Around 2005, Noel Alfonso, coordinator of the Environmental Monitoring Program at the Canadian Museum of Nature, started finding mussel shells strewn around the edges of the pond at the museum’s research and collections facility in Gatineau, Quebec.
In 2015, these large mussels—known by their common name giant floaters—were formally surveyed for the first time by Environmental Monitoring Program summer students. For the second consecutive summer, we have been doing a lap around the pond every few weeks to collect specimens that the local muskrats have fed on and left on the shore.
The regular collection and analysis of specimens can give us approximate population and age estimates.
A giant floater (Pyganodon grandis) collected from the shore of the museum’s pond. Image: Carly Casey © Canadian Museum of Nature
The pond did not exist before 1997; somehow, giant floaters made their way there in a very short time frame, given the age of some of our samples.
The discovery of giant floaters in 2015 was quite remarkable because, among the giant floater specimens (Pyganodon grandis) in the museum’s collection, the nearest was found in Lac des Fées in Gatineau Park, Quebec.
So how does a mussel get up and move several kilometres to a new pond? This is no easy task for animals that move by dragging themselves across the lake or river bed with their extendable “foot”.
The pond is on the museum’s Natural Heritage Campus in Gatineau, Quebec, close to the facility that hosts the museum collections and research activities. Image: Carly Casey © Canadian Museum of Nature
As it turns out, they get a little help from some friends. Giant floaters release larvae into the water, which then attach to the gills and fins of a fish. Once developed into their juvenile form, they detach from the gills and settle into the bottom substrate, where they will reach adulthood.
The giant floater is a generalist species that can sustain low dissolved-oxygen levels and a variety of substrates. The same can be said for its choice of larval-host fish species.
Of the 27 or so potential host fish species, five can be found in the museum’s pond. The most likely scenario is that giant floater larvae hitched a ride on some of these fish, which transported them to the pond.
Fish travelling upstream in the ditch along Pink Road, on which the museum’s research and collections facility is located. Image: Carly Casey © Canadian Museum of Nature
A trip to Lac des Fées renewed our confidence that giant floaters are still present there. In this case, for the fish to get from Lac des Fées to the pond is no easy task because they must make their way up Betty’s Creek against the current.
From Betty’s Creek, they then travel west—again upstream—along the Pink Road ditch before reaching the pond. This ditch is usually flooded, but nevertheless, some parts of the journey are quite difficult to navigate, especially in a dry summer like this one.
The POV (point-of-view) video below shows the route through the “waterway” to the pond.
A Mussel Larva’s Journey Down the Pink Road Ditch
Map showing the most likely route of colonization of the pond by giant floaters. Image: Geoffrey Carter © Canadian Museum of Nature. Sources: Esri, DigitalGlobe, GeoEye, Earthstar Geographics, CNES/Airbus DS, USDA, USGS, AEX, Getmapping, Aerogrid, IGN, IGP, swisstopo, and the GIS user community
Giant floaters can be dated fairly easily and in a similar fashion to trees, each growth season being marked by a dark stripe on the shell of the mussel. This counting technique can underestimate a mussel’s true age by up to half. As a result, some of our specimens could be up to 20 years old. Their potential maxium age means that the period between the creation of the pond in 1997 and its colonization could have been very short indeed.
A culvert along Pink Road that would be difficult for fish to pass through during this dry summer. Image: Carly Casey © Canadian Museum of Nature
What a journey! The lengths that fish and mussels will go to colonize new areas can leave many of us wondering how on Earth that species got there.