Skeletal resorption in bryozoans: occurrence, function and recognition.

Abstract

Skeletal resorption - the physiological removal of mineralised parts by an organism - is an important morphogenetic process in bryozoans. Reports of its occurrence and function across the phylum are patchy, however, and have not previously been synthesised. Here we show that resorption occurs routinely across a wide range of bryozoan clades, colony sizes, growth forms, ontogenetic stages, body wall types, skeletal ultrastructures and mineralogies. Beginning in the early Paleozoic, different modes and functions of resorption have evolved convergently among disparate groups, highlighting its utility as a morphogenetic mode in this phylum. Its functions include branch renovation, formation of branch articulations, excavation of reproductive chambers, part-shedding, and creation of access portals for budding beyond previously formed skeletal walls. Bryozoan skeletons can be altered by resorption at microscopic, zooidal and colony-wide scales, typically with a fine degree of control and coordination. We classified resorption patterns in bryozoans according to the morphology and function of the resorption zone (window formation, abscission or excavation), timing within the life of the skeletal element resorbed (primary or secondary), and scale of operation (zooidal or multizooidal). Skeletal resorption is probably greatly underestimated in terms of its utility and role in bryozoan life history, and its prevalence across taxa, especially in fossil forms. It is reported proportionally more frequently in stenolaemates than in gymnolaemates. Some modes of resorption potentially alter or remove the spatial-temporal record of calcification preserved within a skeleton. Consequently, knowledge that resorption has occurred can be relevant for some common applications of skeletal analysis, such as palaeoenvironmental interpretation, or growth and ageing studies. To aid recognition we provide scanning electron microscopy, backscattered electron scanning electron microscopy and transmission electron microscopy examples of skeletal ultrastuctures modified by resorption.