My Questions about Tradeoffs in Size of Female Joro Spiders (Trichonephila clavata)

 The Joro spider (Tichonephila clavata) is native to Eastern Asia and Japan but in 2013 was found in Georgia and has since rapidly established large populations in much of Georgia. These large, flashy spiders are part of the golden-orb weaver genus and are easily visible in a wide range of habitats from front yards to riparian forests. Over the past 2 years I have been fascinated with this species and regularly observe and collect individuals. Even to the causal observer it will quickly become obvious that the adult females of this species can display a huge range of sizes from smaller than a quarter to having a leg span that is as large as your hand. I regularly asked myself why this is?

As it turns out, a good deal of work on this species has been done in their native range. In 1991, Tadashi Miyashita from Tokyo University did a number of experiments on how food availability affects the size of female Joro spiders. She found that it is highly plastic and that there is a strong correlation between feeding success and maximum size. (Miyashita, 1991). While this answered at least one part of my questions, my observations in the field and our in class discussions have raised many more.

Throughout the course of the summer, I watched a large number of these spiders grow along the North Oconee Greenway here in town. By late August, much of the greenway was dominated by huge (10+ ft) webs inhabited by large females. Often multiple tiny males could be observed in their webs as well. In late August, we had a massive storm come through Athens over the course of a single evening. After that night, I only observed small female Joro (about the size of a quarter) for the rest of the year before winter set in. I am still extremely curious as to why this is and our discussion in class about fitness consequences has caused me to re-evaluate my question.

While larger females might be highly attractive to males and likely can bear the burden of egg laying, is there a trade off to maintaining such a large size? Heavy rain and wind make a large, fragile spider vulnerable to damage that smaller spiders could potentially avoid. That being said, a freak storm like the one we had in late August is not a common occurrence, and these spiders evolved in a climatically similar environment to the southern US and so likely regularly deal with summer thunder storms. 

When we discussed the Wilbur-Collins model in class I immediately thought of spiders, though I know that is a bit of a stretch. The questions I asked were not "am I big enough to metamorphosis" but rather am I large enough and healthy enough to mate and reproduce? If not then keep eating! Often in spiders, egg laying is fatal to females however spiders in the genus Trichonephila are able to consistently survive and lay multiple egg sacs (Harvey et. al 2007). I would like to know where the "sweet spot" is in terms of size and timing of reproduction in this species. How long can you keep growing before a predator or freak weather event results in mortality and no reproduction? Ideally, keeping a life history table of a population of Joro's would give some insight into this question, and is something I plan on doing this upcoming summer. I think Joro spiders are a fantastic study organism and can lead to many insights on environmental plasticity in non-model organisms. 

Citations: 

Harvey, M. S., A. D. Austin, and M. Adams. 2007. The systematics and biology of the spider genus Nephila (Araneae:Nephilidae) in the Australasian region. Invertebrate Systematics 21:407–451.

Miyashita, T. 1991. Direct Evidence of Food Limitation for Growth Rate and Body Size in the Spider Nephila clavata. Acta Arachnologica 40:17–21.