In this post, I will give you a few glimpses of how were the early plankton-human interactions and how we managed to study these creatures. As you will see, there was already evidence from ancient Greece that there were strange beings, including plankton organisms, that did not fit into any known classification. However,perhaps the greatest work was done by a series of amateur and professional naturalists from the 16th to the 19th centuries. Their discoveries were not always published as books but were often contained in correspondence they maintained with recognized institutions, such as the Royal Society of London. Even though many valuable records are certainly lost, a good part of this correspondence is still available, and we have evidence of the exciting progress of those people who had a whole new world to discover. You will see that I give special emphasis to the first illustrations of plankton because, as the popular saying dictates, many times a picture is worth a thousand words.
Logically, the first recorded members of plankton were possibly jellyfish and other larger organisms, particularly those that lived attached to other animals for daily consumption, such as fish. Already in the 4th century BC, the great philosopher and naturalist Aristotle first identified a parasitic copepod. He classified it, like jellyfish and other soft-bodied organisms, such as sponges and ascidians, under the name Zoophyta, something between animals and plants, and that classification lasted for hundreds of years. However, we owe the first image of a parasitic copepod (Figure 1) to Gillaume Rondelet, a French zoologist born in 1507.
Figure 1. Below, on the right and above the tuna’s gills, you can see what could be a parasitic copepod. Illustration by Rondelet (1554).
The free-living copepods had to wait slightly longer to be discovered; in 1688, Stephan Blankaart drew the first recorded free-living (freshwater) copepod (Figure 2). Carl Linnaeus (father of modern taxonomy, 1707-1778) named them Monoculus and classified them as insects. As insects, they remained for many years until the beginning of the 19th century, when Jean-Baptiste Lamarck classified them as crustaceans, along with water fleas, amphipods and isopods.
Figure 2. The first illustration of a free-living copepod, left. Possibly of the genus Cyclops from a sample of freshwater. Stephan Blankaart (1688).
Out of curiosity, the first illustration of a free-living marine copepod corresponds to Gunnerus (1770), a Norwegian zoologist who identified Calanus finmarchicus (rather numerous and important species on the Norwegian coast), although he called it Monoculus finmarchicus (Figure 3).
Figure 3. The first illustration of a free-living marine copepod. Ernst Gunnerus (1770).
We owe the discovery of protists to Antonie van Leeuwenhoek, who with his rudimentary microscope first saw infusoria and other planktonic organisms. Between 1674 and 1716, this Dutchman, recognized as the father of microbiology, described several species of protozoa, mostly ciliates (infusoria, Figure 4), among other planktonic creatures.
Figure 4. Drawings of different infusoria and other organisms made by Antonie van Leeuwenhoek 1702.
He did not pay much attention to planktonic algae, and although he surely saw them, the first diatom (not quite planktonic) was described by an English gentleman, probably Charles King, in 1703 in a note sent to the Royal Society of London. From here, many naturalists devoted themselves to classifying, observing, and drawing protozoa (for example O.F. Müller has a detailed description with drawings of the behavior of tintinnids dating from 1779. Of course, the maximum expression of art depicting these beautiful creatures can be found in the drawings of the German Ernst Haeckel (Figure 5). This artist and scientist suggested in 1866 that all those animals (including bacteria) that he saw under the microscope should constitute by themselves a third independent animal kingdom called Protista (the first or primordial).
Figure 5. Illustrations of different protozoa by Ernst Haeckel.
Many years passed from the discovery and classification of planktonic organisms to the beginning of research into their role in the oceans. The first studies, purely descriptive of the diversity of life forms, were linked to large expeditions, such as those of Captain James C. Cook between 1768 and 1780 in the Pacific Ocean. In those expeditions, there is evidence, for example, of spots of the cyanobacterium Trychodesmium on the surface of the ocean. However, the first nets specifically designed to collect plankton were probably used by the French naturalists Francois Péron and Charles-Alexandre Lesueur during an expedition to Australia from 1801 to 1804. Additionally, during the long voyage of the HMS Beagle (1831- 1836), Charles Darwin used nets to collect samples of plankton. However, we own the creation of modern oceanography to the Challenger expedition (1872 to 1876), as it was the first organized specifically to collect data from the marine environment, including temperature, water chemistry, bottom geology, currents, and marine life. The HMS Challenger was equipped with laboratories and microscopes, as well as a team of six scientists.
In 1887, the physiologist Victor Hensen introduced the term plankton to describe all those animals that drifted in water currents. He was also the first to propose that perhaps marine life was not nourished by what flowed into rivers but by microscopic primary producers. Through his studies, he perfected existing plankton net designs and created one that was truly quantitative and is still used today (the Hensen net). From the analysis of his samples, he came to the erroneous conclusion that plankton are homogeneously distributed in the ocean and that there are too few of them to sustain fisheries. Facts that were refuted immediately afterward. The study of the distribution and abundance of the different plankton groups continued during the late 19th and early 20th centuries with names such as Marie Lebour (specialist in diatoms and dinoflagellates, 1876-1971), Alister Hardy (creator of the continuous plankton capture system, CPR, 1896-1985), Sheina Marshall (a pioneer in the study of copepod feeding, particularly Calanus sp. (Figure 6), 1896-1977), Hans Utermöhl (inventor of the sedimentation chambers named after him, 1896-1984), to name a few of them. The result of those studies is the discovery of daily patterns of vertical migration, for example. In 1817, the French naturalist George Cuvier made observations of the vertical migration of zooplankton, although he did so in a lake, and his study lasted only one day. More complete, marine and long-lasting were the investigations of the German Carl Chun, in 1888, on vertical migration.
Figure 6. The marine copepod Calanus hyperboreus.
Although there were hints of their behavior, what those microscopic beasts actually did in the sea was still mostly a mystery, and we had to wait until the 20th century for estimates of, for example, primary production, first with variations in oxygen measurements under light and dark conditions, and afterward using radioactive carbon. Similarly, the estimation of zooplankton production was established as a technique well into the 20th century. In 1963, Ramon Margalef established the basis for understanding the structure of the ecosystem and the relevance of ecosystem maturity in species succession. By the end of the 20th century, MR Landry and RP Hassett devised a method to estimate the impact of microzooplankton feeding on phytoplankton in seas and oceans. At approximately the same time, F Azam, T Fenchel and other collaborators conceptualized the operation of the microbial network and introduced the term microbial loop, which takes into account the tremendous importance of dissolved organic compounds and the key role of bacteria in the marine food web. All these milestones have been decisive in positioning us into current knowledge. However, although we have come a long way and have modern techniques and disciplines at our fingertips, such as satellite imaging, sequencing, genomics, and gene expression, we are still in the infancy of fully understanding (and predicting) the function of the different plankton groups in the marine food web.