Christian Buschbaum

Mytilid mussels: global habitat engineers in coastal sediments

Dense beds of mussels of the family Mytilidae occur worldwide on soft-bottoms in cold and warm temperate coastal waters and have usually been considered hot spots of biodiversity. We examined intertidal mussel beds at four distant locations around the globe with the same sampling method, to find out whether this “hot spot” designation holds universally. We studied species assemblages within the matrices of byssally interconnected mussels engineered by Mytilus edulis in the North Sea, by mixed Perumytilus purpuratus and Mytilus chilensis at the southern Chilean coast, by Musculista senhousia in the Yellow Sea and by Xenostrobusinconstans at the coast of southern Australia. In all cases, species assemblages inside mussel beds were significantly different from those outside with many species being restricted to one habitat type. However, species richness and diversity were not generally higher in mussel beds than in ambient sediments without mussels. In the North Sea (M. edulis) and at the Chilean coast (P. purpuratus, M. chilensis), mussel beds have markedly higher species numbers and diversities than surrounding sediments, but this was not the case for mussel beds in Australia (X. inconstans) and the Yellow Sea (M. senhousia) where numbers of associated species were only slightly higher and somewhat lower than in adjacent sediments, respectively. In conclusion, although soft bottom mytilid mussels generally enhance habitat heterogeneity and species diversity at the ecosystem level, mussel beds themselves are not universal centres of biodiversity, but the effects on associated species are site specific.

Growth of the mussel Mytilus edulis L. in the Wadden Sea affected by tidal emergence and barnacle epibionts

On the tidal flats in the Wadden Sea mussels Mytilus edulis form extensive epibenthic beds. Near the island of Sylt (German Bight, North Sea), these extend from mid intertidal to shallow subtidal zones and are often heavily overgrown by barnacles (Semibalanus balanoides and Balanus crenatus). The effects of tidal emergence and barnacle epibionts on growth in M. edulis were investigated by field experiments and surveys. The results showed that mussel cohort lengths increased from the mid intertidal (25‐35 mm shell length) over low intertidal (30‐45 mm) to the subtidal zone (45‐60 mm). In the low intertidal zone M. edulis were heavily overgrown by barnacles (2.2 ^ 0.4 g dry weight mussel 21 ), while balanid epigrowth was significantly lower subtidally (0.9 ^ 0.4 g mussel 21 ) as well as in the mid intertidal zone (0.7 ^ 0.3 g mussel 21 ). Cross-transplantation experiments of clean mussels (25‐35 mm length) showed that from July to September growth was about 3 mm in the intertidal zone and significantly increased to about 10 mm subtidally. Mussel origin had no influence upon growth. Further field experiments showed significantly lower growth in mussels with barnacles than without. Living and experimentally sealed barnacles did not differ in their effect, indicating that food competition was not responsible. This investigation suggests that fluctuations in the extent of barnacle epigrowth may modify growth patterns in M. edulis which are otherwise governed by tidal exposure. q 2001 Elsevier Science B.V. All rights reserved.

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.

Does climatic warming explain why an introduced barnacle finally takes over after a lag of more than 50 years

Invading alien species may have to await appropriate conditions before developing from a rare addition to the recipient community to a dominance over native species. Such a retarded invasion seems to have happened with the antipodean cirripede crustacean Austrominiusmodestus Darwin, formerly known as Elminius modestus, at its northern range in Europe due to climatic change. This barnacle was introduced to southern Britain almost seven decades ago, and from there spread north and south. At the island of Sylt in the North Sea, the first A. modestus were observed already in 1955 but this alien remained rare until recently, when in summer of 2007 it had overtaken the native barnacles Semibalanus balanoides and Balanus crenatus in abundance. At the sedimentary shores of Sylt, mollusc shells provide the main substrate for barnacles and highest abundances were attained on mixed oyster and mussel beds just above low tide level. A. modestus ranged from the upper intertidal down to the subtidal fringe. Its realized spatial niche was wider than that of the two natives. We suggest that at its current northern range in Europe a long series of mild winters and several warm summers in a row has led to an exponential population growth in A. modestus.

Effects of barnacle epibionts on the periwinkle Littorina littorea (L.)

Abstract In a sandy bay with mussel beds in the Wadden Sea (Island of Sylt, eastern North Sea), periwinkles Littorina littorea (L.) were often strongly overgrown with the barnacle Balanus crenatus Bruguière in the lower intertidal zone. Consequences of this epibiosis on mobility, reproduction and mortalityof the snail were examined. B. crenatus growing on L. littorea increased snail volume up to 4-fold and weight up to 3.5-fold and crawling speed of fouled L. littorea was significantly slowed down. The epibiotic structure also caused a decrease in reproductive output. In laboratory experiments, egg production of fouled L. littorea was significantly lower than in snails free of barnacles. Presumably, copulation of the periwinkles is hampered by the voluminous and prickly cover of barnacles. Field studies demonstrated an increased mortality of overgrown L. littorea. A decrease in reproductive output and a lower survival of snails with a cover of barnacles suggest that B. crenatus epibionts may have a significant impact on the population of L. littorea.

Mass occurrence of an introduced crustacean (Caprella cf. mutica) in the south-eastern North Sea

The caprellid amphipod Caprella mutica is indigenous to coastal waters of north-east Asia and was first recorded in European waters in 1995. We now detected mass occurrences of > 3000 individuals per m2 in harbours of the two islands of Sylt and Helgoland in the German Bight, North Sea. Currently, Caprella mutica seems to be restricted to artificial hard substrata but we expect it to become a new species in natural hard bottom assemblages of that region as well.

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

Abstract. 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.

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

Abstract. Balanids are the numerically dominant epibionts on mussel beds in the Wadden Sea. Near the island of Sylt (German Bight, North Sea), Semibalanusbalanoides dominated intertidally and Balanuscrenatus 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.

Sample pooling obscures diversity patterns in intertidal ciliate community composition and structure.

The aim of this study was to assess the effect of sample pooling on the portrayal of ciliate community structure and composition in intertidal sediment samples. Molecular ciliate community profiles were obtained from nine biological replicates distributed in three discrete sampling plots and from samples that were pooled prior to RNA extraction using terminal restriction fragment polymorphism (T-RFLP) analyses of SSU rRNA. Comparing the individual replicates of one sampling plot with each other, we found a differential variability among the individual biological replicates. T-RFLP profiles of pooled samples displayed a significantly different community composition compared with the cumulative individual biological replicate samples. We conclude that sample pooling obscures diversity patterns in ciliate and possibly also other microbial eukaryote studies. However, differences between pooled samples and replicates were less pronounced when community structure was analyzed. We found that the most abundant T-RFLP peaks were generally shared between biological replicates and pooled samples. Assuming that the most abundant taxa in an ecosystem under study are also the ones driving ecosystem processes, sample pooling may still be effective for the analyses of ecological key players.

Long-Term Ecological Change in the Northern Wadden Sea

The Wadden Sea is a shallow coastal region in the south eastern North Sea. Karl Mobius started ecological research in the northern Wadden Sea about 150 years ago studying the extensive oyster beds. With the foundation of a field station of the Biologische Anstalt Helgoland in List/Sylt in 1924 biological research in the Wadden Sea was continued to date. Several time series were initiated between the 1970s and the 1990s including a bi-weekly phytoplankton and zooplankton program and an observation program on macrobenthos. Three factors dominating the changes observed during the past decades are a rise in temperature, decreasing nutrients, and increasing invasions of non-native species. Phytoplankton blooms gradually decrease due to the combined effect of decreasing nutrient loads and increasing winter temperatures. Mean annual zooplankton abundance is stimulated by higher winter temperatures. Recently, invading species are increasingly dominating native mussel beds. For several invaders, a positive effect of temperature was shown. We expect that major pressures of change during the next years will be further species introductions, temperature increase, and reduced nutrient loads. On the long run (21st century), we expect sea level rise to be the key factor of coastal change through a loss of habitats with fine-grained sediments and intertidal sediments in general. A major challenge for coastal research will be to disentangle the interactive effects of these pressures on the long-term development of the Wadden Sea.