Waiting for an answer to a Macromite Electron Raster Challenge is a bit like waiting for a warn spring day in Edmonton: patience may be rewarded, eventually. But warmth is predicted for today (although along with a brisk wind) and I decided to take the day off work and catch up at home. First up is awarding points, or rather first up is fitting a mite into this somehow, and I offer a red Queensland Charletonia mite eating spider eggs while baby spiders look on in horror in honour of the Queenslander with the best answers. In the mite’s case, UV-protection and perhaps an advertisement of bad taste are thougth to be the reasons it is red.
Without a doubt, Snail’s Eye View was there the firstest with the mostest (although Adrian should get some kind of prize for ketchup). So, congratulations to Bronwen Scott from the beautiful and diverse Atherton Tablelands where I used to spend many a happy day plucking mites from rainforest canopies.
No one really knows why Polymorphus marilis sequesters carotenoids and no one got the species correct – hardly surprising since the worm, a specialist on its definitive host the Lesser Scaup Aythya affinis (Bush & Holmes 1986), is nowhere near as well known as P. minutus, P. paradoxus, or a host of other carotenoid blazing acanthocephalans that adorn the pages of various scientific journals. Most cystacanths of Acanthocephala lack colour, but the more famous ones shine through their intermediate hosts’ cuticle as bright yellow-orange to red spots.
Photoshop aside, one could claim that the reason these cystacanths are red is that they sequester carotenoids from their intermediate hosts, and this does seem to be true, but why? Well warning colouration and protection from ultraviolet light are two common uses of carotenoids, but neither makes sense here – the worms want to get eaten (at least by the definitive hosts) and one would think (perhaps incorrectly) that they are protected from UV under the cuticle of the amphipod intermediate host. So what is going on?
Well, one definite effect of this red pigmentation has been to cause quite a few scientists to paint red spots on uninfected amphipods to see if they are more likely to be eaten by definitive hosts (usually ducks or fish) than unpainted amphipods. The results have been mixed (see Ted’s link and the Bakker et al. paper listed below for two contrasting fishy examples, and Bethel & Holmes and the citations therein for the fowl truth). Peter links to a paper that reports that European P. minutus appear to cause sterility in an another amphipod, and perhaps, the sequestering of carotenoids is related to failure to produce eggs. Zohar & Holmes (1998) also demonstrate that Polymorphus-infected Gammarus lacustris males are less likely to pair up or guard mates. But, more interesting associations between red acanthocephalans and their various hosts are changes in amphipod colour (loosing their camouflage brown) and behaviour (losing their fear of light so that they swim near the surface, tending to cling to anything they touch).
Amphipods that are not infected by an orange acanthocephalan maintain carotenoids in their cuticle, and thus, acquire UV protection, a brownish hue that blends in with lake bottoms, and the typical reddish shell of a boiled crustacean when they die. This is wonderfully illustrated by a concise and informative letter to Nature by Ole Hindsbo – again on P. minutus, but this time in New Zealand. In less than one page, Hindsbro elegantly demonstrates that most (but not all) infected Gammarus lacustris are pale because their blue blood shows through their carotenoids deficient cuticle, that the pale amphipods are more likely to swim in sunny spots where they would be easily seen by a definitive host, and that in the lab a duckling is more likely to eat the pale, infected amphipods. He also cites M. Denny’s PhD thesis (from UA no less) showing that Gammarus lacustris infected with Polymorphus paradoxus tend to cling to floating objects (see mallard picture). Dabbling ducks like a mallard may not be proficient at hunting arthropods in the water column, but Bethel & Holmes (1977) demonstrated that clinging to a mallard bum is a good way to get a mallard to eat amphipods infected with P. paradoxus. Scaup do hunt and consume a lot of amphipods, so one might hypothesize that those infested with P. marilis would be less likely to cling and more likely to paddle palely through the water column.
So why are some acanthocephalans red? I vote for ‘because they make their hosts blue’.
Thanks to all those students of red acanthocephalans who have provided me with so many hours of interesting reading. Special thanks to John Holmes for a sparking discussion on worms, ducks, and muskrats at the Strickland dinner and to Leo Balanean for many interesting stories about amphipods and their worms on those early morning bus rides and for the use of his picture. When Leo’s thesis is finished, the world will know just how red Polymorphus marilis and paradoxus really are.
References (also see links in comments):
Bethel WM & Holmes JC. 1977. Increased vulnerability of amphipods to predation owing to altered behavior induced by larval acanthocephalans. Can. J. Zool. 55: 110-115.
Bush AO & Holmes JC. 1986. Intestinal helminths of lesser scaup ducks: patterns of association. Can. J. Zool. 64: 132-141
Bakker TCM, Mazzi D & ZalA S. 1997. Parasite-induced changes in behavior and color make Gammarus pulex more prone to fish predation. Ecology 78(5): 1098–1104.
Hindsbo, O. 1972. Effects of Polymorphus (Acanthocephala) on colour and behaviour of Gammarus lacustris. Nature 238: 333.
Zohar S & Holmes JC. 1998. Pairing success of male Gammarus lacustris infected by two acanthocephalans: a comparative study. Behavioural Ecology 9: 206-211.