K. T. Jensen

The role of biotic factors in the transmission of free-living endohelminth stages

The transmission success of free-living larval stages of endohelminths is generally modulated by a variety of abiotic and biotic environmental factors. Whereas the role of abiotic factors (including anthropogenic pollutants) has been in focus in numerous studies and summarized in reviews, the role of biotic factors has received much less attention. Here, we review the existing body of literature from the fields of parasitology and ecology and recognize 6 different types of biotic factors with the potential to alter larval transmission processes. We found that experimental studies generally indicate strong effects of biotic factors, and the latter emerge as potentially important, underestimated determinants in the transmission ecology of free-living endohelminth stages. This implies that biodiversity, in general, should have significant effects on parasite transmission and population dynamics. These effects are likely to interact with natural abiotic factors and anthropogenic pollutants. Investigating the interplay of abiotic and biotic factors will not only be crucial for a thorough understanding of parasite transmission processes, but will also be a prerequisite to anticipate the effects of climate and other global changes on helminth parasites and their host communities.

The importance of some epibenthic predators on the density of juvenile benthic macrofauna in the Danish Wadden Sea

Abstract Recruitment of Cerastoderma edule (L.), Mya arenaria L., Mytilus edulis L., and Macoma balthica L.) on an intertidal flat in the Danish Wadden Sea was very successful in a year following a severe winter. Initially, densities of Cerastoderma edule up to 60000 ind · m −2 were observed but shortly after settling declining densities occurred. Among the main epibenthic predators on juvenile animals on the flat, small individuals of the shore crab, Carcinus maenas (L.) were found to be important. Thus, laboratory experiments demonstrated that juvenile crabs in particular were preying on Cerastoderma edule. The juvenile crabs arrived simultaneously with the spat of C. edule. In July and August, the production of juvenile crabs was estimated to account for 26.1% of the elimination of cockles. Later the cockles were protected from predation by crabs by size. Analysis of stomach contents showed that another dominating predator on the flat, Crangon crangon (L), was mainly feeding on meiofauna. We suggest that the juvenile individuals of Carcinus maenas play a key role in structuring the benthic community in the Wadden Sea by preventing the development of cockle beds.

Mass mortality in two common soft-bottom invertebrates,Hydrobia ulvae andCorophium volutator-the possible role of trematodes

Two littoral macrofaunal invertebrates,Hydrobia ulvae (Prosobranchia) andCorophium volutator (Amphipoda) suffered mass mortality on an intertidal mudflat in the Danish Wadden Sea in May–June 1990. Dissection of collectedH. ulvae individuals revealed a considerable increase from March to May in numbers of infected individuals by microphallid trematodes that useH. ulvae andC. volutator as first and second intermediate host, respectively. The numbers of infested snails were hereafter reduced by an amount equal to the observed mortality rate of snails. At the same time, theC. volutator population became extinct. Since other conceivable mortality factors could be ruled out, parasites are suspected to be the causative agent. Apart from the expected effects on potential predators by the decline in the two invertebrate populations, the benthic community changed and destabilization of the substratum occurred probably because of the die-off inC. volutator. Meteorological data suggest high temperatures as a triggering factor of the massdevelopment of the studied trematodes.

Ambient fauna impairs parasite transmission in a marine parasite-host system

To understand possible factors controlling transmission of trematode larvae between first and second intermediate hosts we examined the impact of ambient fauna on parasite transmission in a marine intertidal parasite-host association. Cockle hosts (Cerastoderma edule) kept together with selected co-occurring macrozoobenthic species in mesocosms acquired a lower parasite load compared to cockles kept alone, when targeted by cercariae of the trematode Himasthla elongata. The reduction of parasite load in the cockles differed between the 7 macrozoobenthic species tested and was between 35 and 91%. Three different types of reduction could be distinguished: (1) predators (Carcinus maenas, Crangon crangon) actively preying upon cercariae, (2) non-host filter feeders (Crepidula fornicata, Mya arenaria, Crassostrea gigas) filtering cercariae but not becoming infected and (3) alternative hosts (Mytilus edulis, Macoma balthica) becoming infected by the cercariae and thus distracting cercariae from the target hosts. In addition, interference competition may occur in the form of disturbance of cockles by ambient organisms resulting in lower filtration rates and subsequently lower parasite loads. Our results suggest that the species composition and relative abundance of the ambient fauna of parasite-host systems play an important role in controlling trematode transmission rates in benthic marine systems.

Enhanced erodibility of fine-grained marine sediments by Hydrobia ulvae

The common mud snail Hydrobia ulvae is a widespread and dominant deposit feeder on fine-grained substrata along the European Atlantic coastline. Previous studies have shown that mud snail activities such as grazing, faecal pellet production and mucous production may influence physical properties of the surface sediment layer and thus depositional and erosional processes. To quantify the influence of Hydrobia density on key parameters such as erosion threshold and erosion rate, a shortterm laboratory experiment was conducted. Snails were placed on fine-grained sediment at densities of 10000 and 50000 ind m 2 and erosion experiments were carried out one, three and five days after establishment of the sediment beds. Controls without H. ulvae were treated the same way. The presence of H. ulvae significantly increased the erosion rate and decreased the erosion threshold compared to snail-free control plots. The erosion rate was increased by a factor of 2 to 4 when H. ulvae were present and showed stronger influence by the snail than the erosion threshold. Snail density did not affect the erosion threshold, but the erosion rate doubled with an increase in density from 10000 to 50000 ind m 2 . The erosion rate was only marginally different after one day but the difference increased over time and the erosion rate was significantly different for all treatments after five days. No significant time dependence was observed for the erosion threshold. The results generally confirm results obtained in situ and differences can be related to different hydrodynamic conditions under field and laboratory conditions. D 2002 Elsevier Science B.V. All rights reserved.

The effect of larval trematodes on the survival rates of two species of mud snails (hydrobiidae) experimentally exposed to desiccation, freezing and anoxia

Digenetic trematodes are widespread among mud snails (Hydrobiidae) living in coastal lagoons and estuaries, but knowledge is generally lacking on their impact on these host organisms. We examined the survival rates of infected and non-infected experimental populations of two mud snail species,Hydrobia ventrosa (Montagu) andHydrobia neglecta Muus, exposed to desiccation, freezing and anoxia in the laboratory. Our experiments indicated that non-infected groups of both species had similar survival rates after being subjected to desiccation and anoxia, whereasH. ventrosa survived freezing better thanH. neglecta. However, infected groups ofH. neglecta specimens subjected to desiccation showed significantly lower survival rates than non-infected groups. Infected and non-infected snails of both species exposed to freezing and anoxia exhibited similar survival rates. The possible mechanisms by which parasites influence their hosts are discussed. It is unlikely that the parasites in the present case mediate the coexistence of the twoHydrobia-species, because the snail with the highest reproductive effort-H. neglecta-showed lower infection rates in situ than its congenerH. ventrosa.

Climate influences parasite-mediated competitive release

Parasitism is believed to play an important role in maintaining species diversity, for instance by facilitating coexistence between competing host species. However, the possibility that environmental factors may govern the outcome of parasite-mediated competition has rarely been considered. The closely related amphipods Corophium volutator and Corophium arenarium both serve as second intermediate host for detrimental trematodes. Corophium volutator is the superior competitor of the two, but also suffers from higher mortality when exposed to infective trematode stages. Here, we report parasite-mediated competitive release of C. arenarium in an intertidal habitat, in part triggered by unusually high temperatures linked to the North Atlantic climate oscillation (NAO). The elevated temperatures accelerated the transmission of cercariae from sympatric first intermediate hosts (mud snails) to amphipods, causing a local collapse of the parasite-sensitive C. volutator population and concordant increase in the abundance of the competitively inferior C. arenarium.

Trematode fauna of Hydrobia ulvae (Gastropoda: Prosobranchia) in a eutrophic temperate estuary

Digenean trematodes infecting the mud snail Hydrobia ulvae were studied at two key sites of the Mondego Estuary (Portugal), from January 1993 to September 1995: a mud flat covered by the seagrass Zostera noltii , and a bare sand flat where seasonal macroalgal blooms occurred as a result of eutrophication. Digeneans belonging to Microphallidae, Notocotylidae, Haploporidae and Heterophyidae were recorded in snails from both sites whereas representatives from Echinostomatidae were only found in snails from the seagrass bed. The density of infected snails was higher at the seagrass bed than at the eutrophic area partly reflecting the difference in population structure of H. ulvae between sites. The Zostera noltii bed supports an abundant and well-structured mud snail population, with all size-classes represented, in contrast to a less abundant and juvenile dominated population in the eutrophic area. In the Z. noltii bed no clear seasonal and interannual patterns emerged in the infection densities. In the eutrophic area, the density of infected individuals increased in the presence of algae, and decreased during the occurrence of occasional flood events. Small scale spatial and temporal patterns in the prevalence of digenean trematodes in mud snails were therefore demonstrated within the Mondego Estuary that has experienced a mixture of anthropogenic impact and climate instability.

A New Video and Digital Camera System for Studies of the Dynamics of Microtopographic Features on Tidal Flats

A newly developed video and camera system for tidal flat microtopographic studies is presented. It consists of a SONY handy cam placed in an underwater housing mounted on a frame about 70 cm above the sediment surface. A rectangular surface area of 30 ×40 cm is imaged by the camera. The camera records video sequences and/or digital images at predetermined time lapses. The total number of images is about 540, and a similar number of 10-second long video sequences can be recorded. The camera is programmed with a PC before deployment, and the total deployment time depends on the time lapse between recordings. The camera is connected to an external power supply (12 volt), and a halogen projector pointing towards the sediment surface ensures that the system is operable on a 24-hours scale. The system has been tested in the Danish Wadden Sea. It has proved to be a very useful tool in studies of topographic effects of erosion and deposition sequences, and for studies of benthic organisms-sediment interactions. The test site was further equipped with sensors for water and seabed measurements, which proved to be indispensable regarding the interpretation of recorded image time-series.

Growth Performance of Indian Major Carps and Their Hybrids in Polyculture in Bangladesh

To explore the effect of hybridization on carp production, survival rates and growth patterns were studied in three Indian major carps—catla, mrigal, and rohu—and their hybrids in polyculture over 2 years. Hybrids are frequently observed among fingerlings. Fry of pure species and of F1 hybrids used for the grow-out phase were produced from six parental fish, two of each species. The fry were used for three setups, one with the three pure species, one with three hybrids, and one with a mixture of pure and hybrid fry. Mrigal had the highest survival rate (77%–81%), whereas only 44%–47% of catla survived. Rohu × catla and catla attained the highest final weights (604 ± 32 g and 587 ± 34 g fresh weight) after 2 years’ growth, whereas mrigal × catla showed the lowest final weight (283 ± 42 g). For all species and hybrids, the highest growth rates were observed during the wet season (April–October). Generally, the yield was low and the hybrids mrigal × catla and rohu × mrigal had the lowest production, indicating that hybridization should be avoided in commercial production.