host fish for only one or a few mussel species. For some mussel species, the host fish species remains unknown.
To propagate mussels successfully, some hatcheries “infest” the host fish with glochidia and then release the host fishes with glochidia attached. Alternatively, infested host fishes remain in the hatchery until the juvenile mussels to drop off. The juveniles can then be released into an appropriate stream.
Even though some of the host fish are relatively common, they can be difficult to capture in quantities sufficient to meet the needs of the mussel hatcheries. CFI has spent the last two years propagating two species to be used as mussel hosts, the whitetail shiner (Cyprinella galactura), and the common logperch (Percina caprodes).
Top: Whitetail shiner parents in a CFI aquarium.
Bottom: CFI's captive-propagated logperch juveniles.
We had worked with several Cyprinella species and close relatives in the past and suspected that we could produce thousands of whitetail shiners. That did indeed turn out to be the case. Spawning them turned out to be very similar to spawning the spotfin chub (Erimonax monachus) that we have worked with for years. These guys spawn in crevices, often along bedrock runs in the stream. In our hatchery we successfully use stacks of ceramic floor tiles to create artificial crevices with the spotfin chubs. This also worked quite well for whitetail shiners. The resulting larvae are small and a bit tricky to feed for a few days, but quickly grow and graduate to larger foods. Consequently, we have been able to produce whitetail shiners in quantities limited only by the availability of grow-out space.
Hundreds of baby whitetail shiners at the hatchery.
Based on our prior experiences with blotchside logperch (Percina burtoni), we questioned our ability to produce sufficient numbers of logperch to help with mussel propagation efforts. Although we were successful in producing blotchside logperch, they were relatively difficult to spawn and did not produce large numbers of viable eggs and larvae. Our hope was that the common logperch would be easier to produce in larger numbers. Happily, this turned out to be the case.
Logperches bury their eggs in loose substrate. Thus, our dilemma was whether to try to extract eggs from the substrate or allow them to hatch undisturbed.
We housed about a dozen adult logperch in a 135 gallon tank. The tank was part of a larger hatchery system and was connected to the rest of the system by way of several overflow drains drilled into one upper corner of the tank. This water drained into a central sump where it was filtered and dispersed back to the rest of the hatchery system. The tank contained a substrate of mixed gravel and sand approximately 2-3 inches in depth and large rocks for cover.
The logperch were collected late in 2006. We over-wintered them by exposing them to shortened day-length and cool water temperatures. As the water temperatures warmed in the spring and day-length was artificially increased, we began to notice increased activity among the adults. Once we suspected they might be spawning, we began vacuuming the substrate in search of eggs. The first eggs and larvae were vacuumed from the tank in mid-March. We had a difficult time with these earliest collections and we suspected that the rigors of vacuuming them from the substrate may have contributed to the problems we had rearing them.
When possible, and when actively swimming (pelagic) larvae are our subjects, we prefer to collect larvae passively. The overflow drains of the spawning tank are diverted from the sump into a collection tub. Water drains from this tub through a fine mesh screen that will not allow the larvae to pass. The tub remains part of the flow-through central system, ensuring good water quality. When the larvae hatch and first swim up, they usually find their way to the overflow. With the overflow located along the tank wall, presumably a large portion of the larvae eventually find their way down the overflow.
This
larval collection tub receives overflow from the logperch spawning tank.
As logperch spawning reached its peak, we would sometimes find 100 or more larvae in the collection tub first thing in the morning. More larvae seemed to be collected either during the night or perhaps early dawn.
Logperch larvae are small and require fine powdered foods and live rotifers. These foods were dispersed to the larvae primarily through an automated feeding system we manufactured using timers and solenoid valves. There is always a balancing act between supplying a sufficient amount of appropriate foods and fouling the water quality by adding too much. We were able to reach a good balance. Growth was rapid. By mid-July, most of the logperch babies were over an inch in length!
Logperch babies.
Click here for a video
clip of logperch babies
in one of our hatchery aquariums.
We have now transferred over 1000 logperch and several thousand whitetail shiners to a mussel hatchery in Virginia. Hopefully, in a future newsletter, we can report on the progress they have made using our fish to rear rare mussels.