Great Barrier Reef Bryozoa

based at the Museum of Tropical Queensland, Townsville

Introduction continued (2)

Colonies are made up mostly of feeding zooids (autozooids), but bryozoans are justly famous for exhibiting a higher degree of polymorphism ("many forms") among individuals than almost any other invertebrate group including siphonophores (planktonic cnidarians). Whereas autozooids have polypides, comprising a ciliated feeding apparatus (lophophore) and a U-shaped gut, kenozooids and avicularia lack them. Polypides are everted through an opening (orifice) in the body wall that merely puckers inwards upon closure or is variously modified as a pleated collar, as a sclerite-strengthened pair of folds, or as a lid-like operculum. Kenozooids are simply closed coelomic chambers, albeit in organic continuity with autozooidal neighbours, that may serve as attachment rootlets (rhizoids, stalks), spines, stolons, space-fillers, or part of the support structure for large erect colonies. Avicularia, on the other hand, may have a feeding polypide or, in most cases, a highly reduced vestige of one that may serve as a glandular or sensory organ. In these morphs, the operculum is modified as a jaw-like mandible or even as a long bristle-like seta. The latter can move in concert and, in the case of bushy colonies of Caberea, can lash simultaneously along a branch, presumably deterring settlement by larve of other sessile organisms. In the case of free-living colonies of Selenaria, bristles around the periphery of lentil-like colonies on sand can move in a weakly coordinated fashion to give the power of mobility to the entire colony.

            Usually, zooids are hermaphroditic, their sperm maturing before their ova. Some bryozoan species have separate female and male zooids that differ in shape, size, and polypide morphology from feeding zooids. Purely reproductive zooids may lack a gut and have tiny non-ciliated lophophores with reduced numbers of tentacles whose sole function is sperm release. This surprising use of feeding tentacles as vasa deferentia (sperm ducts) appears to be true for all bryozoans. Since the lophophore also serves as a gill for respiration, it is a truly a multi-purpose structure.

Scratchpads developed and conceived by (alphabetical): Ed Baker, Katherine Bouton Alice Heaton Dimitris Koureas, Laurence Livermore, Dave Roberts, Simon Rycroft, Ben Scott, Vince Smith