![]() Contractions can effectively restrict or stop the water flow through a sponge ( Reiswig, 1971 Riisgård et al., 2016 Kumala et al., 2017 Ludeman et al., 2017 Goldstein et al., 2019) and have been suggested to protect the filter-pump, for instance during periods with high loads of re-suspended sediment ( Elliott and Leys, 2010 Bannister et al., 2012 Leys, 2015 Grant et al., 2019). Ambient water is drawn through numerous inhalant openings (ostia) into an incurrent canal system, where it enters many choanocyte chambers ( CCs) through prosopyles, passes microvillar collar-sieves, exits the CC through apopyles and leaves the sponge via an excurrent canal system that finally leads to an exhalant opening, the osculum ( Larsen and Riisgård, 1994). Sponges are sessile filter-feeders that pump water by means of choanocytes ( Reiswig, 1975 Asadzadeh et al., 2019) which are specialized flagellated cells that efficiently retain particles down to ∼0.1 μm on their microvilli collars ( Riisgård and Larsen, 2000). panicea, including osculum, ostia, in- and excurrent canals and apopyles. Our findings suggest that contraction-expansion affects the entire aquiferous system of H. ![]() The effects of GABA-induced contraction-expansion events on the aquiferous system were investigated using scanning electron microscopy (SEM) on cryofractured explants. ![]() panicea as observed in untreated explants. The neurotransmitter GABA triggered similar contraction-expansion dynamics in H. We show that contraction-expansion dynamics can occur spontaneously (in untreated explants) and can be induced by exposure to chemical messengers such as γ-aminobutyric acid (GABA, 1 mM) and L-glutamate (L-Glu, 1 mM), or to inedible ink particles (4 mg L –1). Here, we document the external and internal morphology of contracted and expanded single-osculum explants of the demosponge Halichondria panicea. Yet, purpose and underlying pathways of contractile behavior have remained largely unclear. During contractions, however, the water flow is being reduced and eventually shut down. As sessile filter-feeders, sponges rely on water with suspended food particles being pumped through their aquiferous system. 2Department of Biology – Genetics, Ecology and Evolution, Aarhus University, Aarhus, DenmarkĬontractile behavior is common among sponges despite their lack of nerves and muscles.1Marine Biological Research Centre, Department of Biology, University of Southern Denmark, Kerteminde, Denmark.Josephine Goldstein 1,2* Nicklas Bisbo 2 Peter Funch 2 Hans Ulrik Riisgård 1
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