Readers’ wildlife photos

Today we have a historical/natural history post by reader Lou Jost, who works as a naturalist and evolutionary biologist at a field station in Ecuador.

A diatom sample from the HMS Challenger expedition of 1872-76

The HMS Challenger was a British naval ship equipped with both sail and steam power. At the urging of scientists, and riding the wave of popular curiosity about our then-poorly-known planet, the ship was converted by the Royal Society of London to become the world’s first specialized oceanographic vessel. It went on a mission from 1872 to 1876 to systematically explore the world’s oceans, especially the scientifically almost completely unknown Southern Ocean near Antarctica. This mission was the 19th century equivalent of a trip to the moon or to Mars (except that this  HMS Challenger mission had a much more interesting and diverse subject region!).

One of the navigators, Herbert Swine, made contemporaneous drawings and paintings on site, including the two HMS Challenger images I have shared here (though these were probably polished somewhat for publication). He also published his lively diaries of his time on the expedition, in two volumes, just before he died of old age. He was the last survivor of the crew.

The voyage of exploration went 80,000 miles, lasted 1250 days, and circumnavigated the globe. They made systematic chemical, temperature, and depth readings across the globe, taking biological specimens along the way. They discovered over 4000 new species, from vertebrates to phytoplankton, and lost several lives along the way. They were the first to systematically explore the mid-Atlantic Ridge, and by pure chance they also discovered the Marianas Trench,  the deepest part of the Pacific Ocean. In 1950-1951 a modern vessel, again bearing the name Challenger in a homage to the original, found the deepest part of any ocean, the “Challenger Deep”, just 50 miles from the HMS Challenger’s deepest depth record.

The immense number of samples obtained by the crew of the Challenger took 19 years to analyze and publish, in 50 volumes. Specimens were sent to many scientists of the time, and some of these still circulate today. Among the most interesting organisms they sampled are diatoms. Diatoms are single-celled organisms that make up much of the oceans’ phytoplankton, and their most notable features are the finely sculpted glass cases called “frustules” that enclose them. These glass frustules are often preserved intact for tens of millions of years, sometimes forming enormous deposits of pure frustules known as “diatomaceous earth” on the beds of ancient lakes and oceans. Some of these deposits are so big that millions of tons of diatom frustules thousands of years old are whipped up by the wind in dry parts of Africa every year, and then cross the Atlantic by air and rain down on the Amazon basin in South America.

The expedition of the HMS Challenger launched the most systematic study of the 19th century on the diatoms of the Southern Ocean. They sampled at regular intervals during their voyage, and at multiple depths, including very deep water that had never before been studied, discovering new species of diatoms such as Asteromphalus challengerensis, named after the vessel (using bad Latin unfortunately). The samples were distributed to diatomists around the world, who carefully mounted them on microscope slides using special mountants of high-refractive-index liquid, designed to make the transparent diatom frustule more visible under standard microscopic illumination. Some of these Challenger diatom slides come up for sale periodically, and I could not resist buying one that appeared in eBay.

Increasing zooms of the diatoms on the slide:

This one slide, from 1873 during an Antarctic visit, has hundreds of individuals consisting of maybe a couple of dozen species. There are also many broken diatom fragments. Among the individuals, I was lucky enough to find several examples of what appear to be the aforementioned A. challengerensis. This is a rare species which is found only in water that is within 1 degree Centigrade of freezing. The taxonomy of this species and its relatives is in flux as we learn more about how the structures change with age.

Two slides of the species A. challengerensis:

Some of the taxonomic problems of these diatoms is caused by their weird way of replication. Diatoms can’t grow like a normal organism because they are in a glass case, so instead they shrink, each half of the frustule making a new matching half that is slightly smaller than the parent half-frustule, so that the two new halves each nest inside their parent half-frustule. Then they separate. Here is a nice illustration of this:

The population thus has a large spread of different sizes, and it appears that some frustule features may change as they get smaller, causing taxonomic confusions in the case of A. challengerensis and others.  By the way, eventually the smallest ones go through a sexual reproductive phase that builds a new full-sized frustule, so that the cycle can start over. This is really weird. Later I hope to write long post about the utterly astounding, almost unbelievable biology of diatoms.

Darwin published his theory of evolution just 13 years before this expedition, and evolution was on everyone’s mind, and the commander of the ship was an “early adopter” of the theory. At the time there was still not much clarity about the predictions of the theory. It was widely believed that the cold dark oceans would preserve “living fossils” similar to the earliest forms of life on earth. The expedition did not find this to be true, and so it actually was a slight setback for evolutionary theory. They unfortunately missed the hydrothermal vents which do indeed shed light on the origins of life.

I wrote at the beginning of this post that the HMS Challenger expedition was the 19th Century analogue of space exploration. So it was fitting that NASA decided to name one of the space shuttles “Challenger”, after the two scientific ships which carried that name. The photo above shows Challenger orbiting over the ocean 110 years after the original HMS Challenger sailed that same ocean. Unfortunately, as in the original Challenger expedition, people died on that space shuttle in the name of science, a reminder that exploration on the margins of what is known will always be risky, and the participants are real heroes of their age.