DARWIN'S WARM LITTLE POND. A candidate place for the origin of life on earth*.

The warm little ponds I consider here are rain pools of a particular type - that is those on rock surfaces as apposed to those on soft substrata which are quite different (Jackson & McLachlan, 1991). Furthermore I consider only those found in Malawi, which is in tropical Africa and consider only the principal inhabitants - midge larvae of three remarkable species at prodigious densities and strictly only one of the three in any particular pool (McLachlan & Ladle, 2001). Aspects of the ecology and adaptive biology of inhabitants are discussed in published work and in my web site (google.atholmclachlan.blogspot.com). I repeat only sufficient to set the scene for my present endeavour which is to emphasise some aspects of the pools that may not have been considered previously or that have not been fully appreciated previously. To start I will attempt to meet comments I have had from editors and others that have lead me to believe that not everyone grasps what they are seeing in photographs of these rain pools. Perhaps they are expecting something bigger and more conspicuous. So, pictures of selected pools are provided in pairs, those on the top are unadorned, while those below have been crudely coloured using a computer app. My classification of pool types depends primarily upon the shape of the depression on the rock surface that accommodates the pool. Pool depth is the principal determinant of pool duration after rain which in turn largely determines which midge species inhabits the pool as larvae (Cantrell & McLachlan, 1982). Thus, in terms of duration from shortest to longest we have - Polypedilum vanderplanki (Fig. 1), Dasyhelia thompsoni (Fg.2), and the two species of Chironomus. The Chironomus species are themselves dived by proximity to permanent water (rivers). C imicola inhabits pools remote from rivers (Fig. 3), while C. pulcher occurs only near rivers where there is shade (Fig. 4). Some C. pulcher pools will be created when filled with river water rather that rain water as the river recedes during the dry season. But, it seems improbable that there will be any ovipositing females to colonise them during the dry season. Or am I wrong? P. vanderplanki alone is able to survive as a completely desiccated larva when its home pool dries though D. thompsoni larvae can survive in specially constructed cocoons against the dry rock bottom. Chironomus larvae cannot survive desiccation and must emerge as adults and leave the home pool before it dries (Cantrell & McLachlan, 1982)

Fig. 1, Pools on a rock surface occupied by the larvae of Polypedilum vanderplanki.

Fig. 2. A pool inhabited by larvae of Dasyhelia thompsoni pools.

Fig.3. Chironomus imicola is found in sunny pools, typically on hill tops, such as the one in Fig. 2.

Fig. 4, Typical pools, p1 and p2, inhabited by larvae of C.pulcher, near a receding river, r. v, vegetation and e, rock surface. Sandra McLachlan on the right again provides scale. To conclude I consider some potentially important interactions with other animals. These include birds, mammals, frogs, ants and rotifers. Birds, notably crows, are frequent visitors to drink and may be important in transporting inhabitants between pools. Birds were cited by Charles Darwin in the role of transporters (Darwin, 1859), and it would be interesting, if a way could be found of doing it, to investigate their role in the present context. Mammals, notably civets and genets, play an unexpected part for they use pools as lavatories (McLachlan, 1981a). Thus it may be a mammal’s choice of pools, rather than pool duration that is the primary determinant of the pools suitability for the larvae of the midge D. thompsoni. Larvae of this species may be able to tolerate the presumably toxic condition following genet activity. Some specialised frog species use the pools as breeding sites (Patterson & McLachlan, 1989). There is an interesting twist to the feeding behaviour of the tadpoles of these frogs because they feed on the water surface where algae and wind-born pollen are to be found. Faecal pellets resulting from their feeding activities fall to the pool bottom where they are accessible to the Dipteran larvae. Thus tadpoles are effectively feeding the dipteran larvae by providing food otherwise inaccessible to larvae (McLachlan, 1981b). Furthermore, when a pool dries, tadpoles are often trapped and die. But there is more because some midge larvae, notably those of D. thompsoni, are able to use dead tadpoles as food when the pool refills after rain (McLachlan, 1981b). The larvae of P. vanderplanki are not safe even when entering a desiccated when their home pool dries because in this condition they provide ideal biltong for scavenging ants who carry them off to their nests (McLachlan & Cantrell, 1980). The abundant rotifers in P. vanderplanki pools do not turn up in P. vanderplanki gut samples so may not interact directly with dipteran larvae, as food at least. I have largely neglected the question of the role of adults emerging from a pool in choosing new pools suitable for oviposition. It is the adult that is responsible for pool choice and we do not know how this is done. Perhaps water chemistry, conditioned by the previous generation of larvae plays a role, but would this work when flushing by thunderstorms presumably removes water ‘conditioned’ by larvae. Nevertheless, an hypothesis that conditioning by larvae triggers oviposition by the appropriate female is, in principal, readily tested. I would create artificial pools, all of the same dimensions, one half with conditioned water and, as control, the same number with unconditioned water. The prediction here is that females would only oviposit in pools conditioned by larvae of the same species as the female. There is much to be learned about the role of adult midges in pool choice.

Fig. 5. These pools in Botswana are about the same depth (duration), as those inhabited by Chironomus in Malawi but are inhabited by crustacea instead. Botswana lies in the subtropics while Malawi is in the humid tropics so it is biogeography to which we must look for an explanation of this difference. Pools inhabited by crustacea have attracted their fair share of attention (Vanschoenwinkel et al., 2011). References *Cited by Dawkins, R. (2004), p465. The Ancestor's Tale. A Pilgrimage to the Dawn of Life. Weidenfeld & Nichlson. Jackson, M. J. & McLachlan, A. J. (1991). Rain pools on peat moor land as island habitats for midge larvae. Hydrobiologia, 209, 59-65. Cantrell, M. A., & McLachlan, A. J. (1982). Habitat duration and dipteran larvae in tropical rain pools. Oikos, 38, 343-348. Darwin, C. (1859). On the Origin of Species by Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. (6 ed.). London: John Murray. McLachlan, A. J. (1981a). Food Sources and Foraging Tactics in Tropical Rain Pools. Zoological Journal of the Linnean Society, 71, 275-277. McLachlan, A. J. (1981b). Interaction Between Insect Larvae and Tadpoles in Tropical Rain Pools. . Ecological Entomology, 6, 173-182. McLachlan, A. J., & Cantrell, M. A. (1980). Survival Strategies in Tropical Rain Pools. Oecologia, 47, 344 - 351. McLachlan, A. J., & Ladle, R. (2001). Life in the puddle: behavioural and life-cycle adaptations in the Diptera of tropical rain pools. Biological Reviews 76, 377-388. Patterson, J. W., & McLachlan, A. J. (1989). Larval Habitat Duration and Size to Metamrphosis in Frogs. Hydrobiologia, 171, 121-126. Vanschoenwinkel, B., Mergey, J., Pinceal, T., Waterkeyn, A., Vandewaerde, H., Seaman, M., et al. (2011). Long Distance Dispersal of Zooplankton Endemic to Isolated Mountain Tops - an Example of a Ecological Process Operating on an Evolutionary Scale. PLos one, 6, 1-10.

SUNLIGHT

Exposure to sunlight appears to be an underappreciated aspect of habitat. Among non-living features of habitat, it is water and temperature that usually come to mind. This note attempts to show that ignoring sunlight is probably a mistake. Indeed sunlight, it appears, is often the main aspect of habitat that determines the distribution of animals and plants. I recently came across a discussion of the role if sunlight in Helena Cronin’s book, The Ant and the Peacock (1991), pp91-92. There she discusses the findings of such distinguished zoologists as Arthur Cain and P. M. Sheppard, on the degree of exposure to sunlight in determining the distribution of the iconic snail Cepea nemoralis. She goes on to show that Alfred Russell Wallace himself took a deep interest in the role of sunlight. Their work together provides context, which I had not at first fully appreciated, for my own effort to identify habitat selection in the two rain-pool dwelling species of chironomid midges, Chironomus imicola or Chironomus pulcher, Life in the Puddle (1988). Why are some rain pools consistently occupied by either C. pulcher or C. imicola, presumably for thousands or even millions of years? Evidence is accumulating that it is the degree of exposure to sunlight responsible here too. Some typical rain pools recently filled by rain, on rock surfaces in Africa are shown above.