Kelp Encrusting Bryozoan     Membranipora serrilamella



In late spring, summer and fall you can find large white oval blotches that range in size from an inch (about two inches in the photograph) up to several inches in diameter on the surface of large brown seaweed such as the sugar kelp .  This is not a fungus but a colonial animal called a Moss animal or Bryozoan.  It is one of a number of marine bryozoans that range from encrusting forms to upright, branching growth forms that resemble algae or coral and hydroids (The brown tree-like growth in the middle of the white patch is a hydroid. Corals and hydroids are relatives of sea anemones and sea jellies).  


Experts have recognized two species of kelp encrusting bryozoa occurring in the eastern Pacific Ocean from Alaska to Baja Mexico.  The common one is serrilamella, while a close relative, membranacea, that occurs mainly in the Atlantic Ocean, can also occur here.   Both are  low intertidal to subtidal species that occur on several species of kelp.


Unlike the more simple-bodied hydroids and corals which have only one opening to the body through which both food and waste products pass,  bryozoa have two openings.  If you look closely at the photograph (or if you look closely at a patch in the field) you will see a series of connected boxes that radiate out from a central location.  This is because the colony grows by cloning of the original settler.  Each of the calcareous boxes houses a separate individual called a zooid which is composed of an opening surrounded by tentacles, a gut and an anal opening just outside the ring of tentacles.  The tentacles lack the stinging cells of hydroids and corals, instead use cilia to trap and pass to the mouth small bits of debris and plankton on which it feeds.  It is fed upon by several species of sea slugs and by the kelp crab.


Sexual reproduction in these species occurs in the spring when the sperm from another zooid enters and fertilizes an egg.  The resulting fertilized egg is released to the open water where it grows into a larva that lives in the plankton for a long period and finally settles on kelp blades.  Other species may brood their eggs in specialized structures.