A Sea of Glass Read online

  The seventy-one medusae or jellyfish are found in three distinct classes: the scyphozoans, the hydrozoans, and the cubozoans. The scyphozoans are the true ocean-going jellies; some, like the lion’s mane jellyfish, reaching sizes as big as a diver. Some species never touch the bottom in any part of their life cycle. One exception are the stauromedusae, which live attached to the bottom as a kind of upside-down jellyfish and have been recently elevated to their own class, the Staurozoa. The cubozoans are the rather lethal venomous box jellies, smaller than a mouse. But it’s the hydrozoans that are the most exciting group to me, because they include colonial forms made up of both the medusae and the polyp. They also include the siphonophores, famed as superorganisms and beloved by the Blaschkas for their complicated division of labor, as discussed in chapter 3.

  Hydrozoa: Leuckartiara octona in glass, showing both the polyp and attached medusa. Note the tiny “actual size” model in the lower left, which reveals how magnified the detail model is. Photo courtesy of the Corning Museum of Glass.

  The hydrozoans are the pinnacle of complexity and coolness, and the hydrozoan medusae are the most intricate models in our collection. Think about how a bottom-dwelling hydrozoan colony starts from a single planktonic baby, much smaller than a ladybug, called a larva. This larva snuffles along the bottom until it finds a very good spot, and then it begins to bud. First it buds more polyps, which are good at catching food to fuel further expansion of the colony. After making enough food polyps, our growing colony makes special defensive polyps, and it finally ends by budding reproductive polyps. Here is the big surprise, the stuff of science fiction, and the reason I love this group so much: the reproductives are sometimes not polyps. They turn out as jellyfish that can stay attached to a branch. Or they can be polyps, but inside they grow tiny jellyfish that are released and swim away to release eggs and sperm to make new babies. What this means is that over evolutionary time, the genetic blueprint for making both a stationary polyp and a medusa has become integrated into a single animal’s genetic code.

  The anemones are in the class Anthozoa and also exist in several forms, starting with one simple polyp, like an anemone. The anthozoans are then divided into polyps with six- or eightfold symmetry. The anemones and reef-building corals are in the sixfold symmetry group (subclass Hexacorallia), and the soft corals with no calcareous skeleton and eight pinnate tentacles are in the eightfold group (subclass Octocorallia). Members of the Hexacorallia go from the form of an anemone to a reef-building coral simply by budding a lot of polyps that stay attached to each other, share food, and secrete a massive skeleton.

  PLATYHELMINTHES, NEMERTEA, PRIAPULIDA, AND ANNELIDA

  The next major group to place on our tree of life includes the four phyla of worms. The extraordinary thing about the worm is that it is a life form that has evolved multiple times in different phyla, so worms are scattered all over our tree. The Blaschkas were extravagant in their efforts to instruct and inspire us about worms, which are all soft-bodied, and they included what were then five phyla of worms in their creations: Platyhelminthes (flatworms), Nemertea (ribbon worms), Priapulida (penis worms), Echiura (spoon worms), and Annelida (segmented worms). The flatworms have the simplest of worm body plans and are just worm-shaped sacs, a crawling stomach with a mouth and no exit, no anus. The Blaschkas seemed to like these simple flatworms and we have twenty-one in our collection.

  Platyhelminthes: the flatworm Acanthozoon ovale in glass. Photo by David O. Brown.

  Next in complexity are the ribbon worms in the phylum Nemertea. They also tend toward flatness but are greatly elongated and stretchy. They are characterized by having a large eversible proboscis studded with huge venom-filled jaws. What more can I say about the appearance of the priapulids, also known as penis worms? They are a very small group but significant in that they molt and so are more closely related to the phylum containing crabs than to the other worms. I’ll bet most people are not familiar with the penis worms, nor with the spoon worms in the group Echiura. Back in the Blaschkas’ day, these were their own separate phylum. We now know they are one twig on the largest, most spectacular branch of worms on earth, the segmented annelid worms. Some scientists, such as my good friends and worm experts Sally Woodin and Rachel Merz, would say the most fabulous of the Blaschka pieces—for their intricacy and sheer brilliance—are the marine segmented worms, within the class Polychaeta. The Blaschkas may well have agreed, since we have twenty-two of these very intricate worms in our collection, many painted or crafted from mixes of colored glass.

  Echiura: the spoon worm Bonellia viridis in glass. Photo by Elizabeth R. Brill.

  Worm biodiversity is, paradoxically, one of my favorite themes in teaching invertebrate biodiversity, because of the very consistent, logical layout of the worm body plan and the absolutely delightful variations on the theme. How much fun it is to compare the variations in massive jaws and bristly parapodia among the powerful, mobile hunting worms and then move on to consider the evolutionary modifications of the array of sessile tube- and burrowdwelling worms. Worms do indeed come in these two types, the errant mobile worms and the sedentary ones (page 73). The great common families of mobile worms include the Nereids, the Glycerids, and the Phyllodocids. These are accompanied by the Blaschka models of the rarer and wonderful errant families the Syllids and the Hesionids. Some of the clam worms (nereids) can reach three feet long, and when picked up or attacking their prey, extrude a long proboscis with very visible black, curved teeth at the end. I dream there could be a human-sized one lurking in the deep somewhere. The diet of these mobile hunting worms includes other worms, corals, and small crabs.

  The other great arm of worm diversity encompasses the sedentary tube-dwelling worms, characterized by bright plumes of feeding tentacles, or by tentacle lines strung across the bottom of the ocean. I think of these in turn as being of two types, depending on whether they filter plankton from the water column with large ciliated plumes or dredge endlessly through the sand and mud with ciliated long strands.

  MOLLUSCA

  The Mollusca are a huge, beloved phylum of spineless animals treasured for the spectacular shells made by snails and clams and the culinary heights reached by meals featuring oysters, clams, snails, and squid. They are closely related to the annelids in the large group Lophotrochozoa, which is characterized by shared patterns of embryonic and larval development, cementing our certainty that they are kin. Embryonic lophotrochozoans all develop by spiral cleaving, which means each new cell of the embryo comes off at a twist from the one below, and they can be asymmetrically sized. Both molluscs and annelids develop through a common larval stage called a trochophore, which can be either free swimming in the plankton or embedded in an egg case.

  Molluscs are divided into seven classes, the best known of which are the big five: the gastropods (including snails and sea slugs), bivalves, cephalopods (including octopus and squid), chitons, and tooth shells. Here is where we have a funny disconnect between the artistry of the Blaschkas and our tree of life. The Blaschkas focused on soft-bodied critters without shells, so they pretty much skipped the numerically dominant groups of molluscs like the bivalves, tooth shells, and chitons. They skipped most of the shelled snails, but focused in exciting detail on many soft-bodied molluscs within the gastropods, depicting the less-known sea slugs, including the brightly colored nudibranchs in their full glory. The Blaschkas produced so many of these different groups of soft-bodied sea slugs that it’s important to describe their relationships in some detail, although this repeats some of what I said in chapter 5.

  Sea slugs differ from shelled snails in having no shell and undergoing a mysterious process called detorsion. Shelled snails undergo torsion during development, so the back end with the anus ends up facing forward, on top of the body mass. This seems to help with positioning the body to carry a shell. Detorsion is an unwinding of this process and is a sign of an evolutionary transition. During development, a sea slug first undergoes normal torsion like a g
astropod, and then undoes it in detorsion. The sea slugs include the sea butterflies, sea elephants, sacoglossans, and nudibranchs. The nudibranchs, so-called naked gills because the adults have no shell and their gills are often exposed as great plumes on their backs, are the single largest group in the Blaschka collection. As a group, nudibranchs are divided into families characterized by specific body forms and prey types, which are described in chapter 5. The prey taken by this group of fierce predators defies characterization and includes tube-dwelling anemones, soft corals, hydroids, and even, in the case of the hooded nudibranchs, small mobile crustaceans. The aeolids, often the most brilliantly colored, with fields of bright plumes on their backs called cerata, are well known for feeding on anemones, hydroids, and even jellyfish. They are master chemists and feed on the most chemically defended, toxic animals.

  Included in the sea slug chapter are other snail relatives that are closely related to the nudibranchs and share their absence or near absence of a shell. This includes the cephalaspids, or headshield slugs, the plant-eating sacoglossans, like the bright green Stilliger ornatus and the spotted Caliphylla tricolor, which are solar-powered by virtue of the algal cells they steal from their prey, and the plankton-dwelling pteropods, the sea butterflies and sea elephants.

  Nudibranchia: the sea slug Plocamopherus imperialis in glass. Photo by Kent Loeffler.

  The pteropods are a group of sea slugs that include both shelled and unshelled forms. The lovely unshelled pteropods, called sea angels and represented in our collection by Clione limacina, are in fact vicious predators that fly through the water on a mission of death and literally tear apart shelled pteropods. As discussed in chapter 8, the shelled pteropods—the sea butterflies, in the genus Limacina—are the canary in the coal mine when it comes to the effects of ocean acidification. The sea elephant, Carinaria, I have never seen in nature. With its reduced shell and a special type of radula, it is actually closely related to shelled gastropods, as opposed to the shell-less opisthobranchs. This strange foot-long mollusc is pelagic, with a muscular ventral fin, but it swims upside down, hunting for prey in the plankton. It feeds on some unusual invertebrate chordates, arrow worms, and fish.

  The Blaschkas also went completely wild with the cephalopods, still within the phylum Mollusca. Although I know some things about invertebrate biodiversity, my specialty is corals, and I’m pretty good with nudibranchs, but as I wrote this book, I learned much about cephalopods from the Blaschkas. The cephalopods are divided into squid, octopus, cuttlefish, and nautilus. They are separated by their numbers and types of arms. The octopuses are distinctive in having eight arms and usually lacking fins. Then there are three groups that are quite closely related and to my eye overlap somewhat. The teuthid squid, as decapods, have ten arms—eight arms plus two specialized hunting or reproductive tentacles—and they have fins. The sepiolid squid are the adorable, puppy-like bobtail squid, in an order separate from the teuthid squid and having reduced fins, but still with ten arms or tentacles. The cuttlefish (order Sepiidae) also have eight arms and two tentacles and are distinguished by having an internal shell or cuttlebone. I’ve called the cuttlefish and octopus shape-shifters because of their surprising ability to transform from looking like a rock to a piece of drifting algae. Cuttlefish in particular are called chameleons of the sea because of their transformative color shifts.

  I am pleased to report we did it! Without your even realizing it, we just completed a survey of most of the great arm of invertebrates called the lophotrochozoans. We left out quite a few small groups, but that’s okay, since the Blaschkas did also. The Lophotrochozoa and the Ecdysozoa are the huge basal branches of the tree of life that form the earliest group of spineless animals, the protostomes. So to complete our look at protostomes, we need to consider the Ecdysozoa, the group defined by the necessity to molt or shed an exoskeleton. The Ecdysozoa include the very largest of all invertebrate animal groups, the arthropods, which is of course home to the insects. Here is my favorite part: the Blaschkas did not include any arthropods in their collection! There are no crabs, insects, shrimp, copepods, or barnacles. Leopold and Rudolf simply excluded the crustaceans, the largest, most biodiverse group of spineless animals. They also left out the nematode worms, another large group of ecdysozoans. We assume the reason is that they wanted to focus on soft-bodied invertebrates, which are difficult to preserve. Perhaps they felt the world was already overrun with models of insects, crabs, and shrimp? At any rate, if you look at our evolutionary tree, the big branch for the ecdysozoans is devoid of glass models except for one, the penis worm, Priapulis caudatus, representing a rather diminutive phylum of wormlike invertebrates. Here is one seemingly small piece of soul that I share with the Blaschkas, but it feels like everything: We are rebellious and anachronistic and unruly and eclectic, driven by passion for what we love, and we love jellyfish, corals, nudibranchs, cephalopods, and even worms. We do not actually love crustaceans, and we are willing to leave them out. But I digress.

  Cephalopoda: the squid Chtenopteryx sicula in glass. Photo by David O. Brown.

  Now that we have finished with the protostomes, we can launch into the group that humans belong to, the deuterostomes, the second major arm, or superphylum, that also encompasses spineless animals. Deuterostomes include echinoderms and chordates; the latter include us humans as well as the other vertebrates. Deuterostomes likely evolved from very early protostomes. They differ in the hidden but important details of their embryonic and larval development. In their very earliest development, the gastrula stage, the first opening into a protostome embryo will form the mouth, and the second opening will be the anus. The opposite is true for us deuterostomes: our butt forms first. (In Greek, deuterostome means “mouth second.”) This means that as deuterostome embryos, we are upside down relative to protostomes: our dorsal surface is their ventral surface. It’s probably okay if this revelation does not rock your world.

  ECHINODERMS

  The Blaschkas selectively depicted animals from two of the deuterostome phyla, the chordates and the echinoderms. The echinoderm classes include sea stars, brittle stars, sea cucumbers, sea urchins, and feather stars. “Echinoderm” comes from the Latin echin- and -dermata, meaning spiny skin, and that’s what unites the group. The echinoderms are also united in having a water vascular system that controls the many tube feet they use for running around, or that the stars use for pulling open clams to eat. Showing their depth of knowledge, the Blaschkas focused on the lesser known soft-bodied part of this group and skipped the iconic sea urchins and made very few sea stars. The sea cucumbers look like worms but have rows of tube feet for mobility (except for a small group of apodous cucumber that are bereft of tube feet) and the same water vascular system as the other echinoderms. The sea feathers, or crinoids, are distinguished by having a mouth on their top surface as opposed to underneath, as with most echinoderms. They have many arms, although these usually occur in multiples of five. The brittle stars, or ophiuroids, always have five long, jointed, whiplike arms. Many of the echinoderms are brightly colored and beautiful underwater, and the Blaschkas of course captured this in both glass and some rather magnificent watercolors.

  Crinoidea: the sea feather Antedon mediterranea in glass. Photo courtesy of the Corning Museum of Glass.

  CHORDATES

  The evolutionary apex of the deuterostome lineage includes the chordates and that “small” branch of vertebrates, which we won’t bother with here. The non-vertebrate branches of the chordates are mostly spineless, contained in a subphylum called the Urochordata. You may think that sea squirts do not hold any wonder for you, but wait until you observe how gloriously beautiful the sea squirts are and see how extraordinary the differences are among the three classes: the Ascidicea, the Larvacea, and the Thaliacea. An adult sea squirt gives few clues of its relatedness to us, but the larval form, called a tadpole larva, is a dead ringer for an adult salamander or a frog larva, also called a tadpole. Over evolutionary time, we humans lost most of our t
ail, but we do have a vestigial tailbone, a nerve cord, and a notochord in early development. A tadpole also has gills and gill arches, characteristics we share in early development. This is the imprint of evolution; these linkages show our shared ancestry. For those who balk at the idea of our close relationships with the apes, to be related to sea squirts has to be an even more bitter pill to swallow. There can be no denying, however, that these larvae show our shared ancestry. I see these signs of relatedness as truly comforting; we really are a part of this great kaleidoscope of creatures spun by evolution over eons of time.

  The sea squirts are the one group with a siphon on each side of this big divide of being spined or spineless. The cool part of their biology is that when the plankton-voyaging larva is ready to become an adult, it undergoes metamorphosis (like when a frog larva changes into an adult): using special suckers, it attaches to the sea bottom in a kind of headstand, sucking in its tail as the notochord dissolves and expanding the gill basket underneath its mighty new siphons, which will be used for processing millions of liters of water. Voilà, most vestiges of the chordate are gone, and we have a spineless sea squirt.