Henry Gosse was at the forefront of the growing interest both
in sea life and in aquaria during the Nineteenth Century. In The Romance of Natural History (1860) he wrote:
“Almost as little known as the heart of Africa are the depths of ocean[s]. The
eye penetrates in the clear crystalline sea a few fathoms down, and beholds
mailed and glittering forms flitting by; the dredge gathers its scrapings;
divers plunge out of sight, and bring up pearls; and the sounding-lead goes
down, down, down, hundreds of fathoms, and when it comes up, we gaze with eager
eyes to see what adheres to the tallow ‘arming’; the tiny shells, the frustules
of diatoms, even the atoms of coral sand,-curious to learn what is at the
bottom of the deep. But, after all, it is much like the brick which the Greek
fool carried about as a sample of the house he had to let.”
How fascinated he would be by our ability to view the deep
sea today, albeit just a tiny fraction of it. There is a huge volume of water
in the oceans and most of it is in darkness, descending several kilometres to
the ocean floor. Using submersibles, we can venture down to take a look, but it
is very different to our ability to explore land, where travel is easy and
there are no problems with pressure, or with a supply of air, or with lack of
light. To extend Henry Gosse’s analogy, we certainly know much more about the
heart of Africa today than we do about the deep sea.
It was only decades ago that we first discovered the
presence of hydrothermal vents on the ocean floor and we now know they take various forms: http://www.bbc.co.uk/nature/habitats/Hydrothermal_vent All produce plumes of hot water, with the hottest having
“black smokers” which exude water rich in chemicals from the Earth’s crust, the
temperature of the plume being up to 400oC. “Black smoker” is a good
description, as the column of water contains many small sulphide-rich particles
which are precipitated as the water cools, forming a chimney around the vent and shooting
a column of what looks superficially like smoke into the water.
One of the characteristics of regions with black smokers is
the abundance of animals that they contain and I will describe just one example
- the vestimentiferan worms - that have come to characterise these communities
(there are also many bivalve molluscs, crustaceans, and other types of
animal).Vestimentiferan worms are long as adults (up to 2 m) and the anterior of
each worm can be extended from its tube, and also withdrawn: http://www.noc.soton.ac.uk/chess/science/images/riftia_fish_hq.jpg
What would Henry Gosse have made of them? I think he would
have been thrilled to discover these extraordinary examples of God’s Creation
existing in such a remote location. They have characteristics of worms which
live in tubes elsewhere in oceans, but the tube worms of ocean margins have
feeding organs, covered with secreted mucus, which allow them to capture tiny particles.
These particles (together with anything else which sticks to the mucus) are conveyed
to the mouth by tiny moving hairs (cilia) and are then ingested. The package of
particles and mucus passes through the digestive tract and it is from this that
the tube worms gain their nutrition. Some of the particles will be algae and
bacteria, some made up of dead organic matter, but all will have resulted
originally from solar energy trapped by photosynthesis.
Close examination of vestimentiferan worms shows that they
do not have a digestive tract, so what do they feed on and how do they gain the
nutrition needed to achieve their large size? These are questions that would
certainly have puzzled Henry Gosse and it took Biologists some time to provide
answers. Like many soft-bodied marine creatures, the worms are able to absorb
dissolved matter across their body wall and this is rich in sulphides from
the water which emanates from the black smokers. These chemicals are concentrated
in some hydrothermal vents, but will not supply much of the nutritional needs
of the vestimentiferans. The secret lies in having large numbers of bacteria
living within the tissues of the worms. These bacteria lie at the base of hydrothermal
vent food chains and they coat all surfaces; explaining why sulphide particles
are ingested by bivalves and also by crustaceans (both stripping off these
nutritious coatings inside their digestive tracts). The bacteria use sulphides as a source of energy, just as
surface-dwelling organisms use light in photosynthesis. It is termed
chemosynthesis and the bacteria living and growing within vestimentiferan worm
tissues are able to secrete organic compounds that the worms use for their own
growth.
Of course, this leads to further questions: how did the
association between bacteria and vestimentiferan worms arise and how did the relatives of surface water tube worms end up at
hydrothermal vents? Henry Gosse would repeat that this was by the act of
Creation and that is a convenient answer. If one doesn’t believe in this
explanation, then how did hydrothermal vent communities come into existence and
how did their association with chemosynthetic bacteria come about? I think I
have the answers, but I’m not sure. That’s the problem with evolution; there
are so many developments that we can only speculate about, as they happened over
such a long time period so long ago.
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