A couple of weeks ago, Ron and I went to the Bone Room to hear UC-Berkeley entomologist Rosemary Gillespie discuss her research on the spiders of the Hawai’ian Islands.
Gillespie and her students get to spend a lot of field time in Polynesia. At least one, a weevil specialist if I recall correctly, decided to stay there.
Gillespie has worked with several groups of Hawai’ian spiders, including the spectacular evolutionary radiation of long-jawed orbweavers. Her marquee species, though, is Theridion grallator, better known as the happy-face spider. It’s a small, furtive, nocturnal creature, found in primary forest on all the major islands except Kaua’i. (The only spiders we encountered during our visit there were exotics, including a sizable huntsman that shared the kitchen of our vacation rental.) In one color morph of the happy-face, the arachnid’s abdomen has two black dots above a curved red smile.
Not all happy-face spiders sport happy faces, though; see Gillespie’s web site for variations. On all the islands where she’s studied them, two-thirds of the happy-faces have plain unmarked yellow abdomens. The others are patterned with spots and blotches of red, black, and white. Each of the 20 or more morphs is genetically determined, although the mechanism in Maui spiders is different from that in Big Island spiders. One of her graduate students is teasing out the details.
Although Maui and Big Island individuals can interbreed, there’s normally no movement between islands. The whole array of color morphs has evolved independently on Oahu, Maui, and the Big Island. For a lucid explanation, with diagrams, go to UC’s Understanding Evolution site
What’s the point of all this polymorphism? Gillespie and Geoff Oxford of the University of York hypothesize that it has to do with predator avoidance. These spiders co-evolved with birds that feed on small arthropods, like the famous Hawai’ian honeycreepers and the ‘elepaio flycatcher. Happy-face spiders hide under leaves; against that background, the all-yellow forms are the most cryptic.
But rather than rely on that, the spider has hedged its bets–diversified its portfolio of colors and patterns. The variation keeps avian predators from developing a consistent search image; as shown in experiments with blue jays, visual multitasking makes for less efficient foraging. So more spiders go uneaten, and the genetic variety is preserved.
Yes, there is a California connection here. During her Bone Room talk, Gillespie said that a similar case of defensive polymorphism had been documented close to home–in Tilden Park, among other places. This involves another spider called Theridion californicum (no common name.) First described from Mill Valley in 1904, T. californicum occurs in coastal habitats from British Columbia to southern California. Like grallator, it lives under leaves, frequently those of poison oak.
As he reported last year in the Biological Journal of the Linnean Society, Oxford collected californicum specimens from several sites in Tilden and from parks in San Mateo and Santa Barbara counties. He catalogued their pattern variations, analyzed how the patterns were formed, and attempted to rear the spiders in the lab.
Although it does not have a happy-face morph in its repertoire, Oxford found that californicum was similar to grallator in several interesting respects.
As in grallator, plain yellow was the most common morph in californicum. The others were patterned with overlays of red, black, and white. Oxford had limited success with captive breeding, but did rear 30 broods collected in the wild. Of these, at least some offspring inherited their mother’s pattern. None of the morphs appeared to be limited to a single sex, as is the case for happy-face spiders on the Big Island (as opposed to Maui.)
Oxford concluded that the same process that created the happy-face variations had been at work in the California species: “..over a certain frequency Yellow becomes disadvantageous in comparison with patterned, and possibly less cryptic, morphs. In this way, polymorphisms may be maintained as a result of shifts in search images by predators with respect to differently coloured prey.”
What makes this more interesting is that, according to Oxford, grallator and californicum are not closely related; grallator is probably closer to the Central and South American genus Exalbidion. That means that the two polymorphic spiders didn’t inherit that set of traits from a common ancestor: they were evolved independently. In one case, Hawai’ian honeycreepers were the shaping force; in the other, North American insect-eaters like warblers and vireos.
It’s also worth noting that you don’t have to go to Polynesia, or the Galapagos or other exotic locales to see evolution in action. It’s happening right next door.