Quantifying Growth Rates Using Growth Checks in Colonial Organisms: A Case Study on Erect Bryozoans From the Southern Ocean

As environmental change accelerates, marine invertebrate growth rates may change in response to a variable environment. Some colonial invertebrates manifest annual growth checks in their skeletons, as somatic growth slows or stops in winter. Here we describe a morphometric measurement methodology to quantify growth rates using annual growth checks visible on X-rays and SEM images. We tested our method on three rigid erect perennial cheilostome bryozoan colonies belonging to the marine species Cellarinella nutti, Melicerita chathamensis, and M. obliqua. They were collected from 150 to 500 m water depth and from 48° S to 78° S latitude. The growth checks in all three species were identified visually in X-ray images. We trialed the use of morphometric data from composite SEM images, including measures of branch width, number of autozooids across branch, autozooid length, and autozooid width, but they did not significantly correlate with growth checks from the X-ray images in all species. In the two polar species, the winter growth checks were associated with slightly longer autozooids. Assuming that growth checks were annual, growth rates were calculated to be from 2.81 to 3.54 mm/year, decreasing over time. Our hypothesis is that a combination of X-ray imaging and morphometric data collection on composite SEM images shows promise for application to other bryozoan species and marine invertebrates from temperate/polar regions.