Bowerbankia imbricata (Adams 1798)

General description: 

Colonies are often very dense and typically between 25-80 mm in length. They appear transparent, white, cream or yellowish in colour. In the creeping form, colonies closely adhere to the substrate, whereas erect colonies are raised off the substrate. The zooids form feathery or bushy clusters along a thick cylindrical stolon (up to 0.3 mm). The species is commonly found in the lower intertidal and shallow subtidal on algal or hard substrata throughout the NE Atlantic, Mediterranean and east coast USA. 

Diagnostic description: 

This species seems most nearly allied to the Setularia cedrina.Linn Syst Nat. Ed.13 p.1313.n.28 Pallaf. Zooph. p. 139, from which however it differs in the vesicles not surrounding the stem in any regular series and in their shape. Height, from one to three inches. Young shoots closely imbricated to their base, but older ones often naked: the smaller branches, which proceed from a main stem, have the vesicles placed bisariously, but their apex resume the imbricated form (Adams 1798)

Growth: 

Colonies grow through asexual budding

Morphology: 

The colony can be creeping (densa) or erect and may vary between these two forms according to season; secondary erect zooids often develop in the late spring to summer. Zooids are arranged in dense, overlapping and irregular clusters on either one or all sides of the stolon to give the colony its feathery appearance. In the densa form, clusters are discrete and irregularly spaced with six to twenty zooids per cluster. In the erect form or on older parts of the colony there may be up to 50 zooids per cluster and often clusters merge to form a continuous series. The zooids are transparent with small star-shaped black pigment spots. The zooids are cylindrical and tend to be bulbous at the proximal end (closest to the stolon). They are unstalked and bud directly from the stolon, which is itself wider (up to 0.3 mm) than the width of a zooid. Zooid size ranges from 0.8-1.5 mm long and 0.25-0.27 mm wide. Zooids tend to have a square terminal office, but can be rounded when the polypide is regressing.

The stolon has regular septa and numerous secondary branches (dichotomous, cruciform or irregular) which may be raised from the substrate. On occasion the stolon develops basal rhizoids which serve as anchorage structures for the colony.

The lophophore has ten tentacles and an average diameter of 0.246 mm. This distinguishes B. imbricata from other British species within the genus which all have eight tentacles. The retracted lophophore and other muscular structure may be observed through the transparent wall of the zooids.

Associations: 

Frequently grows attached to intertidal and shallow subtidal seaweeds of the species Fucus vesiculosus, Fucus serratus and Ascophyllum nodosum. Common on Corralina officinalis. B imbricata may also grow epizooically on other Bryozoa such as Flustra, Vesicularia and Alcyonidium

Distribution: 

Common throughout the NE Atlantic and Mediterranean. Range extends north to the Barents Sea and it has been reported from the Black Sea.

Habitat: 

Commonly grows on algal substrata, but is also frequently found on the underside of boulders or rocks, or on concrete constructions. B. imbricata is known to range from the intertidal zone down to depth of ~ 55 m. In sheltered areas, large erect colonies may develop. This species is tolerant of fluctuations in salinity and is known to occur, often in abundance, in brackish water.

Reproduction: 

Embryos are deep yellow in colour and are brooded within the tentacle sheath. Embryos have been observed from May to October in the British Isles, Iceland and the USA. Larvae are coronate.