Dive Dry with Dr. Bill

#288: The Staghorn Bryozoan

The subject of today's column, the staghorn bryozoan, has me thinking. Dangerous thing for me to do, I know. We have a number of marine critters and algae which are named after land animals... the elk kelp, the sheep crab, the sea mouse, the bat star, and the staghorn bryozoan among many others. How many terrestrial beasties are named after marine critters? I don't remember learning about a barracuda bat, shark-toothed rattlesnake or sea urchin hedgehog in my biology classes. There is the kelp fly, but that name makes sense based on where you find those often annoying insects. I guess it must have something to do with the fact humans first experienced the land before they really explored the sea in any "depth." They related things they saw underwater with critters they had known from their own habitat.

Recently instructor Tim Mitchell asked me what all the yellowish-orange, coral-like critters were in the dive park. At first I didn't know what he was referring to, but later guessed it might be this species. The staghorn bryozoan usually forms small colonies less than two inches across, but this year they are not only far more abundant than usual, they seem to be forming much larger colonies. Perhaps it is the colder water at depth that is bringing more nutrients in, building up the food chains they feed on.

When I learned marine invertebrate zoology back in the Dark Ages, these critters were in a phylum known as Bryozoa. There may be as many as 4,000 species worldwide. Nearly 250 are known from California waters, although most of them are found in deeper water. They have since been divided into two different phyla based on, of all things, whether their anus is located inside their ring of feeding tentacles or outside of it. This species is in the latter group, known as Ectoprocta. Don't worry, there won't be a test on this as I can barely remember all the changes myself!

The Ectoprocts are almost all colonial forms. The tiny individuals, known as zooids, are grouped in colony known as a zoarium. The individual zooids are about 1 mm in size, or about 1/25th of an inch. Because their structures are made of calcium carbonate, and are often fairly ornate, they may be mistaken for tiny corals. However, they are not even closely related. The individual zooid has a hard exoskeleton known as a zoecium. The organism extends its feeding structures through a small hole in the zoecium.

Their feeding structure is known as a lophophore. It bears a circle of tiny feeding tentacles which surround the mouth. Bryozoa extend the lophophore into the water column to capture food including bacteria, particles of organic matter, plant plankton, small animal plankton and other invertebrates. These are digested in a U-shaped gut which ends in the anus, located outside the ring of feeding tentacles.

The southern staghorn bryozoan is found from British Columbia to Costa Rica. A major field guide states they are common off the west coast of Mexico and in the Sea of Cortez. They are always present in our island waters as well, although not usually in the numbers we have been seeing this year.

The coral-like colonies are usually about one inch in diameter, but are known to form masses up to about four inches tall. These larger zoaria have been common this year off Catalina. They show a branching pattern with the exoskeletons of several individual zooids fused together to form each branch. There are open spaces between the branches which allow water currents to enter and the interior zooids to feed. These spaces also provide hiding places for small invertebrates. I often see the feeding arms of very small brittlestars extending out to feed from the protection of the bryozoan colony.

I've never observed their mating since my aging eyes are not able to focus on things that small. However, I've read in the scientific literature that their reproductive patterns are often quite complex, and differ in different groups of bryozoa. They are able to utilize both asexual and sexual reproduction, but do so to meet different goals. Sexual reproduction generally produces larvae which can disperse with the currents to form new colonies and colonize new areas. In some species, there are individuals modified strictly for egg production and brooding of the eggs. Individuals may be of the two different sexes, or they may be sequential hermaphrodites... beginning life as one gender and changing into the other.

In the group the staghorn bryozoan belongs to, fertilized eggs undergo an interesting phenomenon known as polyembryony. That's a big word scientists made up to describe the process whereby a fertilized egg undergoes division to form two or more separate embryos. Sounds like they are using fertility drugs. Each egg develops into a free-swimming larva. It eventually reacts negatively to light near the surface, which drives it toward the bottom where it attaches and begins the growth of the colony.

Now asexual reproduction comes into play. However, to form a large colony of many individuals, the newly settled individual must begin dividing into new ones... and the new ones into more new ones... for the colony to grow. So they fulfill the essential task of increasing the numbers of their species using both reproductive mechanisms. Of course I prefer my "reproduction" the old fashioned way, but my goal is not to produce hundreds of offspring! My son Kevin is just the right number of offspring for me!

© 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 "Munching and Mating in the Macrocystis," "Great White Sharks of Guadalupe," "Calimari Concupiscence: Mating Squid, " "Playful Pinnipeds: California Sea Lions," "Belize It or Not: Western Caribbean Invertebrates, Fish and Turtles," "Gentle Giants: Giant Sea Bass," "Common Fish and Invertebrates of the Sea of Cortez" or the new "Sharks and Rays of Southern California" DVD's. Yes, take Dr. Bill home with you... we'll both be glad you did!

Staghorn bryozoan colonies of different shapes; the underside of a colony showing
branching and a microscopic view of the individual zoaria.

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