CHIPOLA SLABSHELL                                                                             

Elliptio chipolaensis

 

SPECIES CODE: F03O I01

 

STATUS: On March 16, 1998, the Chipola slabshell was designated as Threatened throughout its range (USFWS 1998).  A recovery plan addressing the Chipola slabshell was finalized on October 1, 2003 (USFWS 2003).

 

SPECIES DESCRIPTION: The Chipola slabshell is a medium‑sized species that reaches a length of about 8.4 cm (3.3 in). The shell is ovate to subelliptical, somewhat inflated, and with the posterior ridge starting out rounded, but flattening to form a prominent biangulate margin.  The periostracum is smooth and chestnut colored.  Dark brown coloration may appear in the umbonal region and the remaining surface may exhibit alternating light and dark bands.  The umbos are prominent, well above the hingeline.  As is typical of all Elliptio mussels, no sexual dimorphism is displayed in shell characters.  Internally, the umbone cavity is rather deep.  The lateral teeth are long, slender, and slightly curved, with two in the left and one in the right valve.  The pseudocardinal teeth are compressed and crenulate, with two in the left and one in the right valve.  Nacre color is salmon, becoming more intense dorsally and somewhat iridescent posteriorly.  The Service currently recognizes Unio chipolaensis Walker, 1905, as a synonym of Elliptio chipolaensis, Frierson, 1927 (USFWS 2003). 

 

Like other freshwater mussels, adults are filter-feeders, orienting themselves in the substrate to facilitate siphoning of the water column for oxygen and food (Kraemer 1979).  Mussels have been reported to consume detritus, diatoms, phytoplankton, zooplankton, and other microorganisms (Coker et al. 1921, Churchill and Lewis 1924, Fuller 1974).  Juvenile mussels employ foot (pedal) feeding, and are thus suspension feeders (Yeager et al. 1994).  Foods of juvenile freshwater mussels up to two weeks old include bacteria, algae, and diatoms with amounts of detrital and inorganic colloidal particles (Yeager et al. 1994).  Specific food habits of the Chipola slabshell are unknown, but are likely similar to those of other freshwater mussels.

 

REPRODUCTION AND DEVELOPMENT:  Little is known about the life history of the Chipola slabshell.  A unionine, it is suspected that this species expels conglutinates and is a tachytictic summer releaser.  Southeastern congeners of the Chipola slabshell have been documented to use centrarchids (sunfishes) as host fish (Keller and Ruessler 1997), although a relationship between cyprinids and tachytictic brooders has been documented (Bruenderman and Neves 1993).

 


RANGE AND POPULATION LEVEL:  The type locality is Chipola River, Marianna, Jackson County, Florida.   The Chipola slabshell was thought to be endemic to the Chipola River system (van der Schalie 1940, Clench and Turner 1956, Burch 1975, Heard 1979, Williams and Butler 1994) until Brim Box and Williams (2000) located a museum lot (single specimen) from Howards Mill Creek, a Chattahoochee River tributary in southeastern Alabama.  The historical range of this Apalachicola-Chattahoochee-Flint (ACF) Basin endemic is centered throughout much of the Chipola River main stem and several of its headwater tributaries.  The Chipola slabshell is one of the most narrowly distributed species in the Apalachicolan Region.

 

The Chipola slabshell is no longer known from Howards Mill Creek.  Likewise, this species is probably extirpated from Dead Lake on the lower main stem of the Chipola and in two Chipola River tributaries, Cowarts and Spring Creeks, and thus is considered extirpated from Alabama (Lydeard et al. 1999).  Currently, six populations of Chipola slabshell remain in Marshall and Dry Creeks, and from the upper two-thirds of the Chipola River main stem (Table 6, USFWS 2003).  The largest remaining subpopulation appears to be on the Chipola River main stem in the vicinity of (but not in) Dead Lake, where the species remains relatively common (J.D. Williams, USGS, unpub. data).  An average of 3.7 Chipola slabshell specimens per site of occurrence (3 sites) were found during the status survey (USFWS 1998).

 

HABITAT: The Chipola slabshell inhabits silty sand substrates of large creeks and the main channel of the Chipola River in slow to moderate current (Williams and Butler 1994).  Specimens are generally found in sloping bank habitats.  Nearly 70 percent of the specimens found during the status survey were associated with a sandy substrate (Brim Box and Williams 2000).  

 

PAST THREATS:  The abundance and distribution of the Chipola slabshell decreased historically from habitat loss and degradation (Williams et al. 1993, Neves 1993) caused by impoundments, sedimentation and turbidity, dredging and channelization, and contaminants contained in numerous point and nonpoint sources.  A comprehensive review of these past threats is provided elsewhere (USFWS 2003, Brim Box and Williams 2000, Butler 1993, Howard 1997, Frick et al. 1998, Buell and Couch 1995, Richter 1997, Watters 1997, Neves et al. 1997).  These habitat changes have resulted in significant extirpations (localized loss of populations), restricted and fragmented distributions, and poor recruitment of young. 

 

CURRENT THREATS:  Habitat loss and degradation (Williams et al. 1993, Neves 1993) primarily caused by contaminants contained in point and nonpoint source discharges, sedimentation and erosive land practices, water quantity and withdrawal, construction of new impoundments, and alien species are primary threats to the Chipola slabshell (USFWS 2003). 

 

Sediment samples from various ACF Basin streams tested for heavy metals that are known to be deleterious to mussels had concentrations markedly above background levels (Frick et al. 1998), among those were copper (throughout the Piedmont), and cadmium (large Coastal Plain tributaries of the Flint River).  Past episodes of significant heavy metal contamination of ACF Basin streams may continue to impact mussel faunas.  An estimated 950 million gallons of chemical-laden rinse, stripping, cleaning, and plating solutions were discharged indirectly into the Flint River (P. Laumeyer, USFWS, pers. comm., 1994) over a several year period.   Concentrations of heavy metals (e.g., chromium and cadmium) in Asian clam, Corbicula fluminea (Muller 1774), and sediment samples were elevated downstream from two abandoned battery salvage operations on the Chipola River (Winger et al. 1985).  Chromium concentrations found in sediments from Dead Lake downstream in the Chipola River (Winger et al. 1985) are known to be toxic to mussels (Havlik and Marking 1987).

 


Agricultural sources of contaminants in the ACF and Suwannee basins include nutrient enrichment from poultry farms and livestock feedlots, and pesticides and fertilizers from row crop agriculture (Couch et al. 1996, Frick et al. 1998, Berndt et al. 1998).  Nitrate concentrations are particularly high in surface waters downstream of agricultural areas (Mueller et al. 1995; Berndt et al. 1998).  A study by the U.S. Soil Conservation Service (USSCS; now the Natural Resources Conservation Service [NRCS]) in the Flint River system determined that between 72 and 75 percent of the nutrients entering Lake Blackshear were derived from agricultural sources (USSCS 1993).  Stream ecosystems are impacted when nutrients are added at concentrations that cannot be assimilated (Stansbery 1995).  The effects of pesticides on mussels may be particularly profound (Fuller 1974, Havlik and Marking 1987, Moulton et al. 1996, Fleming et al. 1995).  Organochlorine pesticides were found at levels in ACF Basin streams that often exceeded chronic exposure criteria for the protection of aquatic life (Buell and Couch 1995, Frick et al. 1998).  Once widely used in the ACF Basin (Buell and Couch 1995), these highly toxic compounds are persistent in the environment, and are found in both sediments and the lipid reservoir of organisms (Day 1990, Burton 1992).  Commonly used pesticides have been directly implicated in a North Carolina mussel dieoff (Fleming et al. 1995).  Cotton is raised extensively in much of the Apalachicolan Region inhabited by these mussels.  One of the most important pesticides used in cotton farming, malathion, is known to inhibit physiological activities of mussels (Kabeer et al. 1979) that may decrease the ability of a mussel to respire and obtain food.  This chemical may pose a continuing threat to some populations of these mussels.

 

Many pollutants in the ACF Basin originate from urban stormwater runoff, development activities, and municipal waste water facilities, primarily in the Piedmont (Frick et al. 1998).  Urban catchments in Piedmont drainages have higher concentrations of nutrients, heavy metals, pesticides, and organic compounds than do agricultural or forested ones (Lenat and Crawford 1994, Frick et al. 1998), and at levels sufficient to significantly affect fish health (Ostrander et al. 1995).  Within the Suwannee River basin, nutrient concentrations were greater in agricultural areas and nitrates were found to exceed U.S. Environmental Protection Agency (EPA) drinking water standards in 20 percent of the surficial aquifer groundwater samples (Berndt et al. 1998).  Pesticide concentrations were found to exceed criteria for protection of aquatic life mostly in urban areas.  Currently, there are discharges from 137 municipal waste water treatment facilities in the ACF River basin alone (Couch et al. 1996).  Although effluent quality has improved with modern treatment technologies and a phosphate detergent ban, hundreds of miles of streams in the ACF and Ochlockonee basins in Alabama, Florida, and Georgia, as identified in reports prepared by the water quality agencies of these states under Section 305(b) of the Clean Water Act, do not meet water use classifications. 

 


Since approximately 29 percent of the ACF Basin is in agriculture (Frick et al. 1998), sedimentation from agricultural sources is probably significant.  According to USSCS (1993), 89 percent of the sediments entering Lake Blackshear on the Flint River are derived from agricultural sources.  The lower Flint River system serves as the heart of numerous mussel species’ range and is a major agricultural center.  This area has experienced “severe losses of topsoil and nutrient additions to local streams due to agriculture” (Neves et al. 1997), and has profoundly affected the biota of surface and ground waters there (Patrick 1992).  Despite the implications, only a few studies (e.g., Cooper 1987, Stewart and Swinford 1995) have specifically attributed changes in mussel populations to sediments derived from agricultural practices.

 

Many southern streams have increased turbidity levels due to siltation (van der Schalie 1938).   The Chipola slabshell attracts host fishes with visual cues, luring fish into perceiving that their glochidia are prey items.  Such a reproductive strategy depends on clear water during the critical time of the year when mussels are releasing their glochidia (Hartfield and Hartfield 1996).  Turbidity is a limiting factor impeding sight-feeding fishes (Burkhead and Jenkins 1991).  In addition, mussels may be indirectly affected when turbidity levels significantly reduce light available for photosynthesis and the production of unionid food items (Kanehl and Lyons 1992).

 

Water quantity is becoming more of a concern in maintaining mussel habitat in the Apalachicolan Region.  The potential impacts to mussels, their host fishes, and their respective habitats from ground water withdrawal may be profound.  Within the Flint River basin, decreases in flow velocity and dissolved oxygen were highly correlated to mussel mortality (Johnson et al. 2001).  Low DO conditions in stagnating stream pools due to drought conditions are having a disastrous effect on these mussels.  Mussel mortality increases dramatically as DO decreases below 5 mg/L (Johnson et al. 2001).

 

Maintaining vegetated riparian buffer zones adjacent to stream banks is a well-known method of reducing stream sedimentation and other runoff (Allan and Flecker 1993, Lenat and Crawford 1994).  Buffers reduce impacts to fish and other aquatic faunas (Armour et al. 1991, Naiman et al. 1988, Osborne and Kovacic 1993, Belt and O’Laughlin 1994, Penczak 1995, Rabeni and Smale 1995), and are particularly crucial for mussels (Neves et al. 1997).  Riparian forest removal in southeastern streams and subsequent sedimentation has been shown to be detrimental to fish communities (Burkhead et al. 1997, Jones et al. 1999).  Particularly affected in the study by Jones et al. (1999) were benthic-dependent species (e.g., darters, benthic minnows, sculpins), which were found to decrease in abundance with longer deforested patches of riparian area.  Benthic-dependent fishes, themselves disproportionately imperiled (Burkhead et al. 1997), commonly serve as hosts for numerous imperiled mussel species (Watters 1994), probably including the Chipola slabshell. 

 

CONSERVATION MEASURES:     

 


Exposure Scenario Summary Table for the Chipola Slabshell

 

Species

 

Life Stage

 

Habitat

Type

 

Exposure

Route

 

Diet

 

Significant

Interspecies

Relationships

 

Chipola

Slabshell

 

glochidia

 

parasite

 

contact with

water, diet

 

fish body

fluids

 

unknown host fish(es),

centrarchids or cyprinids??

 

 

juvenile/

adult

 

sediment

dweller

 

contact & ingestion of

water, diet,

sediment

 

filter feeder

(bacteria,

algae, detritus,

sediment)

 

 

 

 

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