Prey acceptance and metabolic specialisations in some Canarian Dysdera spiders

Spiders of the genus Dysdera are peculiar for their preying on terrestrial isopods though the preference for this prey type varies between species. We tested prey acceptance of two isopod and two non-isopod prey in 8 species endemic to the Canary Islands, and analyzed growth and metabolic parameters (growth efficiency; dry mass, lipid and N extraction efficiency; lipid:protein consumption ratio) of 6 of these species when fed either house flies (Musca domestica) or isopods (Porcellio scaber) in the laboratory. The species represented four morphological types (unmodified chelicerae, slightly elongated chelicerae, concave chelicerae, flattened fang), supposedly reflecting different specializations to isopod prey. The results showed reduced relative acceptance of non-isopod and increased acceptance of isopod prey in species groups with specialised morphologies compared to the unmodified species group. All species had similar or lower growth and growth efficiency when feeding on isopods than on flies. Extraction efficiency of dry mass and lipid were higher for flies than for isopods, while extraction efficiency of protein was higher for isopods than for flies. All species also utilized isopod protein equally well, but protein utilization of flies was lower in the presumed specialist compared to generalist species, indicating a possible metabolic trade-off from isopod specialisation. Thus, morphological adaptations were associated with increased behavioural preferences for isopods and reduced metabolic ability to handle 'generalist' prey.