The Nature of Animal Light Part 6

[Ill.u.s.tration: FIG. 27.--Sectional view of photogenic organ of _Sergestes prehensilis_ (_after Terao_). _bm_, bas.e.m.e.nt membrane; _cs_, connective strands of photogenic layer; _hy_, hypodermis; _l_{1}_, _l_{2}_, _l_{3}_, layers of lens; _le_, lens epithelium; _n_, nerve; _ph_, photogenic cells; _pi_, pigment layer; _r_, reflector; _th_, theca.]

[Ill.u.s.tration: FIG. 28.--Diagram of photogenic organ of _Nyctiphanes Conchii_, to show pathways of light rays arising in the light cell layer (_after Trojan_). _p_, pigment; _ri_, inner reflector; _lp_, light cells; _rf_, refractor; _f_, focus; _l_, lens; _A-A_, axis; _a_{1}-a_{4}_, _b_{1}-b_{4}_, light rays reflected from _ri_; _c_{1}-c_{4}_, light rays pa.s.sing directly outward; _d_{1}-d_{9}_ and _e_{1}-e_{9}_, light rays which have pa.s.sed refractor and lens respectively.]

All gradations in complexity of light organs may be found from the condition in the shrimp just described to that found among the squid and fish. Figs. 29 and 30 are sections of two of the more complicated types found in squid. The explanation given to the various structures is that of Chun (1903) to whom we are indebted for a careful histological investigation of these forms. It will be noted that in addition to photogenic and lens tissues there are various types of reflector cells and a line of pigment about the whole inner surface of the organ to effectively screen the animal's tissues from the light. In one form (Fig. 30) chromatoph.o.r.es are found about the region where the light is emitted and these no doubt serve as color filters. There are also an abundant blood supply and nerves pa.s.sing to the organ. Figs. 30 and 31 are sections through light organs of fishes.

We thus see that light organs may be very simple and also very complicated. The latter must have evolved from the former, although it is not always possible to point out the intermediate stages. It is not within the scope of this book to discuss bioluminescence in its evolutionary aspects. It may be worth while, however, to point out briefly what is known concerning the use of the light to the animal.

There are four possibilities.

[Ill.u.s.tration: FIG. 29.--Sectional view of photogenic organ of a squid, _Abraliopsis_ (_after Chun_.) _refl^1_, _refl^2_, reflectors; _lac._, lacunar s.p.a.ces; _chr._, pigment screen of chromatoph.o.r.es; _chr.^1_, chromatoph.o.r.e; _phot._, photogenic cells; _l_, lens; _co._, cuticle; _v_, blood vessel; _fibr._, connective tissue.]

(1) The light may be of no use whatever, purely fortuitous, an accompaniment of some necessary or even unnecessary chemical reaction.

This appears to be the case in the luminous bacteria and fungi and perhaps the great majority of forms which make up the marine plankton, _Noctiluca_, dinoflagellates, jelly-fish, ctenoph.o.r.es and even the sessile sea pens.

[Ill.u.s.tration: FIG. 30.--Sectional view of photogenic organ of a squid, _Calliteuthis_ (_after Chun_). _phot._, photogenic cells; _l_, _l^1_, lens; _n_, nerve; _spec._, "Spiegel"; _pg._, pigmented screen; _c.

fusif._, spindle-shaped reflector cells; _chr._, chromatoph.o.r.e color screen.]

[Ill.u.s.tration: FIG. 31.--Sectional view of photogenic organ of a fish, _Stomias_ (_after Brauer_). _p_, pigment screen; _dr_, _dr^1_, photogenic gland cells; _l_, lens.]

We know that luminous bacteria occasionally lose the power of lighting and that on certain culture media they develop as non-luminous forms.

Luminescence is not indispensable to them. The same is true of some of the fungi but _Noctiluca_ and other animals are not known in a non-luminous condition, although we can see no definite value to the organism of this power of luminescence.

[Ill.u.s.tration: FIG. 32.--Sectional view of photogenic organ of a fish, _Argyrophelecus affinis_ (_after Brauer_). _p_, pigmented screen; _dr._, photogenic cells; _r_, _r^1_, reflector?; _l_, lens?; _s_, sclera; _g_, connective tissue.]

In the case of sea pens, however, we might suppose that the light acts as an attraction to small organisms on which the sea pen feeds, although these creatures only luminesce when stimulated in some way, which rather detracts from the above suggestion.

(2) The light may act as a warning to scare away predacious animals which would otherwise feed on the luminous organism. Perhaps this is the case in the sea pens, although these forms possess nematocysts which should serve as adequate protection. The marine worm, _Chaetopterus_, is brightly luminous and lives its whole life in an opaque parchment tube.

If this tube were torn open by a predacious form we might conceive that the attacking animal would be alarmed by the light and refrain from destroying the worm. The _Chaetopterus_, however, could not rebuild another tube and its light would only protect it in the night time.

These cases will suffice to indicate the difficulties and perplexities of the problem. Perhaps we may add one more guess and suppose that the light of certain fishes is actually for blinding or distracting their enemies or blinding the forms on which they feed. Until this use of luminous organs has actually been observed, we can give little credence to it.

(3) The light may serve as a means of recognition or a s.e.x signal to bring the s.e.xes together for mating. It would seem from the work of Mast and of McDermott that this is the case in the common fireflies and it may be the case in the toad-fish, _Poricthys_, which is only luminous in the sp.a.w.ning season and in the worm, _Odontosyllis_, of Bermuda, which is brilliantly luminous while swarming when the eggs and sperm are shed.

It is non-luminous at other times (Galloway and Welch, 1911.)

(4) Finally, it is possible that animals with complex luminous organs, such as squid, fish and shrimp, actually use these as lanterns. It is significant that most of them are deep sea forms, living in a region of perpetual darkness, and it is perfectly logical to suppose that they make use of their light organs for illuminating purposes.

The whole problem of the use and purpose of luminous organs is an exceedingly complex and difficult one. We have, perhaps, said enough to indicate this and may add that in most cases, so far as opinion is based on actual evidence and observation, that of the layman is of as great value as that of the scientist.

CHAPTER V

THE CHEMISTRY OF LIGHT PRODUCTION, PART I

Two experiments, both performed very early in the history of Bioluminescence, are of great importance in understanding the nature of animal light. Boyle (1667), as already mentioned, proved the necessity of air for the luminescence of wood and fish and Spallanzani (1794) showed that parts of luminous medusae gave no light when dried but if moistened again would emit light as before. We see then, that air (oxygen), water, and some photogenic substance are necessary for the light production. Spallanzani's experiment, which has been confirmed for a great many luminous forms, shows also that animal luminescence is not a _vital_ process, in the same sense that the conduction of a nerve impulse is a vital process. A nerve loses its characteristic property of conduction on drying or maceration while luminous cells still possess the power to luminesce after drying or maceration. Using the terminology of the older physiology we may say that "living protoplasm" is not necessary for light production.

The experiments of Boyle (1626-91) are of great interest, especially those in which he studied the behavior of shining wood under the receiver of his air pump. On October 29, 1667, he wrote:

"Exp. I.: Having procured a Piece of _shining Wood_, about the bigness of a groat or less, that gave a vivid Light, (for _rotten Wood_) we put it into a middle sized _Receiver_, so as it was kept from touching the Cement; and the _Pump_ being set a-work, we observed not, during the 5 or 6 first Exsuctions of the Air, that the splendor of the included Wood was manifestly lessened (though it was never at all increased;) but about the 7th Suck, it seemed to glow a little more dim, and afterwards answered our Expectation, by losing of its Light more and more, as the Air was still farther pumped out; till at length about the 10th Exsuction, (though by the removal of the Candles out of the Room, and by black Cloaths and Hats we made the place as dark as we could, yet) we could not perceive any light at all to proceed from the _Wood_.

"Exp. II.: Wherefore we let in the outward Air by Degrees and had the pleasure to see the seemingly extinguished Light revive so fast and perfectly, that it looked to us almost like a little Flash of Lightning, and the Splendor of the Wood seemed rather greater than at all less, than before it was put into the Receiver."

Boyle proved that light from the wood was able to pa.s.s a vacuum and later showed that "shining fish" behaved as the "shining wood," but that a piece of white hot iron would not regain its light on readmitting air to the exhausted receiver and that the iron lost its glow under the air-pump merely because it cooled off. A piece of glowing coal, however, did lose its light in the absence of air and regained it on again admitting air, provided the air had not been removed for too long. Boyle was apparently impressed with the similarity of the light giving process in glowing coal and shining wood as he draws a comparison between the two which brings out the fundamental similarity of combustion processes.

"Resemblances:

VII. The Things wherein I observed a Piece of _shining Wood_ and a _burning Coal_ to agree or _resemble_ each other are princ.i.p.ally these _five_:

1. Both of them are _Luminaries_, that is, give _Light_, as having it (if I may so speak) _residing in them_; and not like _Looking-gla.s.ses_, or _white Bodies_, which are conspicuous only by the _incident Beams_ of the _Sun_, or some other _luminous Body_, which they _reflect_....

2. Both _shining Wood_ and a _burning Coal_ need the Presence of the Air (and that too of such a _Density_ to make them continue _shining_)....

3. Both _shining Wood_ and a _burning Coal_, having been deprived, for a Time, of their _Light_, by the withdrawing of the contiguous _Air_, may presently recover it by letting in fresh _Air_ upon them....

4. Both a _quick Coal_ and _shining Wood_ will be easily quenched by _Water_ and _many other Liquors_....

5. As a _quick Coal_ is not to be _extinguished_ by the Coldness of the _Air_, when it is greater than ordinary; so neither is a Piece of _shining Wood_ to be deprived of its _Light_ by the same Quality of the _Air_....

Differences:

1. The first Difference I observed betwixt a _live Coal_ and a _shining Wood_ is, that whereas the _Light_ of the _former_ is readily _extinguishable_ by _Compression_ (as is obvious in the Practice of suddenly _extinguishing_ a piece of _Coal_ by treading upon it), I could not find that such a _Compression_ as I could conveniently give without losing sight of its operation, would _put out_, or much injure the _Light_, even of small Fragments of _shining_ Wood....

2. The next _Unlikeness_ to be taken notice of betwixt _rotten Wood_ and a _kindled Coal_ is, that the latter will, in a very few _Minutes_, be totally _extinguished_ by the withdrawing of the _Air_; whereas a Piece of _shining Wood_, being eclipsed by the Absence of the _Air_, and kept so for a Time, will immediately _recover_ its _Light_ if the _Air_ be let in upon it again within half an hour after it was first withdrawn....

3. The next _Difference_ to be mentioned is, that a _live Coal_, being put into a small close Gla.s.s, will not continue to _burn_ for very many _Minutes_; but a Piece of _shining Wood_ will continue to shine for _some_ whole _Days_....

4. A _fourth Difference_ may be this: that whereas a _Coal_, as it _burns_, sends forth Store of _Smoke_ or _Exhalations_, _luminous Wood_ does not so.

5. A _fifth_, flowing from the former, is, that whereas a _Coal_ in _shining_ wastes itself at a great Rate, _shining Wood_ does not....

6. The last Difference I shall take notice of betwixt the bodies. .h.i.therto compared is, that a _quick Coal_ is actually and vehemently hot; whereas I have not observed _shining Wood_ to be so much as sensibly _lukewarm_."

It should be clearly borne in mind that if we place luminous organisms, say bacteria or fungi, in an atmosphere devoid of oxygen and find that no light is produced, this may merely mean that certain functions of the cell are interfered with, including light production, but does not necessarily indicate that oxygen is actually used up in the photogenic process. If we find, however, that extracts of luminous cells or luminous secretions devoid of cells cease to light when the oxygen is removed and again luminesce when it is returned, we may be quite certain that the photogenic process itself requires free oxygen. As luminous extracts of fireflies, pennatulids, ostracods, _Pholas_ and others give off no light when the oxygen is removed, we may safely conclude that for these luminescences, oxygen is necessary. Bacteria, fungi, and _Noctiluca_, whose light also disappears in absence of oxygen, although they are whole cells, we may by a.n.a.logy also a.s.sume to require oxygen in the photogenic process.

Some of the earlier workers on fireflies and _Noctiluca_ obtained light even after placing these organisms in absence of oxygen, but they did not realize how low is the amount of oxygen necessary to produce light.

It is difficult to remove traces of oxygen from the water, traces which are nevertheless sufficient to cause luminescence. If the organisms are numerous, as in an emulsion of luminous bacteria, they will themselves use up all the oxygen and the liquid soon ceases to glow except at the surface in contact with air. We may gain an idea of the amount of oxygen necessary for luminescence from an experiment of Beijerinck (1902). He mixed luminous bacteria with an emulsion of clover leaves containing chloroplasts and kept the two in the dark until all the oxygen was used up and the bacteria ceased to glow. If now a match was struck for a fraction of a second, sufficient oxygen was formed by photosynthesis to cause the bacteria to luminesce for a short time.

Exact figures on the minimal concentration of oxygen for luminescence cannot be given. The luminescent secretion of _Cypridina hilgendorfii_ will still give off much light if hydrogen containing only 0.4 per cent.

of oxygen is bubbled through it, _i.e._, a partial oxygen pressure of 1/250 atmosphere (3.04 mm.Hg). However, addition of a fresh emulsion of yeast cells to a glowing _Cypridina_ secretion is sufficient to rapidly extinguish the light, because the yeast is capable of utilizing the last trace of oxygen in the mixture. Light only appears when, by agitation, we cause more air to dissolve. The minimal concentration of oxygen for luminescence of _Cypridina_ lies somewhere between 3.04 mm. and the amount which living yeast fails to extract from solution, a concentration approaching zero. It is probably nearer the latter figure.

As the oxygen pressure is increased from 0 to about 7 mm., the intensity of the _Cypridina_ luminescence increases and at the latter figure the light is just as bright as if the solution were saturated with air (152 mm.O_{2}). Thus, the luminescence requires only a low pressure of oxygen and the similarity to the saturation of haemoglobin with oxygen is obvious. Just as haemoglobin is nearly saturated with oxygen at low pressures and becomes bright red in color, so the luminous material becomes saturated with oxygen at low pressures and glows intensely.

Boyle also made many experiments to show that air was necessary for the life of animals and the germination of seeds and showed that repeatedly respired air was unfit for further breathing. About the same time R.

Hooke discovered the true meaning of respiratory movements and by forcing a blast of air continuously through the lungs with bellows, was able to keep animals alive. He concludes "that as the bare Motion of the _Lungs_, without fresh air, contributes nothing to the life of the Animal, he being found to survive as well as when they were not moved as when they were; so it was not the Subsiding or Movelessness of the _Lungs_ that was the immediate cause of death, or the stopping of the circulation of the Blood through the _Lungs_, but the Want of a sufficient Supply of fresh Air." The cause of death on collapse of the lungs could not be better stated to-day. Thus combustion, respiration and luminescence of flesh or wood were early recognized as related phenomena.

Although the "gas sylvestre" (CO_{2}) of burning charcoal and fermentation of wine was known to van Helmont (1577-1644) and Mayow (1646-1679) in 1674 showed that "spiritus nitroaerens" (oxygen) was responsible for the life of animals and for combustion, a century elapsed before the true significance of these gases became known. In the meantime the phlogiston theory of combustion had been developed, Black (1728-1799) in 1755 had rediscovered carbon dioxide ("fixed air") in the expired air and Priestley (1733-1804) and Scheele (1742-1786) had both rediscovered oxygen ("dephlogisticated air") in 1774. About the same time Lavoisier overthrew the phlogiston doctrine and showed that in the combustion of organic substances water and CO_{2} are formed.

Later it was realized that this slow combustion did not take place in the lungs, or in the blood, but in the tissues cells themselves and respiration in the chemical sense has come to mean this universal slow combustion in the cells of the body rather than the breathing movements of the lungs themselves. In anaerobic respiration, CO_{2} is given off, but no oxygen absorbed. In aerobic respiration, oxygen is absorbed and CO_{2} given off. In addition we know of many substances which oxidize by taking up oxygen without giving off CO_{2}. We have seen that oxygen must be absorbed for luminescence of animals and we may now inquire whether CO_{2} is given off and the relation between respiration and light production.