Moss Animals

​If you have been snorkelling around your shoreline, you may have noticed a strange mass attached to dock structures or fallen tree branches. That mass is actually a colony of animals called bryozoans, or moss animals for their moss-like appearance. Each individual in the colony is called a zooid. Colonies appear as slimy gelatinous masses, 30 cm or less in diameter, with rosettes on the surface consisting of 12-18 zooids. As filter feeders, they remove algae, zooplankton and bacteria like E. coli from the water column. Bryozoans are fed upon by snails, worms, crayfish and insect larvae, and are thus an important component of the benthic food web.

Bryozoans can reproduce sexually or asexually. Asexual reproduction is most common, and is used to expand the colony during the summer, to over winter successfully, or when colonies are stressed. Colonies release statoblasts, which are highly tolerant of adverse conditions, and can form colonies after remaining dormant in the sediment for decades. Once favourable conditions arise, and a statoblast reaches a hard substrate like a concrete wall, fallen tree branch or rock, it becomes attached and develops into a colony.

Although statoblasts are highly tolerant of pollution, developed bryozoan colonies are not, and their presence indicates good water quality with at least 50% oxygen saturation. There are several native species of bryozoans, as well as an exotic species that can outcompete zebra mussels for attachment sites.