In the Footsteps of Linnaeus – Microscopic Imaging in Marine Biodiversity Research

In 2007 Sweden celebrates the 300^th birthday of Carl von Linné (Carolus Linnaeus, 1707–1778), one of the key figures in the history of natural sciences. Linné’s work is an incredible contribution to our understanding of the living world. His great achievement as a scientist was the design of a universal system for classifying living organisms. In this respect, Linné is often regarded as the father of taxonomy and biological systematics. In 1735 Linné published his most famous work “Systema naturae” in which he described a system that divided plants from animals and minerals. The paper was barely twelve pages long, but nevertheless was an outstanding contribution to science.

Before Linnaeus, classifying plants and animals was a rather chaotic enterprise. Naturalists ordered organisms in all kinds of ways, and that was obviously done for practical reasons. For example, plants were often separated in edible and poisonous flora. The method of naming plants and animals was equally chaotic. At the time it was usual to describe a particular species simply by assigning a series of adjectives to them. In this way the same species could end up being described in entirely different ways by two or more people, which made it nearly impossible to know for sure whether the same organism was being identified. This made communication and scientific work so difficult that Linné used to say (Linnaeus 1737, Critica botanica): "If you don’t know their names, knowledge about things is useless."

Linné invented a universal system to describe, identify, and classify organisms, which we know today as the binomial nomenclature. He allocated to each species a first name that defines the genus, and a second name referring to the particular species. For example Homarus gammarus identifies the European lobster. Given this name biologists all over the world will know which animal is described. This system became established very quickly and it made scientific work much easier so contemporaries often said that "God created the world, but Linné has put it in order". It was Linnaeus that coined the name Homo sapiens and grouped humans with apes as early as 1766, almost hundred years before Charles Darwin presented his ideas of evolution. However, like most contemporaries, Linnaeus believed that all species including humans were created by God at some point and since then have not changed.

With his Systema naturae Linnaeus started something wonderful – the discovery of biodiversity. During his lifetime he recognized about 8,000 plants and 6,000 animal species. Today this number has risen up to approximately 1.75 million named species and it surely would make Linnaeus happy to see his legacy growing with such tremendous progress over the past 250 years. However his work is far from being finished and the discovery of biodiversity is still in its infancy. The estimated number of all species on the planet is at least 10 million and scientists at the Global Biodiversity Information Facility in Copenhagen have calculated that it will take us at least another 1,000 years to describe all species if we continue at the same rate as in the past. This confronts us with a serious problem because at the same time biodiversity declines rapidly all over the world due to the destructive influence of human activity, causing species to disappear forever. These are species that possess a unique and valuable potential for our own technological advancement, species from which we can learn and extract drugs or biomaterials, and perhaps most importantly species on which other organisms critically depend. As a consequence of this dilemma, taxonomy, the science of describing and classifying organisms, has been revitalized over the last decade as one of the leading scientific disciplines in modern biological research.

There is still a lot to be discovered, especially in the microscopic world. Most of the organisms in the sea are microscopic with body sizes of less of than a milli-metre, and although they play a central role in marine ecosystem function, their biology is largely unstudied. The Swedish government has recently launched a large scale project with the goal of describing the complete flora and fauna in of its country. The so-called Swedish Taxonomy Initiative (www.artdata.slu.se) is a 20 year assignment and can be regarded as a birthday present to Linnaeus by his mother land. More importantly perhaps is the exemplary role of such a survey, as this project makes Sweden the first country worldwide that attempts to carry out an inventory of its entire multicellular life.

Fig. 6: Light microscopy (left) and clsm (right of the microscopic larva of the marine bryozoan Membranipora membranacea (size ~500 µm). The triple staining shows three organs in one image. Cells of the nervous system are indicated in pink, ciliated organs in blue, muscular structures in green. Photo: Judith Fuchs (left) and Katrine Worsaae (right).

Most of the species recognized in Linné’s time had a reasonable size and could be studied with the naked eye or simple magnifiers. However, today we are able to advance into a world where sand grains are gigantic. This world is inhabited by a bewildering number of small organisms, like flatworms, foraminifers, roundworms, copepods, rotifers, tardigrades, loiciferans, and many other creatures with extraordinary shapes and abilities. It is also home to the minute larvae of most of the larger marine invertebrate animals. By using the modern methods which are available nowadays it is possible to obtain concise and detailed anatomical reconstructions of such organisms. This provides us with essential information in ecological and taxonomic studies that was difficult to obtain before. For example the effects of pollution on minute organisms can now easily be recorded by 3-dimensional reconstructions of the major internal organ systems in the confocal laser scanning microscope.

Fig. 7: Stereo microscopic image (left) of the female of the bone-eating worm Osedax mucofloris. The worm lives exclusively in whale carcasses on the bottom of the deep sea but was recently discovered at the Swedish West coast. The dwarf male (right) shown by CLSM is much smaller than the female and parasitizes in large numbers the uterus of the female. Photo: Thomas Dahlgren (left) and Katrine Worsaae (right).

Such methods are equally helpful when describing new organisms. In traditional practice species descriptions contain hand drawings that assemble all features observed in the microscope. Such drawings take time and even today can be very subjective. Using multiple immunocytochemical staining methods in combination with confocal laser scanning microscopy is a relatively fast and objective process that can assemble information from a number of internal features in just one single image. Likewise, external features are relatively easy be to combine in one image using montage software packages in the stereo microscope. The microcosm is full of mysteries and new imaging methods are powerful tools to unravel these. There is no doubt that Linné would find much to admire in today’s technical possibilities to observe tiny organisms. Let’s hope we are able to study them before they disappear.

Blunt W: Linnaeus. Frances Lincoln Publishers (2004). ISBN-10: 0711223629.
Miller G: Linnaeus’s legacy carries on. Science 307 (2005) 1038–39.