A Sea of Glass Page 9
Nudibranchs also have large value thanks to their potential for housing new drugs. As tiny alchemists with unusual powers, they not only concentrate rare and valuable chemicals from their prey, but they also add new twists in combinatorial chemistry. Nudibranchs make chemicals so unusual in their biological effects that even the most seasoned chemists are left looking like amateurs by comparison. An example is one of the chromodorids that feeds on a sponge containing the chemical manoalide; it can biotransform the manoalide into different chemicals that display antimicrobial activity against human pathogenic bacteria like Escherichia coli, known as E. coli, which is common and can be deadly (Karuso and Scheuer 2002). Other nudibranchs, such as the Spanish dancer, Hexabranchus sanguineus, are a source of novel anti-cancer chemicals. Pawlik et al. (1988) have shown that Hexabranchus sequesters manoalide from its food sponge, Halichondria sp. The dancer then concentrates the chemical in its back, releases it in mucus, and even loads it as a defense into its egg masses. There is an entire field of natural-products chemistry, funded by the National Cancer Institute and other agencies, devoted to finding valuable pharmaceuticals in marine animals. In this search, nudibranchs have been a rich source of new chemicals with novel biological effects.
Those who know about nudibranchs are fascinated and perplexed in turn by their brilliant coloration and clever chemistry and understand that they are, in many ways, comparable to the better-known butterflies on land. Caterpillars are larval butterflies and the ones that do the eating; they feed on particular, often chemically defended plants. For example, monarch butterfly larvae feed on milkweed and extract toxic chemicals (cardenolide agylcones) that defend them against birds that like to eat caterpillars. Like butterflies and caterpillars, the striking colors and shapes of nudibranchs are closely matched to their particular prey species; they are master chemists, using their prey’s toxic chemicals in their own defense (Paul et al. 1990), as well as making up their own toxic brews; and they have elaborate mimicry complexes with lookalike animals. For example, some large flatworms are colored with stripes that match the striking warning coloration of some nudibranch species. The evolutionary story of nudibranchs is again similar to terrestrial butterflies in their capacity for striking evolutionary radiations, guided by the bounds of their prey type. One of my fun finds in Wakatobi was a bright blue, orange, and black striped flatworm that mimicked a chromodorid nudibranch; they were side by side on a steep coral wall. Several species of flatworm mimic nudibranchs that are well-armed with chemical defenses. The flatworm is not chemically defended, but derives protection from fish predation by resembling a toxic nudibranch.
One of the best-armed species among the nudibranchs is the fluorescent blue-striped sea dragon (Glaucus atlanticus, page 102). It’s an open-ocean nudibranch that feeds on such dangerous prey as the Portuguese man-of-war, the lovely Porpita jellyfish, and the by-the-wind sailor. Each year, 10,000 people in Australia end up hospitalized due to a reaction to the neurotoxin in the tiny barbs of the man-of-war. Yet consider how the sea dragon nudibranch, Glaucus, received her name, which refers to the poison dress sent to Glauce by the jealous Medea. Glaucus swallows whole the toxin-loaded harpoons of the man-of-war and actually passes them through her entire digestive system. Once ingested, she mounts them as armature on the feathery projections on her back. This qualifies nicely as a poison dress, although with barbs aimed outward. It took scientists years to figure out the sea dragon’s trick. She selectively ingests and transports immature stinging cells through her digestive system to small end pockets in her feathery projections (Greenwood 1988). They then grow to maturity on her back, to be used as hand-me-down weapons.
A sea dragon (Glaucus atlanticus) in glass (top) and in a Blaschka watercolor. These tiny predators live in the open ocean and prey on the Porpita and Physalia jellyfish, which are also Blaschka matches. Photo by Guido Motofico, courtesy of the Natural History Museum of Ireland. Watercolor courtesy of the Rakow Research Library, Corning Museum of Glass, BIB ID: 95575.
As I navigate the deep waters off Kapota, I consider the threats to our trio as we seek out examples of these tiny weaponized slugs. These reefs are packed with dangerous fish and invertebrates and it seemed like there was a lionfish, moray eel, or sea snake in every ledge and crevice. It’s hard to say which is the most dangerous. Lionfish, which range from mouse sized to Chihuahua sized, have venomous spines. If you are stung, the neurotoxic venom won’t kill you, but it hurts like a man-of-war sting and can trigger lethal allergic reactions. Although moray eels don’t sting and usually prefer to hide in a crevice, they scare me more. If you accidently put your hand near one’s mouth, it can easily flay your skin with its recurved teeth. The big green morays on this reef are a foot wide and ten feet long. Black and white striped sea snakes, though reputed to have the deadliest venom of all snakes, are rather docile, and their mouths are too small to even grasp your finger. But that doesn’t stop me from being startled—they have an unsettling tendency to unexpectedly pop up from the reef into our faces.
The most common danger among the envenomated creatures is the one without a backbone that I haven’t mentioned yet. Remember the cnidarians with their stinging cells? My hand was blistered from the stings of a fire coral colony in an earlier expedition to Raja Ampat and, on this dive, some unknown jellyfish got into my wetsuit and turned my shoulder into a red mass of thirtyfive swollen, super-itchy blisters. Stinging hydroids and jellyfish, by the way, are one more prey favored by the ever-resourceful nudibranchs. In one of the snazziest biological tricks ever, they don’t get blistered stomachs from eating fire coral. Although scientists still have only part of the story, we know that nudibranchs somehow select immature stinging cells as they eat. They pass these through their stomachs and zip them into special glands on their backs. Here, as if they had always belonged to the nudibranch, they continue developing as deadly harpoons. This trick raises very large biological questions. How do the nudibranchs sort immature from loaded nematocysts in their gut? How do the nudibranchs slip these foreign cells past their own vigilant immune systems and adopt them as part of their bodies?
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Finding a small nudibranch in fast water flow on an underwater cliff takes some doing. Filming one takes even more finesse. I’d lucked out in finding and cataloguing a blue and orange Anna’s nudibranch, one of the chromodorid species I hoped to find. Now, David is trying to stabilize his macro lens to capture the tiny creature on film. He gently wedges himself against an outcrop without kicking or elbowing any delicate coral colonies or nudging any lionfish. One wrong move and a head of coral could be dislodged. This is tough duty for a man used to chasing big things like beluga whales and walruses. But unlike whales or even some fish, this animal with glowing gills is blissfully unaware that it’s being filmed as it searches the wall for its favorite food, sponges. David had barely reorganized from the tough job of filming when we heard another tank clang. This time, Jardeen had found Phyllidia, a blue, orange, and black nudibranch that feeds on sponges and exudes toxins deadly enough to kill fish in an aquarium. This would turn out to be the most common of the nudibranchs we found on our trip, and reassuring evidence that these diminutive invertebrates seem to be doing well in the healthier pockets of reef here. Across Indonesia, we have so far seen five different species of Phyllidia, separated by their color patterns, from black and white, to black and green, to blue and white and black, to mixing in yellow. As David lines up to get a macro shot, an ever-present lionfish scuttles beneath his arm.
The next tank clang is mine to sound, as I’d found a couple of beauties. Tucked into an overhang on the wall is a Blaschka lookalike the size of a mouse: Goniobranchus leopardus, with splashy brown spots patterned like a leopard; nearby lurked Goniobranchus kuniei, with iridescent blue spots as bright as a peacock (page 105). Both of these join the ranks of the sponge eaters.
Sponge-eating nudibranchs in the Wakatobi Islands. (Top, from left) leopards of the sea: Goniobranchus leopardus and Kuni’s n
udibranch (G. kuniei) near Kapota Island; (bottom) Anna’s chromodoris (Chromodoris annae). Top photos by Drew Harvell (left) and David O. Brown; bottom photo by Drew Harvell.
Day by day, we catalogue the surprisingly resilient biodiversity of Indonesia’s protected reefs in the heart of the Wakatobi National Park. Every day there are new species of nudibranchs, more clown fish in different anemones, and an impossibly high biodiversity of hard corals, soft corals, sponges, and fish. I had come with a mission, and the results have exceeded my expectations, despite the fringe of denuded reefs I’d stumbled upon at the beginning of my trip, which were outside a protected area.
As we’re settling into Patuno for a long ride back across the bay after one of our last dives in Wakatobi, David asks me a tough question, one he wants included in the film: What is the value of biodiversity? It seems easy enough to answer on the surface, but it’s one that plagues scientists and conservationists who often have to prove to governments, stakeholders, and the public why protecting delicate ecosystems is important. David presses further, asking if the number of species matters. Would it be okay if we had just one species of nudibranch instead of the roughly twelve we have seen and the probably fifty-five that we haven’t seen?
David’s question would be easier to answer if he had posed it about economically important fish or corals, but there is indeed a story for the ecological role of nudibranchs, even if it’s tough to put a price on. Nudibranchs, as we know, are extreme food specialists—each species only eats one food. So different species of nudibranchs are not substitutable. For example, during a dive earlier in the trip, I had led an expedition to survey coral reef health in Raja Ampat and Bali. On the very over-fished reefs with high nutrient inputs, the corals were being killed and overgrown by a platoon of sponges, sea squirts, and algae. Sponges and sea squirts are the absolute favorite food of nudibranchs. So if you lose the one species of nudibranch capable of eating the extremely toxic and fast-growing neon purple sponge or the terrible green sea squirt, your corals might become overgrown with the sponge and the reef will be less sustainable. Indo-Pacific sponges and sea squirts are so toxic that few animals other than nudibranchs, a few specialized fish, and hawksbill turtles eat them. In Raja Ampat, nudibranchs were few and far between. We don’t know if it was because their predators—certain fish, sea spiders, sea stars, and even some turtles—were unusually effective, or whether other aspects of their habitat had become unsuitable.
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In talking about the prospects of sea slugs in the oceans of today, we have to look also to other sea slugs, such as the pteropods, or sea butterflies and sea angels, which get their name from the transparent wing-like projections that row them through the oceans as if in flight. I periodically see the Blaschkas’ exquisite sea angel, Clione limacina (page 108), flying through the water near my lab at Friday Harbor. This tiny but vicious predator, with transparent wings and a red glowing stomach, has keen senses and a sharp radula with which to hunt and tear apart the shelled species of sea butterfly. Ancient whalers called them “whale food” because although each pteropod is small, their flocks can dominate the plankton and are the food that feeds baleen whales. The shelled pteropods, or sea butterflies, are indicator species for the impacts of acidification.
Ocean acidification is the lowering of pH that occurs when there is too much carbon dioxide in the water. The oceans absorb approximately a third of the excess carbon dioxide we emit, thereby slowing adverse effects like warming in our climate system. But the carbon dioxide accumulating in ocean waters has decreased the average pH across entire ocean basins by 30 percent in the last 160 years. This has tipped the scales in acidic hot spots like the Pacific Northwest, making the water actively corrosive to the point that it dissolves the shells of animals like the shelled sea butterfly, which live its entire life in acid waters. The link between decalcification and ocean acidification wasn’t noticed until 2007. An oceanographer from California State University San Marcos by the name of Victoria Fabry, aboard a research vessel in Alaska, noticed that the shells of oceanic pteropods were pitted and thinning from the corrosive waters in her experiment (Fabry et al. 2008). This was the first indication that our climate-changed future was here in today’s seemingly clean oceans, affecting beautiful and important marine life in ways that we could actually see. Since then, the dissolving shells of sea butterflies have been joined by dissolving shells of larval oysters, with big economic impacts as a result. Recent work has shown an increase in the percentage of pteropods with pitted shells in acid-rich parts of the Pacific, and scientists found that the highest percentage of sampled pteropods with dissolving shells were along a stretch of the continental shelf from northern Washington to central California, where 53 percent of those sampled had severely dissolved shells. They suggest ocean acidification will disrupt the great oceanic food chains that start with phytoplankton and end with whales (Bednaršek et al. 2014).
Clione limacina, one of the fierce, predatory sea angels, in glass (top) and alive at Friday Harbor. Sea angels are called naked pteropods, since they do not have a shell. Photos by Gary Hodges (top) and Reyn Yoshioka.
Unfortunately, the nudibranchs aren’t immune from the acid waters either (Davis et al. 2013). Although they lack an adult shell, nudibranchs, like all gastropods, have thin larval shells that are sensitive to levels of acidification. The nudibranch larva is called a veliger and its shell is made of aragonite, which is more sensitive to low pH than other forms of calcium carbonate. Scientists have been finding that the larval forms of many invertebrates, like the nudibranchs, have very delicate calcareous skeletons that are sensitive to the corrosive effects of low pH. No one has checked nudibranch shells, but larval shells of other molluscs begin to dissolve at pH 7.6, a level already reached in some areas of the Pacific Northwest (Byrne 2011). Ocean acidification is a diabolical threat that is increasing unseen, a lurking time bomb that explodes unpredictably in cold, upwelled waters like those of the Pacific Northwest, the Arctic, and the Antarctic.
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By the time our Kapota dive had ended, we had seen eight different species of nudibranchs. David had close-up videos of them all, each recording accompanied by a soundtrack of us laughing and muttering underwater at the ridiculous spots and stripes and colors on our slugs. Even before surfacing, the excitement of a perfect dive was shared as we all three were suspended weightlessly at our fifteen-foot safety stop, floating over the reef edge. We communicated in an underwater language of shaking and nodding our heads, our hands signaling “thumbs up” and “so big” at the riches of this reef. As we surfaced, David couldn’t get his regulator out fast enough to express how amazing the reef, the nudibranchs, and the clownfish anemones were. I was satisfied to see this brilliant whale videographer so pleased with finding the wily sea slugs, and the quiet Jardeen excitedly passing around the Indonesian natural history books to the captain and mate to show them the species we saw. The chatter continued as we warmed up with tea on the deck of Patuno, joined by an Australian couple who had come here knowing it was rich in sea slugs. “It was like a nudibranch circus down there,” the woman exclaimed.
We brought two of our close Blaschka matches aboard and carefully placed them in a glass-walled tank so I could identify the species and snap some photos. Afterward, I returned them to their reef, grateful to have seen them. We had come a long way to find these fierce little sea slugs, and it was reassuring to know they still dominated this small place. I also loved seeing that dive tourists valued the sight of nudibranch circuses and would come to Southeast Asia with that as a must-see goal. However, as a group, the nudibranchs today are no doubt much harder to find than 160 years ago, as their habitat and prey are much reduced, and the practice of blast fishing is leveling the few safe habitats remaining.
6.
OCTOPUS AND SQUID
Shape-Shifters under Pressure
The long-armed squid (Chiroteuthis veranyi) in glass. Photo by Guido Motofico, courtesy of the Natural
History Museum of Ireland.
AS WE HEAD DOWN the Big Island’s Kohala Mountain for a night dive on the reef, David explains that if we’re lucky enough to find an octopus during our dive, he wants to capture footage of me interacting with it. Well, how does that work? I ask. Usually when I see an octopus, which is not often, I watch it for a while and then it jets off, shifts into camouflage mode, and I never see it again. David explains that he wants me to hold it, to herd it between my hands, and gently engage with it. Apparently, this is common among recreational divers, especially with inquisitive octopus who seem just as curious as their human counterparts. My training is to only touch or collect organisms we need for research, so it’s unusual for me to handle something like an octopus.
Me: “But don’t they bite?”
David: “No, they never do—it won’t bite you.”
Me, very dubiously: “So . . . you want me to herd it and then pick it up?”
David: “Yes, you’ll see how easy it is.”
I figure our chance of finding an octopus is remote, so I decide not to worry about it. Our goal is to find the day octopus, Octopus cyanea, a visual match to our Blaschka common octopus (O. vulgaris) but now shown to be slightly different genetically (Kaneko et al. 2011). The common octopus is widely distributed from the Mediterranean to Japan, but the day octopus dominates in Hawaii, where it is well established. Despite their name, day octopuses are active at night and so we might actually find one out hunting at night. We are also hoping to see the rarer and more beautiful nocturnal ornate octopus, which our dive guides, Denise and David from Blue Wilderness Divers, said they’ve seen at a particular reef on recent nights.