Crawling-tracks of trematode infected Macoma balthica (L.)

Increasing global temperature and sea-level rise have led to concern about expansions in the distribution and prevalence of complex-lifecycle parasites (CLPs). Indeed, numerous environmental variables can influence the infectivity and reproductive output of many pathogens. Digenean trematodes are CLPs with intermediate invertebrate and definitive vertebrate hosts. Global warming and sea level rise may affect these hosts to varying degrees, and the effect of increasing temperature on parasite prevalence has proven to be nonlinear and difficult to predict. Projecting the response of parasites to anthropogenic climate change is vital for human health, and a longer term perspective (10 4 y) offered by the subfossil record is necessary to complement the experimental and historical approaches of shorter temporal duration (10 −1 to 10 3 y). We demonstrate, using a high-resolution 9,600-y record of trematode parasite traces in bivalve hosts from the Holocene Pearl River Delta, that prevalence was significantly higher during the earliest stages of sea level rise, significantly lower during the maximum transgression, and statistically indistinguishable in the other stages of sea-level rise and delta progradation. This stratigraphic paleobiological pattern represents the only long-term high-resolution record of pathogen response to global change, is consistent with fossil and recent data from other marine basins, and is instructive regarding the future of disease. We predict an increase in trematode prevalence concurrent with anthropogenic warming and marine transgression, with negative implications for estuarine macrobenthos, marine fisheries, and human health.parasites | disease | global warming | paleoecology | mollusks P redictions for the increase of global temperature and concomitant sea-level rise in the coming decades and centuries have led to concerns about the range expansion or shift of parasites and pathogens to higher latitudes, resulting in an increase in prevalence and severity of diseases. Indeed, environmental factors such as temperature, humidity, and precipitation can strongly influence the infectivity and reproductive output of many pathogens (1, 2); however, the life cycles of parasites can be quite complex, and accurately predicting how their prevalence will be influenced by these factors can be difficult. For example, digenean trematodes are complex-lifecycle parasites (CLPs) with multiple intermediate invertebrate hosts and a definitive vertebrate host. Environmental change may affect each of these hosts to varying degrees, such that a modest increase in temperature could result in an increase of parasite reproductive output and infectivity. However, a more severe increase in temperature could be beyond the thermal tolerance of one of the hosts, thus ultimately resulting in a decrease in prevalence (3)(4)(5). Still other studies have produced quantitative models and compared spatial and temporal (seasonal-to historical-scale) aspects of climatic and other environmental variables with outbreak data i...

The aim of the present work was to assess the effect of digenean trematodes on indirect mortality of the cockle Cerastoderma edule, an infaunal bivalve. The tested hypothesis was that parasites altered the burrowing capacity of cockles and thus exposed them at the sediment surface, where they are more vulnerable to predators. If the predator is the final host, this mechanism, which drives the cockle out of the sediment, is considered as a 'favourization'. Cockle populations from 2 stations in Arcachon Bay (France) -Banc d'Arguin (oceanic situation) and La Canelette (lagoonal situation) -were sampled for 1 yr. At La Canelette, monitoring every 2 d showed that 50% of adult cockles regularly migrated to the sediment surface at a rate of 5 cockles m -2 yr -1 and disappeared in a few days. In the laboratory, 67% of these 'surface cockles' did not burrow again, suggesting that they would die in the field. Moreover, mortality measured after 7 d in the laboratory was 2 to 5 times higher than mortality of 'buried cockles', at both stations and particularly during summer. Species richness and abundance of digeneans from both stations were compared in 'buried' and 'surface' individuals to determine whether parasites played a role in cockle migration and mortality. Ten and 9 digeneans were found at Banc d'Arguin and La Canelette, respectively, with Himasthla quissetensis and Labratrema minimus being the most prevalent and abundant species at both stations. The abundance of H. quissetensis was slightly higher in surface cockles at Banc d'Arguin, but the difference fluctuated with station and cockle age (or size). L. minimus prevalence was only higher in surface cockles at La Canelette. In the latter station, we estimated that L. minimus and H. quissetensis were responsible for the emergence of 9 and 2%, respectively, of the buried cockles. Although this favourization mechanism may induce some mortality in cockles, it does not alone explain the magnitude of the observed mortalities (41 and 57% at La Canelette and Banc d'Arguin, respectively). A correspondence analysis did not show the presence of a particular parasite community in buried or surface cockles, which could explain these high surface cockle mortalities in association with the 2 dominant digeneans.

When groups of freshly collected cockles Cerastodema edule L. were transferred to laboratory conditions and exposed to cycles of tldal emersion and immersion and a light regune of 12 h light. l 2 h dark, up to 40% of the group emerged onto the surface of the substratum during emersion at the onset of darkness. This pattern of emergence decreased in intensity from 37% to 10% of all individuals emerging after 30 d in the laboratory. However, not all the same cockles emerged each day. After emergence, the cockles remained quescent until Immersion, when they actively roamed and ploughed across the sand surface and then reburrowed. During movement the cockles produced furrows up to 0.5 m in length; at hlgh densities cohsions with other buried cockles occasionally occurred, which in turn stimulated emergence and a change of posihon. Groups of cockles previously exposed to a hdal cycle and 1ight:dark regime responded to continuous darkness by emerging onto the substratum for 3 low tides before the activity pattern disappeared. The intensity of the emergence pattern in continuous darkness was less than the nocturnal activlty observed during light :dark conditions. Similarly, under contmuous immersion, emergence during darkness was less pronounced than under sunulated tidal emersion. In continuous illummahon an emergence pattern was absent. The nocturnal pattern of emergence observed in the laboratory can be explained in terms of the cockles' reaction to burial with sediment. In laboratory experiments, cockles which were buned with a covering of sediment at the onset of emersion responded immediately by emerglng onto the sedvnent surface. One group maintained in continuous darkness emerged at low water and continued to roam and plough across the surface when they were immersed, whereas a second group sublected to continuous ~Uumination emerged upon burial and then reburrowed. Sirmlar experiments carried out in the natural environment confirmed the biological sigmficance of the laboratory-induced emergence rhythm. On the shore, replicate areas were covered with sediment during daylight, just prior to emersion of the natural cockle beds. Approximately 1.5 h after burial, cockles were observed to emerge with peak activity at the onset of darkness, and agaln 24 h later. However, cockles &d not emerge in any numbers In control areas in which no sediment was added. The sudden burial of cockles with sediment during a storm w l l stimulate them to move in order to maintain contact with the sand surface for feeding and respuation. The disturbance of cockles dunng collection and the sudden onset of darkness in the laboratory followed by the immediate emergence of the cockles, simulated the cockles' reactlon to deposition of sedlrnent in the field. The observed crawhng and burrowing behaviour of the cockles dunng high tide and darkness represents a situation in which the cockle is continuing to try to emerge as d it was still covered in sediment.