Solar-Powered Bugs and Sun-Gobbling Slugs: Shedding Light on Plantimals
The sacoglossan sea slug is one example of animals that use sunlight as energy. Photo via Wikimedia.
About six or so years ago, I found a book at my local Barnes and Noble that forever changed my life: The Wildlife of Star Wars. As the title suggests, this literary masterpiece cataloged in great detail every creature you’ve never heard of from George Lucas’s Universe, and I was hooked.
One illustration in particular, from the oppressively humid bogs of a swamp planet, utterly captivated me: a bizarre-looking spider, but one whose legs conspicuously resembled the exposed root systems of the trees in the background. During the later stages of its life cycle, the book said, the creature would find a suitable place to settle down, enter a deep state of hibernation, and become one of the countless plants comprising the forest. My mind was blown: a literal walking spider tree! Perhaps the boundaries between ‘plant’ and ‘animal’ could be more fluid than I had initially thought. I decided to call these strange spider trees, and similar creatures that pushed the evolutionary envelope, plantimals.
Illustrations of the ‘knobby white spider’ from The Wildlife of Star Wars book. To 12-year-old me (and present-day me), the thought of a spider-tree hybrid was enthralling. Photos via Wookieepedia.
Generally, plants sit still and metabolize sunlight, with some exceptions. Animals move around and eat food, with some exceptions. What if a creature could do parts of both? Something about the sight of a walking forest, or some ethereal flier that captures the sun’s rays with its wings, has always been incredibly alluring to me, as has the simple thought of being able to make my own food out of pure sunlight. Don’t pretend like that wouldn’t be the coolest thing ever.
Over time, this artistic interest turned into a scientific one. Last I checked, trees don’t lazily saunter over to a river when parched and, much to my dismay, I have thus far been unsuccessful in acquiring energy from the sun through my outstretched arms. What gives? These seem like they’d be pretty useful adaptations. As most college students are wont to do, I sought to do a deep dive into the world of plantimals on Earth, to understand the what’s, if’s, how’s, and why’s of photosynthesis, food, and everything in between.
What I found through my research was rather interesting. Turns out, plantimals are indeed present on Earth (and surprisingly common), but not in the forms people expect them to come in. Nature’s a bit subtler than walking trees, but no less fascinating. I’ve cataloged some of the more striking examples of these strange fusions of different lifestyles.
The anatomy of a coral polyp. Individuals like these link up to form massive colonies, which are what we typically see as the structure of coral reefs. Photo via Snappygoat.com
Let’s start with a (relatively) known example: coral reefs. These stony, branching growths, which are estimated to provide shelter and food to a quarter of marine species are, in fact, a composite organism. What do I mean by that? The hard, slimy, animal-ish part of the organism is, indeed, an animal. The coral itself is actually a colony of thousands of tiny, interconnected creatures called polyps, which are closely related to sea anemones and jellyfish. The polyps lay down calcium carbonate for structural support, but acquire their rich array of colors from live-in algae called zooxanthellae (pronounced zow-uh-zan-theh-lai), which ooze out tasty sugars for their living homes.
It’s actually estimated that these in-house sugar farms provide the coral with upwards of 90% of its daily energy requirements. The rest is acquired from tiny bits of plankton caught in the current; since coral receive sustenance from both food and sunlight, they are what scientists refer to as mixotrophs, and what I would refer to as plantimals. However, without their symbiotic partners, coral wouldn’t be able to grow nearly as quickly, and the ecosystem built upon their bodies would collapse.
Photosynthetic zooxanthellae, as seen under a microscope, give the coral their oftentimes vibrant colors. Photo via Wikimedia.
As much as I hate to be the bearer of bad news, this scenario is actually currently playing out; as climate change warms our oceans, the coral are oftentimes compelled to expel their symbionts due to heat stress in a phenomenon known as coral bleaching. This appears to be a major drawback of some symbiotic relationships; they require stable conditions to maintain themselves for long periods of time, and stress within one partner or the other sees the mutualistic dynamic begin to break down.
As I came to find out, symbiotic relationships were actually somewhat common in the ocean, especially between algae-like zooxanthellae and simple invertebrates like clams, jellyfish, or worms. However, one such creature stood out from the others and more or less blew my mind: enter the sacoglossan sea slugs. Like coral, these marine gastropods seem to acquire part of their nutrition from photosynthesis, but the way they go about accomplishing this is quite extra.
The sacoglossan sea slug Elysia clarki, resting on its favorite algal snack. Notice the green color on its body, which literally comes from the algae parts that it steals. In addition to using sunlight for an energy boost, the slug is also theorized to camouflage itself with its chloroplasts, in a process termed nutritional homochromy. Photo via Wikimedia.
Instead of simply letting algae live within their bodies, it is believed that these sea slugs digest almost all of the plant matter they eat except for the chloroplasts, which they retain in their gut wall cells. If you remember anything from high school biology, the chloroplast is the photosynthetic structure, or organelle, present in plant and algal cells. In essence, the slugs are stealing the plants’ powers of photosynthesis in a process called kleptoplasty. This would be broadly similar to me eating escargot, holding some of the snail’s cells captive in my body and gaining the ability to grow myself a shell.
If you’ve been paying attention thus far, a running theme with all of these plantimals is that they’re not really just one organism. Rather, an animal is housing cells (or cell parts) that do the work of photosynthesis for it. Scientists theorize that this is due to the fact that animals simply never evolved to do as plants do; they are specialized heterotrophs (food eaters), through and through. That’s why most cases of “plantimals” involve actual plants coming in to metabolize the sunlight. However, every rule has its exceptions, and the pea aphid spits in the face of those who think they know what’s possible.
See, instead of getting an assist from algae, the pea aphid, a sap-sucking insect, just decided to make its own photosynthetic machinery. This is actually a pretty big deal, because this tiny bug is one of the only known animals that performs this chemical magic trick. The aphid is capable of acquiring energy from sunlight through a set of orangish-yellow pigments called carotenoids (these also give tree leaves their striking fall colors).
A pea aphid perched on a leaf. Some individuals come in green or white, but this variety clearly showcases the orange carotenoid photopigments. Photo via Wikimedia.
What’s quite notable about aphid photosynthesis is that...it’s not really photosynthesis. Plants draw CO2 out of the air and store it in sugar molecules but the aphids, once again being weird, skip that step entirely and generate ATP directly from sunlight. ATP is the actual energy-carrying molecule our cells use, which they can make from the aforementioned sugar generated from photosynthesis. Unlike plants, however, the pea aphid can’t store up the energy it gets from sunlight, and so simply uses its pigments as a way of capturing solar energy in real-time to power its metabolism. Nonetheless, there are still many questions lingering about why these aphids “photosynthesize” when so many other animals don’t.
Of course, all of these sunlight-harvesting shenanigans among many of the animal kingdom’s less complex members beg the question: why are only a handful of small creatures taking advantage of this bountiful source of energy? Where are the giant walking tree spiders? The simple and rather disappointing answer is that photosynthesis just doesn’t provide that much energy. You heard me. One of the most fundamental metabolic processes that powers almost every ecosystem on our planet is woefully inadequate at converting already dilute sunlight into chemical fuel. For reference, photosynthesis has a maximum theoretical efficiency of 6%. Only a tiny fraction of the light hitting a leaf is transformed into usable energy for the plant, and this assumes optimal conditions.
There’s a reason why every plantimal seen in nature is either a small invertebrate or a completely immobile statue of an animal (looking at you coral), both of which have very low caloric requirements. There’s a reason plants don’t move or do so extremely sparingly. There’s a reason why over three acres (roughly 2 ½ football fields) of cropland are required to produce enough food for one human
To really put the nail in the coffin, scientists estimated just how much energy a person could extract from the sun if their entire body became one big human-shaped leaf, and they sunbathed all day. The answer: one marshmallow. Well, it’s technically the caloric equivalent to a single marshmallow, or roughly 1% of the average person’s daily energy needs. That’s pretty lame, plants. If I were to hypothetically become completely sun-powered, I would need photosynthetic hands the size of tennis courts which, in all honesty, sound just a tad impractical from an evolutionary perspective.
So, there you have it, one of many answers that attempts to explain this discrepancy between what we see in cool sci-fi or fantasy movies, and what we see in reality. As an avid giant walking spider tree enthusiast, I was initially somewhat disappointed by my findings. I’ve always fantasized about the weird and wonderful forms that complex alien life could take, should we ever discover it in our galaxy. Nonetheless, it appears as though the laws of physics and the trends of evolution present a metaphorical headwind to any aspiring plantimal. Unless the photosynthesizers of the world get some bright ideas to make their sun gobbling more efficient, it remains likely that many of our wilder ideas will remain in the dark.