Overview
Crab spiders belong to the family Thomisidae, a group that contains over 2100 species worldwide. They get their common name from their ability to move sideways like a crab. These spiders are ambush predators rather than web-builders and the first two pairs of legs are longer and armed with strong spines. These features enable crab spiders to grab and hold prey before delivering a bite.
Crab spiders are found throughout New Zealand and occur in native forests and modified habitats such as gardens. There are two subfamilies in New Zealand. The Stephanopinae, or square-ended crab spiders, often look rather rugged and gnarled and are rather drably coloured. When viewed from above, the upper surface of the abdomen looks a little like a trapezium. They can be found in leaf litter and on shrubs, grasses and ferns. The Thomisinae, or flower spiders, look rather sleek and are brightly coloured. As the name suggests they are associated with flowers where they lie in wait for insect pollinators. One species (Cymbachina albobrunnea) is rather different. It is often found on lichen and has a colour pattern that allows it to blend in with this background.
As part of my PhD thesis, I looked at the diversity of the New Zealand crab spider fauna. I used DNA to work out if these spiders have been here since New Zealand split from Australia or if they were descended from more recent colonists.
My work on thomisid diversity shows that we have 11 species and this includes three species that were previously unknown to science. Two species appear to have arrived from Australia in the last 80 years, while the remainder are unique to New Zealand. A number of species were known from only one sex, but DNA allowed me to match the males and females for every species. In the case of Sidymella angulata I was able to show the male that had previously been described for this species was actually Sidymella angularis. This work will be published as a taxonomic monograph.
The level of difference in DNA suggests that New Zealand crab spiders are quite distinct from their Australian counterparts, yet appear to have descended from Australian colonists around five million years ago. At the outset of the study, it was thought that stephanopine crab spiders were poor at long distance dispersal, but the discovery of two recent arrivals suggests this may not be so. Specimens in museum collections suggest Sidymella longipes arrived in New Zealand in the 1930s, while S. trapezia was first recorded here in 2003. Both species appear to be well established.
Te Papa's online guide to New Zealand spiders: What spider is that?
References:
Sirvid PJ, Moore NE, Chambers GK, Prendergast K (2013) A preliminary molecular analysis of phylogenetic and biogeographic relationships of New Zealand Thomisidae (Araneae) using a multi-locus approach. Invertebrate Systematics 27 (6): 655-672. Online abstract.
Sirvid PJ (2014) The Taxonomy, Phylogeny and Biogeography of the New Zealand Thomisidae. Unpublished PhD thesis (Victoria University).