THE LIVING MUSEUM • VOLUME 57 • NO. 2 • 1995 19

Illinois Mussels

The Silent Storytellersby Robert E. Warren, Associate Curator of Anthropology

dant in lakes. They also have preferences forcertain water depths (shallow or deep), cur-rent velocities (swiftly moving or standingwater), and substrate compositions (coarsegravel or fine mud). Given the habitat pref-erences of different species, when an aquaticenvironment changes, the species composi-tion of the local mussel community changesaccordingly.

Mussels play several key roles in the eco-system. As filter feeders they help clarifywater and concentrate impurities in theirshells and soft tissues. They also serve as afood resource for raccoons, muskrats, thefreshwater drum fish, and other aquatic andsemi-aquatic animals. In prehistory, mus-sels were used by Native Americans as botha source of food and as a raw material forthe manufacture of shell tools, utensils, andornaments. Mussels were also important his-torically—as sources of pearls during the late1800s and in the lucrative shell-button in-dustry from 1890 to 1950. A new marketfor mussel shell came into being in about1960. Shells are now being exported to Ja-pan, where they are made into small spheresand then implanted into living oysters tocreate cultured pearls.

Historically, mussels were diverse andabundant in the Illinois River. Forty-eightspecies have been documented in it. By1910 it had become the most productivestream (per river mile) in the early historyof the shelling industry. However, whenWilliam C. Starrett of the Illinois NaturalHistory Survey conducted an extensive sur-vey of mussels in the river in 1966, only 23species remained. These numbers representan alarming 52% decline in species diver-sity in a span of little more than 50 years.In some segments of the river the devasta-tion has been especially severe. The upper

or nymph); and a superfamily of freshwatermussels (Unionoidea; formerly Naiadacea).

Biological terminology has changedthrough the years, and today the term na-iad is rarely used as a name for freshwatermussels. However, the metaphor relatingmussels to the water nymphs of Greek my-thology still seems appropriate. Mussels haveno supernatural control over the life andhealth of aquatic environments, but they dorespond to changes in their environments.Old mussel shells provide a record of howour rivers and lakes have changed; they tellthe story of aquatic environments, past andpresent. Mussel communities have docu-mented changes in the Illinois River fromprehistoric to modern times.

Mussels are invertebrate animals in oneof the most diverse phyla in the animal king-dom, the phylum Mollusca. Other mollusksinclude solenogasters, tuskshells, chitons,snails, slugs, clams, scallops, squids, and oc-topuses.

Mussels have soft inner bodies and hardouter shells consisting of two valves, one onthe left side and one on the right. The shellsare joined by an elastic ligament thatstretches across a hinge at the top. The shellsof different species vary in size, shape, thick-ness, color, and in the presence or absenceof sculpturing (ridges or bumps) on theouter surface of the shell. The morphologyof the hinge is also diagnostic; some specieshave interlocking “teeth” that ensure properalignment of the valves, while in other spe-cies the hinge teeth are reduced or absent.

Mussels live in a variety of aquatic envi-ronments, including rivers, creeks, lakes, andponds. Of interest is the fact that many spe-cies have specific habitat preferences. Someprefer large rivers, others are found only insmall streams, and still others are most abun-

The Water NymphThe Water NymphA nymph lives under water

in our rivers, in our lakes.

It is mute but tells us stories

of our past, of our mistakes.

For it sees what’s in the water

and responds accordingly.

It will thrive or it will flounder,

adjusting its diversity.

When we pollute the water,

when we let our topsoil go,

The nymph will keep a record.

If we read it, we will know.

It has lived here through the ages,

since before the dawn of man.

Can it survive another decade?

With your help, perhaps it can.

When you look into the water

of the stream flowing to sea,

Ask yourself: Is this a healthy place

for a water nymph to be?

Who is this nymph I speak of?

It is the naiad Unio.

A bivalve with a pearly shell,

has one foot, but no toes.

In Greek mythology, the world wasinhabited by nymphs of various kinds.They were feminine divinities of

nature, often portrayed as beautifulmaidens, which dwelled in the mountains,forests, meadows, and waters. Each wasresponsible for giving life and perpetuityto its own domain. The nymphs presidingover lakes, rivers, springs, and fountainswere called Naiades (Greek: ;"YU*,H).

Biologists have applied the name “na-iad” to several types of plants and animals,all of which inhabit aquatic environments.These include a family of aquatic plants(Naiadaceae); an aquatic larval stage of drag-onflies, mayflies, and other insects (naïad

20 THE LIVING MUSEUM • VOLUME 57 • NO. 2 • 1995

portion (above Starved Rock) had a richmussel fauna in the 1870s, but until veryrecently, no living mussels had been foundthere since 1912.1 Unfortunately, the dis-appearance of mussels in the Illinois Riverparallels national trends; more than 70% ofNorth American mussel species are nowconsidered endangered, threatened, or ofspecial concern.

Why are they dying? We have altered ourrivers and lakes so extensively—through pol-lution, siltation, and commercial develop-ment—that many species can no longer sur-vive in places where they once thrived. It is

difficult to say which causes are most im-portant, as many interacting factors may beinvolved.

One kind of evidence that deserves at-tention is the testimony of the musselsthemselves. In what ways have mussel com-munities changed? Also, can we learn any-thing by comparing the habitat preferencesof modern communities with communitiesthat lived in the past?

William Starrett’s survey provides goodinformation on the modern composition ofmussel communities in the central IllinoisRiver (the La Grange Pool). Collections ofold shells from archaeological and geologi-cal sites provide useful data on earlier com-munities. One such collection is that recov-ered from the Nancy’s Point site, a prehis-toric occupation located on the west bankof the river in Fulton County. The site is

marked by an extensive bed of shells thatruns along the beach and covers a nearbynatural levee. Prehistoric artifacts have beenfound in association with the shells andmany of the latter have been discolored byfire. The high incidence of charred shell(14%) and the absence of any shell arti-facts suggest that the mussels were collectedas a food resource and then discarded asgarbage. Artifacts dating to the EarlyWoodland cultural period (2100–2550years ago) occur on the site, although theyare not necessarily associated with theshells. We do not yet know the shell bed’santiquity, although we can be certain thatit predates the modern era.

Thirty-three species of mussels are rep-resented in the prehistoric and modern col-lections (Table 1). The prehistoric collec-tion (28 species) is considerably more di-verse than the modern sample (18 species).This drop-off in diversity is probably re-lated to the recent decline of mussel popu-lations during the 20th century. Although13 species found in the prehistoric collec-tion are still common in the Illinois River,four species are rare and 11 others havebeen eliminated (extirpated) from the river.

But which species have been extirpated?

Table 1. Freshwater Mussels from the Illinois River

Prehistoric Modern Modernmussels mussels status1

(Nancy’s (La GrangeScientific name Common name Point Site) Pool, 1966)

Subfamily AnodontinaeAnodonta suborbiculata Flat floater — 1 lArcidens confragosus Rock-pocketbook 2 16 lLasmigona complanata White heelsplitter — 5 lLasmigona costata Fluted-shell 5 — ¡Pyganodon grandis Giant floater — 29 lStrophitus undulatus Squawfoot 21 — ¡Utterbackia imbecillis Paper pondshell — 2 l

Subfamily AmbleminaeAmblema plicata Threeridge 147 363 lElliptio crassidens Elephant-ear 24 — ¡Elliptio dilatata Spike 981 — ¡Fusconaia ebena Ebonyshell 20 — ¡Fusconaia flava Wabash pigtoe 51 1 lMegalonaias nervosa Washboard 5 23 lPleurobema coccineum Round pigtoe 10 — ¡Pleurobema rubrum Pyramid pigtoe 17 — ¡Quadrula nodulata Wartyback 2 5 lQuadrula pustulosa Pimpleback 20 13 lQuadrula quadrula Mapleleaf 4 86 lTritogonia verrucosa Pistolgrip 4 — ¡

Subfamily LampsilinaeActinonaias ligamentina Mucket 63 — ¡Ellipsaria lineolata Butterfly 2 — ¡Epioblasma triquetra Snuffbox 3 — ¡Lampsilis siliquoidea Fatmucket 2 2 lLampsilis teres Yellow sandshell 1 8 lLeptodea fragilis Fragile papershell 2 21 lLigumia recta Black sandshell 1 — ¡Obliquaria reflexa Threehorn wartyback 17 1 lObovaria olivaria Hickorynut 3 — ¡Potamilus alatus Pink heelsplitter 10 1 lPotamilus ohiensis Pink papershell — 12 lToxolasma parvus Lilliput 10 — ¡Truncilla donaciformis Fawnsfoot 34 — ¡Truncilla truncata Deertoe 89 2 l

Total 1550 591

1Conservation status in 1966: l = Present in Illinois River; ¡ = Rare or Absent in Illinois River (data from Starrett 1971).

Prehistoric freshwater mussels from the Nancy’s Point site,length = 94 mm); Lower, threeridge mussel (Amblemathe most abundant species at Nancy’s Point, but they notolerant of pollution and silt, and are the most abundant

1. The long absence of mussels in the upper sectionof the river may have been part of a massive,progressive mussel die-off caused by the diversionof untreated Chicago sewage and industrial wastesinto the Des Plaines and Illinois rivers after theopening of the Chicago Sanitary and Ship Canalin 1900.

THE LIVING MUSEUM • VOLUME 57 • NO. 2 • 1995 21

body types, water depths, current velocities,and substrate types and it produces general“habitat scores” for the collections analyzed.

The habitat scores derived during thisanalysis reveal major differences between theprehistoric and modern mussel faunas. Theprehistoric mussels signify a large river withrelatively shallow water, a swift current, anda coarse gravelly substrate. This is the clas-sic signature of a big-river shoal or riffle, inwhich many mussel species adapted tosmaller streams live in the shallow wateralongside mussels that live primarily in bigrivers. In contrast, the habitat scores of themodern fauna indicate a more specializedbig-river mussel community adapted todeeper water, a slower current, and a finermuddy substrate (Table 2).

Clearly, significant changes in musselhabitat have occurred along the central Illi-nois River. The mussels seem to be tellingus that they have lost an important habitattype, the big-river shoal, which may havebeen crucial for the survival of many spe-cies. What were the main causes of this loss?Two factors are implicated. First, the con-struction of locks and dams along the Illi-nois River has increased water depths andslowed current velocities. Second, erosionfrom agricultural runoff has deposited mil-lions of tons of silt in our streams. Themussels’ message seems to be that by tam-ing our shoals and muddying our waters,we have eliminated an environment wheremany of them once thrived.

And what of the future? Illinois musselsare still threatened by pollution, siltation,impoundments, and other old enemies. Butthey also face a new threat from their dis-tant east-European cousins, the zebra mus-sels (Dreissena polymorpha). These foreigninvaders crossed the Atlantic Ocean in shipsand gained a foothold in the eastern GreatLakes about 1987. Since then, the alienshave expanded throughout the Great Lakes

prehistoric abundance spectrum have beeneliminated—the abundant, the common,and the rare. Two of the four most com-mon species in the Nancy’s Point collectionhave disappeared: the spike (Elliptiodilatata), which ranks first in abundance(63% of the total), and the mucket(Actinonaias ligamentina), which ranksfourth (4% of the total).2 Overall, more thanthree-fourths of the prehistoric shells (77%)are species rare or absent in the Illinois Rivertoday.

In comparison, the modern collectionfrom the Illinois River is dominated by thethreeridge (Amblema plicata). This speciesaccounts for 61% of the live mussels docu-mented by Starrett in the La Grange Pool,and it was the most common species inmany of the areas he sampled. Other com-mon species include the mapleleaf(Quadrula quadrula) and the giant floater(Pyganodon grandis).

To compare the environmental adapta-tions of the prehistoric and modern musselfaunas, I analyzed the shells using a com-puter program that I had previously devel-oped called UNIO. (Unio is a Latin wordmeaning “pearl.” It has been used as a ge-nus name for many species of freshwatermussels in Europe and North America.)UNIO accounts for habitat preferences ofdifferent mussel species in various water-

Table 2. Changes in MusselFaunas Reflect Changes in theCentral Illinois River Environment

Prehistoric ModernHabitat mussels1 mussels2

Water-body type Large River Large RiverWater depth Shallow DeepCurrent velocity Swift ModerateSubstrate composition Gravelly Muddy

1Nancy’s Point site fauna2La Grange Pool fauna, 1966

Just the rare ones that may have disappeareddue to chance or because of minor changesin the river environment? The numbers ofidentified shells from the Nancy’s Point siteindicate that species from throughout the

Illinois. Upper, spike mussel (Elliptio dilatata; right valve;plicata; right valve; length = 91 mm). Spike mussels werelonger live in the Illinois River. Threeridge mussels are morespecies in the Illinois River today. Photo by Robert Warren.

First row (left to right): slippershell mussel (Alasmidonta viridis); rainbow (Villosa iris);hickorynut (Obovaria olivaria). Second row (left to right): plain pocketbook (Lampsiliscardium); mucket (Actinonaias ligamentina). Shown here 0.60 actual size. Photo by Marlin Roos.

2. Recent collections by the Illinois Natural HistorySurvey indicate the mucket may be making acomeback in some sections of the Illinois River.

22 THE LIVING MUSEUM • VOLUME 57 • NO. 2 • 1995

Map of the Illinois River, showing the loca-tions of the Nancy’s Point site and the LaGrange Pool.

Additional ReadingAdditional Reading

Cummings, Kevin S., and Christine A.Mayer. 1992. Field Guide to FreshwaterMussels of the Midwest. Manual 5.Illinois Natural History Survey,Champaign.

Starrett, William C. 1971. “A Survey of theMussels (Unionacea) of the IllinoisRiver: A Polluted Stream.” IllinoisNatural History Survey Bulletin30(5):267–403.

Stolzenburg, William. 1992. “The mussels’message.” Nature Conservancy 42(6):16–23.

Warren, Robert E. 1991. “Freshwatermussels as paleoenvironmentalindicators: a quantitative approach toassemblage analysis.” In Beamers,bobwhites, and blue-points: tributes to thecareer of Paul W. Parmalee, edited by J.R. Purdue, W. E. Klippel, and B. W.Styles. Scientific Papers Vol. 23. IllinoisState Museum, Springfield.

Warren, Robert E. 1992. UNIO: ASpreadsheet Program for ReconstructingAquatic Environments Based on theSpecies Composition of Fresh-WaterMussel (Unionoidea) Assemblages.Quaternary Studies Program TechnicalReport 92-000-3. Illinois StateMuseum, Springfield.

Warren, Robert E. 1995. “PremodernPleurobema rubrum (Rafinesque 1820)from the Illinois River.” Transactions ofthe Illinois State Academy of Science88(1–2): 5–12.

Dr. Warren examining some specimens in the ISM’s collections of fresh water mussels atthe Research and Collections Center, Springfield. Photo by Marlin Roos.

and much of the Mississippi River system.They have become a costly headache forswimmers, boaters, municipal utilities, andnative aquatic wildlife.

Zebra mussels were first detected in theIllinois River in 1991, and by 1993 theyhad attained peak densities of up to 94,000individuals per square meter. They formed

dense colonies on the shells of native mus-sels, limiting their mobility and robbingthem of oxygen and food. By the autumnof 1994, up to 90% of some native musselspecies had been killed at some locations.Some zebra-mussel populations also crashedin 1994, but are expected to rebound be-fore they crash again. If the native musselscan survive these dramatic fluctuations, per-haps the zebra mussels will eventually sta-bilize at a tolerable level. We can only hopethat the next chapter in the water-nymph’ssaga is one of conflict and survival, ratherthan extinction. ❦

Invading colony of zebra mussels attached to a native mussel. Zebra mussels pose a new threat tothe survival of native mussel species. Photo by Scott D. Whitney, Illinois Natural History Survey.