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What is the diet of Palaemon elegans Rathke, 1837 (Crustacea, Decapoda), a non-indigenous species in the Gulf of Gdansk (southern Baltic Sea)?[1]

OCEANOLOGIA, 50 (2), 2008.

pp. 221-237.

Palaemon elegans Feeding

Non-indigenous species Gulf of Gdansk Baltic Sea

© 2008, by Institute of Oceanology PAS.

KEYWORDS

Urszula Janas Anna Baranska

Institute of Oceanography, University of Gdansk,

al. Marszalka Pilsudskiego 46, PL-81-378 Gdynia, Poland; e-mail: oceuj@ug.gda.pl

Received 17 January 2008, revised 30 March 2008, accepted 31 March 2008.

Palaemon elegans, a new component of the Gulf of Gdansk macrozoobenthos, colonised the southern Baltic coastal zone in the late 20th and early 21st century. Analysis of the stomach contents of P. elegans revealed 16 plant and animal taxa that these prawns had fed on. The principal dietary component was detritus, with a mean frequency of occurrence in stomachs of > 80%. The most frequently occurring plant components in the diet were algae from the genus Cladophora and the family Ectocarpaceae, while the most significant animal components were Harpacticoida, Chironomidae, Ostracoda and Gammarus spp. The results of the study show that the dietary composition of P. elegans differed significantly between stations and months. The foraging area consisted of two distinctive regions - the Inner Puck Bay, and the Outer Puck Bay together with the Dead Vistula River; two of the stations - Gdynia and Sopot - were distinct from all the others. However, no obvious seasonality in the food composition could be demonstrated.

Abstract

2. Material and methods

The study area was located in the Gulf of Gdansk (southern Baltic Sea). This consists of the Inner Puck Bay (Puck Lagoon) and the Outer Puck Bay, which are separated by a shallow underwater sandbank known as the Rybitwia Mielizna. The shallow, Inner Puck Bay is covered mainly by medium- or fine-grained sands; gravels and large cobbles occur sporadically and are associated with cliffs (Jankowska 1993). In the littoral zone, the main components of the macrophytobenthos are green algae and brown algae from the family Ectocarpaceae (Plinski & Florczyk 1993, Plinski & Jozwiak 2004). In many parts of the Gulf of Gdansk, e.g. Hel or Sobieszewo, the substrate is artificial - the shore is reinforced with concrete or boulders and groynes. Such localities are particularly suitable for Palaemon elegans, as are timber structures like the Sopot pier, or port areas (Gdynia). The hard substrate down to a depth of 1 m is overgrown, mostly by green algae from the genera Cladophora and Enteromorpha.

Specimens of P. elegans were collected from 10 stations in the littoral zone of the Gulf of Gdansk in July and August 2004 (Figure 1). Further specimens were taken from the yacht marina in Gdynia from June to

54o55' 54o50' 54o45' 54o40' 54o35'

54o30' 54o25' 54o20'

54o15'

54o10'

18o20'        18o30'        18o40' 18o50' longitude E

Figure 1. Location of the sampling stations in the Gulf of Gdansk

December 2004 and in May 2005. The surface water temperature and salinity were measured with a conductometer (WTW, Germany). The animals were caught with a hand net (mesh diameter 3 mm) from a depth of 1 m and immediately deep-frozen.

After thawing in the laboratory, 20 specimens were picked at random from every station and every month; if less than 20 specimens had been caught, then the whole material was investigated. The samples included both males and females. The total length of each specimen was measured from the tip of the rostrum to the tip of the telson. The length of the animals used for the spatial and seasonal variations of dietary composition ranged from 19 to 53 mm. In accordance with Dall (1968), the anterior chamber of the proventriculus (stomach) from P. elegans individuals was dissected with a scalpel and the contents placed on a microscope slide for qualitative analysis. A total of 239 animals with full stomachs were subjected to such analysis. All food items were identified to the lowest possible taxon under a microscope. These included well-preserved fragments of higher plants and algae, which were identified according to Ringer (1972) and Plinski (1988a, b,c,d). The benthic animals were identified from intact body parts according to Hayward & Ryland (eds.) (1996) and Rybak (2000). These parts were mostly thoracic segments, parts of jaws or legs (Chironomidae, Gammarus spp.) or chaetae (Hediste diversicolor (O.F. Muller, 1776)). Larger fragments or whole bodies of prey items were intact where these were small: Ostracoda, Harpacticoida, Hydracarina. Detritus has been defined by Velmirov et al. (1981) as dead material of unrecognisable origin. Into this category was placed a mass of unidentified, comminuted matter, digested food, and all other animal and plant parts too small to permit identification.

A quantitative analysis of the various categories of food was also carried out. The significance of a given dietary component was assessed on the basis of two parameters: frequency of occurrence, and contribution to the stomach content. Frequency of occurrence indicated the percentage of prawns that had eaten the prey at every station and in every month. The percentage of each separate dietary component was determined by microscopic examination of the stomach contents. The volume of the entire stomach contents placed on the slide was taken to be 100%; then, the area occupied by the various organisms or their parts was estimated. This was not done if the parts were very small, e.g. chironomid claws or algal cells.

All multivariate analyses were carried out using PRIMER v. 6 (Clarke & Gorley 2006). Prior to the statistical analyses, datasets were square-root transformed, and similarity matrices were constructed using the Bray-Curtis similarity coefficient. The percentage of the total stomach contents of each dietary category for individual prawns (separately for each individual) from

each station and each month were used to determine whether their dietary compositions differed between stations and/or seasons. For this purpose, one-way Analysis of Similarities (ANOSIM) was performed on the similarity matrices. For each ANOSIMtest, the null hypothesis that there were no significant differences between groups was rejected when the significance level p was < 0.05.

When seeking similarities between stations/months, hierarchical cluster analysis was applied to the food composition/frequency data in order to distinguish groups of samples of similar dietary composition. The SIMPROF permutation procedure was used to test the significance of the clusters. The null hypothesis of no internal group structures in the full set of samples was rejected when the significance level p was < 0.05. Similarity percentages (SIMPER) were used to determine which dietary categories typified particular groups and were most responsible for any dissimilarity between groups.

3. Results

The water temperature at the time of sampling was 4.0-22.5°C. The salinity varied in a very narrow range from 6.2 to 6.9 PSU; in the Dead Vistula River it was 5.6 PSU.

Altogether 15 aquatic plant and animal taxa made up the diet of Palaemon elegans. Detritus and pine pollen grains were also found (Tables 1

and 2).

ANOSIMdemonstrated that the dietary composition of P. elegans differed significantly among the studied stations (global R = 0.40,p < 0.001). Pairwise tests showed a significant difference between pairs of stations (p = 0.001-0.040), except for those between all pairs of three stations: Kuznica, Chalupy and Rzucewo (p > 0.05). Pairwise comparison showed that the greatest differences in diet composition were between the two stations at Sopot and Puck (R = 0.785), whereas the least difference was detected between two adjacent stations: Puck and Rzucewo (R = 0.086).

Cluster analysis indicated a successive split into two groups of stations and two stations: Gdynia and Sopot were distinct from all the others (SIMPROF global test n = 2.82,p < 0.05) (Figure 2). One group consisted of prawns foraging in the Inner Puck Bay (Puck, Rzucewo, Kuznica, Chalupy), the other group sought food in the Outer Puck Bay (Jastarnia, Jurata, Hel) and in the Dead Vistula River (Sobieszewo). The animals from the Inner Puck Bay displayed the greatest variation in their diet, which consisted of 13 components, whereas those from the other areas of the Gulf of Gdansk

Table 1. Frequency of occurrence (F) and mean contribution by volume (V) of the dietary categories to the overall diet of Palae­mon elegans at stations situated in different areas of the Gulf of Gdansk, produced by hierarchical clustering of stations (Figure 2); N - number of stomachs examined

Food items Gulf of Gdansk

Inner Puck Bay Outer Puck Bay and Dead Vistula River

Kuznica    Chalupy      Puck      Rzucewo Hel Jurata     Jastarnia   Sobieszewo     Gdynia Sopot

(N =10)(N =13)(N =17)(N =14)(N =12)(N =18)(N =17)(N =11)(N =15)(N =14)

F[%] V[%] F[%] V[%] F[%] V[%] F[%] V[%]   F[%] V[%] F[%] V[%] F[%] V[%] F[%] V[%]   F[%] V[%] F[%] V[%]

Cyanophyta

10

0.2

46.0

6.5

53.0

1.0

21.0

0.5

0

0

0

0

0

0

0

0

0

0

0

0

Bacillariophyceae

0

0

0

0

70.5

5.0

21.4

1.0

0

0

64.0

7.2

0

0

0

0

0

0

0

0

Dinophysis spp.

00000000

00000000 60.0

2.5

0

0

Cladophora spp.

60.0

31.5

61.5

14.2

35.5

9.0

54.2

16.4

41.6

11.2

88.0

29.0

82.3

16.0

36.4

7.7

53.3

12.0

0

0

Enteromorpha spp.

20.0

2.0

0

0

76.4

20.5

42.8

13.5

25.0

3.3

39.0

16.0

17.6

2.0

54.4

17.7

20.0

1.0

0

0

Ectocarpaceae

40.0

1.5

7.7

0.2

11.7

1.0

21.4

4.6

16.6

1.0

39.0

4.4

64.7

9.0

0

0

66.6

20.0

50.0

15.3

Ceramium spp.

10.0

3.0

15.3

2.0

0

0

21.4

2.8

0

0

0

0

0

0

0

0

6.6

1.5

0

0

H. diversicolor

0

0

23.0

5.7

0

0

21.4

3.0

0

0

0

0

0

0

0

0

0000

Ostracoda

80.0

5.8

92.0

22.5

76.0

12.0

71.4

9.3

0

0

0

0

0

0

0

0

0

0

0

0

Harpacticoida

30.0

1.0

38.5

2.0

41.0

3.4

28.5

2.0

33.3

2.0

22.2

2.7

70.5

6.0

45.5

3.1

20.0

1.5

7.0

1.5

Gammarus spp.

00000000 50.0

10.4

39.0

7.7

41.2

10.5

81.8

31.3

0

0

64.2

13.2

Chironomidae

70.0

6.0

77.0

7.7

82.0

14.3

85.0

10.0

66.6

20.8

50.0

4.0

47.0

7.0

27.3

2.2

0

0

0

0

Hydracarina

00000000 25.0

10

0

0

0

0

0

0

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