Right through a moss animals colony

Bryozoans, commonly known as moss animals, are a group of primitive animals that almost always live in colonies. There are about 5000 species, which occur in both fresh and salt water, at sites that are characteristic for them, such as ponds, lakes, ditches, streams and rivers, as well as water cellars of water companies. The colonies they form resemble plants. They are flat mats or branch-shaped colonies that are very variable in shape, and can grow to about 30 cm high. They need a hard substrate such as aquatic plants, reed stems, sticks, stones, cans, old boots and the like. Stones and especially wood seem more popular than glass and metal. They cling to any surface that provides a grip.

Here we see a cross section of a Plumatella colony that has formed around the stem of an aquatic plant. The colony is compact and zooids contact each other with their lateral walls. The large number and irregular arrangement of zooids means that almost every part of the animal is present in one place or another in the cross section.

The overview

In the center of the image, the cross section of the stem of an aquatic plant is shaded light green. The image of detail A shows some starch grains stored in the plant cells. Around the plant stem there are numerous zooids. Phylactolaemates have monomorphic colonies, i.e. their colonies are composed of similar zooids. Each zooid includes a cystid (immovable part) and a polypide (movable soft tissues: a lophophore, tentacle sheath, gut and associated muscles, and nervous system) A cystid consists of an endocyst composed of living tissues: epidermis, extracellular matrix, muscles and coelomic lining and ectocyst.

Labels on the images of details A, B, C, D and E:

• ca: caecum
• cl: coelomic lining
• dt: distal tentacles
• ec: ectocyst
• en: endocyst
• ecm+m: extracellular matrix plus muscles
• ep: epidermis
• la: distal parts of the lophophoric arms
• mbc: main body cavity
• os: oral surface of the lophophore (close to the mouth)
• ph/oe: pharynx (oesophagus?)
• py: pylorus
• re: rectum
• rm: retractor muscles
• sg: starch grains
• te: tentacles
• ts: tentacle sheath.

Bryozoan polypides have a U-shaped gut which includes pharynx, stomach (divided into cardia, caecum, pylorus) and rectum. On the images of details B and C we see the rectum full of half-digested particles. The mouth is located within a lophophore, the anus opens outside the lophophore and is located at the tentacle sheath. The tentacle sheath encloses the lophophore when polypide is retracted and everts when the polypide protrudes. The polypide retracts due the contraction of strong retractor muscles. The polypide protrudes due the contraction of the muscles within the endocyst.

In the image of detail E we see bacteria or algae and the remainders of a Desmid, Cosmarium most probably.

Bibliography

"Microscopic Anatomy of Invertebrates", Volume 13: Lophophorates, Entoprocta and Cycliophora, pages 45-206, Chapter 3 Bryzoa. Copyright 1997 Wiley-Liss, Inc.

With thanks to

• Dr. Natalia Shunatova, Department of Invertebrate Zoology, St. Petersburg State University, St. Petersburg,Russia.
• Yuta Tamberg PhD, Department of Marine Science, University of Otago, New Zealand.
• Willem Kolvoort, underwater photography: https://www.kolvoortonderwaterfoto.nl/