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Gone Missing, circa 1892

A unique organism sighted only once, more than a century ago, could shed light on the evolution of multicellularity—if it ever actually existed.

By | October 1, 2012

image: Gone Missing, circa 1892
TRUTH OR MYTH? Among Frenzel’s detailed 1892 drawings of Salinella salve’s life cycle is a depiction of a mature individual (labeled 5 at right) composed of a single layer of cells with a mouth on one end and an anal opening on the other. Click on the image to view an extended caption.
Credit: Wikipedia

Two decades ago, Michael Schrödl, curator of the mollusk compilation at the Bavarian State Collection of Zoology in Munich, Germany, attended a lecture on Salinella salve, a mysterious organism described as a single layer of cells, ciliated on both inner and outer surfaces and surrounding a central hollow sac open at both ends. Its body plan, which suggests a central digestive tract with a mouth and an anus, was unlike that of any known life-form. Composed of just one cell layer, Salinella seemed intermediate between single-celled organisms that perform all digestion inside the cell, and multicellular life-forms that excrete digestive juices into a central sac.

Was Salinella a missing link between protozoans and metazoans? A living relic of the transition from single-celled life to more complex forms with multiple cell layers? Schrödl was determined to find out. But there was one major problem—Salinella salve had been collected only once, by German biologist Johannes Frenzel, who claimed to have found it in the salt pans of Argentina in the late 19th century.

Sensing the opportunity for a scientific treasure-hunting adventure, Schrödl and a couple of friends set off for South America with only a vague, century-old description from Frenzel to guide them. “From our own pocket, but in the name of science, we bought a rusty car and went to Córdoba in Argentina,” Schrödl recounts. The first challenge Schrödl and his companions faced on their hunt was locating the source of the salt and soil from which Frenzel had reported culturing Salinella a century earlier. When they arrived at what they thought was the location where Frenzel had obtained the original Salinella sample, they found lush fields and mooing cows instead of salt pans and semidesert.

To make matters more complicated, during the course of the investigation Schrödl discovered that Frenzel had not actually collected the soil samples himself. A geologist friend had given them to him. Yet Schrödl figured that if Frenzel had indeed managed to coax Salinella to grow from dry dirt, then it could have survived as a wind-borne spore or cyst, and therefore it shouldn’t be restricted to one area. So he and his friends collected salt, sand, and earth from each salt lake they encountered across hundreds of miles of Argentina’s Córdoba Province.

According to Frenzel’s 1892 description in the journal Archiv für Naturgeschichte, he left his aquaria uncovered for months, exposing them to dirt, debris, and the occasional plant material. Schrödl attempted to follow Frenzel’s protocol for growing the elusive specimen—to no avail. “Many things were growing, but no trace of Salinella,” Schrödl says. Was Frenzel simply mistaken in his description? Schrödl thinks not, since Frenzel was a keen observer and a meticulous artist and would not have missed additional cell layers had Salinella indeed possessed them. Some have dismissed the Salinella mystery as scientific fraud, but Schrödl finds this hard to believe. Frenzel was a “serious scholar” who made valuable contributions to the fields of anatomy, protozoology, and ornithology, Schrödl says. And then there’s still the possibility that Salinella is “awaiting rediscovery as one of the most interesting creatures for scientific study ever,” Schrödl adds. “It was my dream to find it, and worth a try. Imagine the possibilities of modern molecular or microstructural research!”

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Comments

Avatar of: bensabio

bensabio

Posts: 3

October 12, 2012

Wow, what an interesting article!!  It would indeed be wonderful if this creature could be found again and studied with current techniques, including sequence-based phylogeny.  Great credit (and heartfelt empathy!) is due to Dr.Schrödl for trying so hard to find it. 

However, as well-written as the article is, I must take issue with one of the interpretive comments.  The kind of cellular and organismal organization of this creature does not suggest an "intermediate" between protozoa and metazoa at all.  There's nothing about it that is protistan, and everything about it that is fully metazoan.  In fact, a very likely evolutionary origin of this simple creature is that of descent from a more complex creature (cnidarian or flatworm?) by loss of complexity (e.g., a tissue layers, etc.)  That's why DNA analysis would be so interesting and crucial for understanding its evolutionary affinities. 

By the way, there is an even simpler metazoan creature that is well known and studied, belonging to its own phylum: Placozoa, represented by a single species, Trichoplax adhaerens, studied lovingly by the great protozoologist, Karl Grell.  It's just a "multicellular amoeba", a single flat amoeboid layer of  ciliated cells with no central cavity at all. See the Wikipedia article on this creature for an excellent description and fascinating discussion of several hypotheses for its evolutionary affinities, and the evidence for these: http://en.wikipedia.org/wiki/Placozoa

Avatar of: bensabio

bensabio

Posts: 3

October 12, 2012

...apparently Trichoplax does have a cavity of sorts between the pancake-like outside and underside layers of its uniciliated cells.  Descriptions tend to be a bit confusing on this issue.

Avatar of: vmaldia

vmaldia

Posts: 26

October 16, 2012

even if its just a base metazoan and not a transitional in between protists and metazoans, its still very interesting.

 

more funding for more expeditions!!!

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