Kathryn Clarke                                                                                               osu820

 

The Distribution and Abundance of Ctenophores in the Menai Straits and Eastern Irish Sea in Comparison to the Distribution and Abundance of Their Copepod Prey

 

Introduction

 

     The members of the phylum Ctenophora are known by many other names including comb jellies, sea walnuts (Ruppert and Barnes 1994) and sea gooseberries (Fraser 1962, Hayward and Ryland 1995, Newell and Newell 1973).  However the scientific name derives from the Greek words ktenes meaning combs and ophora meaning carrying and relates to the form of locomotion that the phyla uses, that is comb rows.  Each animal has 8 rows of fused cilia that run vertically around the animal.  These plates of “combs” are of equal distant apart and beat in waves allowing the animal to move (Ruppert and Barnes 1994). 

This phylum is only small and contains about 50 species, the majority of which are planktonic (Fraser 1962) and are found in throughout the water column (Waggett and Costello 1999).  Of these 50 species only 3 are common visitors to British waters (Hayward and Ryland 1995). Pleurobrachia pileus, Bolinopsis infundibulum and Beroe cucumis are all members of the holoplankton (Newell and Newell 1973) that can be found in the coastal waters of the Irish Sea.  They are found in their highest abundance in the summer months but their numbers can vary quite dramatically from year to year (Hayward and Ryland 1995).  Of these species only Pleurobrachia is looked at in this study.

     Pleurobrachia pileus is a ctenophore of 17-20mm in length (Hayward and Ryland 1995) and like all other members of its phylum it is a voracious predator.  It captures its prey by extending 2 long, branched tentacular arms that form a kind of net. Prey gets caught in this net by becoming stuck on the adhesive colloblasts that line the branches.  Once prey items have been caught the comb jelly retracts its arms and pushes the food into its mouth (Ruppert and Barnes 1994).  The diet of this predator consists of copepods (both large and small), fish eggs and larvae, molluscan adults and larvae, cladocerans and even other ctenophores (Falk-Petersen et al 2002, Mutlu and Bingel 1999, Waggett and Costello 1999) so making it a important factor in the regulation of the zooplankton communities in the waters where it is found (Falkenhaug 1996).  As well as this primary ecological importance to the planktonic community it also has a commercial importance to fisheries as it predates on larvae and eggs of economically important fish species (Waggett and Costello 1999).

     The aim of this study is to look at the distribution and abundance of Pleurobrachia pileus in the Menai Straits and Eastern Irish Sea and compare it to the distribution and abundance of 4 common copepod species also found here.  The copepod species that will be looked at are Pseudocalanus elongatus, Temora longicornis, Acartia clausi and Centropages typicus.  The hypothesis is that the distribution pattern of the predator should be the same as its prey.

 

 

 

 

Results

    

     Table 1 shows the mean number of animals per m3 and the standard deviation for Pleurobrachia and the 4 copepod species calculated from samples collected at each of the 5 sampling stations, 1 in the Menai Straits and the other 4 in the Eastern Irish Sea, on the 3 consecutive days that samples were taken, 2nd – 4th October 2002.

 

Table 1: Comparisons of the mean number of animals per m3 along with the standard deviations, SD, (to 2dp) of Pleurobrachia pileus and 4 copepod species, Pseudocalanus elongatus, Temora longicornis, Acartia clausi and Centropages typicus, calculated from samples collected from 5 sampling stations in the Menai Strait and the Eastern Irish Sea on the 2nd, 3rd and 4th October 2002.

 

 

Species

 

 
Pleurobrachia
Pseudocalanus
Temora
Acartia
Centropages

 

StStation

Day

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Mean

SD

Pier

2nd

1.33

1.15

708.27

586.51

1575.3

298.47

323.23

214.93

741.97

369.5

3rd

0

0

303

114.97

474

324.76

56.13

48.30

175.33

87.52

4th

0

0

135.5

120.92

393

434.16

43.5

61.52

106

149.9

Puffin

2nd

0

0

9.6

13.29

113

36.77

13.8

3.96

114.5

34.65

3rd

0

0

24.33

15.82

257

50.09

73.33

29.14

389.67

79.59

4th

0

0

72.5

3.54

334.5

41.72

10.5

6.37

216.5

47.38

Llandudno

2nd

0

0

0.5

0.71

22.5

6.36

1.7

1.84

36

1.41

3rd

2.33

0.58

9.67

3.21

136.67

82.71

6.33

4.04

184.33

95.46

4th

0.1

0.17

21.83

25.36

89.67

25.03

3.87

6.18

27

1

Offshore

2nd

7.5

3.54

168

87.68

358

125.87

39

36.77

156.5

47.38

3rd

9.33

3.51

67.33

7.64

293.33

37.55

88.67

27.97

62

18.33

4th

0.01

0.02

45.33

19.60

125.33

17.04

18

13.23

77

9.17

        Llandonna

2nd

0

0

10.5

6.36

202.5

70

18.5

9.19

105.25

56.21

3rd

0.10

0.17

29

4.58

189.33

34.31

22.33

12.06

196.67

213.91

4th

0

0

50.87

53.82

287.33

267.33

19.33

19.50

109.67

78.5

 

 

     Table 1 shows that Pleurobrachia was usually found in mean highest abundance at the offshore sampling station and wasn’t found at all at the Puffin Island station.  The copepod species have more variable distributions but a quite common pattern seems to be that the mean highest numbers were counted at the Pier sampling station and the mean lowest numbers at the Llandudno station.  It is also apparent from table 1 the large differences in the mean number of animals counted per m3 between the predator and the prey species, sometimes with differences of 100 fold or more.  These differences are obviously explained by the relative size differences of the animals with Pleurobrachia being approximately 10 times longer than Temora longicornis, for example.  Another observation from the data in the table is the large differences in the mean number of copepods per m3 within each species, for example, Pseudocalanus elongatus observations range from 708.27 to 0.5 animals per m3.  These differences could be due either to the patchiness of plankton distribution in the water or due to human error in identification and counting.

     Figures 1, 2 and 3 show the distributions of the 5 planktonic species at the 5 sampling stations for each day that samples were taken.  The log of the mean numbers of animals per m3 was used instead of the actual numbers so that the large differences in the numbers were reduced and the Pleurobrachia line would actually show on the graphs.  The original mean numbers all had 1 added to them before the log was taken because log10 0 is an impossible mathematical function and 0 appears quite regularly in the data for the numbers of the ctenophore.

 

 

 

     Figure 2 shows good similarities in the abundance and distribution of the Pleurobrachia compared to all the copepod species, with peaks at the pier and offshore sampling sites and then declining at the other 3 sites.  By looking at the samples from this day alone it would appear that the distribution of Pleurobrachia relates directly to the distribution of their prey.

     Figure 3 shows the least similarities of all the days in the distribution patterns of the 5 species looked at.  From this graph the only pattern which would encourage the hypothesis is the peak in numbers per m3 of Pleurobrachia, Temora, Acartia and Pseudocalanus at the offshore sampling site followed by the dip at Llandonna.

     Figure 4 shows good similarities between the distribution patterns of the copepod species but not between the predator and the prey.  It is noticeable that this day produced the lowest number of ctenophores caught over the 3-day sampling period but had similar numbers of copepods to that of the 3rd.  The only peak in ctenophore numbers per m3 on this day occurred at the Llandudno sampling station.  This is the complete opposite to the copepod species that saw their lowest numbers on this day collected at this site.

 

Discussion

 

     From the results above it cannot be determined whether the hypothesis can be accepted or not as sometimes the distribution of Pleurobrachia did appear to follow that of its prey and sometimes it didn’t.   However other studies, such as Mutlu and Bingel (1999), found that increased abundance in the ctenophore population did in fact correlate well with increased abundance in Pseudocalanus elongatus and Acartia clausi, and this result would prove the hypothesis stated in this report. Although the expected results were not found here there are many factors that effect the distribution of planktonic organisms, including Pleurobrachia.

     One major factor in the lives of zooplankton communities is advective processes. These processes relate to the changes in community structure of the plankton corresponding to the physical exchange of water masses between sites.  These water masses will contain different planktonic communities and as they are relocated to another site they will effect the planktonic community structure at their new site (Falkenhaug 1996).  This means that animals such as Pleurobrachia may be moved around due to the water mass in which it belongs and may not necessarily be taken to places were its prey is in high abundance.  Although these organisms can alter their position by swimming and can therefore change the water mass in which they live, by swimming up and down into new currents (Barnes and Hughes 1982), they are ultimately at the mercy of the physical forces in the ocean.   In this case the biological factor of predation is overcome by the physical forces (Falkenhaug 1996).

     Another possible reason for the unexpected results could be that the sampling techniques used were not optimum for catching the ctenophores.  For a start these animals are very delicate and can easily be macerated when caught in a net or washed through a mesh so the numbers counted were probably only a fraction of the numbers caught. 

Secondly, only the surface waters were sampled and this may not be where the ctenophores live, as they possibly migrate from depths to feed.  In the study carried out by Mutlu and Bingel (1999) they found that the position of Pleurobrachia pileus was controlled by the density stratification in the waters in which they live, and as the Irish Sea does stratify we can assume that this would effect the ctenophores here as well.  The study goes on to suggest that this distribution due to density allows the animals to congregate in places were they are neutrally buoyant and also allows them to make vertical movements in the water with little energy. 

Finally it has also been suggested that ctenophores occur mainly below the thermocline, were seasonal temperature fluctuations are slight, so therefore below the surface waters in which their copepod prey reside (Gorsky et al. 2000, Mutlu and Bingel 1999).

     To better study the distribution of ctenophores in the Eastern Irish Sea a longer study would probably be required along with more delicate sampling techniques that sampled throughout the water column and not just in the surface waters.

 

References

 

Barnes, R.S.K. and Hughes, R.N. 1982.  An introduction to marine ecology. Blackwell Scientific Publications.

 

Falk-Petersen, S., Dahl, T.M., Scott, C.L., Sargent, J.R., Gulliksen, B., Kwasniewski, S., Hop, H. and Millar, R-M. 2002. Lipid biomarkers and trophic linkages between ctenophores and copepods in Svalbard waters.  Marine Ecology Progress Series. Vol 227, pp 187-194.

 

Falkenhaug, T. 1996. Distributional and seasonal patterns of ctenophores in Malangen, northern Norway.  Marine Ecology Progress Series.  Vol 140, pp 59-70.

 

Fraser, J. 1963. Nature adrift; The story of marine plankton. London G.T. Foulis and Co. Ltd.

 

Gorsky, G., Flood, P.R., Youngbluth, M., Picheral, M. and Grisoni J-M. 2000.  Zooplankton distribution in four western Norwegian fjords.  Estuarine, Coastal and Shelf Science.  Vol 50, pp 129-135.

 

Hayward, P.J. and Ryland J.S. 1995.  Handbook of the marine fauna of North-West Europe.  Oxford University Press.

 

Mutlu, E. and Bingel, F. 1999.  Distribution and abundance of ctenophores, and their zooplankton food in the Black Sea. I. Pleurobrachia pileus.  Marine Biology.  Vol 135, pp 589-601.

 

Newell, G.E. and Newell, R.C.  1973.  Marine plankton: A practical guide.  Hutchinson Educational.

 

Ruppert, E.E. and Barnes, R.D.  1994.  Invertebrate zoology.  Saunders College Publishing.

 

Waggett, R. and Costello, J.H. 1999.  Capture mechanisms used by the lobate ctenophore, Mnemiopsis leidyi, preying on the copepod Acartia tonsa.  Journal of Plankton Research.  Vol 21, No.11, pp 2037-2052.