For a few brief weeks of a mussel’s life, it is a true parasite, taking from its host and giving nothing in return.
“Some mussels live more than forty years,” says USDA Forest Service scientist Wendell Haag. Only the larvae, or glochidia, are parasitic.
“The interesting thing about this host relationship is that different mussel species use different fish species as hosts,” says Haag.
A female mussel can produce hundreds of thousands of larvae, which clamp onto fish gills. If larvae elude the fish’s immune system, they are encapsulated – fish tissue surrounds each larva, which is about the size of a grain of salt. Once encapsulated, the larvae enjoy the host’s nutrients. For two or three weeks, their host is everything to them.
Eventually, their tiny bodies radically transform as they undergo a complete metamorphosis. After metamorphosis, the glochidia fall off their host and spend the rest of their lives as free-living creatures.
Mussel larvae on inappropriate fish species are not encapsulated and are doomed. They’re sloughed off before metamorphosis and die.
Haag recently contributed to a study on host fishes for two mussel species, ebonyshell (Reginaia ebenus) and elephantear (Elliptio crassidens). Michael Hart, a research associate at the Texas A&M Natural Resources Institute, led the study, which was published in the journal Aquaculture Reports.
A century ago, scientists suggested that skipjack herring was a primary host for ebonyshell and elephantear. However, this proposed host relationship was never confirmed because of the difficulty of working with skipjack herring. Skipjack herring are large, migratory river fish that are hard to handle and hard to study.
“Skipjack herring are hard to keep alive in captivity – if they’re in a regular aquarium they go crazy and beat themselves up on the sides,” says Haag.
Haag and his colleagues wanted to confirm whether skipjack herring was truly a host for ebonyshell and elephantear, and whether larvae of these mussels could use other fish species as hosts. The scientists captured 97 skipjack herring, along with 30 Alabama shad, a closely related species. They also captured minnows, bass, and other fishes, 14 species in all.
The smaller fishes were easy to maintain in conventional aquaria. But to keep skipjack herring and Alabama shad alive, the scientists placed them in round tanks that held 5,700 liters of water.
But juvenile mussels are still the size of a grain of salt, even though they now resemble miniature adults.
The scientists set up a pump and vacuum system to sample from the bottom of the tanks. The large volume of water, coupled with large amounts of fish waste and detritus, made it difficult to find juvenile mussels that had fallen off the gills and sunk to the bottom of the tank. But an accidental discovery saved them the trouble.
“Eleven days into the experiment, one of the skipjack herring died,” says Haag. “Its gills were still fresh when we found it. We placed the gills in a small volume of fresh, circulating water, and a few days later juvenile mussels began coming off the gills.”
The mussels were evidence that skipjack herring is a suitable host fish for both mussel species, as suggested in the 1910s.
Haag and his colleagues also recognized a new method of identifying fish species that could host mussel larvae. They excised the gills of other skipjack herrings and found encapsulated glochidia. They kept the gills in fresh circulating water and found that glochidia must spend 11 to 17 days encapsulated on live fish gills before they could complete their metamorphosis on excised gills.
Ebonyshell and elephantear mussels were produced from the excised fish gills in large numbers. The ability of excised gill to produce juvenile mussels reduces the amount of time that fishes must be kept alive and the amount of material that must be examined to find the juveniles.
“The method could be useful for other large or sensitive fishes,” says Haag. “Skipjack herring is just one example of a large, sensitive fish species that does not lend itself well to traditional methods for host identification. Many other such species have not been evaluated for host suitability due to the difficulty of handling them. In addition to host identification, the method could be used to propagate in captivity mussel species that use such fishes as hosts.”
Their research also showed that Alabama shad is a suitable hosts for elephantear mussels. None of the other 12 species were suitable hosts for either mussel species.
Skipjack herring and Alabama shad are migratory, and they have vanished from streams where dams block their spawning runs. Ebonyshell and elephantear mussels have also vanished from such streams, apparently because of the disappearance of their hosts. However, they remain common in rivers that still have skipjack herring or Alabama shad populations.
More concerning, mussels have vanished from streams without dams, streams that continue to support healthy fish and aquatic insect populations. These declines are enigmatic and affect almost all mussel species.
“It’s a major conservation problem, and we don’t know what’s causing it,” says Haag,” who assessed conservation strategies for mussels in 2014.
Mussels are one of the Southeast’s hidden jewels. As Haag points out, people come from all over to see the biodiversity of the region – birds, flowers, trees, and more.
“The southeastern U.S. is also home to the most diverse and unique mussel assemblage on Earth,” says Haag. “Mussels are an important part of the distinctiveness and interest of the region, and we’re learning that they may be vital to the healthy functioning of stream ecosystems.”
For more information, email Wendell Haag at firstname.lastname@example.org.