Skeletal morphogenesis and growth check lines in the Antarctic bryozoan Melicerita obliqua

Skeletal morphology and colony growth pattern of the Antarctic bryozoan Melicerita obliqua from the eastern Weddell Sea, on the Weddell Sea shelf, have been investigated in order to test the hypothesis of a relationship between growth banding and the seasonal variability of the high Antarctic environment. Melicerita obliqua is a common endemic species on Antarctic shelves. The colonies are bifoliate-flattened and sabre-shaped, and are anchored with organic rootlets in the sediment. The most significant feature of the skeletal morphology is the segmentation of the colony caused by the formation of annual growth check lines. Formation of check lines occurs as a combination of thickened walls in the proximal part of the nodal autozooid and an oblong, thin-walled distal part. Both are separated by a back-fold of the frontal cryptocyst resulting in a narrow slit on the cryptocyst. Changes in the length of autozooids and segments indicate seasonal and inter-annual variations in the environmental factors. Within a segment, nodal autozooids are longest, whereas autozooids immediately proximal to the next node are shortest. The length of segments varies within a single colony but shows a slight decrease with colony age. Melicerita obliqua displays a slightly decreasing growth rate with increasing age, hinting at a limited life span fixed in the genetic programme. Individual colonies of M. obliqua reveal a maximum age of about 45 years. Seasonal growth patterns in the colony are expressed in segment formation with nodes and internodes, differences in autozooid length, and polypide degeneration/regeneration cycles. Sizes of segments within colonies are used to calculate colony growth rate. Variations in colony growth rate calculated from length increment of segments depict a pattern of longer and shorter segments indicating phytoplankton variability between the years.