1. food and drink

MarBioShell internal report series 1: 1-15
Density of mussel larvae (Mytilus edulis) in Danish waters
Kim Lundgreen & Hans Ulrik Riisgård
Marine Biological Research Centre (University of Southern Denmark), Hindsholmvej 11, 5300
Kerteminde, Denmark
Abstract: Here we describe the seasonal variation in density of blue mussel larvae in Danish waters
as related to environmental factors (temperature, salinity, phytoplankton biomass) which is basic
information of relevance for future line-mussel farming. The mussel larval density measured by the
environmental authorities in Skive Fjord during a period of 20 years, from 1989 to 2009, along with
corresponding temperatures and chlorophyll a, makes up the most important series of data reported
here. In most years, a pronounced spring density peak and a subsequently lower autumn peak can be
seen in Skive Fjord, but most conspicuous in the period 1993 to 2002. Further, own data on mussel
larval densities recorded in Kerteminde Bay (2008-2011), Musholm Bay, Svendborg Sund, and
Horsens Fjord (2008) are presented. The maximum spring densities in Danish waters are found in
Limfjorden, typically in May, whereas much lower peak densities are found at the other locations
studied, and apparently later in June-July in Kerteminde Bay (Great Belt).
1.0 Introduction
The common mussel, Mytilus edulis L. may reproduce at any time of the year, but in the
temperate zone spawning is most frequent during spring and summer (Seed 1976).
Temperature seems to be the most important factor that determines the annual reproductive
cycle, and a cold winter tend to synchronize spawning the following spring (Savage 1956,
Dare 1976, Jørgensen 1981). The fertilized egg develops within 24 h to a trochophora larva
that soon after secrete the first larval shell to become a free swimming D-shaped veliger larva
(100 to 120 µm shell length) feeding on small 2 to 4 µm particles (Bayne 1965, Riisgård et al.
1980, Jespersen & Olsen 1982, Sprung 1984, Goslin 2003). This stage lasts between 3 and 5
weeks (Thorsen 1961), i.e. time to reach ~300 µm, depending on environmental factors
(temperature, food ration, salinity) until metamorphosis when an extensible foot appears and
the larva becomes a pediveliger. The pediveliger larva seeks out different types of substrates
in preparation for settlement which involves a sequence of swimming and crawling behavior
that culminates in attachment once a suitable substrate is chosen. After settlement,
morphological change (metamorphosis) results in a young post-metamorphic mussel (Bayne
1965, Goslin 2003). Information on density and dispersal of mussel larvae is crucial for
understand recruitment to the adult stock.
The aim of the present study was to describe the seasonal variation in density of blue
mussel larvae in Danish waters as related to environmental factors (temperature, salinity,
phytoplankton biomass) in order to obtain basic information of relevance for future linemussel farming.
2. Materials and methods
2.1. Mussel larval density in Skive Fjord (1989-2009)
1
Data on mussel larval density in Skive Fjord during the period 1989 to 2009 were obtained
from the Danish national monitoring program, Danish Nature Agency, Danish Ministry of
Environment. According to the official monitoring guidelines (Miljøcenter Ringkøbing
2008), 20 l of water were sampled by means of a submersible pump (150 l min-1) resulting in
equally sized subsamples taken through the water column from bottom to surface by hauling
the pump up with a speed of 0.5 m s-1. The sampled water was filtered (60 µm mesh) to
obtain a representative sample of the mussel larvae. Samples were preserved in basic Lugol’s
solution and examined in the laboratory by Orbicon A/S in order to determine the
concentration of mussel veliger larvae (and other zooplankton organisms).
2.2 Mussel larval density in Great Belt region and Horsens Fjord (2008-2011)
Water samples were collected at the 4 locations in 2008: Musholm Bay, Svendborg Sund,
Kerteminde Bay, and Horsens Fjord and subsequently in Kerteminde Bay in 2009-2011 (Fig.
1) by using a small conical zooplankton net (Ø24 cm, KC Denmark) with a mesh size of 80
µm and a 80 ml collection cylinder. The plankton net was dropped to a known depth and then
slowly hauled to the surface. The sample was then transferred a 100 ml bottle containing
Lugol to conserve the mussel larvae. Before counting in the laboratory using a reversed
microscope (Leitz Labovert FS), the sample bottle was turned upside down several times to
ensure an even distribution of the mussel larvae. The total number of counted larvae was
expressed as individuals per liter of filtrated water. The filtrated water volume (V) was
calculated from the circular area of the plankton net (A = 0.045 m2) and the length (L) of the
vertical haul through the water column (V = A×L) which was dependent on the depth at the
sampling location (2 to 6 m).
2.3. Density of settled mussels in Great Belt (2011). Mussel-rope bands (5 m long by 5 cm
wide, hereafter 'mussel-ropes') were set out in May 2011 at the MarBioShell Research and
Demonstration Mussel Farm in Kerteminde Bay, Denmark, and subsequently collected at
different intervals between 14 June and 9 December 2011. The densest part of the top end
(1.5 m) of the farm-rope was located and a suitable piece of the mussel-rope was cut off for
determination of shell length of settled mussels. Mussels on the rope were scraped off and
counted along with the individuals still attached to the farm-rope by using a stereo
microscope. Subsequently, shell length was determined for 3 groups; 1: a random sample of
all mussels, 2: a sample of the 20% biggest mussels in the random sample, and 3: a sample of
the subjectively selected 10 biggest individuals on each mussel-rope.
2.4. Environmental data
Data on chlorophyll a, salinity and temperature in the Great Belt region and Skive Fjord were
obtained from the Danish national monitoring program, Danish Nature Agency, Danish
Ministry of Environment. Temperature and salinity were measured with a CTD at heights
spaced 0.2 m apart, from 0.8 m below the surface down to about 0.3 m above the bottom.
Chlorophyll a (chl a) was determined from water samples taken from a depth of 1 m and
subsequently analyzed at an authorized laboratory according to Danish national standards.
2.3. Size distribution of mussel larvae
The shell length of the 30 first mussel larvae seen in the samples collected in 2008 from
Horsens Fjord, Musholm Bay and Svenborg Sund were measured to determine the size
distribution. The larvae were divided into 3 size groups: 80 to 125, 126 to 200, and 201 to
300 µm.
2.5. Statistical analysis
2
One-way repeated measures analysis of variance (ANOVA) and Pearson product moment
correlation (SigmaPlot, version 12) were used to evaluate seasonal larval densities and
temperature data, respectively. In all cases a significance level of α = 0.05 was used.
3. Results
3.1 Larval densities
3.1.1. Skive Fjord in the period 1989 to 2009. The mussel larval density measured in Skive
Fjord during a period of 20 years, from January 1989 to January 2009, along with
corresponding temperatures and phytoplankton biomass expressed as chlorophyll a are shown
in Figs. 2&3. In most years, a pronounced spring density peak and a subsequently lower
autumn peak can be seen. This trend, however, seems to be most conspicuous in the period
1993 to 2002, and therefore the mean maximum spring and mean autumn mussel larval
densities for this period have been compared to previous (1989 to1992) and subsequent (2003
to 2009) periods in Table 2. The few scattered data for the period 1989 to 1992 are not
adequate for statistical tests, but statistical analysis showed that the spring density of mussel
larvae in the period 1993 to 2002 was significantly different (P < 0.05) from the subsequent
period 2003 to 2009 (F(1,15) = 7.528, P = 0.041). However, the difference in autumn larval
density in the same two periods was not significantly different (F(1,15) = 0.00249, P = 0.962).
3.1.2. Kerteminde Bay in 2008 to 2011. The densities of mussel larvae recorded in
Kerteminde Bay in the period 2008 to 2011 are shown in Fig. 4, and the date of maximum
spring density of 3.4 to 4.6 ind. l-1 appears from Table 3.
3.1.3. Horsens Fjord, Musholm Bay, Svendborg Sund in 2008. The larval density recorded in
Musholm Bay, Svendborg Sund and Horsens Fjord in 2008 are shown in Fig. 5 along with
salinity, temperature and chlorophyll a, and mean values of the latter in the sampling period
are shown in Table 4.
3.2. Larval size distribution. Figure 6 shows the size distribution of mussel larvae, presented
as the percentage of a size group in relation to the total count of 30 individuals, in 2008 in
Musholm Bay, Svendborg Sund and Horsens Fjord.
3.3. Density of settling mussels. Figure 7 shows the growth of settled larvae from settling
which occurred around 14 June 2011 until December where the fastest growing mussels had
reached a length of about 38 mm.
3.4. Temperatures. The mean annual temperatures from 1982 to 2009 (Fig. 8) varied between
9.5 to 13 °C. Statistical test showed that there has been no significant increase in the annual
mean temperatures in Skive Fjord (P = 0.452).
4. Discussion
It is well know that mussel larval densities may fluctuate considerably on both relatively
small spatial (tens of metres) and short temporal scales (days) depending on the hydrography
(e.g. Larsen et al. 2007). Nevertheless, only few and scattered data have up to now been
reported from Danish waters. Larsen et al. (2007) investigated the temporal patterns of
3
bivalve larvae in the Isefjord, Denmark, and found that the total concentration of bivalve
larvae peaked in late June/ early July, with one plankton sample having a much higher
concentration than the other samples with a density of 188 ind. l-1. A small autumn peak with
7.8 ind. l-1 was observed in late August, and between these peaks the density ranged between
1.4 and 6.4 ind. l-1. In an earlier study in the Isefjord, Jørgensen (1981) followed a cohort of
bivalve larvae, mainly (about 90 %) Mytilus edulis, during its residence in the plankton.The
initial mean density of 90 µm shell length larvae was 3150 ind. l-1 in late May, but this very
high density must be considered as "an extreme peak situation in boreal neretic waters" (Fotel
et al. 1999), and has not been report ever since. The abundance of mussel veliger larvae has
also been studied in the embayment of Knebel Vig, Denmark, by Fotel et al. (1999) who
found that the larval density varied from 1.7 to 40.4 ind. l-1 in 1994 and 2.7 to 99.0 ind. l-1 in
1995 which may be compared to the present data for larval densities in Limfjorden (Fig. 2,
Table 2), Kerteminde Bay (Fig. 4, Table 3), and Musholm Bay, Svendborg Sund and Horsens
Fjord (Fig. 5, Table 4). A pronounced spring peak is typical for all locations. The maximum
spring densities in Danish waters are found in Limfjorden, typically in May, whereas much
lower peak densities are found at the other locations studied, and apparently later in June-July
in Kerteminde Bay (Great Belt).
The significant decrease in veliger larval density in Limfjorden after 2002 is not obvious.
Statistical analysis showed that the annual mean temperatures in Limfjorden (Skive Fjord)
ddid not change in the period 1982 to 2010 (Fig. 8), but the decrease in larval density may
perhaps be correlated with overfishing (dredging) and reduction of the mussel stock in
Limfjorden (Dolmer & Frandsen 2002).
Acknowledgements. This work formed part of the MarBioShell project supported by the
Danish Agency for Science, Technology and Innovation for the period January 2008 to
December 2012. Thanks are due to Sandrine Serre, Coralie Barth-Jensen, and Caroline
Vandt-Madsen for practical assistance, to the Danish Nature Agency, Danish Ministry of the
Environment, for providing zooplankton and hydrographical data and Daniel Pleissner for
statistical analysis assistance.
References
Bayne, B.L. (1965). Growth and the delay of metamorphosis of the larvae of Mytilus edulis.
Ophelia 2:1-47.
Dare, P.J. (1976). Settlement, growth and production of the mussel, Mytilus edulis L., in
Morecambe Bay, England. Fisheries Investigations Series II, 28: 1-25.
Dolmer, P., Frandsen, R.P. (2002). Evaluation of the Danish mussel fishery: suggestions for
an ecosystem management approach. Helgol. Mar. Res. 56: 13-20.
Fotel, F.L., Jensen, N.J., Wittrup, L., Hansen B.W. (1999). In situ and laboratory growth by a
population of blue mussel larvae (Mytilus edulis L.) from a Danish embayment, Knebel Vig.
Journal of Experimental Marine Biology and Ecology 233: 213-230.
Gosling, E. (2003). Bivalve molluscs. Biology, ecology and culture. Fishing News Books,
Blackwell, p. 443.
4
Jespersen, H., Olsen, K. (1982). Bioenergetics in veliger larvae of Mytilus edulis L. Ophelia
21:101-113.
Jørgensen, C.B. (1981). Mortality, growth, and grazing impact of a cohort of bivalve larvae,
Mytilus edulis L. Ophelia 20: 185-192.
Larsen, J.B., Frischer, M.E., Ockelmann, K.W., Rasmussen, L.J., Hansen B.W. (2007).
Temporal occurrence of planktonic bivalve larvae identified morphologically and by single
step nested multiplex PCR. Journal of Plankton Research 29: 423-436.
Riisgård, H.U., Randløv, A., Kristensen, P.S. (1980). Rates of water processing, oxygen
consumption and efficiency of particle retention in veligers and young post-metamorphic
Mytilus edulis. Ophelia19:37-47.
Savage, R.E. (1956). The great spatfall of mussels in the River Conway Estuary in Spring
1940. Fisheries Investigations Series II, 20: 1-21.
Seed, R. (1976). Ecology. In: Bayne, B.L. (ed.): Marine mussels: Their ecology and
physiology, pp. 13-65. Cambridge University Press, Cambridge.
Sprung , M. (1984). Physiological energetics of mussel larvae (Mytilus edulis). I. Shell
growth and biomass. Mar. Ecol. Prog. Ser. 17:283-293.
Thorson, G. (1961). Length of pelagic larval life in marine bottom invertebrates as related to
larval transport by ocean currents. Oceanography pp. 455-474.
5
Fig. 1. Map of Denmark showing Mytilus edulis larval sampling sites: (1) Skive Fjord, (2)
Musholm Bay, (3) Svendborg Sund, (4) Kerteminde Bay, (5) Horsens Fjord.
6
(93)
75
T
65
T (°C), chl a (µg l-1 )
(200, 120, 130)
C
55
45
35
25
15
5
L (ind. l-1 )
-5
200
180
160
140
120
100
80
60
40
20
0
(s,787)
s
(s,511)
(s,283)
s
s
s
s
a
a
a
s
s
a
a
a
a
a
Fig. 2. Mytilus edulis. Temperature (T, °C), chlorophyll a (C, µg l-1) (upper graph) and mussel larval density (L, ind. l-1) (lower graph) in Skive
Fjord in the period 1989 to 1999. Spring (s) and autumn (a) peaks are indicated. Values exceeding 75 µg chl a l-1 and 200 ind. l-1 are shown in
brackets. Mean ± S.D. salinity in the period was 24.3 ± 2.0 psu.
7
(110)
75
T
T (°C), chl a (µg l-1 )
65
C
55
45
35
25
15
5
-5
(s, 712)
150
L (ind. l-1 )
120
(s, 267)
s
s
a
a
s
90
s
60
30
a
s
a
s
a
a
s
a
0
Fig. 3. Mytilus edulis. Temperature (T, °C), chlorophyll a (C, µg l-1) (upper graph) and mussel larval density (L, ind. l-1) (lower graph) in Skive
Fjord in the period 2000 to 2009. Spring (s) and autumn (a) peaks are indicated. Values exceeding 75 µg chl a l-1 and 150 ind. l-1 are shown in
brackets. Mean ± S.D. salinity in the period was 23.1 ± 2.1 psu.
8
T (°C), S (psu), chl a (µg l-1 )
28
26
24
22
20
18
16
14
12
10
8
6
4
2
0
-2
9
S
T
C
8
7
L (ind. l-1 )
6
5
4
3
2
1
0
Fig. 4. Mytilus edulis. Salinity (S, psu), temperature (T, °C), chl a (C, µg l-1), and mussel
larval density (L, ind. l-1) in Kerteminde Bay in 2008 to 2011.
9
S
T
C
L
25
25
20
20
15
10
0
0
25
T
C
L
T
C
L
(B)
10
5
S
S
15
5
30
S, T, C, L
30
(A)
S, T, C, L
S, T, C, L
30
(C)
20
15
10
5
0
Fig. 5. Mytilus edulis. Salinity (S, psu), temperature (T,°C) , chlorophyll a (C, µg l-1), and mussel larval density (L, ind. l-1) in 2008 in (A)
Musholm Bay, (B) Svendborg Sound, (C) Horsens Fjord.
10
100
80-125
126-200
201-300
(A)
80-125
126-200
201-300
(B)
80-125
126-200
201-300
(C)
% distribution
80
60
40
20
0
100
% distribution
80
60
40
20
0
100
% distribution
80
60
40
20
0
Fig. 6. Mytilus edulis. Size distribution of mussel larvae in 2008 at 3 locations: (A) Musholm
Bay, (B) Svendborg Sund, (C) Horsens Fjord. No samples were obtained from Horsens Fjord 29
May and 6 June, and from Musholm Bay 29 May.
11
300
Density (ind. cm-1 )
250
200
150
100
50
0
Fig. 7. Mytilus edulis. Density of young mussels after settling of was observed 10 June 2011
larvae on farm ropes at Risinge Hoved, Kerteminde.
12
25
Temperature (°C)
20
15
10
5
0
Fig. 8. Annual mean (±S.D.) temperatures in Skive Fjord in the period 1982 to 2009. Statistical
analysis showed that there was no significant increase in temperature over time.
13
Table 1. Mytlilus edulis. Maximum mussel larval density (L, ind. l-1), water temperature (T, °C),
salinity (S, psu) and chl a concentration (C, µg l-1) on collection dates in Skive Fjord in the
period 1989 to 2009. s/a = spring/autumn.
Year s/a
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
2004
2005
2006
2007
2008
2009
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
s
a
Date
10-Jul
23-May
1-Jun
7-Sep
4-Jun
23-Aug
16-May
5-Sep
27-Jun
4-Sep
13-May
9-Sep
20-May
15-Sep
11-May
28-Sep
21-Jun
11-Oct
10-May
7-May
17-Sep
8-Apr
9-Sep
26-May
1-Sep
27-Sep
17-May
12-Sep
22-May
25-Sep
7-May
23-Jul
9-Jun
6-Jul
6-May
10-Aug
Max. L
(ind. l-1)
22
8
66
11
193
13
145
19
78
7
787
21
511
75
283
77
98
44
117
712
48
267
112
53
100
26
18
22
79
18
114
76
65
14
11
6
T
(°C)
20.6
14.2
20.4
14.3
15.1
15.0
15.1
16.1
20.0
16.1
9.3
16.2
13.5
14.3
12.5
14.3
15.8
12.5
18.4
10.4
14.3
7.6
18.5
13.1
16.9
13.6
12.0
16.8
12.6
16.6
11.0
17.7
20.2
20.1
11.7
20.5
S
C
(psu) (µg l-1)
23.5
19
25.3
17.0
20.2
25.9
24.6
27.6
21.3
24.9
21.4
23.7
21.7
25.3
23.4
26.8
24.6
25.5
23.2
23.4
20.3
22.0
17.0
15.0
2.0
34.0
49.0
27.0
9.0
17.0
13.0
18.0
130.0
17.0
20.0
22.0
14.0
11.0
19.4
23.2
20.4
21.6
21.3
22.9
10.0
15.0
8.1
34.0
19.0
34.0
26.2
24.2
25.5
23.8
25.1
11.0
5.9
5.7
7.7
13.0
20.9
23.0
23.8
23.1
4.8
3.7
2.1
3.5
14
Table 2. Mytilus edulis. Mean ± S.D. maximum larval densities (ind. l-1) in Skive Fjord in spring
and autumn in 3 periods between 1989 and 2009 along with corresponding water temperatures
(°C).
Years
Season
Max. density
Temp.
Salinity
Chl a
(ind. l-1)
(°C)
(psu)
(µg l-1)
1989-1992
Spring
32.0±30.3
18.4±3.6
23.0±2.6
19.3±2.5
1993-2002
Spring
319.1±260.0
13.8±3.9
22.0±1.8
17.9±8.0
2003-2009
Spring
56.7±38.6
13.4±3.4
22.8±1.6
7.9±6.5
1989-1992
Autumn
11.0
14.3
25.9
17.0
1993-2002
Autumn
46.2±35.6
15.3±1.7
24.7±1.9
31.8±39.4
2003-2009
Autumn
37.5±35.8
16.9±1.7
24.3±1.5
11.8±11.5
Table 3. Mytlilus edulis. Maximum density of mussel larvae (peak) in Kerteminde Bay along
with corresponding temperature, salinity and chl a measured at the peak date in the years 2008 to
2011.
Year
Date
Max. density
Temp.
Salinity
Chl a
(ind. l-1)
(°C)
(psu)
(µg l-1)
16.6
1.7
2008
17-July
3.4
16.8
20.8
0.9
2009
22-June
4.6
14.1
13.6
2.4
2010
16-July
3.9
20.1
14.2
1.6
2011
18-May
7.9
10.9
Table 4. Mytlilus edulis. Maximum density of mussel larvae (peak) on 3 mussel larval sampling
locations in 2008 along with corresponding temperature, salinity and chl a measured at the peak
date.
Location
Date
Max. density
Temp.
Salinity
Chl a
(ind. l-1)
(°C)
(psu)
(µg l-1)
Musholm Bay
10-July
2.7
18.4
12.9
0.7
Svendborg Sund
17-June
12.4
15.3
15.8
3.2
Horsens Fjord
17-June
5.7
14.8
20.4
3.2
Table 5. Mytlilus edulis. Size distribution of 3 size classes of mussel larvae sampled in Musholm
Bay, Svendborg Sund, and Horsens Fjord (See Fig. 5).
Location
Size class
14
22
29
06
12
17
26
(µm)
May
May
May
June
June
June
June
Musholm Bay
80-125
63
7
3
93
30
80
126-200
17
70
33
7
20
7
201-300
20
23
63
0
50
13
Svendborg Sund 80-125
53
7
33
77
6
3
17
126-200
47
70
53
20
67
17
50
201-300
0
23
13
3
27
80
33
Horsens Fjord
80-125
77
17
97
30
27
126-200
23
83
0
50
43
201-300
0
0
3
20
30
15