Blaxter Lough

Fig. 1. View of Blaxter Lough, the humic lake habitat studied here.

I want to return to an extraordinary situation discovered by me on the high moors of Northumberland. My intention is to re-examine this purely ecological story but this time in the light of evolution theory. This opens the door to some interesting and testable questions previously hidden. The story concerns the interrelationships among three organisms which allows them to flourish in the nutritionally impoverished environment of a humic lake. Relationship hinges on the faecal pellets produced by larvae of a chironomid midge, Chironomus lugubris, feeding on particles of peat introduced to the lake by wave action. Particles in suspension are rapidly colonised  by decomposer micro-organisms which evidently make otherwise refractory peat nourishing as food for the midge larvae. Faecal pellets are yet more heavily colonised by micro-organisms and  should therefore be attractive to the larvae as food. However, perhaps because they are too hard for their mandibles, the larvae appear not to make use of this seemingly valuable resource.

Fig. 2. Sectional diagram of a tube dwelling Chironomus larva with a swarm of Chydoris  swimming precisely over the faecal end of the tube. 

But now comes the interesting part. Midge pellets, although ignored by the insect larvae, appear to be grazed by a second arthropod, the Cladoceran Chydoris sphericus. These hold a pellet between their valves and rotate it while grazing its surface. In the lab, a cloud of Chydoris can be seen swimming precisely over the pile of faecal pellets produced by larvae, at only one end of the tube (Fig 2). Of course the crustacea also produce faecal pellets. Passing through the fug of a chironomid larva boosts  micro-organism populations so passing through the gut of a chydorid  would be predicted to harbour yet higher loads of micro-organism. Unlike their own pellets, those of Chydoris are of a size which should be easily consumed by larvae. Here there are two gaps in our data which call for formal investigation. First, what is the nutritional actual value of Chydoris pellets and are they actually consumed by Chironomus larvae? Second,  in artificial pools in the laboratory, unless chironomid larvae are added to the culture, Chydoris does not make an appearance. In the wild Chydoris adults appear in large numbers in the summer, supposedly spending the winter as diapausing eggs (ephippia), in the mud. Something, largely unknown, is required to break this diapause (Barnes, Calow, and Olive, 1988; Bronmark and Hansson, 2005). Thus there is a hint here of  some fundamental relationship between chironomids and chydorids beyond that of  food.

This whole interaction between these two arthropods is discussed by Mike Begon and colleagues (Begon, Townsend, and Harper, 2006), pp 340-341, and by Brian Moss (Moss, 2010), p 315. Research details can be found in (McLachlan, 1976, 1978; McLachlan and Dickinson, 1977; McLachlan and McLachlan, 19975; McLachlan, Pearce, and Smith, 1979).

Fig. 3.  Diagram summarising relationships between players, an insect larva, a crustacean and micro-organisms, in a putative mutualism centred on faecal pellets. a, chironomid faecal pellets. b, chydorid faecal pellets. The consumption of 'b' by the chironomid awaits testing.

I turn now to look at this story from an evolutionary perspective. As far as I know this has not been attempted before. The central question is whether the observed relationship between three organism; insect, crustacean and micro-organism, is an economy in the sense of Richard Dawkins (Dawkins, 2004), p 266, or an adaptation in the sense of George Williams (Williams, 1966). Only if it is an adaptation can it be referred to as a mutualism (Boucher, 1992). This is not an easy question to answer because economy and adaptation merge into each other and an economy may be moving, under selective pressure, toward an adaptation. The application of Pittendrigh's principal of teleonomy (discussed by George Williams (Williams, 1966), is useful here. What is required is the identification of a function. For example, in one case of well established mutualism, that of bees and flowers, the question 'what is the function of a flower?', is readily answered - it is to attract bees. Hence the bee/flower relationship is a true mutualism. Turning to the feeding relationships shown in Fig.3., feeding relationships based on faeces are by-product relationships (Dawkins, 2004). Faecal pellets are not organisms and hence cannot respond to natural selection. But they are colonised by bacteria and fungi which can respond. So we may be dealing with a mutualism after all. For a discussion of the relationship between mutualism and reciprocal altruism see (Boucher, 1992).

Turning from feeding to diapause; recall the finding that the presence of Chironomus larvae appear necessary for the appearance of Chydoris. What is implied is that the chironomid larvae somehow break the diapause of  Chydoris. In other words, there  appears to be some kind of intimate  relationship here not based on by-products. A formal, but straight forward experiment is required to explore this idea. A survey of humic lakes and perhaps of other representative kinds of lakes as well, could strengthen an assumption of mutualism if all three players are invariably found together. Thus diapause could be an adaptation if it answers to the principle of teleonomy because a function is readily identified - the function of Chironomus is to break diapause of Chydoris.

To summarise - what I have attempted is to re-examine an old piece of work in the light of evolution theory. This shows up a new set of questions. Key questions are; first that we are dealing with a true mutualism based on faeces and second that Chironomus larvae must be present for Chydoris also to be present. The first question can be tested by determining how consistently the three players are found together. An obligate association supports an hypothesis of mutualism. The second, the diapause breaking,  is readily tested by a simple experiment in the laboratory with replicate aquaria containing winter lake mud with chironomid larvae and a control set of replicate aquaria but without insect larvae. Thus both question are testable. Perhaps some one will pick it up. It could be rewarding.

references

Barnes, R. S. K., Calow, P., and Olive, P. J. W. (1988). The Invertebrates, A new synthesis. Oxford: Oxford University Press.

Begon, M., Townsend, C. R., and Harper, L. (2006). Ecology. From Individuals to Ecosystems. (4 ed.): Blackwell Publishing 

Boucher, D. H. (1992). Mutualism and Cooperation. In E. F. Keller & E. A. Lloyd (Eds.), Keywords in Evolutionary Biology. (pp. 208-211). London: Harvard University Press.

Bronmark, C., and Hansson, L.-A. (2005). The Biology of Lakes and Ponds. (Second ed.). Oxford: Oxford University Press.

Dawkins, R. (2004). A devil's chaplain. London: Phoenix.

McLachlan, A. J. (1976). Factors Restricting the Range of Glyptotendipes paripes EDWARDS (Diptera: Chironomidae) in a Bog Lake. Journal of Animal Ecology, 45, 105-113.

McLachlan, A. J. (1978). Interactions between Freshwater Animals and Microorganisms. Annals of applied Biology, 89, 162-165.

McLachlan, A. J., and Dickinson, C. H. (1977). Micro-organisms as a foctor in the distribution of Chironomus lugubris ZETTERSTEDT in a Bog Lake. . Archiv fur Hydrobiology, 80, 133 - 146.

McLachlan, A. J., and McLachlan, S. M. (19975). The Physical Environment and Bottom Fauna of a Bog Lake. Archiv fur Hydrobiology, 76, 198 - 217.

McLachlan, A. J., Pearce, L. J., and Smith, J. A. (1979). Feeding Interactions and Cycling of Peat in a Bog Lake. Journal of Animal Ecology, 48, 851-861.

Moss, B. (2010). Ecology of Fresh Waters. A view for the Twenty-First Century. (4 ed.). Chichester, UK: Wiley -Blackwell.

Williams, G. C. (1966). Adaptation and Natural Selection. Princeton: Princeton University Press.