Kelp Lace (Membranipora membranacea)

KINGDOM Animalia
PHYLUM Ectoprocta
CLASS Gymnolaemata
ORDER Cheilostomidae
FAMILY Membraniporidae


Bryozoans are also called “ectoprocts,” and the name of their phylum is Ectoprocta. Ectoprocta means “outside anus” and refers to the anus being separate from the mouth, which is surrounded by the lophophore, an organ unique to this and two other small phyla considered related to one another by the presence of this structure.

Ectoprocts form large colonies, all connected to one another, and they may be upright and branched, globs of jelly, or forming a crust on a substrate. This species forms a whitish calcareous (calcium carbonate) crust on kelp and other seaweeds (marine algae). The crust is very fine and lacy, so the colonies remain flexible on the kelp blades swaying in the current.

Each discrete animal within the crust is a zooid, somewhat box-shaped with an opening through which the lophophore projects to gather in food particles. The box around it is called a zooecium, in case you are curious. By being in colonies and growing within the protective crust, the individual animals are much better protected from environmental disturbances than if they merely attached singly to the substrate.

The animals in the colony are intimately linked. They share a primitive nervous system, and nutrients from food taken in by any zooid are distributed throughout.

Like most other sessile animals, ectoprocts feed on plankton that they harvest with the tentacles around the lophophore. The colony is arranged so that water currents sweeping across them bring food to all the individuals.

The colonies have predators, particularly nudibranchs that nip off the lophophores, and when attacked by these predators, some of the individuals of the colony produce chitinous spines that deter further predation. Producing the spines slows the growth rate of the colony, so they might be considered a last resort.

Ectoprocts reproduce asexually by fission (dividing in two), much like more primitive animals, but they also may form new animals by fragmentation. They also may go into a resting stage in a capsule, in which they can drift around like the spore of a fungus, then “come to life” again and reattach.

They reproduce sexually as well, as there are both male and female zooids in the colony. Specialized tentacles on the male zooids shed sperm into the water, where it drifts to the females and fertilizes them. The eggs are then released into the current. The transparent, triangular larval stage, or cyphonautes, is less than one millimeter in length. It drifts in the currents and eventually settles on a kelp blade in a strong current, metamorphosing into the zooid in a few days.

Where abundant, this species has become invasive and is considered a potential pest by covering the fronds of algae and limiting their reproduction.