K. Thomas Jensen

Invaders interfere with native parasite–host interactions

The introduction of species is of increasing concern as invaders often reduce the abundance of native species due to a variety of interactions like habitat engineering, predation and competition. A more subtle and not recognized effect of invaders on their recipient biota is their potential interference with native parasite–host interactions. Here, we experimentally demonstrate that two invasive molluscan filter-feeders of European coastal waters interfere with the transmission of free-living infective trematode larval stages and hereby mitigate the parasite burden of native mussels (Mytilus edulis). In laboratory mesocosm experiments, the presence of Pacific oysters (Crassostrea gigas) and American slipper limpets (Crepidula fornicata) reduced the parasite load in mussels by 65–77% and 89% in single and mixed species treatments, respectively. Both introduced species acted as decoys for the trematodes thus reducing the risk of hosts to become infected. This dilution effect was density-dependent with higher reductions at higher invader densities. Similar effects in a field experiment with artificial oyster beds suggest the observed dilution effect to be relevant in the field. As parasite infections have detrimental effects on the mussel hosts, the presence of the two invaders may elicit a beneficial effect on mussels. Our experiments indicate that introduced species alter native parasite–hosts systems thus extending the potential impacts of invaders beyond the usually perceived mechanisms.

Change of topography and sediment characteristics on an intertidal mud-flat following mass-mortality of the amphipod Corophium volutator

A Corophium-bed characterized by a mosaic of emerged areas and tidal pools during low water was established on an intertidal flat in the southern part of the Danish Wadden Sea. In spring 1990, the plateaux harboured relatively high densities of the amphipod Corophium volutator , whereas the amphipods were almost absent in the pools. Following a parasite-induced mass-mortality in the population of Corophium , summer 1990, the plateaux of the bed were subjected to sediment erosion. The heterogeneous plateau-pool structure disappeared, accompanied by increased median particle diameter and decreased silt content in the former plateau sediment. These events were accompanied by increased chlorophyll- a concentrations in the sediment of both plateau and pool areas. The results suggest Corophium volutator to be the single most important organism stabilizing the plateau sediment, and hence, the heterogeneous topographic structure of the Corophium -bed. It is proposed that the stabilizing effect of Corophium burrows also in deeper sediment strata may outweigh the surface stabilizing influence of epipelic diatoms, as well as the negative effect of amphipods eating the diatoms, in non-cohesive coarser sediments.

A field experiment on competition between Corophium volutator (Pallas) and Corophium arenarium Crawford (Crustacea:Amphipoda): effects on survival, reproduction and recruitment

Abstract The amphipods, Corophium volutator and C. arenarium, occur abundantly on some intertidal mudflats in the Danish Wadden Sea. During a faunal survey, the two species showed a spatial segregation in their distribution along an intertidal gradient. This could not be assigned to differences in abiotic factors. The possible significance of inter- and intraspecific competition amongst C. volutator and C. arenarium was investigated at various densities in experimental cages in the field. Density-dependent mortality of C. arenarium was caused by members of its own species and by C. volutator during a 4-wk period. Neither intraspecific nor interspecific density-dependent mortality could be demonstrated in C. Volutator. Interactions among the two Corophium species also influenced their fertility and recruitment. The fertility of both species was reduced by the addition of C. volutator or C. arenarium to control densities of either species. C. volutator recruits were less abundant in the Corophium -addition cages than in cages without any addition of adults (zero treatment), but the mean abundance of recruits was higher in the C. volutator treatments than in the C. arenarium treatments at the end of the experimental period. A few other species colonized the cages and among these the Nereis diversicolor recruits were less abundant in the C. arenarium treatments than in the C. volutator and the zero treatments. The field experiments indicated that adult C. volutator is competitively superior to adult C. arenarium and it is proposed that the mechanism involved is competition for space amongst adult Corophium individuals. The biotic interactions demonstrated in these experiments may maintain an existing spatial segregation between the two Corophium species. However, without a selective impact from other species C. volutator would probably have been the dominant species in this area. C. volutator has the greatest colonization rate due to its higher egg production and a better dispersal of juveniles compared to C. Arenarium. Measurements of the burrow depths of the two species suggested that C. arenarium is more protected against the most important bird predator, the dunlin Calidris alpina, than is C. volutator, due to its deeper burrow. The coexistence of the two Corophium species on this mudflat is supposed to be maintained as a result of the impact of C. alpina. But other infaunal species may also have negative impact on the abundance of Corophium. As an example, it was shown that Macoma balthica reduced the recruitment of C. volutator.

Reduced survivorship of Himasthla (Trematoda, Digenea)-infected cockles (Cerastoderma edule) exposed to oxygen depletion

Bivalve populations from inshore waters often accommodate a diverse trematode fauna that may have a variety of effects on host specimens. In particular, larval trematodes that grow or reproduce within their host are known to be severe pathogens, whereas trematodes utilising bivalves only for encystment are thought to be relatively benign. Yet this may depend on the environmental conditions, and it can be expected that such trematodes in concert with other stress agents can be detrimental to host organisms. To examine the impact of such larval trematodes on hosts subjected to stress, we studied the digenetic trematode Himasthla elongata and one of its second intermediate hosts, the bivalve Cerastoderma edule. Experimentally infected cockles and non-infected cockles were exposed to oxygen depletion, whereupon we measured their burrowing ability and survivorship. After 30 h of hypoxia, the survival of infected cockles was significantly reduced compared to non-infected cockles, whereas no effect of parasites on cockles under normoxic conditions was found. In addition, parasites tended to reduce the burrowing ability of cockles exposed to hypoxia but the effect was not clear. The effect of parasites and possible ecological consequences are discussed and it is suggested that the combined effects of parasites and oxygen deficiency may explain some hitherto unexplained cases of mass mortalities in bivalve populations.

Effect of intermediate host size (Cerastoderma edule) on infectivity of cercariae of three Himasthla species (Echinostomatidae, Trematoda)

To understand prevalence patterns of parasites in marine host populations experimental infection studies are required. Bivalves are important host organisms to a variety of trematodes species and in our study area (Wadden Sea) three different Himasthla species co-occur in cockle populations. These species are morphologically very similar but differ with respect to various morphometric dimensions. To study the possible functional importance of differences between Himasthla cercariae (the free-living stage shed from prosobranch snails and encysting as metacercariae in bivalves), we experimentally measured the infectivity of the three congeners in regard to different size groups of juvenile cockles. The smallest species, H. interrupta, has a high infectivity in small cockles (optimum around 4 mm), whereas the two other congeners H. continua and H. elongata exhibit low infection efficiencies in cockles less than 6 mm and higher efficiencies in larger cockles. Behavioural experiments were performed to identify proximate causes underlying the observed infection patterns. Parasite avoidance behaviour of the cockle varies in a host–parasite size-dependent manner so that a large cercaria tend to provoke an avoidance response in a small cockle. The possible consequences of the observed host size preferences in relation to definitive host species (waterbirds) are discussed and it is suggested that one or more of the parasite species are adapted to other host species and that their sympatric occurrence in cockles in our study area is a result of a spinoff from their main cycle mediated through migratory birds.

Density-dependent growth in cockles ( Cerastoderma edule ): evidence from interannual comparisons

To examine density-dependent effects on growth in Cerastoderma edule (L.) (Bivalvia), growth rates of individual cockles established by chance at high and low densities on the same intertidal mudflat in two different years (1984 and 1989) were compared. Two-year-old cockles occurring at high densities (>2000 individuals m -2 in 1984) attained mean lengths from 16.1 to 18.8 mm on the lower shore at the end of their third growing season, while low-density cockles ( -2 in 1989) of the same age reached mean lengths in the range of26.5 to 30.3mm. In terms of individual weight the difference between cockles from the two periods was even more striking, as the flesh ash-free dry-weight of a cockle from 1984 constituted only 7% of that from a 1989 cockle. During both periods growth in cockles increased with the duration of tidal submersion, but the interannual growth differences exceeded by far the effect of differences in submersion time.

Macrozoobenthic community structure in a high-arctic East Greenland fjord

Abstract A macrozoobenthic community study was conducted in an East Greenlandic fjord (Young Sound, 74°18′N; 20°15′W) during the ice-free period from July to August in 1996. Grab samples as well as underwater photography were used for quantifying the macrozoobenthos at water depths between 20 and 85 m. Abundance decreased with depth from 2700 ind. · m−2 at 20 m to 900 ind. · m−2 at 85 m. At a time series station at 35 m, abundance increased from 700 ind. · m−2 in mid-July to 1400 ind. · m−2 in mid-August. Polychaetes dominated in grab samples but bivalves constituted an important part of the benthic fauna, especially at the shallow part of the depth gradient. Photographs revealed high abundances of large epifaunal species, especially brittle stars. Diversity was generally high, with around 45 species per 201 individuals, as calculated by Hurlberts rarefaction term. A gradual change in community structure with depth was observed, which could be related to variation in sediment composition and disturbance intensity.

Macrozoobenthos on an intertidal mudflat in the Danish Wadden Sea: Comparisons of surveys made in the 1930s, 1940s and 1980s

To assess whether long-term faunal changes have occurred on intertidal flats in the Danish Wadden Sea, results of faunal surveys in the Skallingen area during the 1930s, 1940s and 1980s were compared. Since the earlier studies, two species have disappeared (Scrobicularia plana andPetricola pholadiformis) and two have invaded the area (Tharyx killariensis andEnsis americanus). This is, however, not a local event as species changes have occurred on a larger scale (Wadden Sea region). Comparison of abundance data did not provide evidence of changes from the 1930s until the 1980s. Spatio-temporal fluctuations in two dominant species, the mudsnailHydrobia ulvae and the cockleCerastoderma edule chosen for closer examination, could be explained by natural processes. Growth data on cockles from the 1930s and the 1980s matched perfectly. During both periods, cockles showed a much lower growth rate than generally reported from the Wadden Sea, while at the same time they occurred at high densities (>2000 ind·m−2). Intraspecific competition among cockles is suspected as being a major cause of the low growth rates. Estimates of secondary production and P/B-ratios of cockles were also in general agreement during the 1930s and the 1980s when corrected for differences in the age structure of the cockle populations. With the possible exception ofMytilus edulis, which according to some observations has extended its range along the low-water line, the present comparison failed to demonstrate long-term faunal changes caused by increased eutrophication. This results is expected to be representative for intertidal flats not exposed to direct impacts from terrestrial run-offs and waste-water discharges.

Digenean trematode species in the cockle Cerastoderma edule: identification key and distribution along the north-eastern Atlantic shoreline.

We describe the digenean fauna of one of the dominant intertidal hosts, the common cockle Cerastoderma edule , in terms of biomass, off north-eastern Atlantic shores. Using published and unpublished literature we have prepared an identification key and provide an up-date of the large-scale distributional patterns of digenean species of the common cockle. At least sixteen digenean species, belonging to seven families, use cockles as intermediate host. Among these species two utilize cockles as first intermediate host only, whereas two species utilize cockles as both first and second intermediate host. The remaining eleven species have cockles as their second intermediate host. Water birds and fish are the definitive hosts to twelve and four species, respectively. Cockles are infected with digeneans along the latitudinal gradient from southern Morocco to the western region of the Barents Sea often with high infection levels. Whereas some of these digenean species occur along most of the latitudinal gradient others show a more restricted northern or southern distribution mostly caused by an underlying latitudinal gradient of host species. Knowledge of digenean species and their large-scale distribution pattern may serve as a baseline for future studies dealing with the effects of climate change on parasite–host systems. For such studies the cockle and its digenean community could be an ideal model system.

The influence of the trematode Microphallus claviformis on two congeneric intermediate host species (Corophium): infection characteristics and host survival

In a laboratory experiment the survival of two amphipod species Corophium volutator and C. arenarium was differentially affected by the digenetic trematode Microphallus claviformis, the former suffering from parasite induced mortality. No clear species specific parasite preference was observed, but there was a difference between the two frequency distributions of parasite number per host, indicating a preference for the competitively superior C. volutator. In both species the parasite metapopulation was overdispersed, but to the highest degree in C. volutator. Formation of a penetration cyst on the Corophium cuticle preceeds the penetration of the M. claviformis cercariae, a common feature of microphallid digeneans, but not earlier reported from this species. Dead individuals of both species carried significantly more penetration cysts than the live ones which suggests that massive invasion of cercariae during a short period of time is the cause of death of Corophium, rather than crude parasite intensity. The potential for M. claviformis to act as a structuring agent in communities of Corophium species is discussed.