J. Great Lakes Res. 24( 2): 442-452
Internat. Assoc. Great Lakes Res., 1998
Distribution of the Ponto-Caspian Amphipod
Echinogammarus ischnus in the Great Lakes and
Replacement of Native Gammarus fasciatus
Ronald Dermott1*, Jonathan Witt2, Young M. Um2, and Maria González3
1
Great Lakes Lab. for Fisheries, Fisheries and Oceans Canada Centre
for Inland Waters Box 5050, Burlington, Ontario L7R 4A6
2
Dept. of Zoology University of Guelph G u e l p h , O n t a r i o N I G 2 W 1
3
Dept. of Biological Sciences Wright State University Dayton, Ohio 45435-0001
A b s t r a c t . The gammarid amphipod Echinogammarus ischnus was found to be
widespread from the south end of Lake Huron, downstream in the St. Clair River and across
Lake Erie to the Niagara River outlet into Lake Ontario. The presence of this exotic species
was first reported in the Detroit River, where it now dominates; this species has been
common in western Lake Erie since the summer of 1995. The species has replaced the
native amphipod Gammarus fasciatus on rocky habitats in the St. Clair, Detroit, and Niagara
rivers, and is the dominant amphipod on rocky shores in western Lake Erie. In one year, E.
ischnus became the dominant amphipod at the Lake Ontario end of the Welland Canal,
although the fecundity of E. ischnus is less than G. fasciatus. E. ischnus has not yet been
reported from the north shore of Lake Ontario or the outlet into the St. Lawrence River but
occurs WO km further downstream at Prescott.
INDEX WORDS: Range expansion, nearshore, exotic species, amphipod Echinogammarus
ischnus, Gammarus fasciatus.
Introduction
The introduction of exotic species into the Great Lakes has occurred at an accelerated pace
in the late 20th century (Mills et al. 1993a), with several species having caused ecological
instability and even economic change. Although the ecological impacts of a few
nonindigenous organisms, such as Dreissena spp., are readily apparent, the impacts of most
invading species are more subtle and difficult to ascertain. Since the mid 1980s, a number of
aggressive species, including the cladoceran Bythotrephes cederstroemi (Bur et al. 1986)
and the ruffe, Gymnocephalus cernua (Pratt 1988), have appeared in the upper Great Lakes.
Four species of Ponto-Caspian origin were discovered in the St. Clair-Lake Erie corridor
within a brief time span.
These include the zebra mussel (Dreissena polymorpha), discovered in 1988 in Lake St.
Clair (Hebert et al. 1989), and D. bugensis present in Lake Erie since 1989 (May and
Marsden 1992, Mills et al. 1993b). In 1990, two gobies (Neogobius melanostomus and
Proterorhinus marmoratus) were found in the St. Clair River (Jude et al. 1992). Recently Witt
et al. (1997) discovered the amphipod Echinogammarus ischnus in the Detroit River during
September 1995. E. ischnus (syn. Chaetogammarus ischnus), originally distributed north of
the Black Sea (lilies 1967, Jazdzewski 1980), is an euryhaline species of larger, slow-moving
rivers with stony substrates, and has been closely associated with Dreissena clumps
(Mordukhai-Boltovskoi 1979, Kohn and Waterstraat 1990).
1
Corresponding author. E-mail: [email protected]
This amphipod migrated to Europe from the Dnieper River via the Pripet-Bug canal system
which connects to the Vistula River in Poland (Konopacka and Jesionowska 1995). It is
present in the middle Danube (Musko 1994) and in the Ems River and Weser-Elbe canal in
Germany. It has recently been discovered in the lower Rhine in the Netherlands (Van den
Brink et al. 1993).
Historically in the lower Great Lakes, the amphipod Gammarus fasciatus has been a
predominant component of the littoral benthic community (Barton and Hynes 1978, Dermott
et al. 1993, Stewart and Haynes 1994). This amphipod is prominent in the diets of numerous
fish species (Price 1963). As a result of the Dreissena invasion, G. fasciatus increased in
abundance (Griffiths 1993, Stewart and Haynes 1994), and now represents one of the most
important components of littoral food webs in the Great Lakes.
The replacement of indigenous amphipods by a related invading species has been well
documented in Europe (Pinkster et al. 1977), and Witt et al. (1997) suggested the possibility
of competitive displacement of G. fasciatus by E. ischnus in the Detroit River. The
replacement of G. fasciatus would not change the structure of the littoral food webs in the
Great Lakes but may change the transfer efficiencies if the two species are not
physiologically equivalent. The purpose of this study was to determine the range expansion
of E. ischnus in the lower Great Lakes, its colonization preferences, and impacts on the size
and age structure of G. fasciatus populations.
Methods
Amphipods were collected between June and November 1996, at depths < 1 m in nearshore
habitats at 30 locations from southern Lake Huron downstream to the upper St. Lawrence
River. The sites in Lake Ontario and the St. Lawrence River were resampled in 1997.
Qualitative sweep, and semi-quantitative kick samples were obtained during the initial survey
of the St. Clair River and western Lake Erie in June 1996. Semi-quantitative kick samples
were collected in two ways, depending on the substrate. A pond net (500 µm mesh) was
used to retrieve rocks larger than about 12 cm; the amphipods on the rocks were removed.
Surface area of the rocks was estimated using a ruler. Where the substrate was small cobble
or gravel, a sweep of 1 net diameter was made covering an area of about 0.06 m2. After July,
quantitative suction samples (area 0.024 m2) were also collected (Dermott et al. 1993).
Samples were preserved in 10% formalin and the amphipods later removed from the debris.
Specimens were examined under a stereomicroscope to separate G. fasciatus, Hyalella
azteca, and the rare Crangonyx gracilis from E. ischnus following descriptions in Holsinger
(1972) and Kohn and Waterstraat (1990). The ratio of E. ischnus to total amphipods in each
sample was calculated. Density (indv./m2) was estimated for the semi-quantitative kick
samples. From the quantitative suction samples, numbers per sample and their blotted preserved mass weighted, after specimens were soaked 1 hour in distilled water, were
converted to density and wet biomass/m2 for each species.
Amphipod samples collected in 1995 from Gibraltar Island, Put In Bay, Ohio (M. Gonzalez,
unpublished data), were examined for the presence of E. ischnus. These included samples
from Dreissena colonies on rocks (with measured surface area) incubated for 1 week in
cages at 0.5 and 5 m depth, and amphipods incubated in the flow-through aquaria system at
the F.T. Stone Laboratory, Gibraltar Island. Previously collected benthic samples from the
littoral of the St. Clair River (1986) and samples collected in 1990 from Lake St. Clair, Detroit
River and Niagara River by Ekman or suction sampler were re-examined for E. ischnus. The
amphipods were also re-examined in a series of 54 petit-PONAR samples collected monthly
in 1993 at three sites in western Lake Erie at depths of 9 to 11 m (Dahl etal. 1995).
The size frequency distribution of the amphipods was examined in the samples from
Gibraltar and Pelee islands, eastern Lake Erie, western Lake Ontario, the Welland Canal,
and Niagara River to determine age structure of the populations by measuring either all the
amphipods present or sub-samples where abundance was greater than 100. Body length
was measured from the rostrum to the base of the telson. In order to compare the reproductive capacity, the size specific fecundity of E. ischnus and G. fasciatus were examined
from the lengths of individual females and the number of eggs in each gravid female. Lengthfecundity regression lines were calculated for both species and the regressions compared
using covariance analysis (Snedecor and Cochran 1971).
Results
Distribution
No E. ischnus was found in samples from the St. Clair River in 1986, Lake St. Clair, the
Detroit River, or Niagara River in 1990, or in the western Lake Erie samples collected in
1993 (in Dahl et al. 1995). E. ischnus was present in samples collected in 1995, from a depth
range of 0.5 to 5 m near the Stone Lab, Gibraltar Island, western Lake Erie (site 10 in Fig. 1).
The earliest record of occurrence was July 1995, when 8 of 19 amphipods were E. ischnus
in a sample from rocks with Dreissena colonies incubated at a depth of 5 m off Gibraltar
Island (41°39.5'N: 82°49.4'W). One was a gravid female E. ischnus (6.37 mm length)
containing 6 eggs; the remainder were young, between 1.5 and 3.5 mm. E. ischnus was also
collected in August and November 1995, and May 1996 from rocks with Dreissena colonies
incubated at a depth of 0.5 to 1 m. Four E. ischnus specimens were also among the
amphipods incubated in the Stone Lab's flow-thru aquaria system in November 1995.
In 1996, E. ischnus was present at 4 of 7 sites sampled between Lake Huron and the mouth
of the Detroit River, and at all shore sites in western Lake Erie, including Pelee Island (Fig.
1). G. fasciatus was present at 7, but common at only 3 of the 13 sites between Lake Huron
and central Lake Erie (Table 1). E. ischnus was also present in the St. Clair River at
Lambton (site 2) in June 1996. During July to September 1996, E. ischnus was present at all
sites sampled in eastern Lake Erie, and at the outlets of the Niagara River and Welland
Canal and one site (Niagara Shores) in western Lake Ontario (Fig. 1). During September
1996, E. ischnus was absent in all samples from the other sites in Lake Ontario and upper
St. Lawrence River between Hamilton Harbour and Brockville (Fig. 1). Total amphipod
densities ranged from 26 indiv./m2 in the Canard River, a tributary of the Detroit River during
June, to 20,680 indiv./m2 at Port Weller, at the outlet of the Welland Canal, in September
(Tables 1 and 2).
In September 1996, E. ischnus was present at two sites in Lake St. Clair and western Lake
Erie (sites 4 and 13), where it was not found during September 1995 (Witt et al. 1997). In
October 1997, E. ischnus was the most common amphipod in samples from the St.
Lawrence River below the Coast Guard dock at Prescott, Ontario (Table 1, site 30;
44°42.4'N: 075°30.9'W) but was not found at the other sites in Lake Ontario and the upper
St. Lawrence River (sites 21 to 29).
Competitive Replacement
Rocks (area 0.037 m2) with attached Dreissena, incubated in cages for 1 week at a depth of
5 m off Gibraltar Island (site 10), and retrieved in July 1995, were colonized by over 64,100
amphipods/m2, of which 29% were E. ischnus. The proportion of E. ischnus on retrieved
rocks incubated at 0.5 m for 1 week at Gibraltar Island was 66% in August, and 74% in
November 1995, but decreased to 25% of the amphipods present in May 1996. In July 1995,
the size frequency of E. ischnus indicated the population at Gibraltar Island was mostly
recently hatched individuals ( < 2 mm), whereas most of the G. fasciatus present were immature and adults of 4 to 7 mm long (Fig. 2). The size distributions of the two species were
more similar in November.
At Port Weller (site 18), the proportion of E. ischnus rose from 52% in July, to 98% in
September 1996, and reached 99.7% the following May, suggesting displacement of the
native G. fasciatus (Table 2). In the lower Niagara River, close to its outlet to Lake Ontario,
the proportion of E. ischnus in samples from cobble at 0.6 m depth rose from 87% in July
1996 to 100% of the 15,600 am-phipods/m2 in September (Table 2). The density of G.
fasciatus at this site in August 1990 was 13,106 indiv./m2 (S.E. 5281; Dermott unpublished
data). At Niagara Shores, on Lake Ontario between the Welland Canal and Niagara River,
the proportion of E. ischnus increased from 3% in July to 15% in late September 1996, and
was 94.2% of the amphipods at this site during the following spring (Table 2). E. ischnus
accounted for 100% of the amphipods at six of the sites sampled in 1996 (Table 1). These
sites had rocky substrates with moderated current in the rivers, or rubble substrates on wave
washed beaches. All except one of these sites were along the St. Clair-western Lake Erie
corridor where E. ischnus had been present for at least 2 years. G. fasciatus was common in
the 1986 samples from the St. Clair River. Densities at a location 3 to 5 km up-river from
Lambton (site 2) had been 336 indiv./m2 in April and 2,120 indiv./m2 in September 1986, on
gravel and sand at a depth of 1.5 m to 3 m. Density of G. fasciatus on Dreissena covered
rocks from the Detroit River in September 1990 was 1,700 indiv./m2, Hyalella density was
200 indiv./m2. In fall 1996, only E. ischnus was collected at these two locations (Table 1).
Three of the sites along this corridor (sites 3, 5, 6) had no E. ischnus; all three sites had a
soft substrate with dense aquatic plants inhabited by H. azteca and /or G. fasciatus. H.
azteca was also the dominant amphipod (4,000 indiv./m2) in the September 1990 sample
from 1 m depth at Mitchell Bay, Lake St. Clair (site 3), density of G. fasciatus, in the 1990
sample was 1,596 indiv./m2.
Lengths and Fecundity
The size frequency distributions indicate that in central and western Lake Erie, both adult
and juvenile E. ischnus were present in mid-summer (Fig. 3). In July 1996, the size
frequency of E. ischnus in the Niagara River and at the Lake Ontario end of the Welland
Canal indicated populations of mainly young (< 3 mm) recently hatched individuals similar in
size to the G. fasciatus present (Figs. 3 and 4).
G. fasciatus females grew larger and produced more eggs than E. ischnus (Fig. 5). G.
fasciatus females produced a maximum of 36 eggs compared to a maximum of 21 for E.
ischnus. The regression of length vs egg number for G. fasciatus was not significantly
different (df 1, 93) than that for E. ischnus. However, E. ischnus apparently begins reproducing at a slightly smaller size (4.8 mm) as compared to G. fasciatus (5.2 mm).
Discussion
In 1996, Echinogammarus ischnus was widely distributed from southern Lake Huron
downstream to the mouth of the Niagara River. This species was often the sole amphipod
encountered at sites in the St. Clair River to western Lake Erie corridor, close to the Detroit
River location where the species was first discovered (Witt et al. 1997). In the vicinity of the
Niagara River, E. ischnus became the dominant amphipod within a time span of 1 year.
Densities were greatest among mussel colonies near Gibraltar Island, on wave washed
cobble on Pelee Island, western Lake Erie, and on cobble fill in the Welland Canal. In its
native habitat, E. ischnus is listed as part of the lithophilic community of large rivers, where
Dreissena is the characteristic component of the community (Mordukhai-Boltovskoi 1979).
The association with Dreissena clumps may be a coincidence of habitat preference for
cobble by both species. Kohn and Waterstraat (1990) found that E. ischnus was scarce
outside Dreissena clumps, but they did not indicate how common other hard substrates were
in the German lake. In this study, E. ischnus was consistently present on cement slabs used
for shoreline protection on the wave-washed beaches in Lake St. Clair and Lake Erie,
perhaps as the slabs are the only hard substrate present at these locations.
Since its discovery in the Detroit River in 1995 (Witt et al. 1997), E. ischnus has rapidly
expanded its range downstream to Lake Ontario. Strong longshore currents moving
eastward in Lake Erie and Lake Ontario (Simons 1976, Csanady and Scott 1974) likely
swept the amphipods along the shore (Barton and Hynes 1978). The isolated population at
Prescott on the St. Lawrence River probably was established via shipping activities at the
Coast Guard depot, which is responsible for maintenance of navigational buoys from the
Welland Canal area. In Europe, the species dispersed easily through man-made canals first
to Poland and then across Germany to the Rhine (Konopacka and Jesionowska 1995). E.
ischnus will very quickly move downstream in the St. Lawrence River to its estuary, and will
probably enter the Mississippi River and Hudson River basins via the interconnecting canals.
At the latest, E. ischnus must have been in the western approaches to Lake Erie during
1994, one generation prior to its presence at two separate locations in 1995. If so, E. ischnus
was able to migrate from the west to the east end of Lake Erie in perhaps 2 years, the same
rate taken by Dreissena, which has a planktonic larval stage (1987-1989, O'Neill and
Dextrase 1994). The appearance of four Ponto-Caspian species in the St. Clair-Lake Erie
corridor within a short time frame demonstrates the efficacy of trans-oceanic transportation
via freshwater ballast. E. ischnus and especially D. polymorpha have a wide distribution in
Europe allowing transport from a number of ports, unlike the rare tubenosed goby (P.
marmoratus) and D. bugensis, with restricted distributions at a few freshwater ports on the
eastern Black Sea. The discovery of another freshwater exotic species in the Great Lakes
further strengthens the argument for rigorous ballast water controls and /or treatment.
Specimens of E. ischnus may have been misidentified as G. fasciatus, which it resembles
superficially. The elongate, spiny third uropods of E. ischnus, with minute inner rami, is the
distinguishing feature of the genus (Barnard and Barnard 1983, Witt et al. 1997). When alive,
the red-colored antenna and uropods of adult E. ischnus can easily be used to separate the
species from G. fasciatus. Failure to separate the species from native gammarids also
occurred in Germany, where specimens were misidentified for 2 years as Gammarus lacustris (Kohn and Waterstraat 1990). This reinforces the need for adequately trained
taxonomists in an era of concern over diminishing diversity and the impact of exotic species
on native communities. Sites in Lakes St. Clair and Erie with dense submerged vegetation or
quiet, turbid water generally had low densities of E. ischnus. G. fasciatus is well equipped to
cling to vegetation (Clemens 1950), and appears to be able to compete with E. ischnus in
these habitats, such as the turbid Canard River, and areas with abundant submergent
aquatic vegetation (Thames River mouth and Mitchell Bay, Lake St. Clair). Densities of G.
fasciatus increased in Lake St. Clair following the increased abundance of aquatic plants
(Griffiths 1993). G. fasciatus generally inhabits lakes and slow moving, often turbid rivers
(Bousfield 1958). However, Krecker and Lancaster (1933) found G. fasciatus most common
in western Lake Erie on angular rubble in water 15 cm deep. Rocky substrates with current
are not a secondary habitat for G. fasciatus; populations often reach 5,000 indiv./m2 on wave
washed cobble beaches in western Lake Ontario and the Bay of Quinte (Dermott et al. 1992)
and can exceed 20,000 indiv./m2 in Lake Erie on the recently created habitat of zebra mussel
colonies attached to cobble (Dermott et al. 1993, and site 16). E. ischnus is agile and
capable of walking across firm surfaces, supported upright by its uropods and antenna. As a
result, E. ischnus may have a competitive advantage over G. fasciatus on the hard
substrates.
Both length and fecundity of E. ischnus in Lake Erie were very similar to that reported in
Poland. However, fecundity in Lake Erie (22 eggs per brood) was much lower than the 40
eggs per brood in populations of the Dnieper or Don rivers (Konopacka and Jesionowska
1995). Clemens (1950) found that the average brood size of G. fasciatus was 17 eggs, with
a development time of 9 to 22 days at temperatures of 20 and 15°C, respectively. At 21°C,
time to grow to maturity for G. fasciatus was 53 days (Clemens 1950), the maturation time
for E. ischnus was 55 to 65 days (Konopacka and Jesionowska 1995). Differences in
fecundity and generation time do not appear to favor E. ischnus compared to G. fasciatus.
With a lower fecundity and having evolved in a region with many more amphipod species, E.
ischnus is more of a specialist on rocky habitats compared with the generalist G. fasciatus.
Competition with G. fasciatus, where both species were present, reduced the frequency of
immature G. fasciatus in the population as compared to sites where E. ischnus was not
common.
G. fasciatus has become rare on rocky habitats, especially those with moderate currents,
such as in the St. Clair, Detroit, and Niagara rivers, where E. ischnus is now often the only
amphipod found. It is likely that G. fasciatus will be replaced by E. ischnus on wave washed
cobble beaches, rubble armored shorelines, and breakwalls, and especially on the rocky
substrates in the interconnecting rivers and larger tributaries of the Great Lakes. Research is
needed on the mechanisms of competition between the two amphipods on rocky substrates
where G. fasciatus is still common.
It is uncertain how successful E. ischnus will be in the tributary streams where it will compete
with G. pseudolimnaeus, and several Crangonyx species. A displacement of several
Gammarus species by the introduced North American G. tigrinus has been recorded in
Europe (Pinkster et al. 1977). In the lower Great Lakes G. fasciatus constitutes more
biomass than any other littoral benthic invertebrate, with the exception of Dreissena and
possibly crayfish (Stewart and Haynes 1994). Thus, it is an important food-web component
as well as a major link in the contaminant pathway in the Great Lakes (Bruner et al. 1994).
Comparative studies with G. fasciatus on the substrate preferences, growth efficiencies,
production rate, as well as contaminant transfer, are required to determine if E. ischnus will
fill the role currently occupied by G. fasciatus. Although E. ischnus is now well established in
the Great Lakes, its impact on the ecosystem of the lakes may be minor, and certainly less
dramatic than that of Dreissena.
Acknowledgments
We wish to thank John Leslie and Bud Timmins for assistance with the collections between
Sarnia and Lake Erie. Amy Downing and Scott Roush assisted with the collections at
Gibraltar Island. A. El-Shaarawi assisted with the regression analysis. This study was
supported by funding from Fisheries and Oceans to R. Dermott and J. Leslie. Funding was
provided to M. Gonzalez by the National Sea Grant College Program (grant # R/ER-23140).