Do “Piggyback Rides” Play an Important Role in Invasive Species Dispersal?

by Neil Coughlan and Andy Stevens
Late one night, Andy Stevens, then a Center for Limnology graduate student, pulled a minnow trap out of the water off of the Hasler Lab pier in Lake Mendota. In the glare of his flashlight, Stevens could see the already familiar clusters of zebra mussels colonizing the wire mesh of the trap. But he also spotted something less routine – a dragonfly nymph giving a couple of zebra mussels a piggyback ride.
This field observation prompted Stevens to contact his colleague Neil Coughlan at the School of Biological Sciences at Queen’s University Belfast. Together the two put together a research project with a few other collaborators and a seemingly obvious question – what role do animals play in moving invasive species around? Are invasive species just a hop, skip, and a jump away from making a splash in new ecosystems?
The answer to those questions led to a study published online in the journal Knowledge & Management of Aquatic Ecosystems last month, that showed their query was not as obvious – and certainly not as well-studied – as they had initially thought.
We’ll let Neil and Andy take it from here:

“That’s a fool’s experiment. But I love fools’ experiments. I am always making them.”
Charles Darwin (1809-1882): In E. R. Lankester, ‘Charles Robert Darwin’, 1896.

Two adult zebra mussels on a dragonfly nymph and a crayfish encrusted in mussels. Photos Andy Stevens and Minnesota DNR.

Charles Darwin is often credited with pioneering the study of long-distance dispersal (LDD), having dedicated two chapters of his work, The Origin of Species, to the subject. Darwin recognised the importance of LDD as a mechanism for connectivity between isolated island habitats, and was fascinated with zoochory, or the idea that transport of one organism by another more mobile animal can facilitate the dispersal of species to new ecosystems.
Pursuing his inquisitiveness, Darwin went as far as to demonstrate that ducks could potentially disperse freshwater snails, and even suggested the possibility of fish-eating birds distributing plants and invertebrates obtained via their catch. Notably, in his essay Transplantation of shells (1878), Darwin documented the discovery of a living adult mussel, Unio complanatus, found attached to the toe of a duck – specifically a blue-winged teal that had been shot on the wing.
Separated by a ‘sea’ of terrestrial habitats, freshwater systems can be viewed as isolated islands of water. Yet, despite this apparent natural seclusion, isolated freshwater sites have been shown to be at high risk from biological invasions.
Overland long-distance dispersal (A), short-distance dispersal (B) and ‘stepping stone’ dispersal (C) of invasive species (dot clusters) between isolated freshwater sites. When sites are hydrologically connected (D) additional vectors, such as fish, may facilitate dispersal.

Undoubtedly, human activity is the primary mechanism for the LDD and short-distance dispersal (SDD) of most aquatic invasive  species (AIS). However, as Darwin predicted, many organisms, particularly propagule stages (e.g., seeds, spores, eggs, etc.) can be transported both internally, via the gastrointestinal tract or upon the exterior surfaces of other animals. In particular, it is now known that birds play an important role in the dispersal of plants, animals, microbes and fungi.
In the modern era, the spread of invasive bivalves represents a major threat to the function and biodiversity of freshwater ecosystems worldwide. For example, zebra mussels, quagga mussels, and Asian clams are prolific invaders, whose presence can have damaging ecological and economic consequences for invaded habitats. Despite management efforts to reduce invader spread within both European Union and United States of America territories, these species continue to spread.
Our studied searched numerous on-line scientific databases for material on the issue of animals moving non-native or invasive species around.
We found examples of blue catfish internally transporting adult zebra mussels in their gut and, in its larval stage, the zebra mussel adhering to the external surfaces of water birds. This small collection of available scientific literature suggests that zoochorous dispersal of invasive bivalves is possible, but likely a rare occurrence. However, even the establishment of a few individuals can, over-time, grow into a substantial population.
We concluded that the ability of freshwater fish to disperse invasive species strongly merits further investigation. Greater knowledge of fish gut retention times could be used to mitigate against further mussel spread, such as by developing minimum quarantine times for fish caught and relocated for restocking purposes.
Possible dispersal of mussels and clams by other freshwater inhabitants such as crayfish, freshwater turtles, and dragonfly nymphs – or indeed, large semi-aquatic and/or mud wallowing vertebrate species like otters, boars, and muskrats – should also be examined in greater detail. While unknown, it seems not unreasonable to us that an animal trapped at an invaded site and relocated to a new site could potentially transport aquatic invasive ‘hitch-hikers’.
In particular, the simple awareness of zoochory, and the importance of data collection, needs to be promoted. A variety of nature enthusiasts, photographers, ecologists, conservationists, game hunters, wildlife and fisheries officers, bird ringers and field ornithologists come in contact with, deliberately observe, and often handle a variety of wildlife. It is not unlikely that instances of one species bringing another along for a ride have been observed but remain undocumented.
Citizen science initiatives to increase the collection and cataloguing of such observations across all potential vector taxa could also be encouraged by AIS managers and research groups.
Additionally, a growing body of research suggests zoochory may contribute to the LDD and SDD of a wide variety of AIS, including gastropoda, amphipoda and freshwater arthropoda (e.g. juvenile crayfish).
For example, the New Zealand mud snail, an emerging freshwater invader across the USA which is very prevalent in nearby Badgermill Creek, has been shown to survive gut passage through several fish species. Therefore, the incorporation of zoochory biosecurity measures like quarantine times is urgently required within AIS management strategies to mitigate against local invader spread.
We feel that, for now, the possible zoochorous dispersal of freshwater bivalves remains an overlooked vector in biological invasions. Although many AIS may have a limited ability to self-disperse between isolated freshwater sites, a hop, skip and a jump aided by a more mobile organism may end with a splash into a new habitat.

Top Photo: While they prefer hard substrate and hiding places, this dragonfly nymph is proof that zebra mussels will grow wherever they’re able to attach. Photo: Andy Stevens
The full report can be found here: