Membranipora membranacea is a very widely distributed species of marine bryozoan known from the Atlantic and Pacific Oceans, usually in temperate zone environments.

It may be known colloquially as the coffin box,[1] sea-mat or lacy crust bryozoan and is often abundantly found encrusting seaweeds, particularly kelps.

[1] Membranipora membranacea colonies consist of individual organisms called zooids, each with a chitinous exoskeleton which is secreted by the epidermis.

[6] 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.

[9] 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.

[9] 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.

[7] 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.

[10] 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.