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Rayed Bean
Written by Martin McAllister
Saturday, 23 April 2005
 Rayed
Bean
(Villosa fabalis)
State endangered and a federal candidate species, the rayed bean
is a very rare mussel in Ohio. Scioto Brush Creek is just one of
twenty-one streams in the world that still have populations of
this small mussel.
The rayed bean is
a small mussel usually less than 1.8 inches in length. The shell
outline is elongate or ovate in males and elliptical in females.
Adult freshwater mussels are filter-feeders, siphoning
phytoplankton, diatoms, and other microorganisms from the water
column. As a group, mussels are extremely long-lived, living
from a couple to several decades, and possibly up to 100 to 200
years in extreme instances. The rayed bean' small size probably
indicates a shorter life span.
The rayed bean has
separate sexes. Age at sexual maturity is unknown, but in other
species is estimated to occur after a few years. Fertilization
takes place internally, and the resulting zygotes develop within
the gills into specialized larvae termed glochidia. Glochidia
must come into contact with a specific host fish(es) to survive.
Without the proper host fish, the glochidia will die. The
Tippecanoe darter (Etheostoma tippecanoe) has been identified as
a host fish for the rayed bean; other potential hosts include
the greenside darter (E. blennioides), rainbow darter (E.
caeruleum), mottled sculpin (Cottus bairdi), and largemouth bass
(Micropterus salmoides). Rayed bean glochidia parasitize fish
gill tissues for a few weeks. Newly-metamorphosed juveniles drop
off to begin a free-living existence on the stream bottom.
Unless they fall into suitable habitat, they will die. Thus, the
complex life history of the rayed bean and other mussels has
many weak links that may prevent successful reproduction and
recruitment of juveniles into existing populations.
The rayed bean is generally known
from smaller headwater creeks. They are usually found in or near
shoal or riffle areas, and in the shallow wave-washed areas of
glacial lakes, including Lake Erie. Substrates typically include
gravel and sand. The species is oftentimes associated with
vegetation in and adjacent to riffles and shoals.
Historically, the
rayed bean occurred in parts of the upper (e.g., Lake Michigan
drainage) and lower (Lakes St. Clair, Erie drainages) Great
Lakes system, and throughout most of the Ohio and Tennessee
River systems. The rayed bean was formerly known from 106
streams, lakes, and some man-made canals in 10 states. Extant
populations of the rayed bean are known from 22 streams and a
lake in 5 states.
The decline of the rayed bean in
the Great Lakes drainages and the Ohio and Tennessee River
systems is primarily the result of habitat loss and degradation.
Chief among the causes of decline are impoundments,
channelization, chemical contaminants, mining, and
sedimentation. Bourgeoning human populations will invariably
increase the likelihood that habitat loss and degradation will
continue to impact extant rayed bean populations.
The rayed bean is not a
commercially valuable species, but may be increasingly sought by
lay and experienced collectors because of its rarity. Most
states with extant rayed bean populations prohibit the taking of
mussels for scientific purposes without a State collecting
permit, although enforcement of this permit requirement is
difficult. The occurrence of disease in mussels is virtually
unknown, and the overall threat posed by invertebrate, piscine,
and other vertebrate predators of the rayed bean is not thought
to be significant.
The majority of
the remaining populations of the rayed bean are small and
geographically isolated. The patchy distributional pattern of
populations in short river reaches makes them much more
susceptible to extirpation from single catastrophic events, such
as toxic chemical spills. Furthermore, this level of isolation
makes natural repopulation of any extirpated population
impossible without human intervention. Population isolation
prohibits the natural interchange of genetic material between
populations, and small population size reduces the reservoir of
genetic diversity within populations, which can lead to
inbreeding depression.
The likelihood is high that some
populations of the rayed bean are below the effective population
size required to maintain long-term genetic and population
viability. Recruitment reduction or failure is a potential
problem for many small rayed bean populations rangewide, a
potential condition exacerbated by its reduced range and
increasingly isolated populations. If these trends continue,
further significant declines in rayed bean population size and
consequent reduction in long-term viability may soon become
apparent.
The non-native
species that poses the most significant threat to the rayed bean
is the zebra mussel (Dreissena polymorpha). The primary means of
impact is direct fouling of the shell. Fouling impacts include
impeding locomotion (both laterally and vertically), interfering
normal valve movements, deforming valve margins, and locally
depleting food resources and increasing waste products.
The Nature
Conservancy considers the rayed bean to be a G1G2 species; the
majority of sub-national ranks are S1. The American
Malacological Society and American Fisheries Society consider
the rayed bean to be threatened. This species is state-listed in
all five of the states that are thought to harbor extant
populations. Numerous parcels of public land (e.g.,
state parks, state forests, wildlife management areas) occur
along historical and extant streams of occurrence for the rayed
bean or in their respective watersheds. Yet, vast tracts of
riparian lands in rayed bean streams are privately owned. The
prevalence of privately held riparian lands in streams with
extant populations somewhat diminishes the level of importance
afforded by public lands. Riparian activities that occur outside
or upstream of public lands may be pervasive and have a profound
impact on their populations. Habitat protection benefits on
public lands may therefore easily be negated by detrimental
activities upstream in the watershed.
Types of
conservation activities that benefit the species are funding
programs, research and surveys, outreach, and habitat
improvements and conservation. Management the species include
implementing existing laws and regulations, prioritizing streams
and watersheds, involving local communities, seeking additional
funding, implementing Best Management Practices on riparian
lands, initiating more habitat restoration programs, adjusting
numerical criteria for pollutants, monitoring populations and
habitat conditions, reducing impacts of mining, increasing
public outreach and education, conducting stress analyses, and
establishing a Geographic Information System database to monitor
and address these activities and objectives.
Adequate information is available
on the distribution, population trends, status, and threats to
accurately assess the spectaclecase for consideration for
candidate status. Research, survey, and monitoring tasks that
are needed include determining additional host fishes,
developing propagation technologies, researching life history
and habitat needs, monitoring zebra mussel populations,
investigating criteria necessary for population viability,
developing parameters for species augmentation, developing
parameters for reintroduction, investigating reasons for
rangewide differences in survival, surveying for additional
populations, investigating the potential taxonomic distinctions
of populations, and developing and implementing cryogenic
techniques. |
