For a long time, sponges on coral reefs had a fairly simple reputation. They stayed in one place and fed on whatever the water brought past them. That picture now appears incomplete. A new study of reefs off Curaçao reveals that many sponges also produce their own food from sunlight—the same fundamental process that fuels plants. The findings were published in the journal Functional Ecology.
Michelle Achlatis from the Institute for Biodiversity and Ecosystem Dynamics (IBED) at the University of Amsterdam studies life that remains attached to the seafloor, rather than life floating above it. Sponges have been at the center of her work for many years.
“We call them a classic example of filter feeders, because they not only filter but also absorb all sorts of edible materials from seawater,” said Achlatis.
Specialized cells pump seawater through their bodies and trap any edible substances along the way. For many years, this filtering role defined biologists’ view of the entire sponge group. It portrayed the sponge as a creature with a feeding apparatus, performing no other function.
A plant-like ability comes from tiny dormant organisms. Some sponges host photosynthetic microorganisms in their tissues, mainly cyanobacteria, which turn sunlight into food and release oxygen.
“Together with their symbiotic microbes, sponges are capable of photosynthesis, similar to plants,” said co-author Jasper de Goeij from IBED.
This ability itself was not entirely new to science. Earlier studies had confirmed it, but only in a small number of species.
Achlatis and her colleagues studied 24 of the most common sponge species in the Curaçao area. They conducted the most detailed measurements on eight of them. The team tracked the organisms’ responses to oxygen and light in the lab and on the reef.
The researchers then scaled these data up to the entire ecosystem level, allowing them to compare sponges with other reef organisms that synthesize food from light. The eight species studied in detail showed a wide range of responses to light, from almost no reaction to a high dependence on it. Comparing oxygen readings with chlorophyll fluorescence revealed how actively each sponge worked as light levels increased.
The result caught the team off guard. Half of the studied species responded to light and performed photosynthesis. Achlatis had expected perhaps a quarter or a third would do so. She also did not anticipate such a significant contribution to the reef’s food base.
“In previous studies of food chains or when modeling large coral reefs, this group was always seen as pure consumers, not producers,” she said.
Most of these sponges still consume more energy than they produce. Sunlight covers only a small portion of their daily energy needs, ranging from 10 percent in some species to 40 percent in others.
“But they are quite undemanding when it comes to food and can supplement their diet through photosynthesis,” said Achlatis.
The reef itself partly explains the surprise. Earlier studies counted sponges using flat photographs taken above the seafloor. This approach misses a lot. Nearly half of the reef’s living material is hidden inside cracks, caves, and the framework beneath corals.
“A previous study showed that if you look at a three-dimensional reef, there are far more sponges than you would expect,” noted Achlatis. “By taking more detailed measurements, you can calculate their volumes.”
With the increased sponge numbers, they shot up in the ranking of reef ecosystem producers. Half of the studied species accounted for about 11 percent of the total primary productivity of the reef. This result places them in fourth place overall.
Only large seaweeds, hard corals, and gorgonians produce more of such algae, and sponges even outrank crustose coralline algae, a group long known for producing food from light.
The method of measuring productivity matters a lot. If measured by flat surface area, the contribution of sponges appears minimal. But if measured by biomass, which better reflects their true size, their contribution increases many times over.
Measurements were taken at a depth of about 10 meters. Some photosynthetic sponges live at much greater depths, in dim waters reaching nearly 150 meters, and they still retain their light-capturing partners.
This has real implications for ocean carbon accounting. The authors argue that sponge-rich habitats deserve a place in assessments of how reefs absorb and store carbon. On the leeward reefs of Curaçao alone, these sponges may absorb carbon almost twice as slowly as all the island’s mangrove forests.
“The main idea of this paper is that people should understand that sponges not only process carbon and nutrients they find on the reef, but also have their own ways of producing them, even if in relatively small amounts,” said Achlatis.
The study covered one tropical region. However, sponges in similar locations often share the same partners, so this pattern may extend far beyond Curaçao.
Professor de Goeij believes the findings reveal something broader about life in the ocean.
“We need to look differently at many organisms in the ocean, because they do not follow the strict division into plants and animals like most land organisms do,” he said.
Sponges have survived hundreds of millions of years of ocean changes. Records of light-feeding species already span most of the world’s coastlines, from the tropics to cold polar waters. As this study shows, treating them as mere consumers has always been unfair to them.
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