Section 2: Body Plan and Functional Morphology

Colony Structure and Zooid Morphology

Bryozoans are unique among colonial invertebrates in that their colonies consist of modular, physiologically connected individuals called zooids. Each zooid is an individual unit, yet it functions as part of a larger, cooperative system. Bryozoan colonies grow by budding, expanding in size and complexity over time. The shape and structure of a colony depends on the species, environmental conditions, and whether the colony is encrusting, arborescent, erect, or gelatinous.

Encrusting colonies grow as thin, sheet-like layers over surfaces such as rocks, shells, or seaweeds. These colonies are common in marine environments and are particularly successful at colonizing hard substrates.
Arborescent (tree-like) colonies have branching, upright growth that extends into the water column, increasing exposure to food particles. These colonies are often found attached to submerged structures such as docks, reefs, or ship hulls.
Erect colonies consist of rigid, upright forms, often with a calcified or chitinous exoskeleton that provides additional support in turbulent waters.
Gelatinous colonies are soft-bodied and lack a rigid exoskeleton. Instead of calcified structures, they are composed of a flexible, proteinaceous matrix that allows for greater movement and flexibility. Gelatinous bryozoans are primarily found in freshwater species, where a lack of wave action reduces the need for structural rigidity.

The Cystid: The Basic Unit of the Bryozoan Colony

Each individual within a bryozoan colony, called a zooid, is composed of two major parts:

The cystid (kystis, "bladder") – The outer exoskeletal chamber, which provides structural support and protection.
The polypide – The internal living tissue, which includes the lophophore, digestive system, and nervous system.

The cystid is the permanent part of the zooid, remaining intact even after the polypide degenerates and is replaced. The exoskeleton of the cystid can be chitinous, gelatinous, or calcareous, depending on the species. In species with rigid exoskeletons, the cystid forms a protective box or tube, from which the polypide extends and retracts during feeding.

Each zooid in a colony is housed within its own cystid but remains physiologically connected to neighboring zooids. This interconnection allows for resource sharing, coordinated defense, and collective behaviorwithin the colony.

Frontal Membrane and Zooid Opening

The frontal membrane is a flexible, cuticle-like structure covering part of the cystid’s opening. This membrane allows the polypide to extend and retract, providing flexibility while maintaining the structural integrity of the zooid.

In some species, the frontal membrane is heavily calcified, forming a protective lid that only opens when the polypide is extended.
In more flexible species, the membrane remains soft and transparent, allowing for greater movement of the lophophore.

Operculum and Protective Mechanisms

Each feeding zooid has an operculum (operculum, "little lid"), a chitinous or calcareous plate that covers the zooidal orifice when the lophophore is retracted. The operculum acts as a protective barrier, preventing predators and debris from entering the zooid.

In some species, the operculum is modified into a trapdoor-like structure, which actively snaps shut when disturbed. This adaptation is particularly useful in high-energy environments, where water currents could otherwise damage the delicate feeding structures.

Some specialized opercula have evolved defensive functions:

In certain species, the operculum is modified into a spine-like projection, deterring predators such as grazing gastropods.
In others, the operculum functions as a trigger mechanism, allowing for rapid closure in response to stimuli.

Musculature and Movement

Although bryozoans are sessile as adults, they possess muscular structures that allow limited movement of their feeding apparatus. The main muscles involved include:

Polypide Retractor Muscle – Contracts to pull the polypide back into the cystid, withdrawing the lophophore for protection.
Parietal Muscles – These muscles work opposite to the polypide retractor muscle; when they contract, they increase internal pressure, which pushes the polypide outward, extending the lophophore for feeding.
Sphincter Muscles – Close the operculum when the zooid retracts.

This muscular system ensures that bryozoans can quickly extend their lophophores when feeding conditions are favorable and retract them when threatened.

Lophophore Function and Feeding Mechanism

The lophophore is the defining feeding structure of bryozoans. This ciliated, tentacle-bearing organextends from each feeding zooid and functions as a highly efficient suspension-feeding system. The lophophore varies in shape:

Circular or horseshoe-shaped in marine bryozoans.
U-shaped in freshwater bryozoans, where it is adapted to calm water conditions.

Water Flow and Particle Capture

The lophophore’s effectiveness is due to its three types of cilia that create a directional water current:

Lateral Cilia – Generate a current that draws water into the lophophore.
Frontal Cilia – Move suspended food particles toward the zooid’s mouth.
Rejection Cilia – Expel unwanted debris or non-nutritious particles.

This active filtration system allows bryozoans to feed efficiently on phytoplankton, bacteria, and suspended organic material. The ability to reject non-nutritious particles prevents clogging and maximizes feeding efficiency.

Nutrient Transport and Intracolonial Communication

Because zooids in a colony are physiologically connected, they share resources through a network of funicular cords. These cords transport:

Nutrients from feeding zooids to non-feeding zooids (such as reproductive zooids).
Chemical signals that coordinate colony-wide responses to stress.

This system ensures that even non-feeding zooids contribute to the colony’s survival, reinforcing the cooperative nature of bryozoan life.

Types of Zooids and Their Functions

Bryozoan colonies are made up of different types of zooids, each specialized for a specific function. This polymorphism allows colonies to efficiently divide labor.

1. Autozooids (Feeding Zooids)

These are the primary zooids responsible for feeding.
Equipped with a lophophore, which extends from the cystid to filter food from the water.
Autozooids supply nutrients to the rest of the colony through intracellular connections.

2. Heterozooids (Non-Feeding Zooids)

Some zooids lack a lophophore and do not feed, instead serving supportive or defensive roles in the colony.

Avicularia ("Little Birds") – Small, beak-like defensive zooids that prevent fouling organisms from settling on the colony.
Vibracula ("Whip Zooids") – Long, flexible zooids that sweep debris away, keeping the colony clean.
Kenozooids – Structural zooids that function like a colony’s scaffolding, helping to reinforce the structure of an arborescent or erect bryozoan colony.

By dividing labor among different types of zooids, bryozoan colonies can function efficiently and competitively in their environments.

Brown Bodies: Internal Waste Management

Bryozoans lack specialized excretory organs, meaning that nitrogenous waste must be stored and managed internally. To achieve this, bryozoans periodically form brown bodies, which are compacted remnants of old, degraded polypides.

Formation of Brown Bodies

As a zooid ages, its internal organs degenerate and are compressed into a small, dark mass, forming the brown body.
The brown body remains inside the cystid while a new polypide regenerates, allowing the zooid to function again.
In some cases, multiple brown bodies can accumulate inside a zooid before it eventually dies.

Ecological and Physiological Significance

The formation of brown bodies serves several functions:

Prevents toxic waste buildup in the absence of dedicated excretory structures.
May reduce microbial infection by isolating decayed tissue.
Contributes to overall colony maintenance, ensuring that non-functional zooids do not pose a threat to the health of the colony.

In some species, the accumulation of brown bodies may contribute to the eventual senescence of a zooid, leading to its replacement by newly budded zooids. This process highlights the dynamic nature of bryozoan colonies, which constantly renew themselves to maintain efficiency and function.