Ports rank among the most disturbed marine environments. Pollutants like trash, oil, and toxins from antifouling paints tend to accumulate in enclosed harbors. Invasive species arriving in ballast water may displace native wildlife, while ship propellers, sonars, and dredging cause noise pollution which can interfere with animal communication. There is thus a great need for effective ways to clean up harbors. One promising method is renaturalization, where hardy species that can survive under these conditions are deliberately introduced to act as pioneers.
"Here we show that some sponge species have exceptional potential for use in the restoration of degraded harbor habitats," said Dr Manuel Maldonado, group leader at the Center for Advanced Studies of Blanes in Girona, Spain. "In our new study in Frontiers in Marine Science , we tested five species, and among them the chicken liver sponge Chondrosia reniformis demonstrated an outstanding ability to adapt to these challenging conditions and even thrive."
Sponges are true ecosystem engineers. Sponge populations filter thousands of liters of seawater daily, removing heavy metals, pathogenic bacteria, viruses, and organic particles while recycling nutrients through their symbiotic microbes. This doesn't just improve water quality, but also delivers energy and organic matter to the food web. Sponges also provide shelter for worms, crustaceans, and juvenile fish.
Sponging the pollution off
As a first step towards using sponges for renaturalization of Mediterranean ports, Maldonado set out to identify the most pollution-tolerant species. To host their experiments, they chose Girona's recreational port 'Marina Palamós'. The authors had shortlisted five candidates, all abundant at low depths off Santa Anna Point, 30km from Marina Palamós: Corticium candelabrum, Crambe crambe, Scalarispongia scalaris, Ircinia oros, as well the chicken kidney sponge. These species are only distantly related and differ in key functional and anatomical traits, such as the presence or absence of an endoskeleton, its mineral composition, and the density of microbial symbionts in their tissue.
After collecting the sponges, the scientists transported them to the nearby Blanes marine research center and kept them, grown attached to ceramic plates, in 500-liter tanks with unfiltered running seawater. Over the course of 2024 and 2025, they transplanted these plates bearing a total of 42 sponges onto 'artificial reefs', made of metal grids coated in calcium carbonate and fixed to the underwater seawall at Marina Palamós. An exception was C. candelabrum, as every collected individual had failed to attach to the tank substrate and died in the laboratory. After transplanting the sponges, the authors regularly monitored their health and survival through underwater photography and visual close-up inspection.
'Creepy' sponges
All individuals of C. crambe, S. scalaris, and I. oros gradually shrank in size and lingered between 20 and 165 days before dying. In contrast, C. reniformis thrived and even reproduced asexually: 91.7% of the original animals survived in the port throughout the entire monitoring period of 455 days. The remainder of the original individuals of this species, as well as several offspring clones, were lost to follow-up through letting go from the substrate or 'creeping', a natural behavior where sponges reorganize their body into fragments for asexual reproduction and dispersal.
The authors concluded that chicken liver sponges are particularly resilient and an excellent candidate for the next phase of harbor restoration trials.
"Unlike many sponges, the chicken kidney sponge can 'creep' away if local conditions deteriorate – for example, when nearby organisms compete. It can also relocate from the exposed upper surface of a rock, where it is subjected to UV radiation, polluted sediments, and predators, to sheltered crevices where these stresses are greatly reduced. It also reproduces asexually by fission: this allows populations to expand rapidly and later reproduce sexually, increasing the chances of successful fertilization."
"Our next step is to monitor the transplanted sponges over several years. We are particularly interested in tracking shifts in their communities of symbiotic microbes – highly abundant in tissue of the chicken kidney sponge, a so-called 'high microbial abundance' species – to find out if these help them to tolerate pollutants," said Maldonado. "But we will also continue testing more sponge species."
