Seasonal changes in the response of oystercatchers Haematopus ostralegus to human disturbance

The response of foraging animals to human disturbance can be considered as a trade-off between the increased perceived predation risk of tolerating disturbance and the increased starvation risk of not feeding and avoiding disturbance. We show how the response of overwintering oystercatchers Haematopus ostralegus to disturbance is related to their starvation risk of avoiding disturbance. As winter progresses, oystercatcher energy requirements increase and their feeding conditions deteriorate. To survive they spend longer feeding and so have less spare time in which to compensate for disturbance. Later in winter, birds approach a disturbance source more closely and return more quickly after a disturbance. Their behavioural response to disturbance is less when they are having more difficulty surviving and hence their starvation risk of avoiding disturbance is greater. These results have implications for studies which assume that a larger behavioural response means that a species is more vulnerable to disturbance. The opposite may be true. To more fully understand the impact of disturbance, studies should measure both behavioural responses and the ease with which animals are meeting their requirements. Conservation effort should be directed towards species which need to spend a high proportion of their time feeding, but still have a large response to disturbance.     

Optimal foraging on perilous prey: risk of bill damage reduces optimal prey size in oystercatchers

Intake rate maximization alone is not always sufficient in explainingprey size selection in predators. For example, bivalve-feedingoystercatchers regularly select smaller prey than expected ifthey aimed to maximize their intake rate. It has been proposedthat to these birds large prey are "risky," in the sense thatbirds may damage their bills when feeding on large bivalves.Large bivalves yield more energy, but according to this hypothesisthis is achieved at the expense of energy yield in the longterm when (1) the risk of bill damage increases with prey sizeand (2) foraging with a damaged bill is less effective. In accordancewith this hypothesis, we show that captive oystercatchers feedingon large cockles experienced a high probability of bill tipdamage, while bill damage was absent when cockles were small.Moreover, among free-living oystercatchers the prevalence ofbill damage was correlated with mean cockle size near the capturesite, and the data on captive birds fit in this pattern. Foodintake of captive oystercatchers feeding exclusively on cockleswas reduced by 23% after bill damage, and free-living birdswith damaged bills had 14 g lower mass. Because lower body masswas associated with higher mortality probability, these resultsindicate long-term costs associated with feeding on large cockles.We conclude that the risk of bill damage can potentially explainwhy oystercatchers avoid large bivalves and that oystercatchersmay maximize long-term intake rate by selecting prey sizes thatare "suboptimal" from a short-term rate-maximizing point ofview.     

Larval growth, juvenile size and heterozygosity in laboratory reared mussels, Mytilus edulis

Studies with marine bivalve juveniles have shown a positive correlation between growth and allozyme multi-locus heterozygosity (MLH), and, in some cases, between larval growth and juvenile growth, but there has been little research on the relationship between allozyme heterozygosity and larval growth. Larvae of M. edulis from different mating systems (half-sib families with a single female, or a single male parent, a reciprocal cross of two malesxtwo females and two mass matings of 13x13 and 8x17 females and males, respectively) were reared in the laboratory and selected into fast and slow growing groups when about 10-30% were undergoing metamorphosis. Offspring were reared to the juvenile stage (>3.00 mm) and both groups of each mating were electrophoresed and genotyped at up to 12 allozyme loci. There was generally good agreement with Mendelian inheritance (half-sibs and reciprocal cross) or the Hardy-Weinberg model (mass matings). Null alleles were detected at the Odh and Lap loci but there was no evidence that null allele heterozygotes grew slower than other genotypes. Over all cohorts, juveniles from the fast growing larval group were not significantly larger, or smaller, than juveniles from the slow growing group which suggests that larval growth rate may be independent of juvenile growth rate. This observation agrees with some, but not all, earlier studies and has commercial relevance. Tests of heterozygosity and juvenile shell length indicated no association between average heterozygosity across all allozyme loci and the size of juveniles in any cohort regardless of the mating system used or their larval growth rate. The association between MLH and juvenile growth in bivalves is seldom detected in cohorts from a limited genetic background. The lack of an association between heterozygosity and size might therefore be expected in the half-sib and reciprocal cross cohorts, but not in the mass matings. The results argue against any significant association between heterozygosity and larval size in mussels.     

Egg size, egg composition and reproductive success in the Oystercatcher Haematopus ostralegus

We investigated the relationship between egg size and composition and their subsequent effects on hatching and fledging success in Eurasian Oystercatchers Haematopus ostralegus on the island of Schiermonnikoog (53̀30'N, 06̀10'E) in the Dutch Wadden Sea between 1986 and 1990. Egg size variation was considerable. The ratio in egg volume of the largest over the smallest egg was more than 1.5 in each of five years. Differences between females accounted for, on average, 61% of the total variance of egg volume. Individual females produced similarly sized eggs from one year to another. Nevertheless, average egg volume per clutch declined by 0.2 cm³ per year as females aged, but this explains little of the observed variation. Female size and food supplementation had no significant effect on egg volume. In absolute terms, large eggs contain more lean dry matter and lipid than small eggs, but the proportion of both constituents decreases with egg size. Consequently, the combustible energy content of eggs increases less than proportionally with egg size. Hatchability averaged 87% and was not correlated with egg volume. Hatchling weight increased with egg size, averaging 65% of fresh egg weight. Chick survival until fledging did not increase with egg volume. Consequently, within the size range observed, large and small eggs are of comparable quality. Since both the costs and benefits of large eggs compared with small eggs seem small at best, we propose that, within the size range encountered in this population, egg size can be considered an evolutionarily neutral trait.     

The influence of sediment type on the aggregative response of oystercatchers, Haematopus ostralegus, searching for cockles, Cerastoderma edule

Models that describe the dispersion patterns of predators between a series of patches that vary in prey density frequently assume that predators, in the absence of interference, will aggregate in patches with the highest prey density, at any point in time. This assumption has important implications for patterns of prey mortality, and the extent to which prey mortality is density dependent. In natural predator-prey systems, it is likely that environmental factors interact with spatial variation in prey density to influence the aggregative response of predators. We show data consistent with this idea on a population of overwintering oystercatchers foraging on cockles. There was no evidence that birds aggregated in patches with the highest biomass density of cockles. The biomass density of cockles was highest in muddy patches at the start of winter, and birds aggregated in patches that switched from being muddy at the start of winter to being sandy at some point during the winter. We argue that sediment type influences foraging costs experienced by the birds, so birds avoid feeding in muddy patches unless the fine sediment is removed from a patch, as happens during winter storms. When this happens a high biomass density of cockles suddenly becomes available and the birds aggregate in such patches. The rate of biomass loss was greatest in patches used intensively by birds for feeding, suggesting that the birds' aggregative response influences cockle mortality. We discuss the implications of our results for ideal free models.     

Anatomical observations on the bill of the Oystercatcher (Haematopus ostralegus occidentalis) in relation to feeding behaviour

Oystercatchers have recently increased as inland breeders in Northern Britain. Concurrently, they have been observed feeding in coastal fields in winter. It is suggested that the bill of this species possesses dual characteristics enabling them to feed on shellfish adn also to probe in the soil for terrestrial invertebrates.
The gross internal morphology of the bills of adults and young is described. A bony core contains large nerves running the length of the bill. These break up towards the tip where numerous sensory corpuscles are seen. It is concluded that the possession of a strong bone core together with numerous sense organs at the tip has been of considerable importance in enabling the Oystercatcher to exploit inland situations.     

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.     

Amino-acid uptake by mussels,Mytilus edulis,from natural sea water in a flow-through system

Natural Wadden Sea water taken from the North Sea (island of Sylt) was pumped at rates of 150 and 300 l h^–1 through a 4 l plexiglass tube mounted on a wooden tripod on the beach. The tube was densely filled with numerous cleaned mussels,Mytilus edulis. HPLC analysis of sea water showed that total dissolved amino acids are patchily distributed, varying by 100 % within 15 min, though proportions of individual amino acids were remarkably constant. Total amino-acid concentrations were 1528±669 nM (N=3) in October 1983 and 1198±597 nM (N=7) in July 1984. Samples taken at the entrance and the outlet of the experimental mussel bed revealed that the mussels had taken up 29 to 66 % of the amino acids dissolved in sea water. Uptake was observed for all amino acids detected in the chromatograms. 78 % of uptake resulted from the 5 most concentrated amino acids: serine, alanine, glycine/threonine, ornithine, aspartic acid. The nutritional profit obtained from uptake of dissolved amino acids amounted to 12 % (N=5, range 5–23 %, flow rate 150 l h^–1) and to 24 % (N=3, range 13–38 %, flow rate 300 l h^–1) of metabolic rate. The present data suggest that amino-acid concentration predominantly determines the magnitude of the nutritional profit obtained from uptake, and to a smaller extent the flow rate. These findings are in contrast to results of previous studies onAsterias rubens, interacting in small-volume closed systems with the natural bacterial sea water flora (Siebers, 1982). In these experiments, bacteria, due to rapid uptake, outcompeted the sea stars in absorption of dissolved amino acids. The present results suggest that bivalve mussels, can, due to their large gill surface areas and the great amounts of water pumped through their mantle cavity, successfully compete with bacteria in uptake of dissolved organic matter. Mussels, therefore, suggestedly play an important role in cycling dissolved organic matter.     

Two analogs of azaspiracid isolated from mussels, Mytilus edulis, involved in human intoxication in Ireland

Two new analogs of azaspiracid, azaspiracid-2 and azaspiracid-3, were isolated from mussels collected at Arranmore Island, Ireland in 1997 as additional causes of human intoxication. Their structures were determined to be 8-methylazaspiracid and 22-demethylazaspiracid, respectively by NMR and negative ion FAB CID MS/MS experiments.     

The role of disturbance in the evolution of life history strategies in the intertidal mussels Mytilus edulis and Mytilus californianus

Summary The intertidal mussels Mytilus edulis and M. californianus compete for space on the west coast of North America. An analysis of differences in size, growth rate, age at first reproduction, life span, mode of reproduction, niche and habitats between these two species demonstrates that their life history strategies are different in several important ways. M. californianus is a larger, sturdier, slower growing mussel which has effective predator-deterring mechanisms and is an overall superior competitor for space in the intertidal. Its reproductive strategy is based on continually spawning at a very low level throughout a yearly cycle. M. edulis is a classic fugitive species which rarely attains a large size but matures early and is characterized by a single massive reproductive output each year. The differences noted in their life history strategies are very likely the result of either a) a pre-adaptation for coexistence, or b) a competitively-induced regional evolutionary divergence.In Washington a band of small M. edulis (removed from most predation and competition) exists in a high intertidal spatial refuge, and contributes relatively little to the gene pool. Larger individuals exist in disturbed areas lower in the intertidal zone. These lower zones represent a relatively high risk, and usually only temporary, habitat for M. edulis because of mortality due to competition and predation. Individuals, however, colonizing these two intertidal patches contribute 43 times as much as their high intertidal counterparts in terms of reproductive fitness (calculated on a populationwide basis).     

Effect of salinity on filtration rates of mussels Mytilus edulis with special emphasis on dwarfed mussels from the low-saline Central Baltic Sea

The effect of salinity on the filtration rate of blue mussels, Mytilus edulis, from the brackish Great Belt (Denmark) and the low-saline Central Baltic Sea, respectively, was studied. First, we measured the effect of long-term (weeks) constant ambient salinities between 5 and 30 psu on the filtration rate of M. edulis collected in the Great Belt where the mean salinity is 17 psu. At salinities between 10 and 30 psu, the filtration rates did not vary much, but at 5 psu the filtration rates were significantly lower. Next, we studied dwarfed M. edulis (<25 mm shell length) from Central Baltic Sea (Askö, Sweden) where the mean salinity is 6.5 psu. The maximum filtration rate (F, ml min^?1 ind.^?1) as a function of shell length (L, mm) and dry weight of soft parts (W, mg) were found to be: F = 0.003L ^2.71 and F = 0.478W ^0.92, respectively, and these results indicate that the filtration rates of dwarfed Baltic Sea mussels are comparable to filtration rates of Great Belt mussels of similar size exposed to salinities >10 psu. When Baltic Sea mussels acclimatized to 20 psu in the laboratory were exposed to 6.5 psu this caused a drastic reduction in the filtration rate, but after about 2 days the previous high filtration rate was regained at 6.5 psu, and further, a similar pattern was observed when the 6.5 psu exposed mussels were finally re-exposed to 20 psu. The observed lack of Great Belt mussels to completely adjust to 5 psu, in contrast to the ease of Baltic Sea mussels to adjust back and forth between 6.5 and 20 psu, is remarkable and may perhaps be explained by different genotypes of Great Belt and Baltic Sea mussels.     

Fate of Francisella noatunensis, a pathogen of Atlantic cod Gadus morhua, in blue mussels Mytilus edulis

Francisellosis, caused by the bacterium Francisella noatunensis, is one of the most severe diseases affecting farmed cod, and has caused great economic loss for the cod farming industry in Norway. We studied the fate of F. noatunensis in the marine environment, focusing on the role of blue mussels. In experimental challenges, waterborne F. noatunensis was rapidly filtered by the blue mussel and transported to the digestive diverticulae. The bacteria passed through the entire digestive system. Intraperitoneal injection of cod with suspensions prepared from faeces collected from challenged mussels resulted in the development of francisellosis in the recipients, demonstrating that some bacteria were alive and infective when shed in mussel faeces. Bacterial clearance from the mussels was relatively fast, and no evidence was found, suggesting that the bacterium is capable of persisting or multiplying in the mussel tissues. A cohabitation experiment with cod and mussels previously exposed to F. noatunensis did not lead to infection in cod. A direct transmission from contaminated mussels to cod was thus not demonstrated; however, faeces particles with infective bacteria may play a role in the transmission of the bacterium in marine food chains.     

A comparison of variability in size dimensions of intertidal and subtidal mussels (Mytilus edulis) (Mollusca: Bivahia)

Variability in size dimensions of bivalve species has previously been claimed (1) to decrease with size and (2) to be greater for individuals in an intertidal habitat than in a subtidal habitat. Results from this study showed that the decrease in variability with size was an artifact of the techniques previously used, and that variability was the same for all sizes of Mytilus edulis. The previous study utilized an interspecific comparison to examine the effects of habitat on variability, which is considered unacceptable because differences in variability are likely to be a consequence of species differences rather than the effects of different habitats. Results presented here from the intraspecific comparison of M. edulis showed that variability was the same for mussels from an intertidal habitat and from a subtidal habitat.     

Microbiological quality of blue mussels (Mytilus edulis) in Nunavik, Quebec: a pilot study

This pilot study was aimed at documenting the presence of fecal indicators and enteric pathogens in blue mussels (Mytilus edulis) from 6 communities in Nunavik, Quebec. One to four 2?kg samples of mussels were collected at low tide in each community. Samples were investigated by enumeration methods for the fecal indicators enterococci, Escherichia coli, F-specific coliphages, Clostridium perfringens, and by molecular identification for the pathogens norovirus, Salmonella spp., Campylobacter jejuni, Campylobacter coli, and Campylobacter lari, verocytotoxin-producing E.?coli (particularly serovar O157:H7), Shigella spp., and Yersinia enterocolitica. In 5 communities, the presence of Giardia duodenalis and Cryptosporidium spp. was also tested by microscopy and molecular methods and that of Toxoplasma gondii was tested by molecular methods. Apart from small quantities of Clostridium perfringens in 2 samples, no bacterial or viral pathogens were detected in the mussels. Toxoplasma gondii was also not detected. However, G.?duodenalis and Cryptosporidium spp. were present in 18% and 73% of the samples investigated for these pathogens, respectively. When considering the indicators and the viral and bacterial pathogens investigated, the mussels examined were of good microbiological quality, but considering the presence of potentially zoonotic protozoa, it should be recommended that consumers cook the molluscs well before eating them.     

Ultrastructural characterisation of Marteilia species (Paramyxea) from Ostrea edulis, Mytilus edulis and Mytilus galloprovincialis in Europe

A focused ultrastructural study of Marteilia spp. found in cultured Ostrea edulis, Mytilus edulis and Mytilus galloprovincialis from France and Spain was conducted with emphasis placed on haplosporosomes, striated plate-like inclusions and spore wall morphology. Two types of haplosporosome were identified, sphaeroid and oblate, which were common to the parasite in all 3 host species. A total of 492 haplosporosomes were measured; those from the Marteilia sp. in Mytilus spp. were marginally smaller than those in Ostrea edulis. Spore wall morphology was found to vary depending on the state of maturity of the parasite--the more mature the parasite, the thicker the wall surrounding it. It is suggested that the current criteria used to distinguish M. maurini from M. refringens are invalid and that M. maurini was relegated to a junior synonym of M. refringens.     

Observations on the ultrastructure of large cells associated with putative neoplastic disorders of mussels,Mytilus edulis,from Yaquina Bay,Oregon

Large cells, generally thought to be associated with neoplastic disorders in bivalve mollusks, were studied with the electron microscope. The atypical cells had an average diameter of 15 μm and a nuclear to cytoplasmic ratio of about 1–1.5. Nuclei were extensively pleomorphic and bizarre shapes were common. Nucleoli were prominent and often multiple. In the cytoplasm of the large cells, there was a wide range of variability in anomalous organelles. Two cell types were originally thought to exist; however, it is now thought that the two cell types represent the two extreme morphological expressions of a single cell line. Their varying appearance is correlated with the density of ribosomes and abundance of cellular organelles. The large Mytilus edulis cells possess many ultrastructural properties that are characteristic of certain malignant vertebrate cells. However, alternative explanations for their structure and function are also possible.     

DNA adducts as indicators of genotoxic exposure in indigenous and transplanted mussels, Mytilus edulis L. from Icelandic coastal sites

Indigenous mussels, Mytilus edulis, were collected at sites with supposed different amounts of pollution; Reykjavík harbour, Keflavík harbour, Grafarvogur and Hvalfj?rdur (reference), along the south-western coast of Iceland in March 2000. Mussels from Hvalfj?rdur and Reykjavík harbour were also collected in August the same year. Additionally, mussels were transplanted from the reference site to Reykjavík harbour for 6 weeks during both winter and summer for comparison. DNA adducts were analysed by 32P-post-labelling in gills and digestive gland. Highest adduct levels were found in gill tissue from indigenous mussels collected in Reykjavík harbour. Adduct levels in both tissues from mussels collected at the reference site were below or very close to the detection limit during winter, but seemed to increase a little during summer. Mussels from sites with supposed intermediate pollution had intermediate levels of DNA adducts in gills but did not differ from Reykjavík harbour in digestive gland. No increase in adduct levels was observed in mussels transplanted from the reference site to Reykjavík harbour, except for a slight increase in digestive gland during winter. This study shows that 32P-post-labelling analysis of DNA adducts is sensitive enough to be used on indigenous mussels from relatively pristine areas and that adduct levels are increased in harbours/urban sites. However, transplantation of mussels from a clean site to the harbour for 6 weeks did not result in increased adduct levels in gills, the tissue with the highest adduct levels. The results also indicate that seasonal variation in adduct levels may occur.