By John Kerr 14/10/2016


Species of coastal New Zealand spiders likely rafted here as a part of an eight million year round-the-world trip, suggests a new study.

The Amaurobioides genus of spiders has species dotted around the Southern Hemisphere, including New Zealand, Australia  and South America. These spiders eke out a tough life living in the coastal ‘spray zone’ on rocky shores.

The genus includes the sea shore spider (Amaurobioides maritima), common on southern New Zealand coastlines. In 1887 P. Goyen described maritima specimens discovered at Shag Point in North Otago, admiring their resilience:

Seen thus, its body covered with silvery globules of air, it is a very handsome object. …. It is a plucky little animal, and fights very stoutly to retain possession of its nest.

Although Amaurobioides spiders all appear to share a common ancestor, the various species are now separated over continents. A new study published in the journal PLOS One aims to answer the question: when did they become separated?

It is possible that the spiders were once one ancient population, but became split up by the slow breakup of the super-continent Gondwana. Those stuck on each part of the tectonic puzzle went on to become the different species we find today.

An alternative hypothesis is that the species’ diverged much later, having traveled across the oceans to come to rest where they now live.

Sara Ceccarelli from the Argentinian Museum of Natural Sciences and her colleagues investigated the history of the spider genus by comparing similarities and differences in the DNA of spiders collected from around the Southern Hemisphere. A total of 45 Amaurobioides specimens and 60 specimens from related spiders were used to construct an Amaurobioides evolutionary tree.

Read more about the research on Scimex.org.

Spiders: Species tree of Amaurobioides using *BEAST shown on the left. Blue node bars represent 95% Highest Posterior Density intervals for node ages. Bayesian posterior probability values are shown at nodes. Codes in brackets next to terminal taxa names correspond to their areas of distribution (AM = South America; AF = Africa; AU = South Australia; AT = Tasmania; NN = North Island and northern part of South Island of New Zealand; NS = central and southern part of South Island of New Zealand). Photos to the right of the tree are of Amaurobioides maritima female (top left; photo M.J. Ramírez), two A. maritima retreats from Jackson Bay, South Island, New Zealand (top right; photo B.D. Opell) and typical habitat of A. maritima, Waikawa, South Island, New Zealand (bottom; photo M.J. Ramírez)
Species tree of Amaurobioides. Photos to the right of the tree are of Amaurobioides maritima female (top left; photo M.J. Ramírez), two A. maritima retreats from Jackson Bay, South Island, New Zealand (top right; photo B.D. Opell) and typical habitat of A. maritima, Waikawa, South Island, New Zealand (bottom; photo M.J. Ramírez). From Ceccarelli et al. (2016).

World Tour

The researchers suggest that, rather than a Gondwana-era split, an ancestor of Amaurobioides may have dispersed eastward from South America to South Africa during the much-later Miocene era, likely aided by the establishment of the Antarctic Circumpolar Current.

trip
Map detailing Amaurobioides long eastward journey. From Ceccarelli et al. (2016).

The Amaurobioides genus then originated in South Africa and dispersed further eastward to Australia and New Zealand in the early and mid-Pliocene.

Finally, Amaurobioides species recolonized South America at the end of the Pliocene, completing the eastward circle of long-distance dispersal around the Southern Hemisphere.

Rafting spiders

While the research does indicate that the spiders travelled over oceans, their mode of transport is not entirely clear. However, the authors make a strong case that the spiders may have travelled on rafts of seaweed and other vegetation.

While some species such as the sheet-web weaver spider are known to travel far on web constructed sails, the authors note that the Amaurobioides  genus are not big on flying, preferring stay on terra firma even when under threat:

This behaviour is in contrast with the usual escape strategy of most entelegyne spiders and anyphaenids in general, of jumping away or dropping while leaving a security dragline.

The very narrow habitat range of these costal spiders is a likely factor, say the authors. By literally throwing caution to the wind these spiders could easily end up in inhospitable environments for which they are not adapted. Thus the authors conclude:

With such reluctance to lose grasp of a solid substrate, we find it much less likely that they would adventure in ballooning.