Wednesday, 29 April 2015

Earthworms play “a more important a part in the history of the world than most persons would at first suppose” (Charles Darwin)

Charles Darwin’s last major work, published when he was 72 years old, was on earthworms and the way they transform vegetable matter and soil profiles. He understood that earthworms ingested as they burrowed and the main beneficial effect was provided by their faecal material, as well as the soil aeration that resulted from their burrows. The faecal material is sometimes egested on the surface to provide casts, but it is mostly retained within soil profiles and earthworms also bring organic matter from the surface to be eaten, and then egested, below ground.

There was no doubting Darwin’s enthusiasm for his subject, and the hours of observation that he devoted to it. In the Conclusion of The Formation of Vegetable Mould, he writes [1]:

Worms have played a more important part in the history of the world than most persons would at first suppose.. ..Worms prepare the ground in an excellent manner for the growth of fibrous-rooted plants and for seedlings of all kinds. They periodically expose the [vegetable] mould to the air, and sift it so that no stones larger than the particles which they can swallow are left in it. They mingle the whole intimately together, like a gardener who prepares fine soil for his choicest plants.. ..They allow the air to penetrate deeply into the ground.. ..The plough is one of the most ancient and most valuable on man’s inventions; but long before he existed the land was in fact regularly ploughed, and still continues to be thus ploughed by earth-worms. It may be doubted whether there are many other animals which have played so important a part in the history of the world, as have these lowly organised creatures.

It is impossible not to disagree with Darwin’s comment on the significance of earthworms, although we now recognise that their efforts are aided by bacteria and fungi that release nutrients during the re-packaging activities of the worms.  During their feeding, worms only assimilate a fraction of the organic matter that they ingest, so their role in transformation is a major one, as they are numerous and feed continuously as they burrow. Any gardener making a compost heap is familiar with the changes to organic matter brought about by worms, aided by a range of other invertebrates that are similarly ingesting, assimilating little, and egesting compacted faecal material. This compaction ensures that micro-organisms that break down complex organic molecules to release nutrients are packed close to potential substrates.In soil, everything becomes mixed with mineral particles.

Although there are at least 6000 species of earthworms, the majority conform to the same body plan and we are hard pressed to tell one species from another. When we look closely at earthworms, we see that they are segmented and this is not just an external feature, as the body is separated into segments internally, with sheets of tissue called septa separating each segment from that preceding and that following. The body wall of the earthworm is made up largely of muscle and a resistant covering that protects the worm from abrasion, together with an epidermis containing secretory cells. The long tubular gut runs along the length of the worm, passing through the septa, and the space between the gut and the muscles is filled with coelomic fluid to form a hydrostatic skeleton. This fluid is the secret of the earthworm’s characteristic method of locomotion, together with the use of small spines called chaetae (or setae) that are projected through the body wall by muscles.

Movement, and burrow formation, is achieved by means of the alternate contraction of the circular and longitudinal muscles in each segment (see below, taken from [2]). When the circular muscles contract, the segment becomes narrower and the pressure on the coelomic fluid causes it to lengthen. Conversely, contraction of the longitudinal muscles shortens the segment and makes it broader. By having a sequence of contractions of each muscle set in succeeding segments, the result is a wave of contraction and expansion that passes along the worm's body. The most anterior segments act as a "battering ram" during circular muscle contraction and the worm’s body is anchored by means of the segments where longitudinal muscles are contracting, the pressure exerted on the burrow walls being enhanced by the pushing out of the chaetae that act like crampons. A summary of the sequence is shown in the second diagram below, and the worms use a similar method to move over surfaces, on the few occasions when they are not burrowing. This use of hydraulic pressure provides an elegant solution to moving without the use of limbs, and lubrication is aided by mucus secreted over the body surface from glands in the epidermis. It's another wonder of evolution.

As mentioned earlier, we take the soil moving, and composting, activities of earthworms for granted and they are indirectly of great importance to us; but there are other ways in which earthworms are useful to humans. For example, they provide bait for fishing and earthworm oil is used to entice fish to attack plastic lures [3]. Further uses of earthworms come in medicine, where extracts have been shown to enhance the healing of skin wounds [4] and in the reduction of cell death in heart muscles after infection [5]. 

One way of exploiting earthworms that has been little explored in Western culture is as food for humans, despite worms being an important part of the diet of several other mammals and some birds. The cross-sectional view of a body segment (shown above) reveals the size of the two sets of muscles used in locomotion and it is a surprise that this source of animal protein has not been investigated more thoroughly. There are recipes available, and a few can be given here, but a warning is required before trying them, as the gut contents of earthworms may be gritty and it is probably best to ensure the worms have egested as much mineral material as possible before using them in cooking. Experiments with feeding worms on sage plants, other culinary herbs, and grains such as corn meal, might be worthwhile before preparing the worms for cooking: there are also interesting possibilities for flavour combinations from such experiments, with the worms providing their own stuffing. Here are some links to dishes you might like to try:

Earthworm sauté (and other recipes):

- or eat them just as they are (the preferred method of young children....)

Darwin was right to emphasise the importance of earthworms in the history of the world and that includes the extensive period of pre-human history. It’s a pity they aren't more recognised – and utilised.

[1] Charles Darwin (1881) The Formation of Vegetable Mould, through the action of worms, with observations on their habits. London, John Murray.

[4] M. Grdiša, M. Popović and T. Hrženjak (2004) Stimulation of growth factor synthesis in skin wounds using tissue extract (G-90) from the earthworm Eissenia foetida. Cell Biochemistry and Function 22:373-378.

[5] Ping-Chun Li and 9 authors (2015) Impact of LPS-induced cardiomyoblast cell apoptosis inhibited by earthworm extracts. Cardiovascular Toxicology 15:172-179.

Monday, 20 April 2015

Cosmology, theistic force and human imagination

I was drawn to a headline in The Independent newspaper: "'Dark matter' not as dark as first thought: Scientists find it interacts with forces other than just gravity" [1]. The scientists referred to in the article were a team of 23 headed by Richard Massey, and their findings were published in the Monthly Notices of the Royal Astronomical Society [2]. Although I have little understanding of astrophysics, I was interested in the closing sentence of their Conclusions section where, having described their findings of the "possible stripping or deceleration of dark matter associated with the infalling galaxies [in the 10kpc core of Abell 3827]", they write:

Detailed hydrodynamical simulations of galaxy infall, incorporating dark matter physics beyond the standard model, are needed to predict its behaviour within a cluster environment, and to more accurately interpret high-precision observations.

I take this to mean that our interpretation of dark matter needs to be re-examined; so what is our current view of this matter and what is a 10kpc core? As a kpc (kiloparsec) is approximately 3.26 light years, the core of the infalling galaxies observed by Massey et al. (and shown below in an illustration from the BBC [3]) is over 300,000,000,000,000,000 km across, if my arithmetic is correct. It is in this minute part of the Universe that the new observations have been made; dark matter making up, in total, about 27% of the material in space, with everything that we can observe (planets, stars, galaxies, etc.) making up less than 5% of the total. The rest is dark energy [4]. 

To define dark energy, I quote from the NASA site [4]:

We know how much dark energy there is because we know how it affects the Universe's expansion. Other than that it is a complete mystery.

Do we have a better understanding of dark matter? Again, a quote from the NASA site is helpful:

We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see.. ..Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter.

We can therefore conclude that the Universe is vast and beyond any scale that we can comprehend and that it is made up of a fraction of matter that we can measure. The rest is explained by our powers of imagination and the predictions made by experts in mathematics and physics.

Recently, I took a course in Cosmology to try and get an understanding of what we know about the Universe and its origins. The course was taught well, but I found myself hopelessly lost with the scales involved and with the levels of imagination required. Our existing knowledge has been gained from Earth-bound observations and, recently, those from satellites and devices that record signals over mind-boggling distances. All the interpretation of these observations has necessarily been Earth-bound and based on human analytical skills, aided by the high-powered computers we have devised. Given the scale of the Universe, these tools seem inadequate, but it is a human characteristic that we feel we can eventually explain all that is currently unexplained.

During the Cosmology course, I also thought about another model that seems equally plausible – that everything was designed by a theistic force. Just as I was overwhelmed by the study of the Universe from Earth, so I was overwhelmed by the idea that a God that created the Universe chose an infinitesimally small planet on which to locate humans that were then given unique powers of reasoning and the belief that, given time, they could understand everything. It is tempting to accept that the idea of such a theistic force is, in turn, a result of human imagination. 

Will we ever understand the Universe? Almost certainly not without the development of a "better" mathematics (of which our wonderfully powerful mathematics is a subset), but now I'm veering off into science fiction.

[2] Richard Massey + 22 authors (2015) The behaviour of dark matter associated with four bright cluster galaxies in the 10kpc core of Abell 3827. Monthly Notices of the Royal Astronomical Society 449:3393-3406.

Thursday, 9 April 2015

The blindness of a Creationist

Philip Henry Gosse, the Nineteenth Century Natural Historian, was one of the pioneers of the craze for parlour aquaria. In addition to popularising aquatic life through his wonderful books, lectures and field courses, he was an acute and enquiring observer of aquatic organisms and his research led to him being appointed a Fellow of the Royal Society.

A visit to South Wales resulted in Tenby: a sea-side holiday [1] that Henry describes as: 

..a faithful narrative of how the author was engaged for about six weeks at that pleasant little watering-place.. ..He hopes..that the approbation so kindly bestowed on his former works will be continued to this, and that it may be accepted as another Lesson in the important art of "How and What to Observe"

The former works include A Naturalist's Rambles on the Devonshire Coast, and Tenby follows a similar format, demonstrating Gosse's skill, both as an observer and as a communicator. There was another side to Henry Gosse and, to him, very much the most important. As a member of the Brethren, he had a strong Christian faith and this extended to a belief in the literal truth of The Bible. On the Title Page of Tenby [1] is a quote from Cowper (shown below) that mirrors Henry's view that observation of all creatures leads to our having a sense of wonder in God's Creation. This standpoint was so important to him that he wrote a book, Omphalos [2], to defend it and this was published in 1857, a year after Tenby and two years before Darwin published The Origin of Species. Darwin's great work revolutionised thinking about evolution, but the concept of geological time scales had already been accepted by most, although not by Henry, who believed that all strata, and the fossils that they contained, were created by God some thousands of years ago [2]. Henry Gosse would certainly have been aware of ideas surrounding the evolution of life forms in 1856, even if he didn't agree with them.

In Tenby, Henry describes an observation that he made on two marine organisms that were placed into fresh water [1]:

The power which some animals have of sustaining with impunity circumstances which are fatal to others closely allied to them, is remarkable. To most marine animals immersion in water is instant death. Some young Crabs in the Megalopa stage which I found in luminous water, and which remained several days alive and well in a bottle of sea-water, died instantaneously on being dropped into a tumbler of fresh water. On the other hand, a specimen of Eurydice pulchra,-one of two that occurred in the same dip,-manifested not the slightest inconvenience at the change, but continued for several days to dart about the vessel, with intervals of motionless, death-like repose on the bottom, and behaved in every respect in a manner identical with that of his fellow that was allowed to remain in his native sea-water.

Henry was observing the effects of osmosis on the crab larvae, with water passing into the body from the weaker medium to the more concentrated body tissues. The result was rapid death, as will be the case with all marine animals having a permeable coating. Eurydice pulchra (see below) is a crustacean that lives in the intertidal zone, buried in sand, and individuals come to the surface and swim when the tide covers them [3]. Having an external skeleton protects them from abrasion from the sand and, as it is impervious to water, it also serves to protect them against changes in salinity and exposure, as might occur after a rain storm, or when they are caught on the surface as the tide goes out.

Image © Hans Hillewaert.

I wonder what went through Henry Gosse's mind when he was making his observations on E. pulchra. A Natural Historian with an open, and enquiring, approach may have wondered whether it was the presence of a resistant cuticle in ancestral organisms that allowed the colonisation of rivers from the sea by migration from estuaries, something that soft-bodied marine creatures cannot do. Or perhaps the thought would arise that the cuticle of intertidal crustaceans allowed the conquest of land by their ancestors, leading to the insects and the subsequent re-colonisation of fresh waters, where insects are abundant? A present-day Natural Historian accepts that the presence of a resistant cuticle is an example of pre-adaptation, but further questions arise about the marine/fresh water transition? For example, how do eels and salmon survive in fresh water before/after the marine phase of their life cycle? The answer lies in their having resistant coverings, but also efficient kidneys to expel water and, importantly, a coating of slime on permeable surfaces that acts as a barrier [4]. Of course, Henry Gosse, for all his powers of observation, could not ask questions such as these, or, if he did, he had to dismiss them as something mischievous that was sent to tempt him away from his anti-evolutionary stance. He was a great observer, but had the blindness of a Creationist.

There are still some Christians with similar views to those of Henry Gosse, although the majority now accept evolution and a looser interpretation of the Biblical account of Creation. That raises another question: how does one separate the literal from the figurative in The Bible?

[1] Philip Henry Gosse (1856) Tenby: a sea-side holiday. London, John Van Voorst.

[2] Philip Henry Gosse (1857) Omphalos: an attempt to untie the geological knot. London, John Van Voorst.

[3] D. A. Jones (1970) Factors affecting the distribution of the intertidal isopods Eurydice pulchra Leach and E. affinis Hansen in Britain. Journal of Animal Ecology 39:455-472.

[4] Roger S. Wotton (2004) The essential role of exopolymers (EPS) in aquatic systems. Oceanography and Marine Biology, An Annual Review 42:57-94.