Monday, November 26, 2012
Bryozoans are tiny colonial invertebrates that feed with a tentacled structure called a lophophore that filters food particles out of sea water. Bryozoan colonies grow either laterally as broad encrusting mats or vertically as upright, branching bushes. In the Gulf of Maine, encrusting species include the MIS invasive species Membranipora membranacea (February, 2012) and the native Electra pilosa (March, 2012). Several upright, branching Byozoan species can also be found, including several beige native species of Bugula (e.g., Bugula simplex and Bugula turrita) and the wine-red Pacific coast species Bugula neritina, which is commonly found in floating dock communities (October, 2012) growing among algae and other invertebrates.
The branching colony is formed by the upright growth of feeding individuals called zooids that are enclosed in a calcareous box called the zooecium. Each branch of the colony is made up of a double row of zooecia and all the zooecia face in the same direction. The two rows in a branch are staggered, so that the top of one zooecium comes to about the middle of the one next to it. A single zooecium has a flexible membrane, and it bears no spines, although the upper, outer corner of the zooecium is pointed. Other species of Bugula bear distinctive, bird-head shaped structures with a jaw-like element that opens and closes, that are called avicularia. However, Bugula neritina has none.
Underwater Photograph of a Colony of Bugula neritina
Bugula neritina with purplish-red color showing branching pattern of the colony. The branches have serrated edges from alternating zooids and bifurcate at regular intervals resulting in a uniformly branching structure.
Microscopic View of Zooid Lophophores and White Ovicells
Microscopic view of Bugula neritina showing translucent red zooids and numerous white ovicells. Zooids and ovicells face toward the front. The flaring, wine-glass-shaped lophophore has 23 tentacles that are arranged around the mouth. Each zooid produces a single embryo at a time, which is brooded in the ovicell. The ovicells are conspicuous and often abundant, appearing as numerous small white beads concentrated in the mature parts of the colony.
Diagram of a Generalized Branching Bryozoan showing Anatomy of the Zooid
Diagram of an upright bryozoan showing the relationship between the anatomy of the zooid and the exoskeletal branches. The lophophore tentacles are covered with cilia arranged along the inner faces and sides. When feeding, the lophophore is fully extended. Currents produced by the beating of the cilia carry food particles (primarily microscopic plankton) down along the tentacles to the mouth.
Diagram of Feeding (left) and Retracted (right) Upright Bryozoan
Mechanism of feeding and retraction by upright bryozoans. When feeding, the lophophore is fully extended and moves around in the water. The body remains inside the enclosure. When disturbed, the lophophore rapidly closes into a tube and the retractor muscles attached to the base of the lophophore pull the zooid into its enclosure.
Diagram of the Life Cycle of an Upright Bryozoan
The life cycle of an upright bryozoan. Bugula neritina zooids are hermaphroditic. Zooids release eggs around the middle of their lifespan but don't release sperm until near the end, thus preventing self-fertilization. When released from the ovicell, the non-feeding larvae settle onto hard surfaces within a few hours and metamorphose into the adult form. The initial upright feeding zooid, called the ancestrula, buds off other feeding zooids (autozooids), which in turn bud off others, enlarging the colony. The base of the colony forms a tubular holdfast with specialized non-feeding zooids (heterozooids) that attach to the substrate and also generate new branches.
Branch of a Young Colony of Bugula neritina
A single branch of Bugula neritina like the one shown in the life cycle diagram above. A few ovicells can be seen on branches on the right side of the colony. A short, second branch is growing from the holdfast.
Diagram of the Branching Patterns of Bugula species
Diagram of three different biserial branching patterns found in Bugula species as seen from the basal (rear) side. Left, Branching with no transitional zone. Middle, branching with a single transitional row of 4 zooids. Right, branching with 2 transitional rows of 4 zooids before returning to biserial growth.
Scanning Electron Micrograph of a Branch of Bugula neritina
SEM view of the exoskeleton of a branch seen from the front side. With the soft tissues and frontal membranes removed, the interior spaces of the enclosures can be viewed. Bugula neritina lacks spines but has a pointed outer corner that is well-illustrated by the zooecia on the left branch.
Photograph of Bugula neritina Collected in Provincetown
Two wine-red color variants of Bugula neritina collected from MacMillan Wharf in September, 2012. Most of the colonies on the docks were 2-3 cm in height indicating that the larvae had settle during a similar period of time earlier in the summer.
Stereomicroscopic Image of Feeding Zooids with Lophophores Extended
Stereomicroscopic view of several terminal zooids from a colony collected in Provincetown. The zooids have wine-red lophophores. The blue-orange lophophore is a double-exposure image of a zooid in motion. Individual zooecia are clearly seen alternating along the branches. Stereozoom 4.0 x 10x objective.
Photograph of Beige Bryozoan Species Collected in Provincetown
Beige bryozoan species collected from MacMillan Wharf in September, 2012. The beige colonies were approximately the same size as the red colonies. The colony was not identified down to species. The native Bugula simplex is a common bryozoan and is fan shaped but has thicker, triserially growing branches. Bugula turrita grows in spiral whorls which are clearly evident when colonies are large.
Winston, JE, and RM Woolacott. Redescription and revision of some red-pigmented Bugula species. Bull. Mus. Comp. Zool. 159: 179-212, 2008.
Ryland, JS, JDD Bishop, H De Blauwe, A El Nagar, D Minchin, CA Wood, ALE Yunnie. Alien species of Bugula (Bryozoa) along the Atlantic coasts of Europe. Aquatic Invasions 6:17-31, 2011.
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Posted by Thomas Ermak