It’s well known that many freshwater mussel species in North America depend on fish for survival. Their glochidia, or larvae, attach temporarily to fish before dropping off to develop into juvenile mussels.
But a discovery by Department of Natural Resources and University of Georgia scientists hints that at least for American shad on the Atlantic Coast, that fish-mussel relationship may work both ways.
DNR zoologist Jason Wisniewski was collecting gravid mussels on the Altamaha River in February 2009 when he opened a live mussel to find what looked like a BB-sized pearl. Yet, this pearl was rubbery. Other mussels had the same. Fellow zoologist Brett Albanese suggested they appeared to be fish eggs.
Early last year, Wisniewski and a group including DNR technicians Matt Hill and Deb Weiler returned to the Georgia river to search for mussels with eggs. Their goal: Determine the egg species and whether the 2009 find was coincidence or something that could be documented throughout the basin.
Their surprising results: 6 percent of nearly 760 native mussels sampled from seven sites across more than 150 miles contained one or more eggs. Embryos were moving in many. Molecular analysis at UGA’s Department of Genetics identified all of the eggs as American shad, an anadromous herring valued for sport and commercial fisheries, and a species of conservation concern because of declining numbers.
Although some fishes native to Africa, Asia and Europe use mussels in breeding (video of bitterlings spawning in mussels), these Altamaha “pearls” marked the first time on this continent that native fish eggs had been documented in live mussels.
If further research affirms a mussel-shad bond that helps one side or both, that relationship could boost and blend efforts to conserve each. The profile of lowly mussels would be raised: They not only filter pollutants and indicate clean water, they may also serve as a link in the life of high-profile fish. For American shad, more mussels could translate into a more robust fishery.
“I think it really shows how everything’s connected,” Wisniewski said.
What’s needed next? Determine if the relationship is amensalistic (negatively effecting one species), mutualistic (benefiting both) or commensalistic (benefiting one without harming the other). That includes studying survival rates of shad eggs in mussels – and just how or where do the young fish escape? Also, can scientists find shad or other fish eggs in live native mussels in other Atlantic Coast rivers?
The research that raises those questions appears in the upcoming issue of “Transactions of the American Fisheries Society” in an article co-authored by Wisniewski, Hill, Katherine Bockrath and Dr. John Wares of UGA’s Genetics Department, and Andrea Fritts of the Warnell School of Forestry and Natural Resources.
The project was funded in part through The Environmental Resources Network, or TERN, friends group of the Nongame Conservation Section.
How this works
American shad swim far upstream to spawn in natal rivers, from the St. Johns in Florida to the Sand Hill in Labrador. Males fertilize eggs that are broadcast by females and then float downstream before sinking to the substrate. The eggs found in the Altamaha mussels likely arrived by “passive transport” – they floated, rolled or bounced in through incurrent apertures or between gaping valves.
Flow and dissolved oxygen levels in the mussels is probably suitable for the developing eggs. Fish hatched in mussels in Europe, Asia and Africa are able to escape, indicating that is possible here.
Researchers speculate that eggs sheltered in mussels might have a higher survival rate. In the Coastal Plain, which has dense mussel populations and mainly sand substrates that are less suitable for fish eggs developing, mussels could substitute as substrate for American shad and other fishes.
Most eggs found – 31 – were in Altamaha slabshells, one of the largest and most common mussels in the Altamaha basin.