Dive Dry with Dr. Bill

#309: Jack Frost... The Lacy Bryozoan

At certain times of the year the blades of our giant kelp forests look like they received a coating of white frost. Why, it almost reminds me of those cold, late fall days back in Chicago shortly before the snows started falling. After surviving a day when the mercury hit 81 below in Chicago (with the wind chill factor, coldest day in their history), I don't feel the least bit chilly when I see "Jack Frost's" work on the kelp blades... even in my "Holy" (er, hole'y) wetsuit!

Although there are many invertebrates that will attach to kelp blades, the quarter-sized or larger patches that look like frost... or lace if you prefer, ladies... are a species of bryozoan sometimes called the kelp lace bryozoan. Scientifically they are known as Membranipora tuberculata. The colonies are formed by a single layer of individuals or zooids that grow outward by asexual division. It is easy to see the individual animals and the colony structure through a dissecting microscope. If the colony is alive and in salt water, you may even see the feeding tentacles extending out of the hole in each individual calcium carbonate exoskeleton (zoecium). They are grouped together on a circular structure known as a lophophore.

I'm sure all my readers know by now that kelp is one of the fastest growing plants in the world, achieving a rate of 24" a day under the most optimum conditions. Give it plenty of water (salt), nutrients and light, and even someone with a brown thumb like myself could keep it alive (unlike the cacti in my planter box). However, anything that can grow this fast provides a very unstable platform for critters to attach to. It is only after the blades have reached full size, and stopped growing, that invertebrates can attach. And there is a virtual ecosystem of attached life on the old blades of kelp ranging from tiny stationary hydroids and bryozoa, anemones and tube worms to "free range" critters like kelp snails and newly settled baby starfish.

Since individual kelp blades last only a few months, these attached bryozoa have relatively short lives. They must undertake their full life cycle within the brief time a kelp blade shows it growth and later drops off the stipe. Fortunately bryozoa are also fast growing and colonies may cover a kelp blade in less than a month. Members of the genus Membranipora are the only ones in their group of bryozoa to have plankton-feeding larvae known as cyphonautes. These larvae have a triangular shell and are fairly distinctive when observed in plankton tows. These planktonic larvae allow this species to disperse and colonize new habitats... including new kelp blades

When the larvae land on the substrate, they metamorphose like a butterfly caterpillar into an "ancestula," which forms an entire new colony through asexual budding. It has been estimated that extremely thick growths of these colonies may add from one up to 22 pounds per square meter of extra weight to the kelp. Seriously encrusted kelp blades often sink to the bottom where sunlight is too low for them to photosynthesize. Now that is a serious weight problem for both species!

This species is known from Alaska to Baja California. I first encountered a closely related species, Membranipora membranacea, in New Hampshire when I was a young Harvard undergraduate on a marine biology field trip with my mentor, Dr. H. Barraclough Fell. That species is largely resident in the Atlantic Ocean, although some scientists have thought it might be found here as well. The lacy bryozoan is not limited to attaching to giant kelp. It can also be found on other algae as well as much more stable substrates like rocks and snail shells. If you have salt water toilets like I do, there may even be a few in your bowl!

The individual zooids are about 1/50th of an inch long. My eyes aren't what they used to be, so I definitely need a dissecting microscope to view them. Their exoskeletons or zoecia are rectangular and form an almost honeycomb like pattern in the colony. The rim of the zoecium has a thicker calcium carbonate lip. There are tiny spines projecting from the rim inward towards the individual animal, and each corner has a calcified tubercle.

The lacy bryozoa have a very interesting predator I occasionally observe feeding on the colonies. I'm referring to several nudibranchs (shell-less snails) of the scientific genus Corambe. Unfortunately, these snails are not common enough to have a common name. I'm sure their bryozoan prey has come up with one comparable to Godzilla, Terminator or possibly just a capitalized Predator. These nudibranchs are almost translucent and have a lacy pattern on their dorsal surfaces that makes them look just like the bryozoan colonies. It is a good idea to camouflage yourself to look like your prey so a passing fish won't take a bite. Of course the camouflage is not necessary to fool the bryozoa, because they can't flee when getting munched.

© 2008 Dr. Bill Bushing. Watch the "Dive Dry with Dr. Bill" underwater videos on Catalina Cable TV channel 49, 10:00 AM and 5:00 PM weekdays and on Charter Communications Cable channel 33 at 7:30 PM on Tuesdays in the Riverside/Norco area. Please help me climb out of self-imposed poverty... buy my DVD's (see this link). Yes, take Dr. Bill home with you... we'll both be glad you did!

Images taken through a dissecting microscope of the bryozoan colony at increasing magnifications
with the last showing the calcium carbonate tubercles.

This document maintained by Dr. Bill Bushing.
Material and images © 2008 Star Thrower Educational Multimedia