Orconectes obscurus
Obscure Crayfish
DESCRIPTION
A small crayfish that is olive-
grey-brown dappled with dark brown in
colour with characteristic dorsal crescent
pattern near back edge of carapace. It is
distinguished from the similar Northern
Clearwater Crayfish by absence of rostral
carina and from the Rusty Crayfish by male
copulatory stylet morphology and absence
of rusty markings on body.
HABITAT
Found in lakes, rivers and streams. Little information is available on the ecology of the
Obscure Crayfish, but it appears to occupy the same ecological niche as the native
Northern Clearwater Crayfish.
DISTRBUTION
Canada/Ontario
Canada/Ontario Ð The
species occurs only in Ontario where it was
probably introduced in the same manner as
the Rusty Crayfish since most locations
coincide with cottage, recreational and sport
fishing areas (Crocker and Barr, 1968).
It has been found in several lakes
and rivers from eastern
Ontario (Lucky Lake
near Plevna), to the
Kawartha Lakes
(Chandos, Kasshabog,
Round Lakes and North
River) and northcentral
Ontario (Rock and Cedar
Lakes in Algonquin Park;
Dropledge Lake, Madawaska
River in Renfrew County;
Oldmans, Black, Simmons, Bells,
Himbury and Crawford Lakes east of Parry
Sound) as well as north west in the
Manitoulin region (Cutler and Maple Lakes
near Massey) (Crocker and Barr, 1968; Berrill,
1978; David et al., 1997). There is also an
unconfirmed record for the Obscure Crayfish
in the Mississippi River (East Ontario) at
Ferguson Falls (Schueler pers. comm.).
North America
The natural range of this
species is small, centering on the Ohio
River valley and the states of Ohio, New
York, Maryland, Pennsylvania, Virginia and
West Virginia. It has also been introduced
into Ontario, Maine, Massachusetts,
Vermont and Tennessee (Hobbs et al., 1989;
Taylor et al., 1996).
ECOLOGY
In the Kawartha
Lakes region, it has been found in rivers
and lakes under rocks, gravel and
boulders (Berrill, 1978). Laboratory
studies have shown this species to have
only moderate station holding capacity
(40 cm/sec or 15.6 in./sec) in fast
currents which may prevent it from
colonizing the upstream portions of faster
flowing rivers and streams (Maude and
Williams, 1983).
LIFE HISTORY
Little is known of the life
history of this species in Canada. In the
Kawartha Lakes region of Ontario, the
populations appear to conform to the
general orconectid type life history pattern.
In Ohio and Pennsylvania populations,
copulations occurred from mid-August
through November, eggs were laid in spring
(April), hatching occurred in May and free-
living young of the year were first seen in
late June. (Fielder, 1972; Ortmann, 1906)
One year old crayfish ranged from 18-34
mm (0.7-1.33 in.) CPL, whole 2 year olds
ranged from 38-40 mm (1.48-1.56 in.)
CPL. Males moulted from Form I to Form II
in May and back to Form I in early June
and mature females moulted after their
young left (Fielder, 1972). The average
lifespan was 2 years.
INTRODUCTION STATUS
As is the case
with the Rusty Crayfish, the Obscure
Crayfish appears to be spreading in
Ontario. Since the original records (Crocker
and Barr, 1968), new distribution points
have been found in numerous lakes and
rivers (Berrill, 1978; David et al., 1997). This
includes populations from the Canadian
Shield suggesting that, unlike the Rusty
Crayfish, this species can spread into that
geographical region despite any possible
environmental constraints. The Obscure
Crayfish has been found together with the
Rusty Crayfish, the Papershell Crayfish, the
Appalachian Brook Crayfish and the Robust
Crayfish (Berrill, 1978; David et al., 1997).
Both Berrill (1978) and David et al. (1997)
found that in some of the watersheds they
surveyed the Obscure Crayfish was the sole
species present, suggesting that it may have
eliminated native species. This may be the
result of direct competition but it is also
believed that hybridization between the
Obscure Crayfish and the Northern
Clearwater Crayfish can occur. Although it
does not appear to be as invasive and
prolific as the Rusty Crayfish, it still is
important to monitor the spread and
impact of this species in Ontario. This is
particularly crucial on the Canadian Shield
where the Rusty Crayfish appears to have
little impact to date.
(Used by permission: text by Premek Hamr, modified from "Baitfish of North America"2007
painting by Aleta Karstad)
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