Membranipora membranacea (Linnaeus, 1767)

General description: 

A very common and conspicuous bryozoan throughout Britain and North America’s Pacific coast. Colonies form white or light grey encrusting sheets on algal substrates, with short tubercules or spines protruding from the zooids. Colonies can become very large, covering extensive areas of kelp fronds. Colonies are found from the lower intertidal down to the limit of the kelp forests

M. membranacea is recognised an invasive bryozoan species and can have serious ecological impacts on kelp forests.

Look alikes: 

M.membranacea bears a superficial resemblance to Electra pilosa, which has a similar colony form and colour. M. membranacea can be distinguished by its rectangular zooids, compared to the rounded zooids of E.pilosa. Zooids of E.pilosa also have a characteristic large central spine.

Species of Membranipora are easily confused with those of the related genus Conopeum. Membranipora species may be distinguished by the presence of a twinned ancestrula (the founding zooid), compared to the single ancestrula of Conopeum species. However, the ancestrula region is frequently missing from colonies, making this character insufficient to distinguish species. Conopeum reticulum has thicker calcification than Membranipora species. C. reticulum may also be distinguished from other littoral and sublittoral species in the same family by the presence of paired triangular chambers (kenozooids) at the end of each zooid.


Conopeum and Membranipora species also differ in their ecology. M. membranacea is the only species of its genus to occur in British waters, where it forms extensive colonies, normally on Laminaria. Membranipora tuberculata, which colonises Sargassum, and Membranipora tenuis, a tropical species, are, on rare occasions, washed up on south-western shores of the UK. Both of the British species of Conopeum colonise hard substrata or estuarine plants, but would not be expected to occur on marine algae.

Morphology: 

Colonies form extensive, lacey sheets on algal substrates. Zooids are rectangular with tubercules (spines) protruding from the corners. The lateral walls of the zooids are calcified, but incorporate two vertical uncalcified bands which provide flexibility to colony as the aglal substrate moves in the waves. Large chitonous spines are produced rapidly in response to waterborne cues from the trophically specialised nudibranch, Doridella steinberae.

Size: 

Zooids are typically 0.42 by 0.13mm

Associations: 

M. membranacea is commonly predated on by nudibranchs including Polycera quadrillineata and Doridella steinberae.

Colonies attach to shallow sub-tidal seaweeds, particularly on the fronds of kelp (Laminaria digitata and Laminaria hyperborean in the UK, Ecklonia, Macrocystis, and Saccharina longicruris elsewhere). Smaller colonies also form on Fucus serratus.

Distribution: 

M. membranacea is native to the North Pacific, but occurs throughout the temperate NE Atlantic and Pacific. Populations have established along the coasts of Nova Scotia, New Brunswick, Newfoundland and the Gulf of Maine. M.membranacea is present on all British coasts and is the most commonly occurring bryozoan in Scotland. This species is the only member of its genus in British waters.

Habitat: 

M. membranacea most commonly grows on kelp from the lower shore down to the lower limit of the kelp forests. It may also colonise Fucus species in rock pools. Common on sheltered rocky shores, colonies will grow best in areas of fast flowing water or high tidal influx.

Life cycle: 

Embryos develop from early spring to early summer. Planktonic larvae are long-lived. They feed and grow throughout summer, and in some cases continue to November. Settlement occurs from summer and peaks in early autumn (September-October), after which larvae metamorphose into a twinned ancestrula (the founding zooid). The ancestrula buds asexually to form young colonies which are approximately circular. Growth is perennial and will occur throughout spring and summer. The colony will tend to grow towards the youngest part of algal substrates at the base.

Trophic strategy: 

Like all bryozoans, M. mebranacea is a suspension feeder. It feeds on small phytoplankton using ciliated tentacles of the lophophore. 

Reproduction: 

Sexually-produced embryos develop once a year, in early spring. Breeding continues through the early summer. The larvae of M.membrancea are planktonic cyphonautes larvae which feed and grow up to 0.6 mm in height and 0.8 mm across the base. Larvae are triangular in shape and laterally compressed. They are common in coastal plankton from February to November, and are especially abundant between June and August.

Risk statement: 

The species is recognised as invasive throughout Europe and the NW Atlantic. The presence of M. membranacea colonies on kelp reduces the algae’s survival, and can exacerbate the natural seasonal declines of kelp beds in autumn. Colonised kelp tends to become brittle and may be more prone to breakage during periods of intense wave action.  Defloliation of kelp beds by M. membranacea facilitates the introduction and establishment of opportunistic green algae and hence alters the habitat of ecologically and economically important species such as sea urchins and lobsters.