December 18, 2013
Pollen-Munching Spiders May Be Reclassified As Omnivores
[ Watch the Video: Pollen On The Menu For Orb-Weaver Spiders ]
Brett Smith for redOrbit.com - Your Universe Online
Spiders have long been thought of as creepy-crawly predators, but a new study from a couple of European researchers has found that these arachnids will munch on pollen from time to time as well.
According to their report in the journal PLOS ONE, the study team found that the diet of younger, common orb-weaver spiders includes around 25 percent pollen. The spiders were even found to eat pollen when insect prey was readily available. While spider webs are most famously used to capture prey, they can also ensnare aerial pollen and fungal spores.
"Most people and researchers think of spiders as pure carnivores, but in this family of orb web spiders that is not the case,” said study author Dirk Sanders, an ecologist at the University of Exeter. “We have demonstrated that the spiders feed on pollen caught in their webs, even if they have additional food, and that it forms an important part of their nourishment.”
To make their discovery, the researchers performed feeding experiments and a stable isotope analysis on juvenile orb-weaving spiders to determine if they include plant resources in their diet.
“The amount of pollen consumption was quite similar in the laboratory and in the field, indicating that orb-weaving spiders actively feed on pollen for optimal nourishment, with all essential nutrients delivered by insect prey and resources provided by plants, at least at early life stages,” the researchers wrote in their report.
“Most pollen grains are too large to pass through the spider’s pharynx and therefore cannot be swallowed accidentally but have to be actively consumed,” the researchers concluded. “Spiders dissolve the outer coating of a pollen grain via extra-oral digestion and suck up the dissolved nutrients afterwards.”
“The pollen grains used in this study were of different shapes and sizes,” they added. “It is likely that other araneid species feed on different pollen. “
The research team found that providing their spiders with different diets did not affect weight gain in the juveniles.
“This suggests that when insect prey is also available, the supplement through pollen might not be important for spider growth, but it could have an effect on long-term fitness, i.e. reproduction and survival, as indeed pollen has been shown to increase the survival of otherwise starving spiderlings,” the scientists wrote. “Therefore, it is likely that pollen availability is crucial for spider survival at the early juvenile stage in spring, when insect prey is scarce in contrast to pollen.”
The research team concluded that the amount of pollen eaten by their spiders was so high that they should be reclassified as omnivores instead of carnivores.
“The question remains whether spiders choose their web building location based on pollen availability in the environment,” the researchers said. “The juvenile (orb-weaving spiders) we observed in this study had built their webs in the branchwood of different wind-pollinated trees. This could be due to the fact that these are optimal locations for successfully capturing flying insect prey; however it could also be possible that webs are positioned according to pollen availability and spiders select these locations as juveniles.”
“Further studies investigating pollen feeding at different life stages and in different habitats are needed to increase our understanding of the importance of herbivory for spider fitness, behavior and their role in ecological communities,” they concluded.