Particle loads, contractile responses and cleaning in the demosponge Halichondria panicea

Sponges exposed to high concentrations of particles during filter-feeding may become overloaded, triggering closure of the osculum and contraction of the sponge, thus preventing too many particles from entering the aquiferous system. However, the minimum particle concentration of small versus larger particles that triggers contractions has not yet been described. Here, based on microscope-video recordings, we report our observations of explants of the demosponge Halichondria panicea. Explants contracted within <1 h in response to high concentrations of cyanobacteria (Cyanobium bacillare; 1 × 106 cells mL−1), small 2 μm beads (1 × 107 particles mL−1), larger algal cells (Rhodomonas salina), and 10 μm beads (1 × 105 mL−1, respectively). No contractions occurred in sponge explants exposed to these particle types at concentrations <0.5 × 104 mL−1. Likewise, inorganic marl and ink particles induced osculum closure in sponge explants at concentrations ranging from 1 × 105 to 1 × 106 particles mL−1. When the explants were subsequently transferred to clean water, they either expelled the ingested inorganic particles in mucus-entangled clumps with the exhalant jet, or translocated the particles to the sponge surface along with local peristaltic-like contractions. The trigger levels of inorganic particles were high compared to reported re-suspended sediment concentrations in the sea, and the observed contractile responses may thus primarily serve to protect sponges from clogging during extreme environmental events.