Ambiguous wonders: the nature of corals, part 1

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By Dimitra Mylona and Roxani Margariti.

Corals. Stunningly beautiful, in all shapes and colors, alive or petrified they have much impact on our planet and our imagination. Not only are they beautiful and mysterious, they also create underwater structures known as reefs, which fringe shores and islands, construct maritime barriers and ridges, and even build entire coral islands or atolls.  Biologists call coral “biogeomorphic agents” because they shape the environment in a profound and large-scale manner.  But what are corals?

Corals build beautiful and infinitely complex ecosystems, hubs of life and biodiversity. In the photograph a coral reef at the Palmyra Atoll National Wildlife Refuge in the Pacific Ocean, by Jim Maragos/USFWS

The coralline conundrum

The definition of a coral has always been very slippery and the more we know about them the harder it becomes to define them. Even today, with our developed taxonomic knowledge, biologists find it difficult to provide a brief and clear answer.  In a review paper on corals, in the Encyclopedia of Islands no less, Daphne Fautin and Robert Buddemeier begin their article by stating this clearly: “The term “coral” is neither scientific nor precise”.  After a short and rather cryptic definition (at least to non-specialists), according to which corals are “cnidarian polyps capable of secreting skeletons that contribute to formation of reefs,” they go on for two densely printed pages to explain what a coral is and what it is not among the swarm of “sessile” oceanic life, meaning those sea creatures stuck to the seabed. For every definition they offer they also provide numerous exceptions creating a taxonomic maze.  Indeed, the term coral is neither scientific nor precise.

The “coral of life”. This drawing shows the major taxonomic groups of “corals” and their evolutionary relationship. The famous precious corals, including the Mediterranean red coral or Corallium rubrum, belong to the Octocorallia branch. The original drawing by Ryan Cowan is here reproduced from Malcolm Shick’s splendid 2018 book Where Corals Lie. A natural and cultural history.

In short, corals are relatives to stingy jelly fishes and sea anemones and they all belong to a large animal group called cnidarians (those that sting). There are solitary corals and corals that form colonies; there are reef builders with hard skeletons, soft corals, false coral, corals that resemble musical instruments (see, for example, the entry for Tubipora musica in the World Register of Marine Species), antlers, hammers, stars, cups, cushions, leather, ivory, feathers, fingers, brain, fire, lettuce and many more!

This magnificent organ pipe coral also goes by the Latin name Tubipora musica and is a soft coral with a hard skeleton of calcium carbonate (upper image) that forms a series of interconnected organ-like tubes. In each tube live several polyps, each having eight feather-like tentacles. The skeleton of this coral is bright purple in color but the colony itself may appear blue, green or purple depending on the color of is polyps (lower image). This is just one example of the endless combination of shapes, colors and life forms in the coral world.

Thinking hard about corals in the ancient Mediterranean

One of the earliest allusions to corals is by Aristotle, the 5th c. BCE Greek philosopher who tried to describe the physical world in an orderly fashion.  He grouped living things based on physical characteristics, way of life and habits, and for that he is considered the grandfather of modern taxonomy. His efforts ended up with a group of creatures that did not fit larger categories and corals were among them.  Aristotle was certainly familiar with Mediterranean corals, but which ones exactly?  Marine biologists Eleni Voultsiadou and Dimitris Vafidis, in their fascinating study on the diversity of marine invertebrates (animals with no bones) in Aristotle’s zoology, suggest that Aristotle described a small coral called sea pen or more formally Veretillum cynomorium, while historian Geoffrey E.R. Lloyd, in his 1996 book Aristotelian Explorations, suggests that Aristotle was talking about Alcyonium palamatum or else “robber’s hand” or “dead man’s hand”.  

Sea pen or else Veretillum cynomorium is considered by Voultsiadou and Vafidis to be Aristotle’s Holothourion (Ολοθούριον),  a creature like sponges, that lives on the sea bed like a plant but is unattached to the soil.  Photograph by Javier Santiago,
Geoffrey E.R. Lloyd considers Robber’s hand or else Alcyonyum palmatum to be the coral Aristotle was familiar with.  The term coral, per se, does not occur in Aristotle’s works, although later students of the natural world, including medieval Arab authors, believed that he was the source of all relevant knowledge. Photograph by Di Albert Kok, in public domain.

In sum, Aristotle never used the word coral and it is uncertain what coral species he may have tried to describe.  Even so, corals must be seen among those aquatic creatures that Aristotle categorized as dualisers (επαμφοτερίζοντα) that were neither quite one thing nor another.  Nicola Carraro (see bibliography) explains this notion in detail. In this scheme, some sessile (immovable) or little moving aquatic creatures, among them the corals’ close relatives, sea anemones and sea squirts, are intermediate between animals and plants (Aristotole, Parts of Animals, IV.5,681b1-2).

Theophrastos, a Greek philosopher from Lesvos island who lived in Athens (late 4th – early 3rd century BCE) and a pupil of Aristotle was a prolific writer of studies on a wide range of themes, from ethics and logic to botany and zoology.   Image from here.

Aristotle’s successor Theophrastos (371-287 BCE) kept changing his mind about corals.  In his work De Lapidibus (On Stones), he described corals as stones that grew like a root in the sea.  Later, in his Historia Plantarum (Inquiry into Plants), he appears certain that corals were well-known sea plants growing near the Pillars of Hercules (Straits of Gibraltar).  The ambivalence continued and, as we shall see, Aristotle’s taxonomy of species along a plant-animal continuum, and especially Theophrastos’s perception of the mineral and plant nature of corals, had a lasting impact on the medieval naturalists of the Islamicate world.  They, too, puzzled over corals, creatures which they could readily observe in the warmer seas of the Middle East and the Western Indian Ocean. 

In Hellenistic and Roman times, these warmer seas, with their greater variety of coral species, were becoming increasingly familiar to Mediterranean people.  The Red Sea, one of the world’s coral reef hot spots with 333 reported species today, became from a Mediterranean perspective a corridor to the Indian Ocean, and there the Mediterranean naturalists’ informants could observe a larger variety of those so-called underwater trees, or lithodendra (stone trees), that could rip apart ship hulls.

After Alexander’s campaign to the East, Greeks’ perception of the world widened. Trade with those distant places by land or by sea brought scholars in contact with what were to them marvelous phenomena, exotic peoples, and wondrous plants and animals. This 1883 schematic map shows the world as was known to Eratosthenes, a Greek polymath who served as the head of the Library of Alexandria. The Red Sea, the Persian Gulf and the western Indian Ocean closest to the Mediterranean received conventional Greek names in this period. Drawing from the work of E.H. Bunbury, (1811-1895), A History of Ancient Geography among the Greeks and Romans from the Earliest Ages till the Fall of the Roman Empire, London, p. 667. Image from Public Domain.

The coral enigma was persistent: how could the supple and soft underwater plants, harden into rocks when exposed to air?  What science of the time could not fully grasp, mythology enthusiastically embraced. 

Ovid, a Latin poet born in 43 BCE (died in 17/18 AD), in his work Metamorphoses describes the birth of the coral in a story that we encountered before, when we talked about the mermaids and mermen. Perseus, after killing Medusa, carrying her severed head with him in a sack, stopped at a coastal location in Ethiopia to confront the sea monster Ketos and save Andromeda.

Detail from Perseus and Andromeda (1572) by G. Vasari now in Palazzo Vecchio, Florence, Italy. The severed head of Medusa turns the sea plants into hard, petrified red corals.  The corals depicted in this work are those known to Vasari and other Renascence Europeans, as the precious coral, Corallium rubrum, that were fished in the Mediterranean.  Corals of this type adorn many paintings of this era.  Image from the Public Domain.

Ovid tells us (Book 4, lines 706 ff) that Perseus built a nest with leaves and seaweed for Medusa’s head to rest on while washing his hands from the blood of the slayed Ketos. The still living branches of that nest absorbed the power of Medusa’s head and turned to stone. Surprised, the Nymphs threw the petrified branches back into the sea. In the water, they were soft. Every time they took some out of the water, they turned to stone. It was thus that corals were born! To the ancient Latins, corals were plants that lived underwater rather than on land.  Just like their terrestrial relatives that dried and hardened without water. Corals removed from water turned into stone.

It took scientists two millennia to conclude what corals are animals after all!

In the Alexander Romance of the global Middle Ages, the mythologized Macedonian king descends into the depths of the sea in an adventure that inspired many a visual artist.  In this version, from early 14th century Flanders, the notion of trees growing underwater is fancifully though suggestively linked to the scientific understanding of corals!  Berlin State Museums collections

Corals in Arabic: translating science from the Mediterranean to the Indian Ocean

In the 9th century of the common era, a remarkable scientific revolution got under way.  The Islamic conquests of the 7th and 8th were followed by the establishment of a new transregional and multi-cultural state.  A new political, commercial, and intellectual urban epicenter was born: Abbasid Baghdad. The scientists of the Abbasid polity and its successor states inherited the knowledge of the ancient world with enthusiasm and built on it with gusto!  Working primarily in Arabic, they embarked on a movement of translation and scientific discovery radiating out of Baghdad. Soon this intellectual trend bloomed in other metropolitan centers of the Islamicate world. 

Aristotle was the darling of Islamicate scientists. In this image, preserved in a 13th-century manuscript from Baghdad, a light-skinned royal figure, perhaps to be identified with Alexander, is consulting Aristotle, depicted as a dark-skinned sage. The color-coding here possibly reflects the artist’s sense that Aristotle and Indian sages belonged to the same social category. It parallels the fact that the Islamicate scientific revolution relied on an inclusive and liberal integration of ancient thinkers, both from the Mediterranean and the Persian and Indic milieux.  The work this image adorns is an encyclopaedic bestiary entitled On the Usefulness of Animals.  The anonymous author of this manuscript (British Library Or. 2784) claims to present material from a zoological work of Aristotle combined with that of the Syriac Christian Persian savant Ibn Bakhtisu (10th-11th century).

The naturalists of the medieval Islamicate world were enthusiastic inheritors of Aristotle—and as with other topics, they credited him with a lot of the ancient knowledge they inherited on coral and other “stones.”  Take the book circulating among medieval Arabic-speaking scientists under the title Book of Stones and attributed to Aristotle (hence forth, Aristotle’s Book of Stones).  As the German orientalist and early historian of science Julius Ruska avers, this book is a characteristic example of ancient knowledge summarized, further developed, and transmitted through Arabic science. Another such composite work of Arabic-language science anthologizing and augmenting ancient and late antique knowledge is the alchemical treatise attributed to the legendary and enigmatic Greek philosopher Apolonius of Tyana (first century CE).  The Pseudo-Apolonius, or Balīnūs in Arabic, became a much-cited source on the nature of corals. Both these works aligned with the notions of the plant-mineral spectrum of previous scholarship regarding corals—in other words conveyed the ideas of Theophrastus rather than those Aristotle and his plant-animal continuum.

Perhaps the most comprehensive summary of medieval Islamicate knowledge of corals appears in the work on Aḥmad al-Tīfashī, a man of letters from Tunisia who lived from 1184 to 1253.  Fleeing his homeland in tumultuous times, al-Tīfashī ended up settling in Cairo where he wrote a famous gemological work entitled Flowering Thoughts on Precious Stones (Kitāb Azhār al-afkār fī jawāhir al-aḥjār). By the way, this same man wrote one of the most titillating and celebrated sexual health manuals of the medieval Islamic world…but for now, let’s stick to corals!

The coral reef in the Gulf of Aqaba in the northern Red Sea is perhaps one of most significant surviving coral ecosystems, currently stirring efforts to conservation. Ancient and medieval travelers would have certainly encountered locations such as the one int he photograph in their travels. For the Aqaba coral reef see here

Al-Tīfashī’s book is divided into chapters, each chapter devoted to another precious stone. Chapter 18 presents coral, marjān in Arabic (other terms used across the Islamicate sources include the Persian bussadh and the Arabic zabad al-baḥr, foam of the sea). Al-Tīfashī begins the chapter with a definition of the nature of corals, which he ascribes to Apolonius (Balīnūs).  Corals, he says, partake of two natures, and we should think of them as hybrids existing between the plant and mineral kingdoms.  They grow on the seabed and the cause of their nature is the “marriage” of heat and humidity.  This theory appears to be a new element, that medieval science adds to the ancient knowledge of corals! Al-Tifashi explains that dry earth dissolves in water heated by the sun’s ray and that stew eventually solidifies into a plant-like organism. He then provides the standard explanation: in the water and while retaining their moisture corals are supple; but once they grow in size and become exposed to the air, the moisture and the heat are wicked off and what remains cools and solidifies.  He adds an explanation of the brilliant redness that made Mediterranean coral, Corallium rubrum, so coveted: when there’s excessive heat, the coral turns red!

The mythologizing Alexander was a cross-cultural phenomenon, including visions of him descending into the depths of the Eastern Ocean . This image illustrates the telling of this episode by the renowned Persian Indian poet Amir Khusraw of Delhi (1253-1325) in the fourth part of his poetic Quintet (Khamsa); the manuscript of the work made by the great Mughal emperor Akbar (1556-1605). Although the undersea is hidden, the picture of Alexander’s half-submerged diving bell (this one a large storage jar by the looks of it!) reminds us that in the early modern period the fascination with the marine world, its delights, and its dangers, did not know borders either! Image from https://www.metmuseum.org/art/collection/search/446561

Eastern and western exploration and the cracking of the coralline enigma

The question about the plant, animal, or mineral essence of corals kept bringing on passionate disputes between naturalists until well into the 18th century.  In the 1600’s the question remained: were corals rootless, leafless and flowerless plants; Were they mineral forms spontaneously created under water much like stalactites in caves?  The speculations of ancient and medieval philosophers and naturalists persisted and got even more complex when the so-called Age of Exploration brought new empirical data on corals.  The Cabinets of Curiosities that Dimitra explored in a previous post were filled with corals of all kinds. Eventually, a new term was introduced to describe them: zoophytes.

This illustration pairing plants and corals appears in Marsigli’s Histoire Physique de la Mer (1725), a guide for the navigators and explorers of all kinds in the global oceans of the time. Corals, which were considered marine plants by Marsigli, are shown growing in sea caves and extending downwards, a feature that distinguishes them from “other plants.” This image is a hand-colored variant of a Matthys Pool’s engraving (Tab XXII) from Sir Joseph Bank’s copy of Marsigli’s book, showing us how knowledge was circulating among scientists and explorers of the world’s oceans in the 18th century. Image copied from M. Schick 2018, 52-53 (see bibliography).

François Pyrard de Laval, a French navigator who left us a written account of his adventures South Asian seas, describing his shipwreck in the Indian Ocean in 1602 he says that he met “… with many branches of a certaine thing which I know not whether to terme Tree or Rocke, it is not unlike white Corall, which is also branched and piercing, but altogether polished on the contrary, this is rugged, all hollow and pierced with little holes and passages, yet aides hard and ponderous as a stone” (see bibliography). Clearly the comparative approach between what was known and what was encountered for the first time was not of much help.

In the warm seas of the global south, corals grow covering large expanses of the seabed.  Searching for light they move upwards, eventually building whole islands and atolls like the one in this watercolor by Ernest Grieset (1871) that was intended as a birthday gift for Charles Darwin by his neighbor John Lubbock.  These waters were fraught with deadly traps for the heavy ocean-going European ships of the time, with corals reaching just under the surface, tearing the hull of vessels that tried to pass through. Image copied from M. Schick 2018, 52-53 (see bibliography).

Some seven decades after De Laval, the intrepid Ottoman traveler and erudite gentleman Evliya Celebi ventured down the Red Sea in the aftermath of 16th-century Ottoman conquest in Arabia and East Africa and what one scholar has called The Ottoman Age of Exploration.  Not unlike De Laval but with beguiling literary flair, Evliya describes underwater forests of marvel and danger:

In the sea of Suez there are wonders called şab (Evliya uses this term, clearly foreign to him, that actually comes from the Arabic shi’b, coral reef!), which are well worth seeing.  Praise be to the Creator whose wisdom and marvels befuddles the human mind and understanding.  In this sea are great trees that look like the plane trees of the land of Rum (Asia Minor).  They grow underwater and have no leaves, just trunks and many branches.  They grow tall and reach the sea surface but do not grow beyond it and most of their bulk remains underwater.  God have mercy, when some ships collide with such reef trees and break into pieces.  If a ship is of sturdy construction, it breaks the branches and the trunk of the reef and passes over them making a cracking sound.  Pearls grow at the depths of these reefs.  Indeed we are talking about an entire underwater forest.  In it also live sword-fishes.  They devour the unfortunate who fall from sinking ships and the divers who dive for pearls (translation by Roxani Margariti with the help of Marinos Sariyannis.  This passage is not included in the otherwise very useful new translation of Book Ten of Evliya Celebi’s travelogue, see bilbiography).

The publication of the tenth volume of Evliya’s travelogue includes the edition of a marvelous map that the great traveler penned and offers, for the first time, access to both works in English.

With a futuristic scuba diver’s clairvoyance, Evliya shows us how received wisdom, new horizons, reliance on local knowledge, exploratory fervor, and undying curiosity were common features in the East and West of the so-called Age of Exploration.

Many ships were torn to pieces and wrecked when they sailed on the extended coral reefs that lurk just under the surface in several parts of the Pacific and the Indian Oceans. Some members of such ships’ crews survived to tell the story. Some of them, like François Pyrard de Laval (in the text above) understood the significance of corals in this mishap. Others did not. In this engraving we see the Dutch East India Company ship Batavia that in the morning of the fourth of June 1629,  was wrecked on Morning Reef, on the Houtman Abrolhos (Lat. 28º 29.422S, Long. 113 º 47.603E) off the eastern coast of Australia. Image kept at the State Library of Western Australia (919.412 PEL) and copied from Lydon 2018 (see bibliography).

In the 18th century, corals became a hot topic for European Enlightenment naturalists, who tried to approach the issue through empirical research. Some even went out to sea with coral fishers to observe live corals, the moment they came out of the water. Others, under the pretense of trading or diplomacy, traveled to distant lands in the Pacific to research the issue! Eventually, experimentation and scientific observation yielded new insights and the coralline enigma was cracked.

Naturalists interested in the mystery of corals went out to sea with coral fishermen to observe them while still alive, just out of the water. In this copper engraving of 1569 by Cornelis Galle I from Antwerp, we see Sicilian divers using goggles to raise corals from the seabed, while the text repeats the view of the time that green coral branches turned red when removed from the water. Τhe engraving is kept in the Museum Boijmans van Beuningen in Rotterdam.

Jean André Peyssonnel, another French naturalist and a doctor, placed live coral branches in vases of sea water and observed that what was considered until then the flowers of the coral, i.e. its polyps, reacted to changes in their environment. This could only happen if they had a neural system, in other words if they were animals!

Observation of the reactions of a live coral branch kept in a vase was one of the first steps in scientific approach to the coral conundrum. The print is from the book by Lacaze-Duthiers Histoire naturelle du corail, 1864 Pl. XII

This revolutionary idea, that corals are animals, caused shock and very intense reactions in scientific community.  It even stirred philosophical dilemmas. Voltaire, in his Dictionaire Philosophique in 1762 debated whether one should believe one’s own eyes that see corals as plants or one’s intellect and scientific reasoning that proved corals to be animals.  Zoophytes (animal-plants), lithodentrums (stone trees), anthozoans (flower animals) and their ambiguous nature was at the center of many scientific debates at the time.  In 1758 the Swedish biologist and father of modern taxonomy, Carl Linee, placed things in their proper place, the one we are familiar with today.  In the 10th edition of his Systema Naturalis, his systematic taxonomy of all living things, corals took their final place in the kingdom of – somewhat strange and endlessly fascinating—animals!

While scientists, ancient and modern, debated or even quarreled over the nature of corals, people all over the world went about using them in myriad ways making them part of their cultural universe.  This post is not finished here. A second part will follow, with much to show about the ways in which humans and corals crossed paths.

Medusa’s head by Carlo Parlati II, engraved on Japanese cerasuolo coral, end of 1980s, with diamonds, pearls, gold, malachite and marble (image cropped from Shick 2018, 78).

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