Sometime in April, a friend alerted me to some odd ball-shaped structures attached to a spruce log in Auke Lake. These are bryozoans, a type of minuscule aquatic animals. They constitute their own phylum, quite distinct from all other animals. There are over 5,000 species of bryozoans, exhibiting a great diversity of form and habit. Most of them live in saltwater, especially in the tropics, but some live in freshwater. Almost all of them are strictly colonial, although the size and shape of the colony varies. Those that we observed in Auke Lake make gelatinous balls, with the tiny animals projecting from the surface of the ball.
The word “Bryozoa” translates as “moss animal.” Some early observer probably thought that a flattish colony looked a bit like a moss.
Each little bryozoan creature (called a zooid) is less than a millimeter long, bearing a set of retractable tentacles for filter-feeding. They catch even tinier particles in the water or sweep them in on water currents. Those particles pass through the digestive tract; some species even have a muscular gizzard, for crunching diatoms (unicellular algae that have shells of silica). Waste products are eliminated from an anus that lies outside of the bases of the tentacles. Individual zooids are typically housed in a body wall of chitin or calcified materials or, in some freshwater species a gelatinous layer. There are pores in the body walls and body fluids can pass from one individual to another nearby. The feeding apparatus and digestive tract may periodically degenerate, which gets rid of accumulated waste products (there is no kidney system), sometimes making room for eggs, and are later replaced by regenerated parts.
An Alaska marine bryozoan, showing the contrast of colony shapes. (Photo by Aaron Baldwin)
Many marine species have polymorphic zooids: in addition to the common ones that feed, there are non-feeding ones that are fed by the others as nutrients are passed among individuals. Some of these have sharp little forceps-like beaks, which can be used to pinch and toss off unwanted small creatures that drift in. Others have long “whiskers” for sweeping away sediment and even, in some species with very small colonies, for walking over the substrate (with very small steps).
All freshwater and most marine zooids are hermaphroditic, producing both sperm and eggs, not necessarily at the same time. Thus, there can be fertilization of eggs among colony members, but this is equivalent to self-fertilization because colonies typically form by asexual budding (cloning) of the initial zooid. In most species, sperm swim to a zooid in female phase, fertilize her few eggs internally; the fertilized egg and young larva are brooded either inside her body or in a special external chamber. A larva of freshwater species can contain a few new zooids, which start a new colony when the larva settles someplace. In addition to sexual reproduction and budding, freshwater bryozoans also produce vast numbers dispersal units. This mass of cells, produced asexually, may just fall to the bottom or may float away or stick to passing animals and floating vegetation. When conditions are right, the unit germinates, producing a new zooid and the start of a new colony.
Marine bryozoans form colonies of many shapes. Some are branching, like corals, and some form leaf-like shapes. Many are flat and form crusts on rocks, shells, kelp, and submerged wood. Others sit like tiny bowls on the bottom or bore into a hard, calcareous substrate. Most live in coastal waters but some are recorded from depths of over six thousand meters.
Of course, bryozoans have predators. In marine systems, sea spiders and nudibranchs are important; grazers such as urchins also take some. I have not found much detailed information on predators of freshwater bryozoans, but snails are reported to eat certain ones and raccoons are reported to eat the gelatinous types. Fish and insects are listed as possible predators.
• Mary F. Willson is a retired professor of ecology. “On The Trails” appears every Wednesday in the Juneau Empire.
