Strewn alongside highways that border coastal salt marshes lives an organism, seemingly camouflaged as neon orange plastic waste, that blends in with the road- shoulder trash. This spring, notice this living creature, which is actually a plant, when its color becomes fully saturated, glowing orange-pink. Though it lacks leaves, it will begin to produce tiny white blossoms in May and June. Evolved from the morning glory family of herbaceous vines, dodder goes by many names: from “freaky-looking mesh” and “strangleweed” to “living, jumbled fishing-line accident.” While the genus Cuscuta includes hundreds of species that grow worldwide, California salt marsh dodder (Cuscuta pacifica) challenges our understanding of plant behavior and the community dynamics of parasites in tidal wetland ecologies.
In the Bay Area, specifically, it tends to live near the shore, in tidal wetlands, where nature persists right up against freeways.
What can this mysterious neon-orange creature teach us? That’s a question I’ve been considering while doing interdisciplinary research as a PhD candidate in sensory evolution and biosemiotics at UC Davis. Not only can dodder smell scents and sense light, but in its complex living relationship, one we call “parasitism,” lies a beautiful example of living things exceeding our concepts, labels, and metaphors for them.
Over evolutionary time, dodder has modified its root structures into haustoria: tiny suckers that penetrate the stem and open the inner membrane of a host plant, integrating structurally and chemically with its vasculature. This creates channels between dodder and its host, and it’s the beginning of a complex dance through which water, nutrient-rich compounds, and information-rich signals are sent back and forth across networks of different plants. In the spring, as pickleweeds (Salicornia virginica), dodder’s most common primary host plant in the Bay Area, absorb more fresh water, the dodder plants begin to glow bright orange.
Like many plants that get nutrients solely from a host plant—known as holoparasitic plants—dodder is a unique color. Since holoparasitic plants don’t produce their own energy from light, their cells lack the green pigment chlorophyll, which in most plants is responsible for absorbing red and blue wavelengths of light while leaving behind green light.
Dodder’s bright orange color comes from carotenoids, an orange-pigmented, light-absorbing molecule found in most plant cells. When green chlorophyll is less abundant, the orange carotenoids are more visible. While our bodies can’t make these pigments, we also depend on them. We get carotenoids through our diets, when we eat most orange vegetables like carrots and squash. Carotenoids are crucial to protecting our eyes because they pick up and filter high- energy blue light (emitted by computer screens, for example) and concentrate in our eyes at locations where protection is needed most. We rely on the light-absorbing chemistry of plants, not only for survival by way of energetic calories, but also to support our vision.
Just as dodder, in being connected to a host plant, no longer needs to produce chlorophyll, we similarly, by being connected to plants through our diet, no longer need to internally produce carotenoids for our vision. Over evolutionary time, connections between species allow us to share these molecular tools, shaping and balancing what we need to produce within ourselves with what we can absorb through others. These long-term interspecies connections can change the landscape of the pressures of survival that shape our bodies.
Curiously, even though dodder plants don’t use light for energy, growing it indoors often fails. It turns out, light is more than just energy sources for the plant—the plant relies on bright sunlight for sensory cues: namely, when and where to grow. Depending on the kind of near- and far-red light detected by different light-sensing proteins in the plant cells, dodder will determine when to grow, what direction to grow, and when to attach the haustoria to the host plant. In this way, sensing is also becoming; environmental cues are sensed and linked with developmental cues. Some parasitic plants develop haustoria based on chemical signals in the soil at the root of the host plant. But dodder starts out as a seed sprouting from the soil, then lets go of earth altogether and instead floats in the air, holding only its host plant, seeming to arise out of the light.
It’s not only light that provides sensory cues, but smells too! Dodder seedlings perform a kind of “smell dance,” where the growing sprout rotates in probing circles picking up odors in the air. Botanists call this dance “circumnutation” and have observed that dodder seedlings show clear preferences for scents; they tend to love the fragrance of tomato far more than wheat. Through dodder, scientists have expanded our understanding of the sensory capacity of plants to detect smells. The receptor system in plants for smelling is still being mapped out and is an area of active investigation, but smelling appears to occur both between plants and within a single plant, which can smell different parts of itself.
Once a host plant is identified, the dodder haustoria invade it, absorbing crucial elements like water and carbohydrates. This “invasion” is also the beginning of a continual connection between the plants. Through these channels, not only are nutrients extracted, but a variety of chemical and genetic signals are shared. Dodder can form “bridges” between plants, sometimes across different species of hosts, creating an ecological community. These can share information, such as threat signals sent in response to insect bites and other stressors, across many plants by passing along chemical biomolecules, like defense genes. This means that dodder’s relationship to its host plant is not strictly parasitic. While on an individual scale it can slow down the host plant, it never harms it fully and actually confers many benefits to the plants on a communal, ecological scale.
Relationships like these require us to think beyond the human social, and sometimes economic, metaphors we project onto life. Ecologists often debate how to conceptualize these relationships, which are similarly complex among fungal-plant interactions. While it is not exactly like our metaphors of “the internet” or the “wood wide web”—which scientists use to explain underground fungal-root networks to the public—it also is not simply a parasitic extraction, nor really a tit-for-tat “transaction.” Dodder makes obvious what microbial ecologist and philosopher Kriti Sharma calls “contingentism,” where organisms are defined by their contingent relationships in biodiverse communities and are never really definable as separate species. Maybe if instead of strictly a parasitic behavior, it’s like a dance. But what kind of dance are these connected organisms doing? We can’t yet really know. The dodder plant is like a sensing layer, a threadlike living interface made of contact points that become channels.
If we call it parasitism, are we humans—with our resource-extractive demand for networks of information—not also parasitic and being parasitized? Dodder reminds us that physical resources and informational signals are interwoven. How can we rethink these reductive, sometimes misleading metaphors, to learn from other species about different kinds of information- and resource-sharing connections, both living and human-designed?
In the late spring, hundreds of clusters of tiny white flowers will abundantly appear amid the confusion of orange vines: dodder blossoms. This blossoming is also entirely timed and dependent upon signals from the underlying host plant, which is also dependent upon the ebb and flow of the coastal salt marsh. A stack of contingent blossoming. In what ways are we flowing with the nutrients of information to and from the world around us? I’m reminded that we might in our lifetimes understand the complex range of relationships possible between diverse organisms living in community with one another, and blossom together.
