Why Some Bryozoans Poop Themselves: A Novel Defecation Method in Ctenostome Bryozoans

Feeding in animal colonies is a challenging but important aspect in organisms comprised of numerous zooids. Among bryozoans, general aspects of colonial feeding currents were studied in detail, whereas zooidal defecation has received little attention. In case of bryozoan colonies with closely situated zooids, feeding currents show interzooidal interference, but this also applies to defecation and faeces removal. More recently, it was recognized that the position of the anus among ctenostome bryozoans is variable, located either close to the base of the tentacle crown, similar to other bryozoan clades, or distally at the vestibulum. The latter case, with a so-called vestibular anus, remained functionally unexplored. Here we report first investigations into the defecation process of alcyonidioidean ctenostome bryozoans, which possess a vestibular anus distally of the retracted tentacle crown or lophophore. Defecation occurs with retracted polypides and the faecal pellets are transferred into the tentacle sheath on top of the retracted lophophore. This mechanism allows the bryozoan to release faeces via the atrium by the general polypide protrusion mechanism. Unlike other bryozoans with typical lophophoral anus, this enables defecation without protruding their entire tentacle crowns. This could yield protection of the lophophore, an advantage in predator avoidance and also capability of defecation at low tide for intertidal species. This 'lophophore-push poop removal' might be an efficient way of faeces removal within the colony and appears to reduce interferences with colonial feeding currents. Whereas colonies in this clade of bryozoans always show dense aggregations of zooids, it would be interesting to study bryozoans with vestibular anus and less dense colonies.