Darwin's Backyard

by James T. Costa

  A. Materials

  • Notebook and pencil

  • Barnacles (acorn and/or gooseneck)

  • Limpets (optional, for comparison)

  • Dissecting microscope or good hand lens

  • Forceps or tweezers

  • Pipet or eye dropper

  • 2 in. (5 cm) C-clamp

  • Shallow pan or tray for dissection (a shallow sardine tin with wax bottom works well; just melt some paraffin wax and pour enough to coat the base of a cleaned sardine can with about 1/10 in. [¼ cm] of wax)

  • Glass microscope slides

  • Scissors (small and sharp)

  • Paper towels

  Note on obtaining specimens: If you don’t live near the ocean, barnacles and limpets can be obtained through biological supply houses. Ward’s Natural Science (www.wardsci.com), www.biologyproducts.com, and www.onlinesciencemall.com sell preserved gooseneck barnacles (generally genera Pollicipes or Lepas), and Carolina Biological Supply (www.carolina.com) offers the Carolina™ Barnacle Cluster—live acorn barnacles. Limpets, single-shelled grazing snails, can be bought online from saltwater aquarium suppliers (e.g., www.reefcleaners.org and www.liveaquaria.com).

  B. Procedure

  1. Select an acorn barnacle. Note that the barnacle’s exoskeleton is shaped like a volcano, with a set of six overlapping and rigid calcareous carinal or wall plates surrounding the barnacle within. Can you identify all six plates?

  2. Which way is up? Acorn barnacles are affixed to the substrate on their dorsal side, so the “crater” of the barnacle volcano is its ventral side.

  3. Inside the “crater” are four movable plates called the opercular plates (the door is the operculum). The two larger kite-shaped plates of the operculum, one on each side, are called scutal plates. Adjacent to these are two tergal plates, held more or less vertically and so seen edge-on at the top. The opercular plates are opened and closed with muscles.

  4. The slit-like opening formed when the operculum is open is the aperture. The upside-down barnacle extends its long modified legs (cirri), which are feeding organs, through the aperture when feeding.

  5. Gently push the operculum open with forceps, and observe the interior with a hand lens or dissecting microscope. The space within the aperture is called the mantle cavity, a term associated with mollusk anatomy. (This is a holdover from a time when barnacles were classified as mollusks. Note the superficial similarity of the Acorn barnacle to the limpet, if one is available, which is a mollusk.)

  External and internal anatomy of a generalized acorn barnacle. Drawing by Leslie C. Costa.

  6. Use the C-clamp to loosen the articulations between the adjacent wall plates: carefully place the clamp jaws on opposite sides of the shell and apply pressure slowly until the wall plates give way and separate. Remove the clamp and repeat at intervals around the circumference until all the plates are loose.

  7. Carefully remove one or two of the wall plates, and the opercular plates. Sketch the scutal and tergal plates.

  8. Within the mantle cavity, note the appendages arising along the ventral (upper!) surface of the body; these are the cirri. How many pairs are present? (You should find six pairs.)

  9. Carefully remove one of the cirri (singular: cirrus), and note that it has two arms (biramous). Place a cirrus on a glass slide and apply a drop of water to observe the fringe of hairs, or setae. Three of the paired cirri capture food particles in the water column, while the other three function to scrape the particles into the mouth. There are many excellent video clips online showing how the cirri function in feeding, such as this one at the Snail’s Odyssey website: www.asnailsodyssey.com/VIDEOS/BARNACLE/barnacleFeed.html.

  10. Select a stalked gooseneck barnacle. The first difference to note with respect to acorn barnacles is the peduncle, or stalk. This functions mainly for attachment but does house muscles and ovaries. (The peduncle of some species is covered with small round calcareous plates called ossicles.) Atop the peduncle is the part you think of as the barnacle proper: the capitulum.

  11. Note that the exoskeleton is flexible; it consists of a thin membrane of chitin (the material that forms the exoskeleton of most arthropods) and protein.

  12. Orient yourself to gooseneck barnacle structure starting from the aperture at the top of the capitulum. As in acorn barnacles, the aperture is opened and closed with opercular plates. In gooseneck barnacles, however, these plates are very large and form part of the exterior body wall. The scutal plates are the largest plates, sitting more or less atop the peduncle. The tergal plates are next largest, just above and sometimes slightly to one side of the scutal plates. The largish plate just below the tergum is called the carina.

  External and internal anatomy of a generalized gooseneck barnacle. Drawing by Leslie C. Costa.

  13. Carefully pull open the aperture with your forceps or fingers. Inside is the dark mantle cavity, and you may see the cirri within.

  14. The pointy tergal side of the body is the posterior of the animal. Placing the barnacle on its side, posterior to the right and peduncle toward you, use the scissors to carefully cut the membrane between the large plates, taking care not to thrust the blade of the scissors more deeply than necessary to cut the membrane.

  15. Lift the cut membrane with the forceps. You will find a large muscle running transversely between the scutal plates. This is the adductor muscle, which controls the aperture. Cut this muscle to free the body wall membrane, and remove this to expose the mantle cavity within.

  16. Observe the six pairs of large cirri, each two-branched (biramous). Remove a cirrus at the base and observe the two long and curled branches covered with long setae.

  17. Compare the scutal and tergal plates and cirri of the acorn and gooseneck barnacles. These structures are homologous between the two barnacle groups, their different morphologies reflecting variation on a theme.

  For further dissection, consult the barnacle section of a manual such as Observing Marine Invertebrates by Donald P. Abbot (Stanford, CA: Stanford Univ. Press, 1987) or Invertebrate Zoology: A Functional Evolutionary Approach, 7th edition, by Edward E. Ruppert, Richard S. Fox, and Robert B. Barnes (Belmont, CA: Thomson, Brooks-Cole, 2004).

  See also:

  M. Lowe and C. J. Boulter, “Darwin’s Barnacles: Learning from Collections,” in Darwin-Inspired Learning, ed. M. J. Reiss, C. J. Boulter, and D. L Sanders (Rotterdam: Sense Publishers, 2015), 273–284.

  “Barnacles” at the Darwin Correspondence Project: www.darwinproject.ac.uk/learning/universities/getting-know-darwins-science/barnacles.

  3

  Untangling the Bank

  One of Darwin’s most enduring images from Origin is that of an overgrown bank teeming with life. Think how a river bank, or a country lane meandering through a rolling woodland, displays nature’s exuberance in cross section: imagine low-growing creepers and fragrant herbs and shrubs growing from moist mossy cushions, here and there punctuated by mushrooms, lacy ferns, and tree trunks festooned with lichens and vines. The shrubs and trees cradle birds’ nests and suspend spiders’ webs; and everywhere are scurrying mice, marching ants, leaf-nibbling beetles and caterpillars, nectar-sipping bees—the whole is a living tangle. In the lovely final passage of the Origin, Darwin contemplated just such an “entangled bank,” one “clothed with many plants of many kinds, with birds singing on the bushes, with various insects flitting about, and with worms crawling through the damp earth.”1

  Then he reflected on where the tangled bank came from. “These elaborately constructed forms, so different from each other, and dependent on each other in so complex a manner, have all been produced by laws acting around us,” Darwin declared.2 Those laws are all about reproduction, growth, variation, the struggle for existence, and natural selection. The diverse and interdependent organisms of the bank are each exquisitely adapted and honed by selection to stake their claim, survive, and flourish as best they can. And what’s more, in a lucid ecological vision Darwin saw that the bank as
a whole, its sublime messiness itself, was an outcome of selection. What seems an exuberance of vegetation with its flitting and creepy-crawly denizens, unruly enough to strike terror into the heart of gardeners whose tastes favor the manicured, has in fact a certain order or underlying structure in Darwin’s eyes. “Thus,” he concluded, “from the war of nature, from famine and death, the most exalted object which we are capable of conceiving, namely, the production of the higher animals, directly follows.”3

  Orchis Bank, near Down House, likely inspired Darwin’s enduring image of the “tangled bank.” Photograph by the author.

  Now, that was just it: “from the war of nature, from famine and death”—like some purifying fire, natural selection’s destruction paradoxically produces beauty, adaptation, and order. Darwin knew he had a problem. The principle of natural selection had come to him in a flash of insight in the fall of 1838, and it didn’t take him long to realize the implications: Malthusian population pressure, the “struggle for existence,” could only mean death and destruction on a literally unimaginable scale. Exponential population growth was straightforward enough, and with it the idea of populations outstripping their means of sustenance. But he knew that the logical conclusion of Malthus’s doctrine as applied to nature flew in the face of experience and received wisdom. “It is difficult to believe,” he wrote in his notebook in March 1839, “in the dreadful but quiet war of organic beings going on in the peaceful woods and smiling fields.”4 It is difficult to believe; to our eye nature often appears bucolic, peaceful. Yes, predators catch prey, trees topple, and the occasional insect outbreak strips fields bare, but think about your experience walking in the woods some fine day, or picnicking on a sunny knoll: such scenes are the very picture of tranquility and beauty. Although we know that these places teem with life that can and does reproduce at a prodigious rate, nature seems unchanging, in balance.

  Well, it is in a kind of balance, or perhaps stalemate is a better way to put it; each species can only grow in population to a point before it butts up against the similarly growing populations of other such species vying for the same space, food, light, or other vital resources, or it comes up against ever-ready predators, pests or pathogens trying to use it as a resource and expand their own population, or it hits the resource limits of its environment. Through any or all of these factors, the population of each species is held more or less in check. Individuals, Darwin realized, strive to the utmost to reproduce, but even as reproductive output is prodigious, mortality is equally so. It was not a happy thought, but reality nonetheless. Musing on an entangled bank led Darwin to think about what sorts of evidence would help make his case. This brought out the experimental side of Darwin, and in the process helped give rise to the field of ecology. But first he contemplated how to make a case for the struggle going on all around us, if only we could learn to see it.

  Struggling with Struggle

  Darwin did not have to read Malthus to learn about the struggle for existence. Charles Lyell wrote about it in the Principles of Geology, quoting at length the French naturalist Augustin de Candolle’s Géo­graphie botanique (“all the plants of a given country are at war one with another”), and giving the phrase “struggle for existence” more than once. But the image goes back further than that, to an earlier generation: his own grandfather, Erasmus Darwin, wrote of “one great Slaughter-house the warring world” in his epic poem The Temple of Nature (1803), in turn citing Carl Linnaeus’s description of nature as Bellum omnium in omnes—the war of all against all. It’s important to keep in mind, however, that by Charles Darwin’s time, nature was viewed through the lens of natural theology, a philosophy that de-emphasized epic struggle and instead saw inherent harmony and goodness in nature, a view more consonant with a wise, loving, and benevolent Creator. Thus Lyell, though describing the struggle for existence in the Principles, took pains to stress a prevailing balance and harmony among mostly peaceably coexisting species. Darwin would have imbibed that religious tradition largely through William Paley, a famous son of Darwin’s own Cambridge College, Christ’s. (Darwin is thought to have occupied Paley’s old rooms at Christ’s, ironically). Paley was renowned for his lucid theological writings, including Natural Theology and Principles of Moral and Political Philosophy; the latter was even required reading when Darwin was a student, and he also devoured Natural Theology. The bucolic and pastoral English landscape seemed to almost compel a view of nature as smiling and glad—as surely as the wretched conditions endured by the urban poor compelled the Malthusian view of nature that Dickens captured so well.

  When Vestiges of the Natural History of Creation was anonymously published in 1844, the outrage it provoked had as much to do with its vision of an uncaring pitiless nature, in which species constantly supplant other species in the history of life, as with the transmutationism it promoted. Extinction was much discussed in Vestiges, both of species and individuals, and it was that unsparing vision of the history of life that led the grieving Alfred Lord Tennyson to decry a “Nature red in tooth and claw” in In Memoriam, the poet’s elegy for his friend Arthur Hallam. Despite Darwin’s admiration for Lyell, and the lessons from Paley, he would have none of that natural-theological “balance and harmony.” He took an unsparing view too: not even Malthus’s “energetic language” adequately conveyed the magnitude of the “warring of the species” that constantly goes on, as he put it in one notebook. He cast about for ways to express it himself, and hit upon a powerful metaphor: “One may say there is a force like a hundred thousand wedges trying [to] force into . . . the gaps in the oeconomy of Nature, or rather forming gaps by thrusting out weaker ones.”5 Species or varieties are like wedges struck repeatedly by the mighty hammer of natural selection—the deeper and more securely they are driven into the “economy of nature” the better adapted they are to their niche or habitat, forcing out “weaker” ones in the process. The gaps are soon filled by other forms, and the competition continues. He later inserted a note between the lines: “The final cause of all this wedgings [sic], must be to sort out proper structure & adapt it to change.”

  It was a metaphor he stuck with for many years, varying the number of imagined wedges: in the brief 1842 Sketch of his theory it was “a thousand wedges” forced into the economy of nature, and then “ten thousand sharp wedges” in the long Essay of 1844.6 He stuck with ten thousand in the version that made it into the Origin: “The face of Nature may be compared to a yielding surface, with ten thousand sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.”7 Curiously, after using this metaphor from 1838 right through the publication of the Origin, he dropped it altogether from subsequent editions. Perhaps it was just too much. Only long and hard reflection would bring the reality of this truth home, he realized, but how to get people to even try? He struggled to express the concept—how to get the point across that the “war of nature” is incessantly raging all around us, though most of us are completely oblivious? In the Origin he wrote that “Nothing is easier than to admit in words the truth of the universal struggle for life, or more difficult . . . than constantly to bear this conclusion in mind.” That took imagination. Think, he urged, how the birds that may seem to be idly singing away in the trees must eat to survive. Reflect on the fact that to live and raise their young each and every one must consume prodigious quantities of seeds and insects. Or reflect on the realities of population growth: “There is no exception to the rule that every organic being naturally increases at so high a rate, that if not destroyed, the earth would soon be covered by the progeny of a single pair.”8 Even the slowest-reproducing species will outpace space and resources in short order.

  We saw in the last chapter how Darwin immersed himself in the world of barnacles for an 8-year period beginning in 1846. But all the while he never stopped contemplating the nuts and bolts of his transmutation theory—struggle, extinction, selection, favored variants, diversification, co
mpetition, and so on. Not only did he contemplate all these things, but more importantly he also contemplated how they interrelated. In addition he struggled with struggle very personally with the traumatic loss of his 10-year-old daughter Annie, in 1851, a shock that may have extinguished in him any remaining sense of a purposeful and benevolent Creator. The reality of an indifferent and uncaring nature was brought into unbearably sharp focus by her death; grieving, he and Emma soldiered on, as he immersed himself in his work. They were buoyed, too, by their other children. 1850 saw the birth of Leonard, called Lenny, and Horace came along the year they lost Annie—the Darwins had seven children by 1851, all under the age of 13.

  Divergent Thinking

  The second of Darwin’s monographs on living barnacles appeared in 1854, bearing a heartfelt dedication: “To Professor H. Milne Edwards this work is dedicated, with the most sincere respect, as the only, though very inadequate acknowledgement which the author can make of his great and continued obligations to the ‘Histoire Naturelle des Crustacés,’ and to the other memoirs and works on natural history published by this illustrious naturalist.” Up to the publication of Darwin’s monographs on the subject, the remarkable French naturalist Henri Milne-Edwards (1800–1885) had provided the definitive treatment of barnacles. Of the many great insights Darwin gained from Milne-Edwards, one in particular stands out—an insight with special relevance to Darwin’s anxiety. In 1852 Darwin read key sections of Milne-Edwards’s Introduction à la Zoologie générale, a masterful overview of anatomy and physiology throughout the animal kingdom. He was struck by Milne-Edwards’s interconnected laws of diversity and economy: “Nature is prodigal in the variety of her creations, yet parsimonious in the means of diversifying her works.”9 The beetles provide a good example, perhaps the single largest taxon in the animal kingdom with nearly 300,000 named species, all different yet in another sense all carbon copies of one another with the same beetle-defining characteristics. Naturalists (including Milne-Edwards and Darwin) struggled with the concepts of so-called “highness” and “lowness” of organisms as they tried to map out diversity and classify species—a holdover from Aristotelian ladder thinking where species were arrayed on a scale of supposed perfection (always culminating in humans, needless to say). Milne-Edwards maintained that highness or lowness in the scale of nature was best expressed by structural complexity, measured by the differentiation of cells and tissues. He called this the “physiological division of labor,” a concept that captured Darwin’s imagination.