Sponges are quite flexible feeders and can supplement with photosynthesis
For decades, the sponge has been cast as the ocean's quiet janitor — anchored, passive, consuming what the current delivers. Researchers at the University of Amsterdam have now revealed a more complex character: half of the sponge species studied around Curaçao's reefs are also producers, generating roughly 11% of tropical coral reef productivity through photosynthesis carried out by symbiotic microbes within their tissues. The discovery does not merely add a footnote to marine biology — it asks us to reconsider how life organizes itself when freed from the categories we impose upon it, and how much of the ocean's productive labor we have simply failed to see.
- Decades of ecological models assumed sponges were consumers only, but new data from Curaçao shows half of tested species are actively generating organic matter through photosynthesis — a foundational assumption of reef science must now be revised.
- The scale of the oversight is striking: sponges account for roughly 11% of gross primary productivity on tropical coral reefs, a contribution that had been invisible because traditional two-dimensional reef surveys systematically undercounted them.
- Three-dimensional reef mapping was the methodological breakthrough — by capturing the vertical complexity of reef architecture, researchers found far more sponges than anyone had tallied, and far more photosynthetic output than models had ever assigned them.
- The finding ripples outward: sponge-rich marine environments worldwide may be quietly sequestering and generating carbon in ways that current climate and ocean productivity models have not accounted for, leaving a measurable gap in our understanding of how oceans breathe.
Sponges have long been understood as the ocean's passive filter feeders — anchored to the reef, drawing nutrients from whatever water passes through them. A team at the University of Amsterdam's Institute for Biodiversity and Ecosystem Dynamics has now overturned that picture. Studying 24 of the most abundant sponge species around Curaçao, they found that half of them are not merely consumers but producers, generating organic matter through photosynthesis performed by symbiotic microbes living inside their tissues. Scaled to the ecosystem level, this contribution amounts to roughly 11% of total gross primary productivity on tropical coral reefs — a figure that forces a reckoning with decades of ecological modeling.
The key to unlocking this finding was perspective, quite literally. Previous reef surveys had mapped the seafloor in two dimensions, missing the vertical complexity where sponges cluster most densely. By switching to three-dimensional reef mapping, the researchers discovered far more sponges than traditional counts had ever captured. That fuller accounting made it possible to calculate actual sponge volumes and scale photosynthetic measurements up to the ecosystem level — a step no prior study had managed.
Benthic ecologist Michelle Achlatis, who led the work, had expected perhaps a quarter of sponge species to show photosynthetic activity. Finding half was a surprise. Co-author Jasper de Goeij notes that sponges resist the plant-animal categories that organize most ecological thinking: together with their microbial partners, they photosynthesize like plants while feeding like animals. That metabolic flexibility, long overlooked, turns out to matter enormously for how energy moves through reef systems.
The implications reach beyond coral reefs. Sponges are widespread across tropical marine environments, and their photosynthetic partnerships likely operate wherever conditions allow. Achlatis argues that sponge-rich ecosystems deserve reassessment for their role in carbon sequestration — these animals are not merely recycling existing carbon but generating new organic matter. As climate models grow more refined and the ocean's carbon budget becomes more consequential, accounting for sponges means accounting for something that was always present, simply hidden within the reef's uncharted depth.
Sponges have always seemed like the ocean's passive inhabitants—anchored to the reef floor, quietly filtering whatever drifts past. But researchers at the University of Amsterdam's Institute for Biodiversity and Ecosystem Dynamics have discovered something that upends how we understand these creatures. Half of the sponge species they studied don't just consume nutrients from seawater. They produce them, through photosynthesis, contributing roughly 11% of the total energy and organic matter generated across tropical coral reefs. It's a finding that forces a reckoning with decades of ecological models that treated sponges as mere consumers, not producers.
Michelle Achlatis, a benthic ecologist who studies organisms anchored to the ocean floor, spent years investigating sponge ecology. What she and her colleagues knew was straightforward enough: sponges are filter feeders. Specialized cells pump water through their bodies, trapping and absorbing whatever edible material floats in. But sponges harbor something else—tiny photosynthetic microbes living inside them in a symbiotic relationship. These microbes use sunlight to manufacture their own food and release oxygen. While scientists had observed photosynthesis in sponges before, the research had been limited to a handful of species and never scaled up to measure the ecosystem-wide impact.
Achlatis and her team assessed 24 of the most abundant sponge species around Curaçao, conducting detailed measurements on eight of them. The key innovation was perspective. Previous surveys had mapped reefs in two dimensions, missing the vertical complexity where sponges truly thrive. Using three-dimensional reef mapping, the researchers discovered far more sponges than traditional surveys had counted. By calculating their actual volumes in this fuller picture, they could scale their findings upward to the ecosystem level and compare sponges' photosynthetic output against other reef producers.
The results surprised even Achlatis. She had anticipated that perhaps a quarter or a third of sponge species might be photosynthetic. Instead, half of them were. More striking still was the magnitude of their contribution—11% of gross primary productivity, the measure of how much organic matter an ecosystem generates. In ecological terms, this is substantial. Sponges had been relegated to the margins of reef food webs and climate models, treated as recyclers of existing nutrients. The data suggested something far more active: sponges were not only consuming but creating.
Jasper de Goeij, an associate professor of marine benthic ecology and co-author of the study, frames the finding in broader terms. Sponges, he notes, don't fit neatly into the plant-animal distinction that dominates terrestrial thinking. Together with their symbiotic microbes, they photosynthesize like plants while feeding like animals. The ocean, he suggests, contains many organisms that defy the rigid categories we've imposed on life. This flexibility—what Achlatis calls their "multitasking" ability—matters for how we assess ecosystems.
The implications extend beyond coral reefs. Sponges are common in many marine environments, and those in similar tropical regions likely harbor the same photosynthetic partnerships. Achlatis argues that sponge-rich ecosystems should be reassessed for their role in carbon sequestration, the process by which carbon is captured and stored. Sponges aren't just recycling carbon and nutrients they find on the reef; they're generating new ones, even if in modest amounts. As climate models grow more sophisticated and as we seek to understand how oceans absorb and store carbon, overlooking sponges means overlooking a piece of the puzzle. The next step is recognizing what was always there, hidden in the reef's three-dimensional architecture.
Notable Quotes
They are quite flexible feeders and can supplement with photosynthesis, not only recycling carbon and nutrients but also making them.— Michelle Achlatis, benthic ecologist
We need to look differently at many organisms living in the ocean, as they do not follow the strict plant-animal distinction as most organisms on land.— Jasper de Goeij, associate professor of marine benthic ecology
The Hearth Conversation Another angle on the story
So sponges photosynthesize. But they're not plants. How does that even work?
They don't do it alone. Tiny photosynthetic microbes live inside the sponge tissue in a partnership. The microbes use sunlight to make food and oxygen, and the sponge benefits from that energy. It's symbiosis—both organisms gain something.
And this was unknown until now?
Not unknown, but underestimated. Scientists had studied photosynthesis in a few sponge species. What's new is measuring it across many species and scaling it up to see what it means for the whole reef ecosystem.
Why does 11% matter so much? That sounds small.
Because sponges were never counted as producers at all. They were treated as pure consumers in every food web model. Discovering they generate 11% of reef productivity means we've been missing a significant energy source in our understanding of how these ecosystems work.
Does this change how we think about carbon in the ocean?
It should. If sponges are producing organic matter through photosynthesis, they're also sequestering carbon. Reefs are already crucial for carbon storage, but we've been calculating it without sponges in the equation. That's a gap worth closing.
Could this apply elsewhere, or is it just a Curaçao thing?
The study was done in one place, but sponges in similar tropical regions likely have the same symbiotic partners. The mechanism isn't unique to Curaçao. The broader point is that we need to look at sponge-rich ecosystems everywhere with fresh eyes.