Flow pattern, particle capture and contractile behaviour in the demosponge Halichondria panicea

In the aquiferous system of demosponges, the flow into inhalant canals is driven by the beating flagella of the choanocytes organized in chambers imbedded in the walls and the filtered water is discharged into exhalant canals. Our aim was to characterize the laminar flow pattern and fate of captured algal cells and microplastic beads in the inhalant canals in a sandwich culture of the demosponge Halichondria panicea in contact with the ‘mother sponge’ using microscope-video technique. Algal cells traced near the centreline of an inhalant canal moved with the highest speed and travelled the furthest, and when approaching the canal walls the flow speed decreased and the cells were captured to be subsequently digested. Microplastic particles accumulated also along the inhalant canal walls but were subsequently expelled into the exhalant canals. Adding a high concentration of 2–22 µm fragmented waste plastic particles triggered contractile behaviour. It started immediately with osculum closure in the ‘mother sponge’ lasting about 25 min and it was followed by delayed and temporary contractions of first inhalant and then exhalant canals in the sandwich culture. These results support the theory of how the sponge responds to ultimately ensure the ejection of the foul particles.