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.).
INTERNET: http://www.corpi.ku.lt/nemo/mainnemo.html

TAXONOMY

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

IDENTIFICATION

• Diagnosis:

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,

• Established:

in the entire Baltic Sea ­ ?
in the area of primary introduction ­ Yes

• Origin of the species:

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.

REFERENCES

1. Martin, J.W. and Cash-Clark, C.E. 1995. The external morphology 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.
3. Mordukhai-Boltovskoi, F.D. 1968. On the taxonomy of the Polyphemidae. Crustaceana, 14: 197-209.
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.