Membranipora membranacea

Membranipora membranacea is a very widely distributed species of marine bryozoan known from the Atlantic and Pacific Oceans, usually in temperate zone environments. This bryozoan is a colonial organism characterized by a thin, mat-like encrustation, white to gray in color. It may be known colloquially as the coffin box,<ref name="FOC">{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> sea-mat or lacy crust bryozoan and is often abundantly found encrusting seaweeds, particularly kelps.<ref name = "Coasts & Estuaries">Barnes, R.D. (1982). Coasts and Estuaries pp 114-115. Hodder & Staughton, London.</ref>

Distribution

Northeast Atlantic including the Baltic Sea, English Channel, Mediterranean Sea and North Sea. Also native to the North Pacific coastline of North America from Alaska to California.<ref name = "Marine Life">De Haas, W. and F. Knorr (1966). Marine Life pp 212-213. Burke, London.</ref><ref name = "Underwater California">North, W.J. (1976). Underwater California pp 161. University of California Press. Template:ISBN</ref> The species was first recorded on the Atlantic coastline of the U.S. in 1987 in the Gulf of Maine.<ref name="Marine Ecology Progress Series">Lambert, W.J., P.S. Levin and J. Berman (1992). Changes in the structure of a New England (USA) kelp bed: the effects of an introduced species? Marine Ecology Progress Series 88:303-307.</ref> In Canada's Atlantic coast, it was first observed in Nova Scotia during the early 1990s, and had reached Newfoundland and Labrador by 2002.<ref name="FOC"/> It now commonly occurs along the Northwest Atlantic from Long Island Sound to northern Newfoundland, including coastal Quebec, New Brunswick, and Prince Edward Island.<ref name="FOC"/>

Morphology and physiology

Membranipora membranacea colonies consist of individual organisms called zooids, each with a chitinous exoskeleton which is secreted by the epidermis.<ref name = "Invertebrate Zoology">Barnes, R.D. (1974). Invertebrate Zoology, 3rd ed. Saunders, Philadelphia, PA:695-712.</ref> This exoskeleton, hardened with calcium carbonate, is known as the zooecium, which not only serves to protect the internal structures of the organism, but also keeps the individual permanently attached to the substrate and neighboring zooids.<ref name = "Biology of Invertebrata">Wilmoth, J.H. (1967). Biology of invertebrata. Prentice-Hall, Englewood Cliffs, NJ:381-384.</ref> Zooids within a colony can communicate via pores in their interconnecting walls, through which coelomic fluid can be exchanged.<ref name="Invertebrate Zoology"/>

The internal, living portion of the zooid is known as the polypide, whose walls are formed by the outer epidermis and inner peritoneum.<ref name="Biology of Invertebrata"/> The lophophore, a ring of ciliated tentacles, protrudes from the polypide to feed.<ref name = "Oikos">De Burgh, M.E. and P.V. Fankboner (1978). A nutritional association between the bull kelp Nereocystis luetkeana and its epizooic bryozoan Membranipora membranacea. Oikos 31(1):69-72.</ref> When not feeding, the lophophore retracts into the polypide through the tentacular sheath. The lophophore is controlled by the zooid's nervous system, which consists of a ganglion at the lophophore base. This ganglion is responsible for motor and sensory impulses to and from the lophophore, as well as the epithelium and digestive tract. The lophophore retractor is the muscle which controls the movement of the lophophore.<ref name="Biology of Invertebrata"/>

This species does not have the ovicells or avicularium seen in most other members of this phylum.Template:Citation needed

Natural history

Life cycle

Membranipora membranacea begins its life cycle as a plankton-feeding larva, triangular in shape.<ref name="Invertebrate Zoology"/> After several weeks, the larva attach to a substrate and undergo metamorphosis.<ref name = "Oecologia">Harvell, C.D., H. Caswell and P. Simpson (1990). Density effects in a colonial monoculture: experimental studies with a marine bryozoan (Membranipora membranacea L.). Oecologia 82(2):227-237.</ref> The larvae typically settle on their preferred substrates in May, and then the colony undergoes growth, stasis and reproduction, shrinkage, and senescence around September, except in regions where temperature allows them to persist further into the winter.<ref name="Oecologia"/> The presence of conspecifics may cause a colony to stop growing and begin stasis and reproduction early.<ref name="Oecologia"/> The presence of predators also reduces growth of a colony.

Reproduction

Colonies of M. membranacea are protandrous sequential hermaphrodites, where colonies transition from male to female reproductive stages, allowing fertilization to occur between colonies or within colonies.<ref name="Oecologia"/> Fertilization takes places in the coelomic fluid of female colonies, and the eggs are released through an opening in the lophophore known as the coelomophore.<ref name = "Invertebrate Zoology" /> Reproduction or growth of the colony can also take place by budding in a radial pattern from the first established zooid, the ancestrula.<ref name="Invertebrate Zoology"/>

Ecology

Habitat

M. membranacea prefers shallow marine habitats between the mid intertidal to the shallow sublittoral. It may also be found in brackish water.<ref name = "Coasts & Estuaries"/> It is typically found attached in colonies to seaweed, shells, or artificial substrates.<ref name="Biology of Invertebrata"/>

Feeding

M. membranacea can eat food particles such as bacteria, flagellates, diatoms, and other small, planktonic organisms by extracting them from the water with their lophophore.<ref name="Oikos"/> They can also supplement their diet with dissolved organic nutrients through the absorptive epidermis.<ref name="Oikos"/>

Predation

Nudibranchs or sea slugs are the primary predators of M. membranacea. To defend themselves against these predators, the M. membranacea produce chitinous spines which protrude from the corners of the zooid. These spines make it difficult for the nudibranchs to access the polypide of the zooid. However, the energy and resources needed to produce the spines may result in decreased growth and reproduction of the colony.<ref name = "J. Exp. Mar. Bio. Ecol">Yoshioka, P.M. (1982). Predator induced polymorphism in the bryozoan Membranipora membranacea (L.). Journal of Experimental Marine Biology and Ecology 61(3):233-242.</ref>

Ecological significance

File:Membranipora membranacea 110439662.jpg

M. membranacea on a kelp blade

M. membranacea has become an invasive species in many places, and is believed to have a potentially negative impact on marine ecosystems by limiting the ability of seaweeds to reproduce, specifically by interfering with spore release from the kelp blade. The colonies of this bryozoan are also known to interrupt nutrient uptake by seaweed.<ref>Saier, B. and A. S. Chapman (2004). Crusts of the alien bryozoan Membranipora membranacea can negatively impact spore output from native kelps (Laminaria longicruris). Botanica Marina 47(4): 265-271.</ref>

M. membranacea also decreases density and size of kelp plants within kelp beds<ref name="Marine Ecology Progress Series"/> by increasing tissue loss and blade breakage.<ref name = "Hydrobiologia">Hepburn, C.D., C.L. Hurd and R.D. Frew (2006). Colony structure and seasonal differences in light and nitrogen modify the impact of sessile epifauna on the giant kelp Macrocystis pyrifera (L.) C Agardh. Hydrobiologia 560:373-384.</ref> Additionally, M. membranacea also affects photosynthetic processes in kelp, since their encrustations may result in reduced concentrations of the primary and accessory pigments in the kelp blade tissue.<ref name="Hydrobiologia"/>