Cercopagis (Cercopagis) pengoi (Ostroumov, 1891)
- AUTHOR: Henn Ojaveer
AUTHOR’S ADDRESS: Estonian Marine Institute, Lai S. 32, EE-0001, Tallinn, Estonia
CITATION OF THIS ENTRY: Ojaveer, H., 1997. Cercopages pengoi. In: Baltic Sea
Alien Species Database. S. Olenin, E. Leppakoski and D. Daunys (eds.).
|Suborder||Eucladocera (Eriksson, 1934)|
|Superfamily||Polyphemoidea (Brooks, 1959) (Onychopoda Sars, 1865)|
|Family||Cercopagidae (Mordukhai-Boltovskoi, 1968)|
|Genus||Cercopagis (Sars, 1897)|
|Species||Cercopagis (Cercopagis) pengoi (Ostroumov, 1891)|
|Newly-hatched Cercopagis from the resting egg (magnification 30x). Photo by Mart Simm||
Forwardly-bent tips of barbs on the caudal
appendage of the spring form individual of Cercopagis pengoi (magnification 30x). Photo by Mart Simm|
- Examples of both sexes of the cladoceran are given on Figure 1,
where the following most pronounced parts of the body can be
identified: the head, the second pair of antenna, four pairs of
thoracic legs (the first leg is 3-4 times longer than the second
leg), abdomen, caudal process (only a short anterior section is
shown), and a brood pouch in females. Body length of females varies
from 1.2 to 2.0 mm and that of males from 1.1-1.4 whereas the
caudal process exceeds the main body 5-7 times by length
(Mordukhai - Boltovskoi and Rivier, 1987).
The most distinctive feature of species of the Cercopagididae is
the long caudal process which has a loop-like curvature at the end.
In adults, the caudal process consists of three articles, each of
which bear a pair of ventrally situated stout spines.
The head is essentially composed of a large single eye, where
the amount of black pigment makes less than one half of the
diameter of the eye.
Characteristically for C. pengoi, length of the abdomen is about
equal to that of the remaining body (the caudal process excluded).
The second antenna is a large appendage (situates dorsally on the
specimen shown on Fig. 1) containing of two branches - the endopod
and exopod. The number of setae on each branch has been used as one
of the distinguishing characteristics to separate two very similar
genera: Cercopagis and Bytotrephes. There are 7 setae on each
ramus in Cercopagis but 7 and 8 on the inner (endopod) and outer
ramus (exopod) of all species of Bytotrephes, respectively. Other
discriminative features between these genera include, amongst
others, presence (Bytotrephes) or absence (Cercopagis) of a
gnathobasic process on the first thoracopod, the structure of 2-nd,
3-rd and 4-th thoracopod, and in males, the shape and structure of
penis (Mordukhai - Boltovskoi, 1967, 1968; Mordukhai - Boltovskoi
and Rivier, 1987; Martin and Cash-Clark, 1995).
INTRODUCTION AND DISTRIBUTION
- First record from the Baltic Sea (year, area, reference):
- Year summer, 1992,
Area Pärnu Bay and the NE Gulf of Riga,
- in the entire Baltic Sea ?
in the area of primary introduction Yes
- Cercopagis pengoi prefers, in general, the brackish-water
environment, but the waterflee has also been found in pure
freshwater conditions. Until the invasion into the Baltic Sea,
the distribution area of C. pengoi has been mainly restricted to
the Ponto-Caspian region: the Caspian, Azov and Aral seas together
with lower reaches of the rivers entering to these waterbodies -
Danube, Dniester, Bug, Dnieper, Don and Volga. The animal has also
been identified in the coastal lakes in Bulgaria, and in the
Tsimlyansk and Kahovka reservoirs at Dniepr and Don, respectively
(Mordukhai-Boltovskoi and Rivier, 1987).
- Distribution in Estonian waters:
- The distribution of the animal in Estonian waters is mapped
on Figure 2. The data in the Gulf of Riga base on monthly
zooplankton sampling on the transect Pärnu Bay - Ruhnu Deep - Irbe
Sound in 1994-1996. In the remaining two finding sites in the
northern part of the basin, occurrence of Cercopagis was recorded
in larval fish samples with Hensen net in 1995.
- Population dynamics in area of introduction:
With regards to seasonal dynamics of the development of the
cladoceran, higher abundance and biomass values have been observed
during summer, in a calm weather conditions, when temperature of
the upper water layers exceeds 16-18 °C. Considerable annual
variability in peaking of the population size of C. pengoi
(from July to September) has been recorded in the Gulf of Riga.
Rapid decline in the abundance and shrinkage of the area of
distribution of the animal could be attributed to destabilisation
of the upper water column due to wind force and also to a decrease
in water temperature in fall. The highest concentration values
(795 ind m-3, 23.85 mg-3) have been recorded in the Gulf of Riga
at 12 m on the transect Pärnu Bay - Ruhnu Deep in 1995.
However, we lack of data during the production peak of the animal
when it choked fishing gears of fishermen. In the beginning of
October, 1994, when the water temperature has fallen to 12.7 °C,
still some specimen of C. pengoi (concentration 2 ind m-3) were
present in the zooplankton community. Thus, the newcomer has
successfully adapted to the local conditions in the Gulf of Riga.
Development of C. pengoi population is mainly governed by weather
conditions which causes yearly variations in peaking of the
abundance and biomass values of the species.
However the spiny water flea Cercopagis pengoi finds almost no obstacles in spreading over all the Baltic Sea. The observations carried out so far have allowed the statement that the species inhabits more and more area along the coast of the Baltic Sea, at least 100 km a year. It is already has occured in the Gulf of Gdansk and in the central part of the Gulf of Bothnia. As a reult one may expect its presence in the western parts of the Sea (Zmudzinski 1998).
THE ROLE IN GULF OF RIGA ECOSYSTEM
- The investigations performed during 1994-1996 allow to
conclude that the cladoceran has actively switched into the local
food-web via predation by fish: adult herring (Clupea harengus
membras), three-spined stickleback (Gasterosteus aculeatus)
, nine-spined stickleback (Pungitius pungitius), bleak
(Alburnus alburnus), and juvenile smelt (Osmerus eperlanus
eperlanus). Obviously, the animal is too large to be consumed
by 0-group individuals of the above-named species. Other
commercially exploited fish, whose distribution area overlap to
higher or lesser extent to that of C. pengoi - pikeperch
(Stizostedion lucioperca), vimba bream (Vimba vimba),
white bream (Blicca bjoerkna) and sprat (Sprattus sprattus
balticus) - were not found to feed upon this prey item.
In general, the spatio-temporal abundance and biomass dynamics,
both intra and inter annual, observed in the distribution pattern
of the cladoceran, matches well with its occurrence in the stomachs
of its predators. In August, 1994, Cercopagis made on
average 98.2-100% (by weight) of the total stomach content of
herring in Pärnu Bay and the NE part of the Gulf of Riga whereas
in September this value dropped to 34.8 %. In 1995, the peak
occurred in July: the mean percentage of Cercopagis in the
diet of herring was 52.2 (max. 88.1 %), of smelt 65.9 (max. 78.7 %)
and of three-spined stickleback 50.2 % in the NE part of the Gulf
of Riga. In September, the cladoceran was occasionally found in
high proportions in stomachs of 1-year-old smelt at 12 m - 78.3 %,
while smelt caught at neighbouring stations did not fed on this
prey. In 1996, the maximum occurred in August - September. Then,
stomachs of herring contained an average 21.4 (max. 23.8 %) and
50.4 (max. 91.5 %) of this animal in Pärnu Bay and the NE Gulf
of Riga, respectively. In addition, sticklebacks and bleak fed
very actively upon this prey - it made up to 100% of the total
weight of the stomach content of these fishes in the above stated
region at this time. Thus, it appeared that the newcomer has
gained an important role in fish diet in Pärnu Bay and in shallower
areas (up to 20 m depth) of the NE Gulf of Riga. The waterflee
was not found in fish stomachs in Irbe Sound area, but it appeared
occasionally at low proportions (2.6 %) in stomachs of sticklebacks
in Ruhnu Deep in July, 1995.
Recent invasion of Cercopagis pengoi into the Gulf of Riga
obviously influences dynamics of the abundance and condition of
fish stocks of various origin (freshwater species, fishes of
marine origin and glacial relicts; Ojaveer, 1997) differently.
Warm-water preferring species (sticklebacks and bleak) and
euryhaline planktivorous herring, whose distribution area
considerably overlap to that of C. pengoi, could gain direct
profit from this invasion through improved feeding conditions.
As the distribution area of cold-water species (e.g., smelt),
exhibit only weak overlap to that of C. pengoi, its
abundant occurrence cannot, therefore, have substantial direct
impact on the stock size of fish of this category.
In summer, of the most abundant predators of the waterflee, herring
and also smelt, tend, in general, to avoid the warmest coastal
areas (e.g., Pärnu Bay), where abundance of C. pengoi is the
highest. Therefore, it seems likely, that major part of the
Cercopagis production sinks, at least in the shallowest
areas, to bottom and this organic matter will be switched into
the food-web via inefficient microbial loop.
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of the onychopod ‘cladoceran’ genus Bytotrephes (Crustacea,
Branchiopoda, Onychopoda, Cercopagididae), with notes on the
morphology and phylogeny of the order Onychopoda. Zoologica Scripta, 24: 61-90.
2. Mordukhai-Boltovskoi, F.D. 1967. On the males and gamogenetic
females of the Caspian Polyphemidae (Cladocera).
Crustaceana, 12: 113-123.
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4. Mordukhai-Boltovskoi, F.D, and Rivier, I.K. 1987. Predatory
cladocerans of the world fauna. Nauka, Leningrad. 184 pp.
5. Ojaveer, H., and Lumberg, A. 1995. On the role of Cercopagis
(Cercopagis) pengoi (Ostroumov) in Pärnu Bay and the NE part of
the Gulf of Riga ecosystem. Proceedings of the Estonian Academy
of Sciences, Ecology, 5: 20-25.
6. Ojaveer, H. 1997. Environmentally induced changes in the
distribution of fish aggregations on the coastal slope in the
Gulf of Riga. Proceedings of the 14th Baltic Marine Biologists
Symposium, Pärnu, Estonia, 5-8 August 1995. p. 170-183.
7. Zmudzinski L. 1998. Cercopagis pengoi (Cladocera) conquered the Southern Baltic Sea. Baltic Coastal Zone, 2: 95-96.