Mytilus galloprovincialis (Mollusca: Bivalvia) in a warm-temperate South African estuarine embayment

The distribution, population structure, condition and gonad indices, and the attachment strengths of the invasive Mediterranean mussel Mytilus galloprovincialis are described from the Knysna Estuary, South Africa. Mussels were found in the rocky intertidal at the mouth of the estuary and on all man-made hard substrata as far as 12?km from the mouth in 2013. However, mussel beds were found only in the lower reach of the estuary, where mussel densities were as high as 120 per 0.01?m². When compared to mussels from the embayment regions of the estuary, those at the estuary mouth, where wave action was high, were generally smaller, had significantly lower condition and gonad indices, and greater attachment strength. This suggests that mussels within the sheltered regions of the estuary invest more energy in growth and reproduction, and that this is one reason for their success as an invasive species in the Knysna estuarine embayment.     

Life history and population dynamics of an endangered seahorse (Hippocampus capensis) within an artificial habitat

Species-specific life-history information is critical for successful conservation, particularly in establishing an accurate baseline status. Obtaining such information is challenging for most species, but in particular for rare and threatened marine species. To facilitate future conservation of the endangered Knysna seahorse (Hippocampus capensis) this study aimed to determine important life-history information for this species. Visible implant fluorescent elastomer (VIFE) tags were used to mark 78 seahorses within a residential marina estate in the Knysna estuary, South Africa, in February 2018. Using a mark-resight approach, the size and movement patterns of the population and growth rate of seahorses were determined over a 14-month period. The closed population estimate for H. capensis, within Thesen Islands Marina, was estimated to be 134 (118–152 95% C.I. ) in February 2018 compared to only 72 (48–108 95% C.I. ) in February 2019. The species showed rapid initial growth with males and females having similar rates of growth based on the specialised von Bertalanffy growth function model. The importance of Reno mattresses as a habitat for H. capensis was confirmed based on the high abundance and site fidelity of the population, which emphasises the conservation potential of heavily modified environments for threatened seahorse species. The use of VIFE tags was deemed effective in studying this endangered seahorse and allowed the collection of important information for this species which can be used in future Red List assessments and conservation actions.     

A historical perspective of the genus Mytilus (Bivalvia: Mollusca) in New Zealand: multivariate morphometric analyses of fossil, midden and contemporary blue mussels

The taxonomic status of smooth shelled blue mussels of the genus Mytilus has received considerable attention in the last 25 years. Despite this, the situation in the southern hemisphere remains uncertain and is in need of clarification. Recent work suggests that contemporary New Zealand mussels from two cool/cold temperate locations are M. galloprovincialis. However, the distribution of Mytilus in New Zealand ranges from 35 ° to 52 ° south (～ 1800 km), meaning that large areas of the subtropical/warm temperate north and the subantarctic south remain unsampled, an important consideration when species of this genus exhibit pronounced macrogeographical differences in their distributions which are associated with environmental variables such as water temperature, salinity, wave action and ice cover. This study employed multivariate morphometric analyses of one fossil, 83 valves from middens, and 92 contemporary valves from sites spanning the distributional range of blue mussels to determine a historical and contemporary perspective of the taxonomic status of Mytilus in New Zealand. The findings indicated that all fossil and midden mussels are best regarded as M. galloprovincialis and confirmed that contemporary mussels, with one possible regional exception, are also best regarded as M. galloprovincialis. Contemporary mussels from the Bay of Islands (warm temperate/subtropical) exhibited much greater affinity to M. edulis than they did to M. galloprovincialis, indicating that mussels from this area require detailed genetic examination to determine their taxonomic status. The analyses revealed a significant difference between the fossil/midden mussels and the contemporary mussels, consistent with levels of present day differentiation among intraspecific populations and not thought to reflect any substantive temporal change between mussels of the two groups. The continuous distribution of M. galloprovincialis in New Zealand from the warm north to the subantarctic south indicates that the physiology of this species is adapted to a wide range of water temperature conditions. Therefore, the distribution of this species on a worldwide scale is unlikely to be restricted by its adaptation to warm water alone, as has previously been widely assumed. © 2004 The Linnean Society of London, Biological Journal of the Linnean Society, 2004, 82 , 329–344.     

Growth of <Emphasis Type="Italic">Mytilus galloprovincialis</Emphasis> (mollusca,bivalvia) close to fish farms: a case of integrated multi-trophic aquaculture within the Tyrrhenian Sea

A current practice of marine aquaculture is to integrate fish with low-trophic-level organisms (e.g. molluscs and/or algae) during farming to minimise effects of cultivation on the surrounding environment and to potentially increase economic income. This hypothesis has been tested in the present article experimentally, by co-cultivating fish and mussels (Mytilus galloprovincialis) in the field. Integrated multi-trophic aquaculture (IMTA) experiments were started in July 2004 by transplanting mussel seed at two depths (−3 and −9 m) within 1,000 m downstream to fish cages and at 1,000 m upstream from cages. Mussels were cultured in nylon net bags for 12 months and the growth recorded biometrically. The outcome of our field experiment corroborated the idea of IMTA effectiveness. In fact, in the study area, the organic matter from fish-farm biodeposition caused changes in the chemical environment (i.e. controls and impacted sites were significantly different for organic matter availability and chlorophyll-a) and this induced changes in growth performance of co-cultivated mussels. Mussels cultivated close to cages, under direct organic emission, reached a higher total length, weight and biomass than mussel cultivated far from farms.     

Genetic diversity of Southern hemisphere blue mussels (Bivalvia: Mytilidae) and the identification of non‐indigenous taxa

The taxonomic and evolutionary affinities of Southern hemisphere smooth‐shelled blue mussels are unclear, with studies using different marker types having identified different relationships among various geographic regions. Using an existing and a new molecular assay, the present study builds on previous work to test the distribution of blue mussels native to and introduced to the Southern hemisphere. Populations of Mytilus were sampled from New Zealand, Australia, and Chile. The nuclear‐DNA marker Me 15/16 was used to identify the taxonomic status of 484 individuals. A new restriction fragment length polymorphism (RFLP) assay was used to identify the hemisphere of origin for a subset of Mytilus galloprovincialis. The Me15/16 marker identified 478 pure M. galloprovincialis from Southern hemisphere sites and six Mytilus edulis/M. galloprovincialis hybrids from the Auckland Islands (New Zealand) and Chile. A cytoplasmic RFLP identified Northern hemisphere M. galloprovincialis in almost every Southern hemisphere region. The presence of native M. galloprovincialis at high latitudes (up to 52°S) has implications for our understanding of environmentally induced selective constraints considered to determine species distributions. Widespread occurrence of invasive Northern hemisphere blue mussels in the Southern hemisphere is documented for the first time. Identification of inter‐specific hybrids (M. edulis × M. galloprovincialis) in Chile and in the Auckland Islands (subantarctic New Zealand) illustrates that environments ranging from international ports to remote protected locations are vulnerable to bioinvasion. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 898–909.     

Food supply, grazing activity and growth rate in the limpet Patella vulgata L.: a comparison between exposed and sheltered shores

The limpet Patella vulgata L. is an important microphagous grazer on intertidal rocky shores of north-west Europe, occurring across the wave exposure gradient. Groups of P. vulgata were selected at mid-tide level of two exposed shores and two sheltered, fucoid dominated shores on the Isle of Man, British Isles, and manipulated to equivalent densities and population structure. The level of grazing activity and growth rate were determined over a 1-year period. At the same time, the abundance of epilithic microalgae, measured as the concentration of chlorophyll a, was determined as an estimate of food supply. Microagal abundance showed a seasonal pattern in both exposed and sheltered conditions, with higher levels in winter compared to summer. In both seasons, the microalgal resource was more abundant on the sheltered shore studied. The level of grazing activity in P. vulgata showed a seasonal pattern on the exposed but not the sheltered shores. Averaged over the year, grazing activity on the exposed shores was over double that on sheltered shores. Thus, in sheltered conditions, food supply for limpets was high and grazing activity low; in exposed conditions, food supply was low and grazing activity high. The growth rate of P. vulgata, measured as increase in shell length, showed no significant difference between exposed and sheltered shores. Growth rate was also determined in P. vulgata at natural densities. Although the overall density declined with decreasing exposure to wave action, the density per unit area of grazeable substance was higher in shelter. In these populations, the mean growth rate was over twice as high on exposed compared to sheltered shores.     

Non-annual external annuli in the freshwater mussels Anodonta grandis grandis and Lampsilis radiata siliquoidea*

1. Two species of freshwater mussels, Lampsilis radiata siliquoidea and Anodonta grandis grandis were measured and permanently marked with pointed, plastic tape positioned at the postero-lateral edge of the shell. Mussels were returned to original conditions at two sites in an oligotrophic lake, retrieved at yearly intervals, re-measured, and external annuli that had been added since marking were counted. 2. External annuli were formed much less frequently than annually; the overall median number of annuli formed each year was 0.5. In one of the four populations studied, the rate of annulus formation was >1 in small animals and <1 in large ones. Many mussels showed no new external annuli at all, even several years after marking. 3. Ford-Walford plots of shell annuli did not yield consistent indices of shell growth. Repeated measurement of mussels in successive years showed that estimates of growth based on shell annuli consistently overestimated real shell growth.     

Evidence of Large Change in Unionid Mussel Abundance from Selective Muskrat Predation,as Inferred by Shell Remains Left on Shore

Three species of unionid mussels (Pyganodon grandis, Lampsilis siliquoidea, and Toxolasma parvus) were preyed on by muskrats (Ondatra zibethicus) that left shell remains at feeding sites (middens) along the shore of Lake LaSalle (West) in western New York State, USA. Mussels became established within five years of the construction of this small artificial lake, and individuals of all three species have grown rapidly to large size. The abundance of prey in middens in 1988–1990 and a marked decline in subsequently deposited shells indicate how dramatically muskrats had exploited this food source. They consumed > 2700 individuals of P. grandis (by far the most common prey item) through 1990, yet only 20 newly deposited shells were found in 1993. At a midden where prey depletion was directly catalogued during 1988, larger specimens of P. grandis were consumed first, suggesting size selection. L. siliquoidea has been least affected by predation, and was the mussel most often found alive in the littoral zone. No live T. parvus were ever collected, and this species is known only from middens. Complete collections of shell remains from the shoreline through 1999 suggest that the mussel population may be recovering slowly. Muskrat predation may be more important in influencing mussel population structure than previously realized, and the effects can be dramatic spatially and temporally.     

Paralytic shellfish poisoning (PSP) toxin profiles and short-term detoxification kinetics in mussels Mytilus galloprovincialis fed with the toxic dinoflagellate Alexandrium tamarense

Mussels (Mytilus galloprovincialis) were experimentally contaminated with paralytic shellfish poisoning (PSP) toxins by being fed with the toxic dinoflagellate Alexandrium tamarense, and changes in toxin content and specific composition during the decontamination period were analyzed by high-performance liquid chromatography (HPLC). Toxins excreted by the mussels into the seawater were also recovered using an activated charcoal column and analyzed by HPLC. The predominant toxins in A. tamarense, mussels, and seawater were the N-sulfocarbamoyl-11-hydrosulfate toxins (C1,2) and carbamate gonyautoxins-1,4 (GTX1,4). There were no remarkable differences in the relative proportions of the predominant toxins within A. tamarense, mussels and seawater. Because the relative proportion of the various toxin analogues excreted by the mussels was similar to that within their tissues during detoxification, it appeared that the selective release of particular toxins by the mussels was unlikely. The total amount of toxin lost from mussels was nearly equal to that which was found dissolved in the seawater, suggesting that, at least the early stages of mussel detoxification, most losses can be accounted for by excretion.     

Spatial patterns of abundance and growth of Mytilus edulis on boulders in the Northern Baltic Sea proper

Small scale spatial patterns of abundance, growth and condition of the mussel Mytilus edulis on sub-littoral boulders (approx. 1–2 m high) were investigated at the island of Askö in the northern Baltic proper. The effect of side (exposed/sheltered with respect to wave action and sunlight) of boulder and position (up/down) on boulder was investigated. A large spatial variability in abundance of M. edulis between boulders and between various sites within boulders were found. The highest numbers of mussels were found on the wave exposed side, near the top of boulders. Shell growth was favoured by a sheltered side and a down position. The body condition (meat weight/shell weight) of mussels was, on the other hand, affected only by position, the condition of mussels being better at the up position. Consequently, there seems to be temporal differences in the condition for growth within a spatial position. The body condition of the mussels was best near the top of boulders in the spring, but long-term shell growth was favoured by a sheltered side and a down position. This may reflect changes in the composition and availability of food during the year with phytoplankton as the major food source during the spring bloom and resuspension of benthic production and detritus as relatively more important during the rest of the year.     

Plastic and Heritable Variation in Shell Thickness of the Intertidal Gastropod Nucella lapillus Associated with Risks of Crab Predation and Wave Action,and Sexual Maturation

The intertidal snail Nucella lapillus generally has thicker shells at sites sheltered from wave action, where crabs are abundant and pose a high risk of predation, than at exposed sites where crabs are rare. We studied two populations showing the opposite trend. We reciprocally transplanted snails between field sites and measured shell length, width and lip thickness of those recaptured 12 months later. Snails transplanted to the sheltered site grew larger than sheltered-site residents, which in turn grew larger than transplants to the exposed site. Relative shell-lip thickness was greater in residents at the exposed site than at the sheltered site. Transplants from shelter to exposure developed relatively thicker shells than their controls and relatively thinner shells from exposure to shelter. Progeny of the two populations were reared for 12 months in a common garden experiment presenting effluent from crabs feeding on broken conspecifics as the treatment and fresh sea-water as the control. The crab-effluent treatment decreased foraging activity, concomitantly reducing cumulative somatic growth and reproductive output. Juveniles receiving crab-effluent grew slower in shell length while developing relatively thicker shell lips than controls, the level of response being similar between lineages. F₂ progeny of the exposed-site lineage showed similar trends to the F₁s; sheltered-site F₂s were too few for statistical analysis. At sexual maturity, shell-lip thickness was greater in snails receiving crab-effluent than in controls, indicating plasticity, but was also greater in the exposed-site than in the sheltered-site lineage, indicating heritable variation, probably in degree of sexual thickening of the shell lip. Results corroborate hypotheses that ‘defensive’ shell thickening is a passive consequence of starvation and that heritable and plastic control of defensive shell morphology act synergistically. Shell thickening of juveniles was similar between lineages, contrary to hypotheses predicting differential strengths of plasticity in populations from low- or high-risk habitats.     

Seasonal dependence of cadmium molecular effects on Mytilus galloprovincialis (Lamarck, 1819) protamine‐like protein properties

Mussels have a seasonal reproduction and cadmium is a common stressor in estuarine and coastal environments. In previous studies, we have shown that exposure to subtoxic doses of cadmium produced alterations in the properties of winter Mytilus galloprovincialis sperm protamine‐like (PL) proteins. In this study, it was analyzed the possibility that these cadmium effects may be seasonal. Winter and summer mussels were exposed to CdCl₂, and it was tested the PL‐proteins for cadmium bioaccumulation, electrophoretic pattern, DNA binding, and potentiality to induce DNA oxidative damage. It was found that cadmium exposure did not produce the same effects on PL‐proteins of summer mussels that were produced on PL‐proteins of winter mussels, that is: cadmium bioaccumulation, alterations in the acetic acid‐urea polyacrylamide gels (AU‐PAGE) and sodium dodecyl sulfate‐PAGE pattern, a reduced DNA binding affinity and the ability to induce DNA oxidative damage. PL‐proteins from summer mussels, apart from not being affected by all the abovementioned effects of cadmium, also showed a very low DNA binding affinity, independent of cadmium exposure. This study reveals clock‐associated seasonal responses to cadmium in M. galloprovincialis. Understanding the mechanisms through which environmental signals guide biological rhythms is fundamental to understanding the seasonal sensitivity of this bioindicator, to use M. galloprovincialis in appropriate seasonal periods.     

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     

Impact of Engineered Zinc Oxide Nanoparticles on the Individual Performance of Mytilus galloprovincialis

The increased use of engineered nanoparticles (ENPs) in consumer products raises the concern of environmental release and subsequent impacts in natural communities. We tested for physiological and demographic impacts of ZnO, a prevalent metal oxide ENP, on the mussel Mytilus galloprovincialis. We exposed mussels of two size classes, <4.5 and ≥4.5 cm shell length, to 0.1–2 mg l⁻¹ ZnO ENPs in seawater for 12 wk, and measured the effect on mussel respiration, accumulation of Zn, growth, and survival. After 12 wk of exposure to ZnO ENPs, respiration rates of mussels increased with ZnO concentration. Mussels had up to three fold more Zn in tissues than control groups after 12 wk of exposure, but patterns of Zn accumulation varied with mussel size and Zn concentrations. Small mussels accumulated Zn 10 times faster than large mussels at 0.5 mg l⁻¹, while large mussels accumulated Zn four times faster than small mussels at 2 mg l⁻¹. Mussels exposed to 2 mg l⁻¹ ZnO grew 40% less than mussels in our control group for both size classes. Survival significantly decreased only in groups exposed to the highest ZnO concentration (2 mg l⁻¹) and was lower for small mussels than large. Our results indicate that ZnO ENPs are toxic to mussels but at levels unlikely to be reached in natural marine waters.     

Zebra mussels decrease burrowing ability and growth of a native snail, <Emphasis Type="Italic">Campeloma decisum</Emphasis>

Invasive species can drive native organisms to extinction by limiting movement and accessibility to essential resources. The purpose of this study was to determine if zebra mussels (Dreissena polymorpha) affect the burrowing ability and growth rate of a native snail, Campeloma decisum. Snails with and without zebra mussels were collected from Douglas Lake, MI, and burrowing depths were studied in both the laboratory and Douglas Lake. Growth rates were calculated as the amount of shell growth from 2004 to 2005. Both the tendency of snails to burrow and the depth to which they burrowed into the substrate were significantly decreased by the presence of zebra mussels on snail shells in both laboratory and lake experiments. There was no difference in the percentage of snails that exhibited growth as a function of zebra mussel presence. However, for those snails that grew, there was a 50% higher growth rate for snails without zebra mussels compared to snails with zebra mussels. These negative effects of zebra mussels on growth and burrowing ability will likely lead to decreases in snail population densities in the future. Handling editor: S. Wellekens     

Shell shape and habitat use in the North‐west Pacific land snail Mandarina polita from Hahajima,Ogasawara: current adaptation or ghost of species past?

The endemic land snail genus Mandarina of the Ogasawara Islands provides an excellent model system to investigate adaptive radiation. Previously, it has been shown that coexisting species of the islands segregate by microhabitat, so that they are either predominantly found on the ground in relatively wet and sheltered sites, dry and exposed sites, or else are arboreal. Moreover, shell morphology correlates with microhabitat, so that species in wet and sheltered sites tend to have high-spired shells with a high aperture, and those in dry and exposed sites tend to have relatively low-spired shells with a wide aperture. We have now found that on Hahajima, Mandarina polita have variable shell morphology, and there is a correlation between morphology and the depth of leaf litter, as well as the presence/absence of other terrestrial species. Specifically, when high-spired terrestrial Mandarina ponderosa is present, M. polita tend to be low-spired and have a large aperture, indicative of character displacement. When M. ponderosa is absent, the shell shape of M. polita is much more variable, the overall spire is higher, individuals are found in deeper litter, and there is a strong correlation between litter depth and spire height. We argue that these patterns are due to local adaptation, but it remains possible that they are an artefact due to the 'ghost of species past'.  © 2007 The Linnean Society of London, Biological Journal of the Linnean Society , 2007, 91 , 149–159.     

Mussel dropping by Carrion and Hooded crows: biomechanical and energetic considerations

Dropping live mussels (Mytilus sp.) onto hard substrata by Carrion Crows (Corvus corone) and Hooded Crows (Corvus cornix) to access their flesh is a commonly observed behavior from late summer to spring in the United Kingdom and Ireland. Despite previous studies, several aspects of prey‐dropping behavior remained incompletely understood. From September 2008 to January 2010, we determined the heights of drops, likelihood of shell breakage from drops at different heights, effect of mussel size on breakability, energetic costs of flying to drop heights, and the energetic costs of transporting mussels from mussel beds to dropping sites. We studied Carrion Crows on the Isle of Cumbrae, Scotland, and Hooded Crows in Cork Harbor, Ireland. Initial experiments were carried out with mussels to determine breakability in relation to size and drop height, and to estimate mussel energy content. Sizes of mussel shells at Hooded Crow dropping sites were compared with those of live mussels from source mussel beds. Adult Carrion Crows (N = 10) dropped mussels from a mean height of 4.7 m, and adult Hooded Crows (N = 21) from 4.8 m. These heights were close to the minimum (4–4.8 m; determined experimentally) required to break all mussel shells on the first drop. Dropping mussels from the minimum height that guarantees breakage reduces handling time and, by minimizing the size of the resulting debris field, likely reduces the risk of kleptoparasitism. Juvenile Hooded Crows (N = 13) dropped mussels onto suboptimal substrates (gravelly mud) from variable heights (mean = 6.1 m) with a low success rate (0% on first drop). This inefficiency could reflect either inexperience or exclusion from prime hard‐substrate dropping sites by adults. Foraging Hooded Crows selected larger mussels, dropping no mussels <32‐mm shell length. Energetic calculations indicate that a Hooded Crow lifting a medium‐sized mussel (55‐mm shell length) to a height of 5 m incurs a cost of only 0.3% of energy assimilated from that mussel, whereas travel to and from a mussel bed 200 m away costs 5.8% of that energy. These results suggest that choice of mussel dropping height by crows is determined by shell breakability rather than the cost of flying up to the dropping height.     

On Temporal Dynamics of Morphological Variations in the Black Sea Mussel Mytilus galloprovincialis Lam.

Temporal variations in morphological characters of the shells were studied in 12 samples of mussels M. galloprovincialis Lam. from the Black Sea commercial collectors that belong to different generations. The samples were shown to be diverse in every studied character and the degree of characters' variability in the samples was different. The morphological diversity of samples diminished with age. Different generations from the same habitat had specific features of age-related variability expressed as different rates of age-related changes and direction and degree of variability. This effect is considered as an index of the influence of ecological conditions on the shell growth and formation.     

South African populations of Littorina rudis

Littorina rudis was collected from stands of Spartina capensis in saltmarsh habitats within Langebaan and knysna lagoon, Cape Province. It was absent from rocky sores and from saltmarshes east of Knysna. The shell, radular and penial morphology are described. L. rudis from both localities bore tessellated colour patterns similar to those of British saltmarsh ecotypes. The Knysna population contained individuals with a unique amber background colour. The shells from Knysna and Langebaan were very thin in accordance with the sheltered habitats. The number of embryos carried in the brood puch was similar in both countries. Sexual maturity was attained at about 4 mm shell length; the male and female modal sizes were 4.9 mm and 5.1 mm respectively at Langebaan. No. published records were found for L. rudis in South West Africa. Some possible origins of the South African L. rudis populations are discussed.     

Quantification of mussel (Mytilus edulis) growth from power station cooling waters in response to chlorination procedures

The age and growth history of individual mussels collected from the cooling water culverts of a power station were determined from the growth bands present in acetate peels of polished and etched shell sections. During periods of exposure to the antifouling agent, sodium hypochlorite (0.2mgl^‐1), shell growth was severely reduced, resulting in marked changes in the structure and deposition of the shell. As a consequence of chlorination, the growth rate of the mussel population occurring within the culverts was substantially lower and the mean length‐at‐age significantly smaller than that of a naturally occurring population immediately outside the cooling water intake. The growth patterns present in the shells of mussels experimentally added to the cooling waters during chlorination were compared with, and found to be similar to, the patterns in the shells of mussels that had settled naturally in the culverts. The daily growth of the experimentally exposed mussels (1.1–5.2 μm d^‐1) is two orders of magnitude lower than the growth rate of mussels growing in untreated waters. The use of mussels for evaluating the efficiency and long‐term effects of low level chlorination is discussed.