What makes a fish a good mussel host?

I was reading up about a local endangered species and reading up on the Dwarf Wedge Mussel I noticed something weird. The fish who host its larvae (other than the two closely related darters and the two sculpins) seem to have nothing in common.

It's hosts are tesselated darter, johnny darter, mottled sculpin, slimy sculpin, and juvenile atlantic salmon. This has me scratching me head as a salmon, a darter, and a sculpin are all very different fish. But its choice of hosts is limited. What do these very different species have in common to make them suitable as a host when many other fish cannot? You'd assume the southern part of its range would have other darter species and the north other salmonoids. It ranged from Canada to North Carolina before its range got depleted so must cover alot of fish species in that range. The few it selects seem odd.

Maybe it's behavioral? Those might be the fish species that nose at the mussel and roll it around a bit instead of ignoring it or eating it in one bite.

I am reminded of the snuffbox mussel.
Here is a video of normal logperch behavior, rolling mussels: http://unionid.misso...sma/rollers.wmv
Now here is a video of a snuffbox capturing a logperch: http://unionid.misso..._snuffbox_1.wmv

More videos at http://bogleech.com/...clampirism.html and the Unio Gallery, http://unionid.misso...sma/default.htm

Edited by EricaWieser, 25 August 2012 - 08:42 AM.

Not ever having encountered or studied the species you listed, I'll go out on a limb and guess that they simply share the same habitat type, are benthic, and/or feed on benthic prey. It doesn't seem like the mussel is too specific about a host, so I would also bet that there are other hosts that make up a smaller proportion of hosts or just haven't been documented.

Now I'm waiting for someone who knows what they're talking about to chime in and tell me how wrong I am...

Edited by rjmtx, 25 August 2012 - 09:50 AM.

I'd have said they're all bottom dwellers most likely to encounter the mussel, till you said juvenile salmon.

I'd have said they're all bottom dwellers most likely to encounter the mussel, till you said juvenile salmon.

It seems to me it would actually make sense they are limited to juvenile atlantic salmon. Juvenile atlantic salmon remain in fresh water from 1 to 8 years, depending on location. Whatever host is taken must be present around April, during the release of glochidia. Salmon spawning also tend to be at various times in the fall. This makes the fry available during the glochidia release around April, and these fry will not leave these fresh waters for at least a year. Adult atlantic salmon simply don't appear to be generally available during this release time. Seems to make sense to me, but I am still just guessing.

I'll offer a couple really simple explanations.

1) darters and scuplins are benthic fish, dwarf wedgemussel glochidia are relatively large (150 microns) and therefore sink. Size of glochidia and host strategy matter.
2) both species co-occur throughout most of their ranges and have co-evolved together
3) for the most part they occupy similar habitats, although for both fish and mussels it varies quite a bit. Dwarf wedgmussel streams in the Chesapeake Bay watershed (which I'll be surveying next week) look nothing like those in the Delaware River watershed.

I've heard that dwarf wedgemussel (in Maine) may move mantle tissue and mimic a midge larvae, but well developed lures and active attraction strategies are not something found in that sub-family/tribe of mussels.

Salmon were found suitable in lab experiments. That doesn't mean it actually happens in the wild. Amphibians and tropical fish are often suitable lab hosts because they have no immunity to glochidial attachment. Salmon also don't span the entire historic range of dwarf wedgemussel. Neither does shield darter, which was also recently found to be a suitable lab host.

Matt, Todd, etc -- Has anybody tested to see whether a fish's suitability as a mussel host changes with age or with repeated exposures to glochidia? I would suspect the fish's immune response is a BIG factor in host suitability, and anything that quickens or intensifies the fishes' immune response (previous glochidia exposure, pollutants, ... ???) might cause glochidia to die.

Also, I've wondered why certain streams with huge populations of common Elliptios are totally lacking any "rare" mussel species. Maybe it's got something to do with the extreme exposure to Elliptio glochidia making the fishes hyper-sensitive to all mussel glochidia ???

The Tar and Neuse Rivers in NC (southern limit of dwarf wedgemussel) have 5 or 6 darter species but no sculpins or salmonids. Other common benthic-feeding fish that might be frequently exposed to DWM glochidia include bluehead chub, swallowtail and satinfin shiners, margined and Carolina madtoms, bullheads, redhorses, hogsucker, eel, redbreast, and Roanoke rockbass. Dont know if any of those have been tested as possible DWM hosts.

I guess most if not all of our mussel/fish host knowledge is from lab tests, correct? Has anybody identified DWM glochidia on wild caught fish?

Yes. Fish can build immunity. The health/condition/age of a fish can also influence the number of transformed mussels. Glochidia and especially juvenille mussels are sensitive to acute toxicity of many water pollutants.

I think in part what you are seeing is that Elliptio are (at least in the Mid-Atlantic) habitat generalists and relic populations of dwarf wedgemussel. We actually find dwarf wedgemussel in streams LOADED with Elliptio, I'm talking 1000+ in a couple hundred square meters of stream, and that is just at the surface. Three to six species too (which is DIVERSE here). The Delaware River drainage streams are the opposite. Also, many of the rarer Atlantic species are in different tribes/subfamilies and thus have different hosts because different types of glochidia and infection strategies.

Sculpins and salmonids don't co-occur over a decent part of DWM range. Host use is also something that varies regionally. Unfortunately host suitability studies aren't often conducted regionally. A good example is the manuscript on Northern Lance from NC. None of its confirmed hosts are native to Maryland, yet the type locality of the species is in Maryland. Something else has to be working that didn't in that study or wasn't looked at. They didn't show up with bluegill. Also, I've noticed on the rare occasion that fish taxonomy doesn't always tend to be kept up to date by mussel folks. So the lack of sympatry between mussel and host aren't always real. So I'm guessing one of your 5 or 6 species you have is johnny/tessellated darters, and something closely related to shield darters. Johnny/Tessellated seem to be the common bond and most used in the wild.

Most are lab tests, but wild caught fish have been used and most recently genetic primers have been developed to ID glochidia, which gets really tough in multi-species assemblages.