Ectotherms are vulnerable to environmental changes and their parasites are biological health indicators. Thus, parasite load in ectotherms is expected to show a marked phenology. This study investigates temporal host–parasite dynamics in a lizard community in Eastern Spain during an entire annual activity period. The hosts investigated were Acanthodactylus erythrurus, Psammodromus algirus and Psammodromus edwardsianus, three lizard species coexisting in a mixed habitat of forests and dunes, providing a range of body sizes, ecological requirements and life history traits. Habitat and climate were considered as potential environmental predictors of parasite abundance, while size, body condition and sex as intrinsic predictors. Linear models based on robust estimates were fitted to analyse parasite abundance and prevalence. Ectoparasitic mites and blood parasites from two haemococcidian genera were found: Lankesterella spp. and Schellackia spp. Habitat type was the only predictor explaining the abundance of all parasites, being mostly higher in the forest than in the dunes. The results suggest that particularities in each host–parasite relationship should be accounted even when parasites infect close-related hosts under the same environmental pressures. They also support that lizard parasites can be biomarkers of environmental perturbation, but the relationships need to be carefully interpreted for each host–parasite assemblage.

Sponges interact in various ways with a wide variety of organisms in benthic communities and ecological interactions may influence the distribution, abundance and diversity of these organisms in different sites. Although several studies have already been developed for Demospongiae, knowledge of ecological interactions in the class Calcarea is lacking. Some calcareous sponges are considered weak competitors for space and to have developed strategies to survive in highly dynamic environments, such as exploitation of micro refuges and epibiosis. We aimed to investigate the influence of intra- and interspecific interactions in the abundance, growth and lifespan of the calcareous sponge Clathrina aurea in south-eastern Brazil. Two peaks of abundance and area coverage were observed during the studied year, probably associated with reproductive effort. We found no evidence of intra- or interspecific competition between the sponge and the benthic community. Clathrina aurea was found using parts of other organisms as substrate (epibiosis), such as algae, demosponges and corals. It was seen that the interaction of the sponge with coral polyps positively affects the lifespan of sponge individuals and the interstices of corals probably promote a micro refuge for C. aurea against competitors for space and predators.

We have examined genetic variability in parasites in the context of ecological interactions with the host. Recent research on Echinococcus, Giardia and Cryptosporidium has been used to illustrate: (i) the problems that parasite variability and species recognition pose for understanding the complex and often controversial relationship between parasite and host occurrence; (ii) the need for accurate parasite characterization and the application of appropriate molecular techniques to studies on parasite transmission if fundamental questions about zoonotic relationships and risk factors are to be answered; (iii) our lack of understanding about within-host interactions between genetically heterogeneous parasites at the inter-and intraspecific levels, and the significance of such interactions with respect to evolutionary considerations and the clinical outcome of parasite infections. If advances in molecular biology and mathematical ecology are to be realized, we need to give serious consideration to the development of appropriate species concepts and in vivo systems for testing the predictions and assumptions of theoretical models.