Recruit/algal interaction prevents recovery of overexploited mussel beds: Indirect evidence that post-settlement mortality structures mussel populations

The mechanisms maintaining community structure following an ecosystem shift are poorly understood and we propose that they must inherently be biological. Over-exploitation can provide a “natural experiment” with man as a predator driving a change in community structure, possibly an ecosystem shift. We examined a possible mechanism that maintains algal beds as an alternative state on the east coast of South Africa where the mussel Perna perna has been overexploited. Even on unexploited shores, about 50% of mussel larvae settle onto algae, but it is unclear whether they later recruit into adult beds. On such shores we used two indirect field approaches to understand the fate of recruits, testing whether inhibition of mussel recruitment by macroalgae could constitute a biological mechanism preventing reversion from the algal to the pre-disturbance mussel-dominated state. First, we examined possible ontogenetic migration of recruits from algae to adult mussels, testing the prediction that the ratio large:small recruits in adult beds is greater where algae are liberally interspersed with mussels. Second, we examined whether, like adults, recruits show spatial structure that is related to the distribution of topographic depressions, testing the hypothesis that large and small recruits show different co-variation with depressions, microhabitats where algae commonly occur. We found no evidence that recruits on algae actively move to nearby mussel beds as neither the ratio large:small recruits nor the abundances of small or large recruits showed any relationship with algal cover/variability. Small and large recruits showed different co-variation with topographic depressions on spatially structured transects. Like adults, large recruits commonly exhibited negative relationships with depressions. Thus, large recruits neither occur on algae nor migrate from algae to the primary substratum or onto adult beds. Consequently our results (a) highlight the importance of post-settlement mortality in structuring these mussel populations, and (b) suggest that the interception of larvae by algae forms a biological mechanism that can maintain macroalgal beds that develop following exploitative disturbance by man, thus preventing or at least drastically delaying the natural recovery of mussel beds.     

Effects of point-source nutrient addition and mussel removal on epibiotic assemblages in Perumytilus purpuratus beds

The objective of this study was to evaluate the effects of point-source nutrient addition and mussel removal on species diversity of the epibenthic assemblage of the purple mussel Perumytilus purpuratus. It was hypothesised that mussels cushion the effects of disturbance on the associated organisms and thereby favour species diversity in patches with added nutrients. In order to test this hypothesis, a two-factorial field experiment was conducted at an exposed rocky shore of northern-central Chile in which nutrient addition and mussel removal were manipulated with two levels. Experimental units consisted of 100-cm² mussel plots that were sampled after two months of experimental manipulations. Local nutrient addition was done with coated slow-release fertilisers. Mussel removal treatments consisted in three destructive events applied throughout the experimental phase, and each event consisted in the removal of 20% of the mussel biomass from each plot. While point-source nutrient addition had only minor effects on diversity, mussel removal had significant and negative effects on faunal species richness and the abundance of suspension-feeders and sessile organisms. The abundance of predators/scavengers significantly decreased with mussel removal in the presence of nutrient addition. Grazers and mobile organisms were very abundant in the mussel assemblage but their abundance remained unaffected by treatments. Finally, species richness and total abundance of algae were also not affected by the treatments. Although the duration of our experiment was relatively short, we suggest that the effects of local nutrient addition are of minor importance at the study site in comparison to the effects of mussel removal. Based on our results and previous studies conducted in soft-bottom systems, we suggest that physical and biological stress acting on exposed hard-bottom communities overshadows the potential effects of local nutrient addition.     

Effects of point-source nutrient addition and mussel removal on epibiotic assemblages in Perumytilus purpuratus beds

The objective of this study was to evaluate the effects of point-source nutrient addition and mussel removal on species diversity of the epibenthic assemblage of the purple mussel Perumytilus purpuratus. It was hypothesised that mussels cushion the effects of disturbance on the associated organisms and thereby favour species diversity in patches with added nutrients. In order to test this hypothesis, a two-factorial field experiment was conducted at an exposed rocky shore of northern-central Chile in which nutrient addition and mussel removal were manipulated with two levels. Experimental units consisted of 100-cm² mussel plots that were sampled after two months of experimental manipulations. Local nutrient addition was done with coated slow-release fertilisers. Mussel removal treatments consisted in three destructive events applied throughout the experimental phase, and each event consisted in the removal of 20% of the mussel biomass from each plot. While point-source nutrient addition had only minor effects on diversity, mussel removal had significant and negative effects on faunal species richness and the abundance of suspension-feeders and sessile organisms. The abundance of predators/scavengers significantly decreased with mussel removal in the presence of nutrient addition. Grazers and mobile organisms were very abundant in the mussel assemblage but their abundance remained unaffected by treatments. Finally, species richness and total abundance of algae were also not affected by the treatments. Although the duration of our experiment was relatively short, we suggest that the effects of local nutrient addition are of minor importance at the study site in comparison to the effects of mussel removal. Based on our results and previous studies conducted in soft-bottom systems, we suggest that physical and biological stress acting on exposed hard-bottom communities overshadows the potential effects of local nutrient addition.     

The Dynamics of an Introduced: Balanus glandula Population in the Southwestern Atlantic Rocky Shores. The Consequences on the Intertidal Community*

Abstract. This paper deals with the population dynamics of the introduced barnacle Balanus glandula in a port shelter and a rocky shore exposed of a southwestern Atlantic area. Replicated areas in both zones were denuded monthly and censused in both the high intertidal and the upper mid intertidal zone. The settlement pattern. recruitment, mortality and growth of B. glandula and a comparative successional pattern in both areas were recorded. Recruitment began in winter, i.e., July in the port and August in the rocky shore. In the port, green algae dominate up to a final community of Balanus glandula all along the intertidal, while in rocky shores the red encrusting alga Hildenbrandia lecanellieri establishes a cover that is replaced by the mytilid Brachidontes rodriguezi in the mid intertidal and by B. glandula in the high intertidal and upper mid intertidal. B. glandula densities were greater in the upper mid intertidal 19 600 and 5050 indiv. · m^-2 in the port and rocky shore, respectively than in the high intertidal 13600 and 1100 indiv.· m^-2, respectively. Mortality was greater in the port up to 62%, but heavy settlement allows a final density 4 times greater than in eulittoral rocky shores and 10 times greater than in the high intertidal. The winter reproduction of Balanus glandula, the absence of predators, the neutral or beneficial effect of algae. with spatial and temporal partitioning of substrate allows it to successfully outcompete mussels and other barnacle species from the intertidal community of both sheltered and exposed areas.     

舟山岩相潮间带动物群落结构的研究

本文应用数理统计方法研究了舟山海区岩岸潮间带中26个测站的春季动物群落结构。根据26个站的位置分为近岸区、近外海区和外海区3个区域。各站均以软体动物和甲壳动物为群落主要组成成分,群落种类组成数从近岸到外海呈增长趋势,且优势种亦从低盐种类向高盐种类演替。近岸区群落主要优势种为短滨螺Littorina brevicula、粗糙滨螺L.scabra、僧帽牡蛎Ostrea cucullata、白脊藤壶Balanus albicostatus等;近外海区为短滨螺、粒结节滨螺Nodilittorinaexigua、僧帽牡蛎、鳞笠藤壶Tetraclita squamosa squamosa、白脊藤壶等;外海区为粒结节滨螺,疣荔枝螺Purpura clavigera、条纹隔贻贝Septifer virgatus、日本笠藤壶T.japonica及藻类等。但群落个体密度和生物量从近岸到外海呈减少趋势。从近岸到外海,群落的Shannon-Wiener多样性指数H和Pielou均匀度J均增加,而McNaughton优势度D减少。群落结构从近岸到外海渐趋复杂。但整体上,舟山海区岩岸春季动物群落结构是比较简单的(绝大多数断面H值小于2.50,J值小于0.5。而D值大于0.6)。近外海区的群落结构比较相似于外海区的群落。根据平均聚类分析,这26个群落可以分为三个生态类型,即隐蔽性岩岸群落,半隐蔽性岩岸群落及开敞性岩岸群落。     

Predators on rocky shores in the northern Atlantic: Can the results of local experiments be generalized on a geographical scale?

Experimental manipulations of invertebrate prey and predators on rocky shores have been done by many authors. In the northern Atlantic the predators involved are usually the green crab Carcinus maenas and/or the dogwhelk Nucella lapillus, and the prey species studied are acorn barnacles (balanid Cirripedia), mussels (Mytilus spp.) and winkles (Littorina spp.). Usually the predators are found to have a regulating “top–down” effect on the prey species. In Iceland the acorn barnacle Semibalanus balanoides, the blue mussel Mytilus edulis and the flat periwinkle Littorina obtusata (including to some extent Littorina mariae) are found on rocky shores all around Iceland in what would seem to be near-optimal physical conditions. The predators Carcinus maenas and Nucella lapillus, on the other hand, are relatively southern species that do not thrive on the colder coasts of Iceland. Thus general surveys of different coasts of Iceland would seem to offer opportunities to see whether the results of local experiments can be discerned on a geographical scale (hundreds of km). The roughly 4900 km of the rocky coastline in Iceland was in this study subdivided into four regions, I–IV, according to the commonness or presence of the two predators. With the hope of reducing compounding factors the surveys were confined to sheltered or semi-sheltered fucoid shores, which were further divided into Ascophyllum (more sheltered) and Fucus vesiculosus (less sheltered) shores. Estuaries or other low-salinity environments were avoided. The study was based on 761 stations distributed around the rocky coastline on these two types of shores. The results for barnacles and mussels, being generally more abundant in regions were predators were scarce or absent, and being less common in Ascophyllum than F. vesiculosus shores in contrast to the predatory dogwelks, were in large measure in accord with predictions from experiments indicating “top–down” regulation. The results for the periwinkles, however, suggested that “bottom–up” regulation might be of greater importance, as their density was closely linked to the biomass of macroalgae, while the abundance of predators appeared to have little effect. The periwinkle, although certainly eaten, may not be a preferred prey of the two predators in question in Iceland. This paper forms a link between experimental and observational study approaches.     

Marine invasive aliens on South African shores: implications for community structure and tropillc functioning

Up to 12 marine mollusc and three crab species are thought to have been deliberately or accidentally introduced to South Africa. Of these, only the Mediterranean mussel Mytilus galloprovincialis and the European shore crab Carcinus meanas have become invasive. M. galloprovincialis, probably introduced in the late 1970s, is already the dominant mussel throughout the Cape west coast. As a result, indigenous intertidal mussels Aulacomya ater have been largely displaced, mussel standing stock has increased and the upshore limit of mussel beds has become considerably elevated. Implications include competitive exclusion of large limpets but enhanced recruitment of juvenile limpets, increased habitat availability for mussel infauna, reduction of habitat for algal infauna and enhanced food availability for predators, particularly terrestrial species. C. maenas was first recorded from Table Bay Docks in 1983 and currently ranges from Camps Bay to Saldanha Bay. Although it is a voracious predator, it appears unable to colonize wave-swept shores, so is unlikely to displace indigenous crabs, or to severely impact prey species on the open coast. Valuable conservation areas and mariculture sites in sheltered lagoonal areas are, however, threatened by it.     

Biotic consequences of a shift in invertebrate ecosystem engineers: Invasion of New Zealand rocky shores by a zone‐forming ascidian

The pervasive effects of invasive ecosystem engineers, that is those species that modify their environment, are well documented, but rarely have the broader impacts of one foundation invertebrate species being replaced by another been examined. In New Zealand, green‐lipped mussels, Perna canaliculus, commonly dominate wave‐exposed rocky shores. The recent appearance of an invasive ecosystem engineer, the ascidian Pyura doppelgangera, at the very northern tip of New Zealand now threatens to exclude these bivalves from this habitat. Here, we report major shifts in assemblages associated with the invader and chronicle its continued spread. We examined epibiota associated with clumps of mussels and clumps of Pyura from two rocky shore habitats—pools and emergent substrata at two locations. We detected some differences in species richness in biota associated with the two foundation species, but faunal abundance only differed between the locations. These minor changes were dwarfed by the shift in species composition within clumps of each foundation species. Molluscs, particularly gastropods, and crustaceans dominated the assemblage within mussels. In contrast, tubicolous polychaetes dominated the fauna associated with the ascidian. Sessile epifauna, notably barnacles and calcareous tube‐dwelling polychaetes, were common on mussels, but never encountered on the ascidian. Multivariate analysis revealed marked dissimilarity (>80%) between the characteristic mussel and ascidian faunas with virtually no overlap. This biotic shift overshadowed any differences between habitats and locations. The broader implications of these faunal shifts for local and regional patterns of biodiversity, as well as ecosystem function, remain unclear, but deserve further attention.     

Extending the generality of ecological models to artificial floating habitats

Marine assemblages on natural hard substrata are generally different from those on artificial habitats. There is, however, the potential for certain ecological processes to operate on both types of structures. On the sides of floating pontoons in Sydney Harbour, there were strong patterns of vertical distribution of sessile epibiotic organisms and molluscan grazers across relatively small spatial scales (in three defined zones, namely splash, shallow and deep). Patterns of vertical distribution of the tubeworms Hydroides spp. were reversed depending on the cover of mussels. A manipulative experiment was done to test if patterns of vertical distribution of Hydroides spp. were due to (1) the functioning of mussels or (2) the structure provided by mussels. Neither the functioning nor structure of mussels accounted for the patterns of distribution of Hydroides spp. Mussels increased recruitment of Hydroides spp., in the shallow and deep zones, and this was not due to increased surface area of the mussel shells. Manipulation of numbers of grazers and covers of sessile epibiota showed that the observed negative relationship between grazers and epibiota was due to grazers reducing recruitment of epibiota and epibiota decreasing survival of grazers. Most importantly, processes that accounted for patterns of distribution of mobile and sessile organisms on artificial floating structures were similar to those repeatedly shown to create such patterns on natural rocky shores.     

Deja vu? A second mytilid mussel,Semimytilus algosus,invades South Africa's west coast

A second marine mussel is shown to have invaded South Africa's west coast. Molecular techniques, based on intraspecific gene sequence divergences, prove its identity as Semimytilus algosus, a member of the family Mytilidae, native to Chile. The identity of an older introduced population found in Namibia is also confirmed. The present geographic range of S. algosus in South Africa extends some 500 km, from Groenriviersmond in the north to Bloubergstrand in the south. Together with Mytilus galloprovincialis, another previously established invasive mussel, S. algosus has become a dominant intertidal organism on wave-exposed rocky shores across this region. Both species are now much more abundant intertidally than either of the indigenous mussels Aulacomya ater and Choromytilus meridionalis, which have become largely confined to sublittoral and sand-inundated habitats respectively. The two invasive mussels display strong spatial segregation, with M. galloprovincialis dominating the midshore and S. algosus blanketing the lower shore. Through a combination of its small size and high abundance, S. algosus is likely to greatly increase food availability for a range of intertidal predators, many of which cannot consume mussels above a threshold size.     

Variation in the shell of the dog-whelk, Nucella lapillus (L.) from Sullom Voe and other parts of the Shetland islands

In most parts of southwestern Europe the shape of dog-whelk shells varies in a predictable manner with the exposure of the shore on which the animals are living. As the same relationship also applies on shores near Mongstad in Norway, it was thought that it might be true in Sullom Voe (Shetland) which is at about the same latitude. In fact the relationship only holds for some west coast samples. Exposed shores are indeed colonised by the short squat form expected of such places but the more elongated form, usually associated with very sheltered shores, is also found on all but the really exposed sites: there is accordingly very little difference in the shape of dog-whelk shells on Shetland shores. The species is consequently of little or no use in the assessment of exposure.     

Effects of high pressure,hot water shore cleaning after oil spills on shore ecosystems in the Northern Baltic proper

The use of high pressure, hot water hosing techniques in oil spill clean-up operations on rocky and stony-gravelly shores drastically reduces the shore vegetation and macrofauna. The negative effects are more substantial than on oiled shores cleaned by raking and scraping. After one year the hot water cleaned shores were not restored completely.On rocky shores the high pressure, hot water technique is very efficient in terms of freeing the rocks from oil. However, due to its detrimental effects on shore organisms this type of oil spill clean-up operation can only be recommended for bird or wildlife protection areas. The cleanup method is inefficient on stony-gravelly shores due to penetration of oil into the ground and sediment and direct killing of shore organisms. The method should be avoided on these types of shores.     

A field experiment demonstrating the influence of seabird guano on intertidal primary production

Nutrient concentrations in intertidal rockpools on the shores of seabird breeding islands off the west coast of South Africa are significantly higher than concentrations in similar pools on mainland shores. This is due to the presence of terrestrial and upper intertidal guano deposits which are washed through the intertidal zone. Growth rates of intertidal macroalgae are significantly faster on the rocky shores of seabird breeding islands than on mainland rocky shores, and the extensive, permanent algal mats that develop on island shores are absent from mainland shores. The hypothesis that guano run-off enhances intertidal algal growth was tested experimentally at a site on the west coast of South Africa where no seabird colonies exist. Seawater, artificially enriched with guano, was trickled down an intertidal rocky slope during a period of two months. Algal settlement and growth on artificial surfaces were significantly enhanced by the guano treatment when compared with controls. The conclusion is that increased intertidal primary production on seabird islands, and the consequent modification of intertidal community structure, are due in part to nutrients which are introduced into the intertidal zone by run-off from the guano cap and by direct deposition of guano in the high-shore region by roosting birds.     

Reseeding of mussels on denuded rocky shores: preliminary studies with the brown mussel Perna perna

A method is developed to establish clumps of mussels Perna perna in denuded areas on high-energy rocky shores on the south-east coast of South Africa. A total of 20 small (20–30 mm total length) mussels is placed under a 30 cm half-section of perforated PVC drainage pipe bolted to the rock surface. The pipe is left in position for one month before removal to allow the mussels to attach firmly to the substratum. A pilot study in a marine reserve indicated 76% survival of clumps and individuals over a period of a year. Mussels grew at a mean annual rate of 30.1 mm from an initial size of 31.8 mm. A subsequent study of survivorship and growth of three size-classes: 25–35, 50–60 and >70 mm, showed that greatest survivorship and growth were exhibited by the smallest size-class. Apart from the initial drilling of holes, the procedure is simple and inexpensive. Mussels could be harvested after about 12 months and the holes and PVC pipe re-used for repeats of the procedure. The method may be used as a research tool and as a way of rehabilitating areas denuded by other natural or anthropogenic agencies.     

Effect of an exotic prey on the feeding pattern of a predatory snail

The mussel Brachidontes pharaonis, which invaded the Mediterranean from the Red Sea about 120 years ago, has recently become abundant in many midlittoral and some infralittoral rocky habitats along the Israeli rocky shore. We investigated the influence of B. pharaonis, as a novel prey, on the foraging patterns of the large whelk Stramonita haemastoma in the field, and examined food preferences in the laboratory. S. haemastoma has shifted from indigenous species to feeding on the novel mussel when abundant. The whelk prefers to prey upon the invasive mussel over all indigenous species offered (e.g. barnacles and mussels), probably due to its larger size. In the midlittoral zone, the foraging activity of S. haemastoma is considerably low even where refuges are readily available (incisioned-rocks) and food density is high (mainly B. pharaonis). Higher proportions of whelks are actively foraging in the infralittoral zone but usually on smaller prey, mostly barnacles. We suggest that this differential foraging activity in the two zones is related to the degree of exposure to wave action. The midlittoral is inherently more exposed to wave action than the infralittoral, where sea conditions are more benign and the whelks may enjoy longer activity periods.     

A model describing the interactive roles of predation,competition and tidal elevation in structuring mussel populations

The black mussel Choromytilus meridionalis can survive when exposed in air up to 50 per cent of the time, but it is normally restricted to about 30 per cent exposure by extremes of temperature and desiccation. Growth rates vary greatly with location and tidal elevation, but as they grow, the mussels compete intensively for space, consequent mortality rates being proportional to growth rate. Because growth declines with tidal elevation and as terminal size is approached, losses attributable to competition tend to be greatest in lowshore and sublittoral regions and early in life. There are many predators of mussels, each differing in its feeding rate and technique. The quantities and size distributions of C. meridionalis taken by communities of natural predators at selected sites are estimated. Total predator consumption is compared with rates of loss as individual mussels increase in size and compete for space. Fast-growing populations with large terminal sizes can support dense predator communities with little deleterious effect. Slower-growing ones with a smaller adult size have lower rates of competition-induced mortality and remain vulnerable to predators for longer. They are thus more likely to be predator-controlled. Predators such as gastropods (Natica and Nucella species), which preferentially select small mussels and have low consumption rates, have far less impact than those, such as Octopus spp. and birds, with higher energy demands and capable of taking larger mussels subject to less mortality from other causes.     

Effects of acid mine drainage from an abandoned copper mine, Britannia Mines, Howe Sound, British Columbia, Canada, on transplanted blue mussels (Mytilus edulis)

Juvenile mussels (Mytilus edulis) were transplanted to Howe Sound, British Columbia, Canada, along an apparent pollution gradient of acid mine drainage (AMD) from an abandoned copper (Cu) mine. Cages containing 75 mussels each were placed at a total of 15 stations and were exposed to concentrations of dissolved Cu in surface waters ranging from 5 to 1009 micrograms/l for a period of 41 days. Mussels located at stations closer to the source of AMD at the mouth of Britannia Creek bioaccumulated higher concentrations of Cu and zinc (Zn) in their tissues. Mussel growth was adversely affected by Cu tissue concentrations above 20 micrograms/g dry wt., while declines in survival and condition index occurred in mussels that bioaccumulated greater than 40 micrograms/g dry wt. Cu. Tissue Zn concentrations (117-192 micrograms/g dry wt.) were likely not high enough to have a direct impact on mussel health. Reduced survival of transplanted mussels was supported by an absence of natural mussels in contaminated areas. Phytoplankton was also severely reduced in areas contaminated by mine waters. Based on the weight of evidence, AMD from the Britannia mine had a deleterious impact on mussel survival in a zone extending at least 2.1 km to the north and 1.7 km to the south of Britannia Creek on the east shore of Howe Sound.     

Further refutation of the primary-secondary settlement hypothesis for the brown mussel Perna perna

The primary-secondary settlement hypothesis, that mussels first settle in algae and then move to mussel beds, was rejected as the only mode of recruitment for the brown mussel Perna perna in a previous study at one location over one year on the south coast of South Africa. We investigated recruitment of P. perna into mussel beds and algal turfs over a five-year period (1995–2000) at 3–4 sites at each of four locations on the South and East coasts. Attachment to the two substrata was significantly temporally synchronous at two of the four locations. The disparity among locations could be due to differences in the algal species present or to differences in the importance of turf algae as a facilitator of recruitment. At Zululand, on the East Coast, attachment to mussel beds and algae was measured monthly and found not to be temporally synchronous. At this location, primary settlement into algae was significantly correlated with an increase in the density of juveniles in the mussel beds 2–3 months later, indicating that turf algae can act as irregular but important facultative facilitators of mussel recruitment, supplementing direct input into the mussel beds. However, as amounts of settlers and recruits in mussel beds exceeded those found in algal turfs most years and places, the primary-secondary settlement hypothesis is inapplicable to P. perna under most circumstances.     

Effects of the suspension feeding mussel Mytilus trossulus on a brackish water macroalgal and associated invertebrate community

The importance of suspension-feeding mussels is particularly apparent in benthic communities; however, the role of this feeding strategy on the development of macroalgal and associated invertebrate communities is in general poorly known. The effect of suspension-feeding mussels Mytilus trossulus on benthic communities was studied in an in situ factorial field experiment in the Northern Baltic Sea over one ice-free season. The experiment was performed under different regimes of wave exposure (low and moderate) and on different sedimentary habitats (soft bottom with high organic content, soft bottom with low organic content, and hard bottom). In general the presence of mussels was associated with increased biomass of filamentous algae, herbivores and deposit feeders and decreased biomass of charophytes. The effect of M. trossulus interacted with the effect of exposure and substrate. Stronger responses were observed in moderately exposed than in sheltered areas. The presence of M. trossulus affected charophytes and deposit feeders on sand with low content of organic matter and filamentous algae on pebbles but not on other substrate types. The magnitude of the effects varied between months. The results suggest that (i) even in dynamic coastal systems the biodeposits and excretions of mussels are at least partly assimilated locally and are not flushed away to the open sea, (ii) the accumulation of faecal material induced elevated growth of deposit feeders, (iii) mussels enhanced the growth of ephemeral macroalgae and reduced the growth of perennial macroalgae, and (iv) together with increasing benthic primary production, mussels indirectly increase the production of herbivores.