Deception & manipulation: Argyrodinae spiders as parasites and hosts

Spiders (order Araneae) are a diverse and successful taxon, present across the world in even extreme environmental conditions. Much of this success is due to their production of silk which, among other uses, forms webs with which to effectively capture prey and avoid predators. However, these resource-intensive external structures are vulnerable to exploitation by organisms utilising the silk structures and captured prey for their own benefit, termed kleptoparasites. One such parasite is the small spider Argyrodes argyrodes, within the sub-family Argyrodinae. The species is commonly found in groups on the webs of other spiders across the Mediterranean, West Africa, Canary Islands and Seychelles. A. argyrodes has been observed scavenging, stealing food bundles, feeding alongside the host, eating host eggs and young and even attacking moulting hosts. This thesis investigates A. argyrodes kleptoparasites living on the webs of colonial Cyrtophora citricola in southern Spain. The aggregation of C. citricola to form sizable, long-lasting, web structures makes them an ideal host to provide large, stable habitat patches for kleptoparasites. Research presented here shows the abundance patterns and reproductive efforts of Spanish C. citricola through a year, showing an unusually dominant reproductive period in spring which is closely followed by the reproductive efforts of A. argyrodes. High frequency of C. citricola egg sac infection by Philolema palanichamyi wasps in autumn is postulated here to skew C. citricola reproduction and data supports a resistance to wasps provided by aggregation into larger colonies. Larger colonies also contained lower A. argyrodes numbers per host, providing further selective pressures favouring colonial strategies in spiders. Kleptoparasitic behaviour by A. argyrodes is described within this thesis’ preliminary work and feeding alongside the host was found to be common in large kleptoparasites. A. argyrodes presence is then shown to reduce feeding frequency of Cyrtophora citricola, postulated to have a negative effect on nutritional intake and so fitness. However, no influence was found in web repair or prey dropping behaviour of hosts. Adaptations of A. argyrodes to this kleptoparasitic specialism are also considered. Firstly, it is theorised that, as the parasites produce little silk of their own, relaxation in selective pressures might lead to a higher rate of mutation retention in unused silk protein sequences,. No support for this is found, with A. argyrodes and other kleptoparasitic Argyrodinae showing similar variation to that of free-living Argyrodinae in their major and minor ampullate silk primary structure. Abundance data collected as part of this thesis indicates a female-biased sex ratio in Spanish A. argyrodes. To this end, the microbiome of A. argyrodes is investigated and targeted screening explored infection frequency of manipulative endosymbiotic bacteria across populations. These studies found Rickettsiella, Rickettsia, Cardinium and Spiroplasma bacteria within A. argyrodes and identified strains present to be either unique to the species or to spider hosts through 16S sequence comparison. Cardinium and Spiroplasma population screening found each present in approximately one quarter of samples across populations. One population tested contained neither bacteria and this is expected to be a founder effect brought on by fluctuating and genetically similar populations. This thesis provides an initial snapshot of a system involving colonial and parasitic spiders little explored in the literature, providing a basis with which to continue work on the ecology, behaviour, silk production and microbiome of arachnids with such extreme strategies