BIOMETRICS OF THE SWAN MUSSEL, ANODONTA CYGNEA L., IN THE SOUTHWEST PART OF THE SZCZECIN LAGOON IN 2007

Juliusz C. Chojnacki, Anna Grzeszczyk-Kowalska, Wojciech Buczek
Department of Marine Ecology and Environmental Protection, West Pomeranian University of Technology, Szczecin, Poland

ABSTRACT

The studies were conducted along the shoreline of the southwestern part of the Szczecin Lagoon from July 30 to September 14, 2007. The animals were collected, measured and then released back into the water. The length [L], height [H], and girth [G] of the shells were measured, and age [A] was determined based on annual growth rings on the shell surface. These data were used to attempt to establish the age structure of Anodonta cygnea, and to determine the dependence among the meristic characters of this animal. The age of individuals ranged from 1 to 6 years, and was dominated by individuals aged 3, while individuals aged 1 year comprised only a small share of the population. The mean length of the Anodonta cygnea shells studied ranged from 4.6 to 10.0 cm, height – 2.7 to 5.7 cm, and girth – 1.6 to 3.6 cm. It was determined that both biotic and abiotic (primarily anthropogenic pressure) factors influenced the abundance, distribution, age distribution, and linear meristic characters of the Anodonta cygnea population inhabiting the Szczecin Lagoon. Despite differences resulting from environmental factors, it was established that the individuals collected belonged to one population. This conclusion was supported by high correlation among the characters measured as well as the fact that the individuals belonged to the same age structure.

Key words: Anodonta cygnea L., biometrics, estuary river Odra, swan mussel, Szczecin Lagoon, estuary river Odra.

INTRODUCTION

The swan mussel (Anodonta cygnea) is a palearctic species inhabiting the waters of Europe and Siberia. It is the largest freshwater mussel in Poland. It usually occurs in standing waters, but is also found, though less frequently, in slow-moving rivers and flow-through lakes. The Anodonta cygnea habitats are found in highly silty basins free of chemical pollution. Due to anthropogenic pressure, the swan mussel has become a very rare species in Poland. It is listed in the Red Book of Endangered Species, and has been under protection since 1995 [1]. Its range of occurrence in Western Pomerania has been confirmed in the following habitats: the Oder River from the city of Szczecin to the Szczecin Lagoon; in the vicinities of the Large and Small lagoons; the Międzyodrza catchment near the Lower Oder Landscape Park; in the vicinity of the city of Szczecin; lakes Dąbie Wielkie and Małe and the Green Canal; the lakes Myśliborskie, Białe, Łubie, Miedwie, Będgoszcz, Binowo, Świdwie, Mętno, Jeleńskie [2], Bobolin, Wierzchowo, Spore, and Żerdno [8]. Determining precisely the sizes of these populations is not presently possible. Formerly, this species occurred fairly frequently in small concentrations in lowlands. Currently, it is retreating from many sites it inhabited previously. In Poland, the mean measurements attained by this species are: length 125 mm, width 62 mm [7].

While environmental and anthropogenic threats to this species are not yet fully understood, it seems that water pollution and eutrophication, to which young mussels are particularly susceptible, are particularly dangerous for it. Constructions to regulate rivers contribute to the degradation of habitats of this species [6]. Stocking waters with alien fish species, on which glochidium metamorphosis is not possible, might also be of significance. And finally, there have been reports recently that the swan mussel is being removed from natural habitats and then transferred to ponds and aquaria; this could also help to explain the declining numbers of this species [8].

The aim of the study was to identify the age structure and the meristic characters of the mussel Anodonta cygnea in the Szczecin Lagoon.

STUDY AREA

According to Majewski [4], the Szczecin Lagoon is a flow-through estuary. The bottom topography divides its waters into two parts. The Great Lagoon, with a surface area of  409.7 km², a mean depth of 3.8 m, and a maximum depth of 8.2 m, lies to the east. The Small Lagoon is to the west and has a surface area of 277.2 km² and a mean depth of 3.7 m. The hydrological and hydrochemical properties of the Szczecin Lagoon are influenced primarily by the variable conditions created by irregular inflows of riverine waters and periodic inflows of marine waters through the straits that connect the lagoon with the Pomeranian Bay. With regard to salinity, the lagoon can be classified as limnetic or oligohaline. The degrees of salinity and oxygen saturation undergo seasonal change [5]. The Szczecin Lagoon is still a highly polluted basin, and the main source of pollutants is the waters of the Oder River, which carry pollution loads from the Szczecin agglomeration and the Police Chemical Factory [11]. The Oder River waters also carry loads of agricultural and industrial pollution from the entire catchment area. The principle factors impacting hydrobiont habitation and production processes occurring in the lagoon are: its flow-through character, the high input of inland waters, and shallow depth. There are few Baltic elements, and those that occur, do so most frequently in the waters adjacent to the Baltic Sea [10].

MATERIAL AND METHODS

The study was performed in the southwest part of the Szczecin Lagoon along the shoreline. The sampling sites were designated in areas free of dense stands of common reed (Phragmites australis) and common bulrush (Typha latifolia). The study material was collected from nine sites from July 30 to September 14, 2007. The animals were collected, measured, and then returned to the water.

The individual mussels were measured with a scaled caliper to the nearest mm for length [L], height [H], and girth [G]. The age of the individuals studied was determined based on the concentric rings distributed over the mussel shell surface. Mussel weight was measured with a measurement cylinder with a capacity of 2000 cm³, which was scaled to the nearest 1 cm³. Each individual was placed in a cylinder filled with lagoon water. The water volume displaced by the mussel was measured, and the value was recalculated into units of weight.

RESULTS

Analysis of age structure
Most sites were dominated by mussels aged 3+, which comprised 46% of the studied population. Mussels aged 2+ comprised 23% of all the individuals, 4+ – 17%, and 1+ – 13%. The abundance of individuals in each age class is presented in Table 1.

Analysis of shell length
Measurements were taken from 151 individuals. Individuals from age group 1+ attained an mean shell length of 5.87 cm, 2+ – 6.47 cm, 3+ – 7.74 cm, and 4+ – 7.94 cm (Table 1). The maximum shell length of Anodonta cygnea was 10.0 cm, while the minimum was 4.6 cm (Table 2). Variation in shell length was 1.7, and the standard deviation was 1.3.

Analysis of shell height
The mean shell height of 1+ mussels Anodonta cygnea was 4.00 cm, that of 2+ – 4.06 cm, 3+ – 4.61 cm, and 4+ – 4.88 cm (Table 1). The mean shell height of the whole sample studied was 4.5 cm. The shell height of the individuals studied ranged from 2.7 to 5.7 cm. Variation in shell height was 0.5 and the standard deviation was 0.7 (Table 2).

Analysis of shell girth
The mean shell girth of the 1+ Anodonta cygnea was 2.13 cm, while that of the 2+ was 2.44 cm, 3+ – 2.90 cm, and 4+ – 2.99 cm (Table 1). The maximum shell girth observed during the study was 3.6 cm, while the minimum was 1.7 cm. The mean shell girth for the entire sample studied was 2.7 cm. Variation in shell girth was 0.2 cm, and standard deviation was 0.5 (Table 2).

Correlations among the biometric characters studied

DISCUSSION

Based on the data of the meristic characters of Anodonta cygnea collected in the Szczecin Lagoon, it can be concluded that the population sample studied was dominated by individuals aged 3+ (46%), and sub-dominated by those aged 2+ (23%). Only single specimens aged 5+ and 6+ were noted (Table 1). The reason for this might be disadvantageous habitat conditions, in which the mussel cannot attain the age 7+ or more. It is worthwhile noting that during the studies many empty shells were noted that had attained ages of 7+ and more. This might be evidence that previously the prevailing habitat conditions in the Szczecin Lagoon were more advantageous. The small percentage of the youngest individuals from the 1+ age class (11%) might also be evidence of substantial population fluctuations that might result from the deteriorating physicochemical parameters of the habitat caused by anthropogenic pollution. It is also possible to consider the hypothesis put forward by Zając [12], according to whom the swan mussel attains maximum abundance at a certain stage of basin succession. As the basin ages, the population of this mussel disappears, and the lake studied begins to become overgrown. Another hypothesis might explain the domination of the 3+ age class; Widuto and Kompowski [9], who studied Lake Kortowskie in Olsztyn, demonstrated that the largest aggregation of Anodonta cygnea occurred at depths of 3.5 m. The current study included the littoral zone with a maximum depth of 1.5 m. Thus, it is probable that a certain number of animals were not included in the sampling. Another cause might be the loss of host for the parasitic larvae (glochidium) of the swan mussel (European eel, perch, tench, pike, pikeperch, stickleback, belica). Fish from the Szczecin Lagoon are under high fisheries pressure from commercial and sport fishers as well as by piscivorous birds, in particular the excessive populations of black cormorants and grey herons. Without their hosts, larvae are not capable of attaining the young mussel stage.

The mean shell length of the swan mussel in the Szczecin Lagoon (7.3 cm) is close to that reported by Kraszewski and Zdanowski [3] for specimens from the Konin heated lakes system. In Lake Binowo, which is a eutrophic water body but one without chemical contamination, the mean shell length of the individuals studied was about 8 cm [1]. Reports from other authors indicate the dominance of swan mussels 7-9 cm in length.

The comparison of mussels collected from different study stations indicates there was substantial variation among the mean shell sizes. This is probably evidence of substantial difference in environmental conditions (salinity, trophic conditions in the water, pollution, pressure from tourism) that are more advantageous for development in the eastern part of the lagoon as opposed to those in the western part. The occurrence of 2+ individuals, which have larger shell sizes than do mussels from the 4+ age class, might be confirmation of fluctuating, more disadvantageous trophic and habitat conditions at the various collection sites, as well faster growth of young mussels that are not yet sexually mature.

Among the meristic characters studies, the highest correlations were noted between shell length and height and between length and girth (Figs. 1 and 2). This is evidence of a simple mechanism: individual growth in length, height, and girth is proportional to linear growth. Significant shell thickening in the annual growth rings was noted in older mussel individuals. The weight of individual specimens was noted along with the growth of the linear characters. The high correlation coefficients noted for the individual characters studied (Figs. 1-3) confirm that, despite differences in the size and coloration of shells, the Anodonta cygnea individuals subjected to biometric analysis belonged to the same mussel population from the Szczecin Lagoon. That the individuals exhibited the same age class range is also evidence in support of this.

CONCLUSIONS

• A total of 153 individuals of Anodonta cygnea were collected at nine study sites in the Greater Szczecin Lagoon.

• The age of the studied population ranged from 1 to 6 years, and this population was dominated by age classes 3+ (46%) and 2+ (23%). Age class 1+ individuals comprised just a small percentage of the whole population (13%).

• The shell length of the Anodonta cygnea individuals collected ranged from 4.6 to 10.0 cm, height ranged from 2.6 to 8.3 cm, and girth from 1.6 to 5.2 cm.

• The weight of the mussels raged from 10 to 100 g.

• The abundance and dispersion of the Anodonta cygnea population inhabiting the Szczecin Lagoon, as well as its age distribution and individual meristic characters, indicate clearly that environmental factors, both biotic and abiotic (mostly anthropogenic pressure), have collectively had a significant impact on this species.

• Despite the variations that stemmed from environmental conditions, the individuals collected belonged to one population as was evidenced by the high correlation among the characters measured as well as the fact that the mussels all belonged to the same age range.

REFERENCES

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Accepted for print: 01.10.2009

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