THE BRYOZOA

Almost all bryozoans are colonial, composed of anywhere from a few to millions of individuals. Each individual, or zooid, is enclosed in a sheath of tissue that in some species secretes a rigid skeleton of calcium carbonate.

opening closed by a doorlike operculum , visible on some of the zooids in the picture  below.

 

www.ucmp.berkeley.edu/bryozoa/bryozoamm.html

 

Each zooid is less than a millimeter long and has a ring of ciliated tentacles centered on the mouth called a lophophore ,

www.microscopy-uk.org.uk/micropolitan/fresh/other/bryozoa.jpg

that protrudes to capture small food particles. The lophophore can be retracted very rapidly by specialized retractor muscles.

The mouth opens into a U-shaped gut; the anus is located just outside the lophophore. From this arrangement comes the alternative name for the Bryozoa, the Ectoprocta (Greek, "anus outside"). The body also contains a coelom and gonads; there is a small central ganglion, or "brain," but no specialized excretory or respiratory systems.  Zooids are usually connected to each other by thin strands of tissue.

http://library.thinkquest.org/26153/marine/sketch/772.jpg

In most bryozoans, several different types of zooids coexist in a colony, each one specialized for a particular function. Feeding zooids like the ones described above are known as autozooids , whereas specialized non-feeding zooids called heterozooids. Some heterozooids form a beak-like, snapping structure that deters small predators while others bear long setae, or bristles, and are thought to function in cleaning the bryozoan colony. These zooids that make up the colony are all specialized for certain functions and are connected to each other.

Bryozoan colonies have a variety of forms.

Encrusting bryozoans form flat sheets that spread out over rocks, shells, and other substrates. Forms that grow upwards into the water column may be massive (solid), foliaceous (sheetlike, with zooids on both sides), dendroid (branchlike or treelike), or fenestrate (many branches joining and rejoining to form a netlike or "windowed" shape). Erect forms were much more common in the Paleozoic than they are now; the majority of today's bryozoan species are encrusting. 

A Brief Geobiological History of SE Florida

 

Encrusting Bryozoa can be found on reefs and also may form colonies around the bases of seagrass. About 100,00 years ago we had a shallow water marine area covering most of Miami-Dade County. A shoal composed of oolite (ovoid sand grains- see photo below) was deposited in the southeastern part of the county. It now forms the high ground that is (or was) where Dade County pines grew prior to tracts of houses that are now on it. This shoal deposit of oolite became cemented when it was exposed to air 18,000 years ago and is referred to as the oolitic part or “facies” of the Miami Limestone. The other part of the Miami Limestone formed behind this barrier and was a lagoon-like environment with seagrass, similar to what Biscayne Bay looked like before the Age of Condominiums. This lagoon covered western Miami-Dade County and extended southwest to the end of the Florida peninsula, Florida Bay and the southern Florida Keys. This region includes FIU’s University Park campus. Turtle grass with prominent bryozoan colonies grew here. The bryozoans were so common that the Miami Limestone west of the oolite ridge is referred to as the bryozoan portion or facies of that limestone. Both facies (oolite and bryozoan) were formed at the same time.

Oolite sand that becomes cemented together becomes oolitic limestone.

 

As mentioned above, the oolite was deposited into and around the southern keys (the Keys south of Big Pine Key are all oolitic). As sea level rose, currents pushed sediments into natural depressions and built bars of sediment (oolite, bivalve shells calcareous algae, etc.) that became solidified under water. The Marquesas Keys, west of Key West, are largely submerged and form the base of the reefs there. However, the reef substrate itself was derived from sediment transport, not from corals or other organisms that grew in place. Is this reef a BIOHERM?

At the same time that the Miami Limestone was forming, a coral reef was growing in the northern Keys. In fact, the northern Keys are composed of those reefs. Like the Miami Limestone, the reefs were exposed 18,000 years ago when sea level dropped by more than 100 meters. That reef deposit is called the Key Largo Limestone and it is a 1,000,000 year old reef composed of corals, rather than by bryozoans or oolite.

A coral fossil composing the Key Largo Limestone is 100,000 years old