Patterns of bioconstruction in the cheilostome bryozoan Schizoporella errata: the influence of hydrodynamics and associated biota

In the region of La Spezia (Ligurian Sea, Italy), the bryozoan Schizoporell a errata forms sizeable buildups, termed 'giant colonies' or 'compound colo nies' by previous investigators. Buildups were encrusting (up to 40 cm wide ) with flat or bumpy surfaces in an exposed site, and erect (more than 20 c m high) branching anastomosed structures in a semi-exposed and in a shelter ed site. The buildup architecture was significantly different in the 3 site s, with higher vertical development and number of branches per dm(2) in the sheltered site. The internal structure of the buildups was also different: densely packed, extensive laminated sheets developed in the sheltered site ; loosely packed, concentric sheets alternating with small cavities filled with sediment and shells of associated organisms were found in the semi-exp osed and exposed sites. Functional groups could be recognised among the bio ta associated with S, errata buildups. The constructional pattern was inter preted as resulting from both interactions with other organisms and hydrody namic conditions. While in the exposed site S, errata showed no vertical gr owth, in the sheltered site soft-bodied erect organisms formed the frame ar ound which S. errata deposited a multilayered structure, which eventually c aused the death and disappearance of the immured organisms. Breakage of bra nch tips by episodic disturbance and repeated binding through a complex arr ay of budding patterns by S. errata resulted in tall buildups which preserv ed no trace of the soft-bodied organisms. Because of the role they played, these may therefore be defined as 'ghost frame-builders'.