Reproduction in freshwater mussels centers around very unique methods quite unlike many other aquatic organisms. Hunter wrote, “In a population of molluscs of a single species it seems likely that the annualreproductive cycle is set according to the following pattern. Maturation of thegametes is set off by annual temperature fluctuations and is possibly correlated to some extent with changes in illumination. A combination of factors—thermal, mechanical, genetic, and hormonal—all trigger the actual spawning of gametes…” (130).
Mussels naturally reproduce once per year. The un-communicated event happens everywhere in lakes, rivers, ponds, and streams. The separate males expel their
gametes into the water column. Females have developed two techniques for fertilization; internal and external. For internal fertilization, the awaiting female siphons the sperm inside to fertilize her ova. Alternatively, some species eject their eggs in a mass outside of the shell, known as a
conglutinate. Sperm will then land on the mass and make contact with the eggs. As soon as fertilization occurs, the packet of larvae, glochidia, can now enter the aquatic world, with one caveat.
Glochidia need a bit of host flesh and blood to metamorphose into a juvenile mussel. Most glochidia normally obtain that meal by being short-term
ecto-parasitic hosts on fish (McMahon and Bogan 363). One mussel species is known to use an amphibian host (363).
How would a non-motile internal mussel larvae find potential hosts of swimming fish? Strayer et al. (431) summarized the three strategies that were found to be the most effective in distributing the larvae:
Once the glochidium attaches to the fish scale, flesh, or gill, it silently feeds off the one with a clamped-down bite. This activity does not harm the host because of the tiny size of the glochidium(< 2 µm). Within a couple of weeks, the juveniles fall off of the host and drop to the sediment below to bury beneath it. But the young juveniles face danger and high mortality from microbes, predators and potential infections (Lima et al 34).
- Some recently fertilized females, or
gravid females, can expose their mantle flesh to attract fish, or amphibians. The mantle flesh has been named a ‘lure’ to signify the act of fishing.
- Another mussel’s mantle lure actually mimics the shape of a benthic fish. This attracts both predators and potential fish mates from which the mussel can expel the glochidia.
- Some mussels even construct a mucous web to temporarily restrain fish hosts so the glochidia can land on them.
The average age of mussel maturity (from glochidium to adult) is 5-7 years, and their typical lifespan could reach to 100 (Wetzel 695). The long-lived organisms remain resilient and may produce endless generations with each passing year adding to existing mussel beds in the benthos. All of the life history and reproductive strategies signify a dependency on fish hosts and fish availability. Some mussels’ glochidia are generalists and can find any suitable host and thrive, while others require particular species (McMahon and Bogan 364). Scientists are still trying to identify all fish hosts for the current mussel populations. By knowing the hosts, artificial reproduction in laboratories could rejuvenate dwindling populations, “Therefore, the complex process of mimicking fish infestation in the laboratory, where these negative factors can be reduced only partially, has a limited contribution to the maintenance of natural mussel populations. However, controlled infestation is still the only practical method for doing so” (Lima et al 34-35). Another side to in-vitro reproduction assists with endangered populations where few adults are found with limited sources to mate; either the fish hosts are gone, or there is a low female to male ratio. Other possible explanations still need to be uncovered. Within the past 10 years, artificial reproduction has been very successful but requires more details: “Differences in growth and longevity among species have important conservation and management implications” (Haag and Rypel 245). Once juveniles have been cultivated in situ, they can be placed back into natural habitats. These populations are routinely checked. Survival and recruitment rates of translocated mussels indicate the success of the transplants. Scientists investigate the water temperature, flow rates, pollution, and effects of impoundments in the areas of mussel releases (McMurray et al 57).
Some investigations work to assess the age of mussel populations: “Age structures present important hints for the actual state of the populations (eg. present ability for reproduction), their future stability and development (undisturbed, impaired or overaged populations)” (Weber 180). For these mussel populations to prosper, however, both natural and artificial reproduction needs to be investigated, as well as the ecology of the freshwater systems. One way to examine trends is to map the areas of mussel beds. Brice and Lewis described the process, “Surveys of freshwater mussel communities have been routinely conducted by individuals from academia, industry and governmental agencies since the latter part of the century” (454). Scientists use both historical and current mussel bed maps to examine trends in habitat and population changes. In reality, mussels have been on the decline in the 20th and 21st century for a variety of reasons (Weber 169).
Go To Ecology and Benefits