Historic mussel beds (Mytilus edulis) in the sedimentary record of a back-barrier tidal flat near Spiekeroog Island, southern North Sea

Mussel beds show irregular cycles of appearance, disappearance and reappearance at preferred distinct locations on intertidal flats. These cycles are documented in the depth profile by the presence of shell-rich sediments intercalated with sandy layers. Such mussel bed layers were regularly found over the past 12 years on the Swinnplate, a back-barrier tidal flat south of Spiekeroog Island, southern North Sea. They are characterised by the occurrence of sub-articulated pairs of Mytilus shells. Based on life span considerations, a period of at least 35–40 years over which mussel beds may have repeatedly established themselves is suggested. A survey in spring 2000 revealed a reduced occurrence of old, embedded beds on the Swinnplate. The present results, based on core profiles from 1992 to 2000 and amino acid age determination, show that alternations of shell layers indicative of former Mytilus beds and sediment layers provide insight into the historic development of tidal flat environments. The frequent occurrence of sub-articulated valve pairs in the shell layers documents that the embedding of the mussel beds took place soon after the death of the organisms without prior physical disturbance. Successions of historic Mytilus beds provide evidence that mussel beds are a regular, but not necessarily permanent, faunal feature of back-barrier tidal flats of the North Sea. Electronic Publication     

Predation on barnacles of intertidal and subtidal mussel beds in the Wadden Sea

Balanids are the numerically dominant epibionts on mussel beds in the Wadden Sea. Near the island of Sylt (German Bight, North Sea), Semibalanus balanoides dominated intertidally and Balanus crenatus subtidally. Field experiments were conducted to test the effects of predation on the density of barnacle recruits. Subtidally, predator exclusion resulted in significantly increased abundances of B. crenatus, while predator exclusion had no significant effects on the density of S. balanoides intertidally. It is suggested that recruitment of B. crenatus to subtidal mussel beds is strongly affected by adult shore crabs (Carcinus maenas) and juvenile starfish (Asterias rubens), whereas recruits of S. balanoides in the intertidal zone are mainly influenced by grazing and bulldozing of the very abundant periwinkle Littorina littorea, which is rare subtidally. Thus, not only do the barnacle species differ between intertidal and subtidal mussel beds, but the biotic control factors do so as well. Electronic Publication     

Selective settlement of the barnacle Semibalanus balanoides (L.) facilitates its growth and reproduction on mussel beds in the Wadden Sea

On the unstable sedimentary tidal flats of the Wadden Sea, a suitable attachment substrate for sessile organisms is generally lacking. Epibenthic mussel beds (Mytilus edulis L.) provide the only and strongly limited settlement sites available for the barnacle, Semibalanus balanoides (L.). Field investigations showed that barnacles were non-randomly distributed within a mussel bed. They preferentially occurred near the siphonal apertures of living mussels but rarely grew on dead mussels or shell fragments. Field experiments revealed that this was due to selective settlement of barnacle cyprid larvae. Growth of barnacles was significantly higher upon living mussels than on empty mussel shells. Moreover, a higher reproductive output was obtained by individuals on living mussels which produced twice as many nauplii larvae than barnacles attached to empty shells. This study shows that selective settlement of S. balanoides cyprid larvae on living mussels is adaptive with respect to individual fitness. Received in revised form: 15 January 2001 Electronic Publication     

An analysis of mussel bed habitats in the Dutch Wadden Sea

A habitat suitability analysis for littoral mussel beds in the Dutch Wadden Sea was carried out. The analysis was based on the presence of mussel beds in the years 1960–1970, and a number of environmental characteristics: wave action, flow velocity, median grain size, emersion times and distance to a gully border. The habitat model describes mussel bed appearance quantitatively. It predicts the distribution of mussel beds quite well, as well as the distribution of spatfall in the years 1994 and 1996. From the analysis we found that wave action (maximum orbital velocity) was the main structuring factor. A low orbital velocity was preferred. Neither very low, nor maximum flow velocities were favourable for mussel beds. Very coarse sands or silty environments were not preferred. Sites close to the low water line showed lower mussel bed appearance; when emersion time was above 50% , hardly any mussel beds could be found. The habitat suitability analysis and the construction of a habitat suitability map was performed in the framework of the discussions on a further or reduced exploitation of the tidal flats in the Dutch Wadden Sea by cockle and mussel fishery activities. Electronic Publication     

Documentation of sites of intertidal blue mussel (Mytilus edulis L.) beds of the Lower Saxonian Wadden Sea, southern North Sea (as of 2003) and the role of their structure for spatfall settlement

Field surveys (dating back to 1950) and aerial photograph series (dating back to 1966) were evaluated to determine sites of intertidal blue mussel (Mytilus edulis) beds at the Wadden Sea coast of Lower Saxony. Maps were prepared indicating sites of blue mussel beds during the last decades. A table gives additional information on the presence (or absence) of blue mussel beds at each site at the time of large-scale surveys. Altogether 187 sites of M. edulis beds were recorded in the investigation area. In spring 1996, there were still only 19 sites where mussel beds still occurred, although at 51 sites residual mussel-bed structures were present, e.g. shell bases of former beds or protruding patches (which had been occupied by M. edulis before the beds vanished) and open spaces. At that time, the majority of the sites contained neither mussel beds nor mussel-bed structures. The analysis of recent data confirmed that mussel larvae have preferred to settle in sites of present mussel beds and sites with bases of former mussel beds. There was no preferential selection of one of these categories (settled beds vs. shell bases). On the other hand, the presence of mussel beds or mussel bed structures is not obligatory for settlement, because sites without those structures were also re-settled by the spatfall in 1996, even though on a smaller scale.     

Age-dependent zonation of the periwinkle Littorina littorea (L.) in the Wadden Sea

On sedimentary tidal flats near the island of Sylt (German Bight, North Sea) abundance and size distribution of periwinkles, Littorina littorea L., were studied in low intertidal and in shallow and deep subtidal mussel beds (Mytilus edulis L.). In low intertidal mussel beds, surveys revealed that high densities (1,369±571 m^–2) of juvenile snails (≤13 mm) were positively correlated with strong barnacle epigrowth (Semibalanus balanoides L. and Balanus crenatus Bruguière) on mussels. A subsequent field experiment showed that recruitment of L. littorea was restricted to the intertidal zone. Abundances of periwinkles (213±114 m^–2) and barnacles abruptly decreased in the adjacent shallow subtidal zone, which served as a habitat for older snails (>13 mm). L. littorea was completely absent from disjunct deep (5 m) subtidal mussel beds. Snail abundance varied seasonally with maxima of >4,000 m^–2 in low intertidal mussel beds in October and minima in July, just before the onset of new recruitment. I suggest that the presence of cracks and crevices among the dense barnacle overgrowth in intertidal mussel beds favoured recruitment and survival of juvenile snails. Larger (older) specimens are assumed to actively migrate to the less favourable adjacent subtidal. Therefore, intertidal mussel beds are considered as nurseries for the population of L. littorea in the Wadden Sea. Received in revised form: 25 September 2000 Electronic Publication     

On the dynamics of the stocks of blue mussels (Mytilus edulis L.) in the Danish Wadden Sea

As biological basis for the monitoring programme for the commercially exploited stock(s) of mussels (Mytilus edulis L.) in the Danish Wadden Sea, samples of mussels have been collected regularly since 1986, both from sub-tidal and inter-tidal mussel beds. These samples are the basis for the estimation of total biomass. They also provide data on size frequency distributions, which have been analysed for cohort identification resulting in length at age data, which again have been used for estimating parameters (L and K) for the von Bertalanffy growth equation (VBGE) as well as mortality parameters. By applying these in the Beverton & Holt model, estimates of average biomass and annual production (P) of the mussels have been obtained together with possible fisheries yields from the beds. The growth and mortality parameters and the figures for annual production and P/B are compared with figures from other investigations. These analyses have been the basis for annual assessments of the mussel stocks, which again are used in the current management of mussel fishery in the Danish Wadden Sea.     

Large-Scale Spatial Dynamics of Intertidal Mussel (Mytilus edulis L.) Bed Coverage in the German and Dutch Wadden Sea

Intertidal blue mussel beds are important for the functioning and community composition of coastal ecosystems. Modeling spatial dynamics of intertidal mussel beds is complicated because suitable habitat is spatially heterogeneously distributed and recruitment and loss are hard to predict. To get insight into the main determinants of dispersion, growth and loss of intertidal mussel beds, we analyzed spatial distributions and growth patterns in the German and Dutch Wadden Sea. We considered yearly distributions of adult intertidal mussel beds from 36 connected tidal basins between 1999 and 2010 and for the period 1968–1976. We found that in both periods the highest coverage of tidal flats by mussel beds occurs in the sheltered basins in the southern Wadden Sea. We used a stochastic growth model to investigate the effects of density dependence, winter temperature and storminess on changes in mussel bed coverage between 1999 and 2010. In contrast to expectation, we found no evidence that cold winters consistently induced events of synchronous population growth, nor did we find strong evidence for increased removal of adult mussel beds after stormy winter seasons. However, we did find synchronic growth within groups of proximate tidal basins and that synchrony between distant groups is mainly low or negative. Because the boundaries between synchronic groups are located near river mouths and in areas lacking suitable mussel bed habitat, we suggest that the metapopulation is under the control of larval dispersal conditions. Our study demonstrates the importance of moving from simple habitat suitability models to models that incorporate metapopulation processes to understand spatial dynamics of mussel beds. The spatio-dynamic structure revealed in this paper will be instrumental for that purpose.     

Seasonal variations in the anaerobic metabolism of the mussel Mytilus edulis (L.)

• 1.1. The incorporation of atmospheric ¹⁴CO₂ into the ethanol soluble metabolites of the tissues of the mussel, Mytilus edulis, exposed to the atmosphere, was measured monthly between July 1976 and June 1977.
• 2.2. The total incorporation of ¹⁴CO₂ was found to be greater in the summer than in the winter and these seasonal effects were not temperature-dependent.
• 3.3. The incorporation of ¹⁴CO₂ into the individual metabolites is discussed in relation to possible seasonal variations in the operation of the anaerobic succinate pathway and seasonal variations of tissue glycogen concentrations.

     

Direct and indirect effects of Littorina littorea (L.) on barnacles growing on mussel beds in the Wadden Sea

On the extensive sedimentary tidal flats of the Wadden Sea, beds of the blue mussel Mytilus edulis represent the only major hard substratum and attachment surface for sessile organisms. On this substratum, the barnacle Semibalanus balanoides is the most frequent epibiont. In summer 1998, it occurred on over 90% of the large mussels (>45 mm shell length) and the dry weight of barnacles reached 65% of mussel dry weight. However, the extent of barnacle overgrowth is not constant and differs widely between years. Periwinkles (Littorina littorea) may reach densities >2000 m^–2 on intertidal mussel beds. Field experiments were conducted to test the effect of periwinkle grazing on barnacle densities. An experimental reduction of grazing and bulldozing pressure by periwinkles resulted in increased recruitment of barnacles, while barnacle numbers decreased with increasing snail density. The highest numbers of barnacles survived in the absence of L. littorea. However, a lack of periwinkle grazing activity also facilitated settlement of ephemeral algae which settled later in the year. Field experiments showed that the growth rate of barnacles decreased in the presence of these ephemeral algae. Thus, L. littorea may reduce initial barnacle settlement, but later may indirectly increase barnacle growth rate by reducing ephemeral algae. It is suggested that periwinkle density may be a key factor in the population dynamics of S. balanoides on intertidal mussel beds in the Wadden Sea.     

Secondary production of an intertidal mussel (Mytilus edulis L.) population in the Eastern Scheldt (S.W. Netherlands)

We monitored an intertidal mussel (Mytilus edulis L.) population between June 1981 and June 1982 in the Eastern Scheldt estuary (S.W. Netherlands). Density and biomass of the population remained relatively constant over the study period. The shell length growth was described by a Gompertz growth curve. The parameters of this equation were estimated from a log-log-modified Ford-Walford plot of the growth-ring data. The slope of the relationship between animal weight and shell length is season-dependent, mainly due to the spawning cycle in larger mussels.Secondary production is estimated with the growth rate method. In the calculated growth rates the change in slope of the length-weight relationship is incorporated, as well as differences in length growth rates between summer and winter. Secondary production amounts to 156 g AFDW m^–2a ^–1 (expressed per m² of mussel bank). P:B is 0.50 a^–1. The mussel productivity is probably a limiting factor for the density of overwintering Oystercatchers (Haematopus ostralegus).     

Age-dependence of metabolism in mussels Mytilus edulis (L.) from the White Sea

Age structure, natural mortality and growth, as well as age- and size-dependent changes in parameters of energy metabolism were studied in blue mussels Mytilus edulis (L.) from the White Sea. Mussels were sampled in August (Summer sample, SS) and October (Autumn sample, AS) and contained animals of three size groups, 2-9 years old. Field data showed an increase of mortality of mussels and strong decrease in growth rates after 6 years of age. Absolute tissue growth increment (AI) reconstructed from winter growth marks on the shells decreased with age and was strongly size-dependent, while relative tissue growth increment (RI) did not depend on size of the animals. Respiration rates and citrate synthase activity demonstrated power regression versus tissue weight with regression coefficients -0.231 and -0.170, respectively. After weight correction both parameters showed a decrease with increasing age. ATP and phosphagen levels also showed a pronounced decrease in animals older than 5-6 years despite considerable differences in the absolute values of both parameters in SS and AS. pH(i) in mussels was also age-dependent and decreased with increasing age after 5 years. In air exposed mussels, pH(i) was reduced only at young age such that pH(i) was low and constant within the whole age range. Our data give evidence that aerobic metabolic rate in M. edulis from the studied population declines when animals reach an age of about 6 years. The decrease in oxygen consumption reflects the drop in mitochondrial respiration, which is mirrored by the decrease in CS activity. A concomitant fall in ATP turnover may include a downregulation of the mechanisms of acid-base regulation. pH(i) will then approach equilibrium indicated by lower pH(i) values in older animals. Our data suggest that intrapopulational comparisons of physiological parameters in mussels should take into account age composition of compared samples.     

Effects of ageing on nuclear DNA integrity and metabolism in mussel cells (Mytilus edulis L.)

As the age of a cell increases, so does the potential for DNA damage. Recent theories on ageing suggest accumulative DNA damage is the primary cause of cellular senescence, possibly due to the decreased ability of DNA to act as a template for gene expression.In this paper we investigate the effects of ageing on the level of nuclear DNA damage in tissues of wild mussels of three different age groups; 2–4 years (group I), 6–8 years (group II) and 10 years (group III). In the digestive gland and haemolymph cells, a significant age-dependent increase of DNA damage was observed, as evaluated by the fluorimetric alkaline DNA unwinding technique, which is able to detect both direct single strand DNA breaks as well as alkali-labile apurinic sites.In addition, the rate of DNA polymerase activity was studied in order to determine whether DNA damage was dependent on DNA alteration, or because of a reduced rate of DNA repair. Unscheduled DNA repair synthesis in isolated nuclei of digestive gland cells in older mussels, was significantly decreased in comparison to younger mussels (−42% in group II and −37% in group III, p<0.01). In the digestive gland, salt extraction gives a slight, but significant, decrease of aphidicolin-sensitive DNA polymerase activity in age group III of −25%, p<0.05.Finally, we looked at the age variation in relation to oxidative stress. This was evaluated by measuring malondialdehyde accumulation in mussel cells. Digestive gland cells of group III, showed a significant age-related increase in malondialdehyde content of 170%, p<0.01, indicative of enhanced peroxidative processes.Taken together, these data suggest that the accumulation of DNA damage in group II is mainly dependent on the impairment of DNA repair systems. This is contrary to group III DNA damage, where a possible relationship between oxidative stress and alteration of nuclear DNA metabolism is found, probably deriving from an antioxidant defence decline.     

Organ specific changes in energy metabolism due to anaerobiosis in the sea mussel Mytilus edulis (L.)

• 1.1. Anaerobic energy metabolism was investigated in different organs of Mytilus edulis and the whole animal.
• 2.2. Succinate accumulates to high levels in most organs but remains low in the hemolymph.
• 3.3. After 16 hours propionate accumulation is observed in all organs. Experimental evidence is not sufficient yet to point out organs that produce more propionate than others.
• 4.4. Acetate is a minor end product.
• 5.5. Acetate and propionate are found in the hemolymph in amounts equal to those in the organs.
• 6.6. Animals incubated in oxygen-free seawater accumulate more end products than animals exposed to air, in the form of volatile fatty acids that are excreted into the incubation water.
• 7.7. Alanine and glutamine increase in the posterior adductor muscle. Aspartate decreases in the total animal, posterior adductor muscle and gills, while in the hemolymph decrease in alanine, asparagine, serine, threonine and proline are observed.

     

Subtidal populations of the blue mussel <Emphasis Type="Italic">Mytilus edulis</Emphasis> as key determinants of waterfowl flocks in the southeastern Barents Sea

Ornithological surveys conducted over the Pechora Sea (the southeastern part of the Barents Sea) in the 1990 s revealed huge non-nesting flocks of marine ducks, the largest in the European North. Especially dense waterfowl aggregations are constantly observed at the shallows near Dolgij Island during molting period and migration to wintering places. All the marine ducks flocking there are specialized benthos feeders predominantly consuming mussels Mytilus edulis. At the same time, numerous previous benthic studies in the Pechora Sea did not reveal mussels near Dolgij Island where benthic biomass was somewhat lower than in the adjacent areas (Denisenko in Mar Ecol Prog Ser 258:109–123. 2003) which left the food source for these abundant bird flocks enigmatic. In the course of an expedition in summer 2007 we found subtidal populations of M. edulis in shallows to the southwest of Dolgij Island. These populations were confined to a coastal zone and were characterized by a highly disjunct distribution with the biomass reaching up to 4 kg m⁻². We describe these subtidal populations as well as an intertidal mussel population on the western shore of Dolgij Island.     

Population differences of aspartate aminotransferase and peptidase in the bay mussel Mytilus edulis

This investigation has demonstrated considerable heterogeneity among populations and some heterogeneity within populations in the distribution of alleles at two variant loci of Mytilus edulis. Although the causes of this variation remain obscure, some speculations have been made on the basis of available data. A cline for aspartate aminotransferase (AAT) alleles has been observed on the Pacific Coast. An immigration model has been proposed to explain the atypical ecological and genetic characteristics of large mussels found on Amchitka Island, Alaska. Marked differences were found in the distribution of peptidase alleles among collections from Southern California, the North Pacific Ocean, and New Jersey. Deviations from random distribution of phenotypes observed in comparisons made between large and small mussels from the New Jersey collection may reflect selection operating on these loci in this population.     

Comparative study of predatory responses in blue mussels (Mytilus edulis L.) produced in suspended long line cultures or collected from natural bottom mussel beds

Blue mussels (Mytilus edulis L.) are a valuable resource for commercial shellfish production and may also have uses as a tool in habitat improvement, because mussel beds can increase habitat diversity and complexity. A prerequisite for both commercial mussel production and habitat improvement is the availability of seed mussels collected with minimum impact on the benthic ecosystem. To examine whether mussels collected in suspended cultures can be used for bottom culture production and as tool in habitat improvement, the differences in predatory defence responses between suspended and bottom mussels exposed to the predatory shore crab (Carcinus maenas L.) were tested in laboratory experiments and in the field. Predatory defence responses (byssal attachment and aggregation) and morphological traits were tested in laboratory, while growth and mortality were examined in field experiments. Suspended mussels had an active response in relation to the predator by developing a significantly firmer attachment to the substrate and a closer aggregated structure. Bottom mussels had a passive strategy by having a thicker shell and larger relative size of the adductor muscle. In a field experiment mussels originated from suspended cultures had a higher length increment and lower mortality when compared to bottom mussels. It is concluded that suspended mussels potentially are an alternative resource to bottom culture and can be used in habitat improvement of mussel beds, but that the use of suspended mussels has to be tested further in large-scale field experiments.     

Population differences of aspartate aminotransferase and peptidase in the bay mussel Mytilus edulis.

This investigation has demonstrated considerable heterogeneity among populations and some heterogeneity within populations in the distribution of alleles at two variant loci of Mytilus edulis. Although the causes of this variation remain obscure, some speculations have been made on the basis of available data. A cline for aspartate aminotransferase (AAT) alleles has been observed on the Pacific Coast. An immigration model has been proposed to explain the atypical ecological and genetic characteristics of large mussels found on Amchitka Island, Alaska. Marked differences were found in the distribution of peptidase alleles among collections from Southern California, the North Pacific Ocean, and New Jersey. Deviations from random distribution of phenotypes observed in comparisons made between large and small mussels from the New Jersey collection may reflect selection operating on these loci in this population.