Search

The successful settlement of the African pea crab Afropinnotheres monodi in the Bay of Cadiz was analysed to gain new insights into the evolution of pea crab parasite life history traits. The pea crab lives symbiotically and with high prevalence in the bivalves Cerastoderma glaucum and Mytilus galloprovincialis, and with low prevalence in Scrobicularia plana. A remarkable monopolisation of the host (1 crab per bivalve) occurred irrespective of host species and crab demographic categories (males, hard females, soft females), probably as an optimisation of resources in small hosts. However, there was a clear asymmetry in host use by the different crab categories. A 1:1 sex ratio was found in crabs harboured by C. glaucum and S. plana, with most of the female crabs being small and hard specimens, whereas crabs inhabiting M. galloprovincialis were primarily large reproductive females. Ovigerous females were found throughout the year in M. galloprovincialis, and there was a strong correlation between female size and fecundity, suggesting that the females harboured by this host were the major contributors to the reproductive effort of the studied population. Conversely, most of the new crabs recruiting to the population were harboured by the remaining 2 host species. We hypothesise that such a generalist but asymmetrical usage of bivalve hosts by pea crabs may have clear benefits for species such as A. monodi, that are undergoing geographical expansion, facilitating their dispersal to new locations and their successful settlement in sheltered systems, such as the Bay of Cadiz. © Inter-Research 2014., This research work was funded by remnant funds of the PAI RNM108 research group.

Bacillus thuringiensis (Bt) has been used successfully as a biopesticide for more than 60 years. More recently, genes encoding their toxins have been used to transform plants and other organisms. Despite the large amount of research on this bacterium, its true ecology is still a matter of debate, with two major viewpoints dominating: while some understand Bt as an insect pathogen, others see it as a saprophytic bacteria from soil. In this context, Bt's pathogenicity to other taxa and the possibility that insects may not be the primary targets of Bt are also ideas that further complicate this scenario. The existence of conflicting research results, the difficulty in developing broader ecological and genetics studies, and the great genetic plasticity of this species has cluttered a definitive concept. In this review, we gathered information on the aspects of Bt ecology that are often ignored, in the attempt to clarify the lifestyle, mechanisms of transmission and target host range of this bacterial species. As a result, we propose an integrated view to account for Bt ecology. Although Bt is indeed a pathogenic bacterium that possesses a broad arsenal for virulence and defense mechanisms, as well as a wide range of target hosts, this seems to be an adaptation to specific ecological changes acting on a versatile and cosmopolitan environmental bacterium. Bt pathogenicity and host-specificity was favored evolutionarily by increased populations of certain insect species (or other host animals), whose availability for colonization were mostly caused by anthropogenic activities. These have generated the conditions for ecological imbalances that favored dominance of specific populations of insects, arachnids, nematodes, etc., in certain areas, with narrower genetic backgrounds. These conditions provided the selective pressure for development of new hosts for pathogenic interactions, and so, host specificity of certain strains.

Exploitation of more than one host species may increase fitness of parasites, but it also shapes their life cycle and evolution. During its post-metamorphic stages, the pea crab Dissodactylus primitivus parasitizes the echinoids Meoma ventricosa and Plagiobrissus grandis with high prevalence. However, the present study provides evidence that P. grandis is infected by adult crabs only, attesting to an unusual asymmetry in host exploitation. Because of its low population densities, P. grandis could be a secondarily acquired host, and its addition to the crab life cycle could be adaptive. The latter hypothesis is tested in this study. Choice experiments revealed that adult and juvenile crabs were equally attracted to and able to settle on the 2 hosts. Most adult crab characteristics were not significantly different between the host species, but females living on P. grandis had higher fecundity. This asymmetric life cycle, where adult parasites infecting the 2 hosts show different relative fitnesses, looks like a transitory stage as predicted in models which predict the evolution of parasitic complex life cycles. However, this parasite asymmetric exploitation may also be a stable state, which occurs due to insufficient differences in crab fitness between the 2 hosts and the absence of host preference.

© 2014 The Authors. Many disease systems exhibit complexities not captured by current theoretical and empirical work. In particular, systems with multiple host species and multiple infectious agents (i.e., multi-host, multi-agent systems) require novel methods to extend the wealth of knowledge acquired studying primarily single-host, single-agent systems. We outline eight challenges in multi-host, multi-agent systems that could substantively increase our knowledge of the drivers and broader ecosystem effects of infectious disease dynamics.