SKELETAL STRENGTH OF ENCRUSTING CHEILOSTOME BRYOZOANS

Encrusting cheilostome bryozoans structurally resemble aggregates of small boxes, with both frontal and vertical walls capable of resisting forces generated by water-borne debris or predators. Both the skeletal strength and design of the walls are important in determining the relative ability of the colony to resist damage. Two mechanical tests, puncture and compression, performed on nine species of tropical bryozoans reveal significant differences in skeletal strength both between species and between the outer and inner regions of colonies. Puncture stresses required to break through the frontal walls of zooids range from 0.8 to 291.0 MNm^-2 for edge zooids and from 1.1 to 457.4 MNm^-2 for inner zooids; compressive stresses required to damage the colony range from 4.4 to 16.9 MNm^-2 for edge regions and 6.5 to 27.2 MNm^-2 for inner regions. Ecological implications for these differences in skeletal strength are discussed with particular reference to resisting predation. From the mechanical test results, the material properties of shear strength (2.6-90.5 MNm^-2) and compressive strength (8.2-110.0 MNm^-2) are estimated for the frontal and vertical walls, respectively. Bryozoan wall material appears to be comparable in strength to such biological ceramics as coral, echinoid spine, bivalve shell, and vertebrate bone, but lower in strength than gastropod shell.