How To Form A Minifera

by Richard L. Howey, Wyoming, USA


 
  What's a minifera?  It's a microscopic candelabrum used in Jewish ceremonies.

 Well, it's summer and I don't have a captive audience of students upon whom to inflict my bad jokes, so you're the recipients instead.  (Just wait until I retire!)

 There are many Micscape readers who know much more about foraminifera or forams than I do, but here I would just like to offer a few practical suggestions for handling and observing these wondrous shells and also to make a few general observations about nature's experiments with form (thus offering a pathetically weak justification for the wretchedly paronomastic character of the title.)

 In his landmark work, On Growth and Form, D'Arcy Thompson shows how nature favors certain kinds of patterns in biological systems.  Although nature is often outrageous in its morphological experiments (consider the platypus, the giraffe, and the sea cucumber), there are nonetheless certain types of forms that recur virtually endlessly, because they turn out to be highly efficient means of solving some basic sorts of biophysical and biochemical problems.  Consider the cilium which is found in virtually every major group of organisms, from the protists up through to humans, and in the same structural form.

 As an aside, D'Arcy Thompson, who very early in his career showed signs of a creatively eccentric mind, was, according to one account, invited to join a faculty luncheon group which included the considerably older, and quite cranky, Herbert Spencer, who wanted to write the definitive system about the nature of everything.  D'Arcy Thompson, who had a rich booming voice, was seated next to Spencer at lunch, and when Thompson began to hold forth, Spencer took his plate and cutlery, left the table and stood in a corner of the room as far from Thompson as he could manage.  If this story isn't true, it should be; it encapsulates the character of the two men so well.  One of the reasons that On Growth and Form is a major, if somewhat neglected classic, is that D'Arcy Thompson insisted in the second decade of the 20th Century, that modern biology was going to have to incorporate physics, chemistry, and mathematics into its investigations in order to develop its proper character as a discipline, and he was, of course, right.  The 1913 two volume edition, of over 1,000 pages, has been reprinted in a single paperback volume by Dover.  Some of the facts are outdated, but it is still a wonderful volume.  However, for those who prefer a more concise edition, the distinguished developmental biologist, John Tyler Bonner, has done an excellent edited version which preserves the essence and character of the original.  If you can manage it, get both editions.

 In this work, Thompson recognizes the crucial aspect of symmetry in the investigation of biological systems and he examines not only bilateral symmetry, but radial and pentagonal symmetry, as well as branching and spiral form in plants, animals, and crystals.  Part of the genius of Thomson was his vision of a new biology that was truly an interactive synthesis of scientific disciplines.  It would have been convenient, but much less interesting, if nature had devised only 2 or 3 basic forms for foraminifera.  As it is, forams tests (shells) occur in a staggering variety of form and constantly challenge our ingenuity in describing them.

 When I think of forams, the first shape that pops into my mind is that of the coiled ammonite-like types that can be described in terms of a logarithmic spiral using the numbers of the Fibonacci sequence.  This is a very pleasing form to human beings and it shows up gloriously in the halves of a Nautilus shell that have been sawn open, in which one can see, not only the magnificent spiral, but the chambers as well.  Think then, how many people would be astonished to discover that millions of years before the Nautilus, nature had already tried out this form on a miniature scale, complete with chambers.  If you have the patience, you can confirm this for yourself by taking a fossil foram and grinding it carefully on a hone using your fingertip.  In some forms, such as the fusulinids, the intricacy of the arrangement of these chambers is astonishing.  Nature uses the form of the spiral, and variants thereof, again and again, from forams to mollusks to flowers to the great spiral galaxies.  Recently, I was looking at the operculum of a marsh snail under the microscope.  The operculum is a flat, thin chitinous plate, which the snail uses to tightly seal the opening of the shell.  Under brightfield illumination, it looked almost featureless, with a light amber color, and just a hint of some structure near the center of the plate.  However, when I shifted to polarized light, I was delightfully surprised—coiling out from the center, a spiral was clearly visible and with the addition of a full wavelength compensator, the results were quite splendid.  Nature is perpetually hiding things away in obscure places for us to discover, if we have the wit and tenacity (or just get lucky).

 Another ubiquitous form is that of the sphere which also has a special aesthetic attraction for us.  There are a few types of forams that are a simple sphere, but it is more usual to find spheres that have irregularly textured surfaces or are elongated or flattened or clustered.  There are even some that have an elongated neck and look like boiling flasks.  There are star-shaped forams, ones that look like ornate fans, others that have the shape of a feather, and still others that look like miniature palm leaves.  There are yet others that look like miniature calcareous trees, flat coiled disks, or microscopic trilobites. You are perhaps beginning to get an idea of why the classification of these shells can be so difficult, but this is just the beginning.  Consider for example the composition.  Indeed, most of the tests are calcareous, that is, they are composed of calcite or aragonite.  However, there are some that are composed of silica and others of chitin and, just to make things more interesting, there are agglutinated tests which usually consist of quartz grains held together by calcite, silica, or chitin.  In some instances, bits of other minerals, spicules from soft corals, holothuroids, or sponges may be attached or bits of shell fragments from mollusks or even other forams or diatoms may be incorporated.  What this means is that species which form agglutinated tests may appear significantly different from individual to individual.

 Then one has to consider the type of aperture of the test, whether it's single or multiple; whether it's crescentic, dendritic , circular, phialine, toothed, etc., etc. and not only the shape, but also its location on the test.  Next one has to consider the type of coiling; whether its dorsally trochospiral or evolute or involute, etc. And then there is the issue of surface texture: is it smooth, reticulate, punctate, etc?  And, of course, ornamentation: does it have bridged or limbate sutures, spines, costae, etc., etc.  And on top of that, one must consider the arrangements of the chambers.  So, at this point, one is ready to go off and read a good Reginald Hill mystery.  How on earth can something so small and, at first glance, so simple, be so inordinately complicated?  Further, it's no consolation that we have not even mentioned such morphological detail as the proloculum, septa, septal fluting, protheca, septal pores, chromata, pyknotheca, or spatulate equatorial chambers.  It's almost enough to make one a bowling or cricket fan.

 The positive side is that one can spend a modest amount of money to obtain enough samples to provide a lifetime of study.  If one gets seriously interested in forams (or bryozoa or ostracods), it is possible to specialize in certain subgroups and become something of an expert as one gradually makes one's way through the thicket of morphological terminology.

 Many microfossils are of that awkward size and composition such that viewing them is made difficult. The average stereo dissecting microscope often doesn't provide quite enough magnification to look at the surface sculpturing of a foram test.  It's rather like trying to pass a large truck on a hill when you are driving an inexpensive compact 4 cylinder car—there is a sense of strain and one feels that the vehicle is underpowered.  For general observation, I usually use an Olympus stereo zoom with a zoom range of 0.7x to 4.0x and most often with 20x oculars.  Above 60x, I notice a considerable loss of illumination intensity and sharpness of image, so 60x becomes the practical limit for this instrument no matter how hard I push on the accelerator.  Finally, two years ago, after three years of searching, I located a classic Leitz Greenough stereo dissecting microscope which has 5 sets of fixed objectives and 3 pairs of oculars allowing superb resolution and contrast from 8x to 216x.  This is a splendid instrument for looking at microfossils (and many other kinds of specimens) where a wide range of magnifications as well as a 3-dimensional view are desired.  Unfortunately, these instruments are not inexpensive and are very difficult to come by and modern equivalents are prohibitively expensive for the vast majority of amateurs.

 What this means is that one has to use a variety of techniques to get microfossils such as forams to reveal their secrets.  Without highly specialized equipment, some of the techniques can be quite time-consuming and rather tedious, but when one gets the hang of them, the results can be very satisfying.

1) A very useful and simple trick for small to medium sized forams is to soak them in mineral oil, castor oil, or even immersion oil, although this latter can get somewhat expensive, if you are examining a lot of samples.  With some types of forams, when the oil penetrates properly, you can get a very good view of the internal chambers.  If you wish to use your compound microscope for examining the specimens at higher magnifications, then you will need to use a cover glass to get optimal resolution and to protect your lenses.  If the specimens are thin enough, a good-sized drop of oil and a cover glass of  # 1½ thickness will do the job.  However, if the forams are a bit too thick for this method, then there are a couple of other tricks you can try.

a) Take a shallow depression slide, place the forams in the depression and add just enough oil so that when you place the cover glass, you don't get air bubbles trapped beneath, but not so much that the oil overflows off the slide.

b) Use a deep well slide or make your own using a small plastic or fiber ring.  This will allow you to make use of a variation of the classic hanging drop technique used for studying amoebae, but, in this case, you substitute a small drop of oil for the culture fluid. Place the drop in the center of the cover glass with the forams and then quickly turn the cover upside down and position it on the edge of the ring or well so that none of the oil is touching any surface other than the underside of the cover, thus hanging suspended.  This has the distinct advantage of not putting pressure on the shells which might cause them to break.

2) When sorting through a sample, one likes to select near perfect specimens, but one should also separate out fragments and broken specimens.  Not uncommonly, such pieces will have fractured in interesting ways, giving you glimpse into the inner structure of the shell.  In other instances, such fragments may require a bit of surgery in order for you to see what you want, and a pair of micro-forceps and a micro-scalpel are often very useful for revealing the internal arrangement of chambers in medium-sized specimens.  If you have a good supply of complete shells of a particular species, you may wish to try this method on a few of those as well by carefully crushing a few. By the way, when sorting samples, I use small plastic boxes and always try to include samples of other relevant fauna and sometimes even crystals to provide a frame of reference regarding the environmental context for the forams.

3) Another technique, which has several applications, is the use of dilute acids, acetic acid in particular.  Here, timing is everything.

a) Specimens from some sites may be covered with a chalky or soft limestone deposit which obscures surface detail.  A brief immersion in weak acetic acid can sometimes clean off such deposits in an effective manner, allowing you a much clearer view of the surface.  At first, try only one or two specimens at a time to get an adequate idea of the time required for that particular matrix.  Once you have treated the specimens for the requisite time, neutralize the acid by washing in tap water or by adding a pinch of sodium bicarbonate to a liter of distilled water and rinsing with that.

b) Shell fragments are especially interesting to treat in this way, since some inner surfaces are already exposed and the acid can etch away further material allowing you an even closer look at the internal structure.

c) If you have some nice moderate sized specimens, you can take a very fine brush and coat one side with a quick drying varnish.  Once they are thoroughly dry, immerse them in the acid and treat as above.  The varnished size will be partly protected and allow you somewhat greater control.

4) Some of the larger ammonoid-shaped forams, nummilites, and fusulinids are strong enough to withstand the rather rigorous treatment of grinding.  Professional equipment for making thin sections is prohibitively expensive for the amateur, so instead we have to rely on other methods including finger power.  Some species of nummilites lend themselves to being split in half quite readily with the encouragement of a microscalpel. [Protect your hands!] These are very attractive when mounted and the chambers are easily discernible.  After splitting them, I give them a quick acid bath to get rid of the powder which may have accumulated in the chambers.

 Mostly, however, these larger forams are not so cooperative and one has to resort to the tedious process of grinding.  Some types of test can be glued to a glass slide or a small block of wood.  A variety of substances has been used including balsam and synthetic resins.  The disadvantage is that these often take a considerable time to dry adequately and, as a consequence, I often use a clear cement, such as, Duco, which dries reasonably quickly and can be removed with acetone.  In the initial stages, a small hand-held electric grinder can often be used to advantage.  It is best to clamp the wood block or the slide (using a bit of padding to prevent breaking) in a small vise.  This will help prevent damage to your hands.  Also goggles should be worm to protect the eyes and the entire procedure is best done in an area well away from any optical equipment which might be seriously damaged by the fine rock dust produced by the grinding.

 There are two basically different kinds of preparations which one can strive for here:

a) A section which can be viewed with incident illumination to reveal the chambers, but is not transparent, is the first type. (These are much easier by far than the second type, the thin section for transmitted illumination.)  After initial preparation with the hand grinder, the finishing is done by hand.  For example, in preparing a nummilites section, I use first a medium and then a fine diamond whetstone (a small one costs about 5 dollars).  Using my index finger (you may want to protect yours with a band-aid), I grind the specimen, checking it frequently.  When you are close to the point you wish to achieve, use canned air to remove the powder from the chambers and ,when you are satisfied, give it a quick bath in dilute acetic acid, followed by a rinse in tap water.  The specimens must then be allowed to dry thoroughly—several days is best.  They can be mounted on a slide within a chamber.  I sometimes use a brass or nylon washer, or if the specimen is quite thick—2 nylon washers glued together.  When the shell has been glued to the slide, one can then cement a cover glass to the washer to protect the specimen from dust.  When I use brass washers, I use a clear gel cement and when I use the black nylon washers, I use the same gel cement, but I add some India ink to it and, over the next few days, ring the washer and edge of the cover glass with 3 coats of such gel.

b) Producing a thin section by hand is a much greater challenge.  Basically the same procedures are involved, but now you have to grind both sides and do so in a manner such that you can get a properly flat cross section or longitudinal section.  Furthermore, this section needs to be thin enough that light will pass through it to reveal the detail of the inner structure.  Anyone who wants to try this out might wish to prepare by reading the book of Job to learn about the virtue of patience in the face of adversity.  It is indeed cause for celebration, when you manage to produce a thin enough section which doesn't fracture before you complete it.  The final polishing, to get rid of the minute scratch marks from the grinding, should be done with an abrasive paste of very fine grit and this polishing should be done very carefully between two glass slides.  For the sake of your own sanity, it is usually preferable to buy thin sections which, though not cheap, are considerably less expensive than therapy.

 With some foram tests, it is desirable to increase contrast by staining.  A very simple means is at hand in your kitchen—food coloring.  This neat little trick has long been used by micro-paleontologists and green is the usual choice, as the eye is most sensitive to that range of contrast, although I have seen specimens where red, blue, orange or yellow have been used.  Many biological stains can also be used and a study of their relative effectiveness by an amateur with access to a wide variety of stains would make an excellent contribution to amateur microscopy when published in Micscape.  My own preference is for a fairly easy silver staining technique which I developed and described in a previous article for Micscape.

 In preparing micro-fossils, it is usually best to begin with the most conservative procedures.  Much depends upon the kind of matrix from which the specimens have been extracted.  If there is simply a light coating of clay or chalky or powdery limestone, a simple soaking in water may suffice.  If that doesn't do it, agitate the flask or beaker in which they are soaking and this may loosen up the undesired material.  A small soft brush may also be of aid in removing the material.  If you have a magnet stirrer, you can drop the magnetic stirring bar into the flask or beaker and become a major magnetic micro-agitator. (Ah, if only Gilbert and Sullivan were still around, we could have our very own amateur microscopy theme song.)

    If none of this works, then it's time to move to phase 2.  Go to your local supermarket and buy 1) a bottle of carbonated water and 2) a package of denture cleaning tablets.  Carbonated water is a weak solution of carbonic acid and will react slowly with the unwanted deposits on your specimens.  A denture tablet when dropped into ordinary tap water immediately begins to effervesce vigorously and this activity may be sufficient to dislodge the debris.

 If you still do not have the desired result, then it's time to move to the judicious used of acetic acid as mentioned above.  If the specimens are siliceous (for example, Radiolaria or if you have forams which in the process of fossilization have undergone a replacement with silica, then you can use stronger and more concentrated acids to remove carbonate material.  Such acids must, of course, be handled with great care.)

 In addition to forams, fragments of bryozoa, coral, ostracod shells, gastropods, small bivalves, and bits of crinoid stems can all be treated with these techniques.

 All comments to the author Comments to the author sent via our contacts page quoting page url plus : ('rhowey','')">Richard Howey are welcomed.
 

 ENDNOTE

 An interesting range of samples of micro-fossils can be obtained from Geological Enterprises, Inc., P.O. Box 996, Ardmore Oklahoma 73402; telephone (530) 223-8537; FAX (580) 223 6965; E-Mail: geoent@ardmore.com

 Many of the samples range in cost from $2-$3.  Their catalog is available for $5.  Forams, bryozoans, and ostracods are the most abundant of the special micro-fossil samples.. For those of you with interest in larger and unique fossils specimens, you can buy a trilobite for $5,000 or if you want something impressive as a lawn ornament, you can get a cast of an entire skeleton of an Acrocantosaurus atokensis for a mere $120,000 and it is approximately 13 meters long!

 
 
 

Microscopy UK Front Page
Micscape Magazine
Article Library


© Microscopy UK or their contributors.

Published in the February 2002 edition of Micscape Magazine.

Please report any Web problems or offer general comments to the Micscape Editor,
via the contact on current Micscape Index.

Micscape is the on-line monthly magazine of the Microscopy UK web
site at Microscopy-UK



© Onview.net Ltd, Microscopy-UK, and all contributors 1995 onwards. All rights reserved. Main site is at www.microscopy-uk.org.uk with full mirror at www.microscopy-uk.net.