Monday, 10 December 2007

A tangled web ...

There was a Redback on the toilet seat when I was there last night.
I didn't see him in the dark but, boy, I felt his bite.

Slim Newton


Just about every garden in Australia has a collection of redbacks (Latrodectus hasselti Thorell 1870). The spiders might not be obvious but they're around, lurking in brickwork, woodpiles, letter boxes and dunnies. My redbacks favour the bell-shaped cover for the water meter. But when I checked it today to take a portrait shot of the big female that usually lives there, I couldn't find her. I don't know whether she's shuffled off this mortal coil or just shuffled off to find a cooler spot for summer.

Redbacks and their relatives are notorious for their venom, which contains the neurotoxin α-latrotoxin. Their habit of hanging around houses increases the likelihood of encounters with humans.

Their affinity for human habitats had allowed them to hitchhike around the world in cargo. Combined with their uniform appearance — one black and red Latrodectus looks much like another — this eight-legged diaspora had led to confusion. Is Species A different from Species B? And is it really native to this location? So what about Species C, clever clogs? You didn't consider that one, did you? Ha! No one expects the Spiderish Inquisition.

Sorry.

For a while, it was thought that Australia's beloved redback might have been an overseas ring-in. Levi (1959) regarded it as Just Another Black Widow (L. mactans) but other workers were not so enthusiastic about lumping the species.

That Thorell did not describe it formally until 1870 raised suspicions. A species with such a strong association with people should have been spotted sooner, were it native. Because it was first recorded from Australian ports, perhaps it been brought in by ship? Was it a holidaying New Zealand katipo* (L. katipo)? Or had Levi been correct and it truly was JABW? The plot thickened …

The conundrum was solved in 2004 when Garb, González and Gillespie examined molecular evidence. Latrodectus fell into two distinct groups — the geometricus clade and the mactans clade.

The first includes the brown widow (L. geometricus), which has been introduced to many locations (I've collected them in Townsville), and L. rhodesiensis from southern Africa.

The second clade is made up of black and/or red species, including our redback. So what does the analysis say about it? Well, it's not closely related to the black widow or any other North American species so we can cross those off this list. (The study also confirms that Levi was a bit too keen in lumping everything into L. mactans.) It is, however, the sister species of the katipo from across the Tasman. (But — just to make it a little bit more complicated — there are two species of katipo*, which are presumably more related to each other than they are to the redback. That means the redback is sister species to the katipo clade … Oh, let's move on.)

So the suspicions about the redback's origin were not correct. Just as well, because we need to keep up our catalogue of home-grown venomous fauna. How would it look if it were revealed that we'd imported one of the best known?

There is one more twist in this story. As suspected, the redback is an accomplished traveller but it hasn't come into the country — it has headed out. It is now established in several localities, including New Zealand, where it occupies inland areas. Endemics and invaders rarely meet though, because katipos live on beaches and among associated dune vegetation.

If my garden redbacks reappear, I'll post a picture.


References
Garb, JE, González, A & Gillespie, RG. (2004). The black widow spider genus Latrodectus (Araneae: Theridiidae): phylogeny, biogeography, and invasion history. Molecular Phylogenetics and Evolution 31: 1127 – 1142. Abstract.

Levi, HW. (1959). The spider genus Latrodectus (Araneae: Theridiidae). Transactions of the American Microscopical Society 78: 7 – 43. First page.
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* PDF files <300 KB

4 comments:

Duncan said...

Interesting post Snail, good info.

boobook48 said...

Thanks so much for sorting that out Snail. Now I can win arguments at the dinner table.

Sherrie said...

The telly was on this afternoon as I puttered around the house... it seemed as though every time I walked past, the host was speaking in a dramatic voice about yet another venomous Australian critter. (And I don't mean politicians or academics.) How do you people sleep at night when everything's out to get you? :-)

Snail said...

I've been meaning for ages to look up the info about the redback. I'm glad I finally got down to it!

I suspect we like it that way, Sherrie. We get to do the Crocodile Dundee thing ... on a very small scale. Redbacks can be a problem because they live in such close proximity to people. Same with Sydney funnel webs, which carry house-breaking equipment so they can get you while you're asleep. The good news is that our scorpions are not particularly nasty. I can vouch for this, having been stung by a scorpion that was on a bath towel in a B & B on the Great Ocean Road ...