Moseley says that ‘probably all is dark below 200 fathoms excepting in so far as light is given out by phosphorescent animals,’ and Wyville Thomson speaks of the ‘utter darkness of the deep-sea bottom.’
Within the last few years a few authors have maintained that it is quite possible that a few rays of sunlight do penetrate even to the greatest depths of the ocean—a view mainly based on the fact that so many deep-sea animals possess extremely perfect and complicated eyes and very brilliant colours. Verrill says: ‘It seems to me probable that more or less sunlight does actually penetrate to the greatest depths of the ocean, in the form of a soft sea-green light, perhaps at 2,000 or 3,000 fathoms equal in intensity to our partially moonlight nights and possibly at the greatest depths equal only to starlight. It must be remembered that in the deep sea far away from land the water is far more transparent than near the coast.’ Packard is of a similar opinion.
There seem to me to be very slight grounds for this view. The fact that, comparatively speaking, shallow-water fish avoid nets that are rendered phosphorescent by entangled jelly-fish does not justify us in assuming that deep-sea fish avoid regions where there are phosphorescent Gorgonians or Pennatulids. It is not by any means certain that fish avoid sunken nets on account of their phosphorescence. Most fish possess, as is well known, a very acute sense of smell, and it is very probable that they avoid such nets on account of the putrid odours of the dead animals that remain attached to them.
Nor is there much strength in the further argument that it can hardly be possible that there can be an amount of phosphorescent light regularly evolved by the few deep-sea animals, having this power, sufficient to cause any general illumination, or powerful enough to have influenced, over the whole ocean, the evolution of complex eyes, brilliant and complex protective colours, and complex commensal adaptations.
We have no sound information to go upon to be able to judge of the amount of light given off by phosphorescent animals at the bottom of the deep sea. The faint light they show on deck after their long journey from the depths in which they live to the surface may be extremely small compared with the light they give in their natural home under a pressure of 2½ tons to the square inch. The complex eyes that many deep-sea animals exhibit were almost certainly not evolved as such, but are simple modifications of eyes possessed by a shallow-water ancestry.
The more recent experiments that have been made, tend to show that no sunlight whatever penetrates to a greater depth, to take an extreme limit, than 500 fathoms.
Fol and Sarasin, experimenting with very sensitive bromo-gelatine plates, found that there was no reaction after ten minutes’ exposure at a depth of 400 metres on a sunny day in March.
But although it is highly probable that not a glimmer of sunlight ever penetrates to the depths of the ocean, there is in some places, undoubtedly, a very considerable illumination due to the phosphorescence of the inhabitants of the deep waters.
All the Alcyonarians are, according to Moseley, brilliantly phosphorescent when brought to the surface. Many deep-sea fish possess phosphorescent organs, and it is quite possible that many of the deep-sea Protozoa, Tunicates, Jelly-fish, and Crustacea are in their native haunts capable of giving out a very considerable amount of phosphorescent light.
If we may be allowed to compare the light of abysmal animals with that of surface forms, we can readily imagine that some regions of the sea may be as brightly illuminated as a European street is at night—an illumination with many very bright centres and many dark shadows, but quite sufficient for a vertebrate eye to distinguish readily and at a considerable distance both form and colour.