It is common knowledge that the deep sea is teeming with a wide variety of flora and fauna that are specially adapted to live in the extreme conditions. It is prime time TV, featuring on the BBC’s Blue Planet, but prior to the 20th century nothing was known about life in the deep sea. From a 19th century perspective, it seemed entirely logical that life could not exist in the freezing cold, highly pressurised, and pitch black environment. Most scientists presumed that the environment was devoid of life. In the 5th century  Socrates wrote   “…Only caverns and sand and measureless mud, and tracts of slime wherever there is earth as well, and nothing is worthy to be judged beautiful by our standards.” In 1843 this was confirmed by deep-sea dredging in the Aegean sea done by Edward Forbes, who concluded that there was no life bellow 300 fathoms (550 meters).  Forbes wrote “As we descend deeper and deeper in this region its inhabitants become more and more modified, and fewer and fewer, indicating our approach towards an abyss where life is either extinguished, or exhibits but a few sparks to mark its lingering presence.” Despite Forbes’ scientific approach the conclusions he reached were false, so how did he reach this conclusion and how was it disproved?

Poor Experimental Design

One of the reasons Forbes failed to recover samples for the deep collected data during his expedition is because water in the Aegean sea is very oligotrophic (nutrient poor) therefore the depths are particularly barren. This could have been avoided by sampling in more oceans before universally applying the conclusions. Another problem with the design was that the scientific sampling for the deep ocean at the time was poorly adapted from commercial fishing gear.  Forbes adapted oyster fishing baskets with canvas bags that would quickly fill up with sediment.

Early dredging equipment. a) Otto Frederick Müller’s dredge; (b) Edward Forbes’ equipment adapted from oyster dredging equipment with a narrow mouth an canvas bag that rapidly filled up with mud; (c) John Ross’s equipment that was more efficient than Forbes’ as it was less likely to be clogged up with sediment; and (d) The dredge used on the HMS Porcupine with hemp tangles that  swept behind the net entangling the organisms. Credit: Anderson & Rice (2006)

Ignoring Other Evidence

Before Forbes went to the Aegean sea there was already evidence that life existed bellow 300 fathoms. Risso found life in the Gulf of Genoa at 1,000 m 70 years earlier. But this was seemingly ignored by Forbes when he was forming his hypothesis.

After Forbes’ expedition more evidence to the contrary was collected by Captain John Ross who was instructed to collect samples from as deep as 850-2000 meters whilst captaining the HMS Erebus and HMS Terror between1839 and 1843. He was able sample more effectively than Forbes as he had created a device known as a “Deep Sea Clam,” in today’s terms it would be called a grab sampler. Using this Ross was able to collect annelid worms and starfish from below 550m.

Why was this evidence ignored by the scientific community? Forbes did examine Captain John Ross’s samples but his notes have been lost. Perhaps Forbes dismissed these samples as Captain John Ross was known to be not very reliable: He reported a completely fictitious range of mountains blocking Lancaster Sound and he also reported his findings to be from 800 fathoms (approximately 1465 meters) but they were actually from 600 fathoms (approximately 1095 meters) due to poorly managed equipment.

The azoic hypothesis was accepted by scientists but many non-academics disagreed with it. Sir James Clarke Ross, a navy captain,  wrote “Although contrary to the general belief of naturalists, I have no doubt that from however great a depth we may be enabled to bring up the mud and stones of the bed of the ocean, we shall find them teaming with animal life; the extreme pressure at the greatest depth does not appear to affect these creatures…”

The starfish Gorgonocephalus arcticus first described by Leach in 1819. This sample was retrieved from the deep ocean from one of Ross’ expeditions where the organism had entangled itself in one of the soundings around the net. Credit: National History Museum.

Disproving the Hypothesis

25 years after Forbes published, Norwegian zoologist Michael Sars dredged off the Scandinavian coast and discovering hundreds of new species between 200 and 300 fathoms, the deepest scientific samples ever recovered from the deep at the time. This data did not directly contradict Forbes as he said life would be diminished at this depth but not completely extinguished until bellow 300 fathoms. The next scientific dredging expedition was by George Charles Wallich and Captain Sir Leopold McClintock on the HMS Bulldog dredging at 1260 fathoms near Iceland. The samples they took clearly disproved the azoic hypothesis, however the scientific community seemingly just ignored this data and the hypothesis was still widely accepted. 

Wyville-Thomson and Carpenter dredged in deep waters off the coast of the UK, Spain, and The Mediterranean. They modified Forbes’ dredges by adding ropes and making the mouth of the net much larger. They claimed “beyond question that animal life is varied and abundant, represented by all the invertebrate groups, at depths in the ocean down to 650 fathoms at least.” This work and that of Michael Sars George Charles Wallich and Captain Sir Leopold McClintock proved that the findings of Edward Forbes did not apply to the whole ocean.

It seemed so illogical that life would exist so deep in the ocean that evidence contradicting the hypothesis was ignored due to prejudice and biases within the scientific community. This concealed one of the greatest scientific questions of the time: How does life exist in such extreme conditions? It took until the Challenger expedition of 1872–1876 for an explanation to explain this.

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