Mastigonemes appear to exist and remain in the beat plane and are thought to reverse the thrust generation by the flagellum ( Holwill and Peters, 1974).
Vanes differ from mastigonemes, which are extracellular nanometer-thick fibers that protrude perpendicular to the flagellum ( Bouck, 1971 Fenchel, 1982), both morphologically and functionally. In both cells, presence of a flagellar vane, a sheet-like structure along the length of the flagellum, has been observed ( Afzelius, 1961 Fjerdingstad, 1961 Mehl and Reiswig, 1991 Weissenfels, 1992 Leadbeater, 2006 Mah et al., 2014). Therefore, understanding how the sponge filtration system came about would go a long way to helping resolve whether sponges evolved from a colonial unicellular ancestor (Porifera first hypothesis) or from a tissue-grade metazoan (as in some interpretations of the Ctenophora first hypothesis).īoth choanocytes and choanoflagellates, a group of free-living unicellular and colonial flagellates, share ancestory to animals, and possess similar collar-flagellated cells, to generate adequate feeding currents. One of the greatest modern puzzles is whether Porifera or Ctenophora branched first in the evolution of multicellular animals ( Whelan et al., 2015 Telford et al., 2016 Feuda et al., 2017).
It is unknown what gave rise to the leuconoid forms of the other three sponge classes. In leuconoid forms, the holes in the surface of the body are connected to a complex canal system with a multitude of small chamber pump units.Īlthough these are often depicted as a progressive gain in complexity in the evolution of leuconoid from ascon and sycon forms, phylogenetic analyses indicate that in Calcarea both leucon and sycon type sponges most likely arose independently from ascon forms ( Manuel, 2006), and some evidence points to the opposite process in which ascon and sycon forms may have derived from a rigid leucon body architecture ( Dohrmann et al., 2006). Water enters through short canals or directly through openings (ostia) into the chambers and leaves through a single outlet (apopyle) to the atrium and from there out of the osculum. Syconoid forms, in turn, possess many ascon-shaped cylindrical chambers branching off a central cavity (atrium). Insets show zoom-in water path (blue arrows) into the chambers in sycons and leucons, where in the latter, water is forced to go through collar filters (black) of choanocytes (red, cc) due to the presence of a physical gasket (g).Īsconoid forms are organized as a single tube in which the choanocytes form a single layer on the inner surface of the tube wall. Schematic of sponge body types in ascon (A), sycon (S), and leucon (L) with evolutionary view of grades of morphological complexity in calcareous sponges (black arrows).
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