Bryozoan Genus: Membranipora Species: tuberculata ITIS: 155828 Primary Common Name: Kelp encrusting bryozoan Common names (s): Lacy crust bryozoan, Kelp lace bryozoan, Encrusting bryozoan, sea mat Similar Species: Membranipora villosa, Membranipora membranacea General grouping: (pull down on site) Zooplankton Geographic Range Range description: Kelp encrusting bryozoans are prevalent in warm, temperate, and tropical waters of the Atlantic, Pacific, and Indian Oceans, wherever floating Sargassum or Fucus is found. Brief range description: (this should always be included in above description) Atlantic, Pacific, and Indian Oceans Habitats (pull down on site) Habitat notes: Commonly found encrusted on floating brown algae, especially the kelps Macrocystis and Cystaseira, and also on smaller plants in shore, especially the red algae Gelidium, occasionally on shells or wood, low inter-tidal zone to shallow subtidal depths. M. tuberculata is found on gulfweed and rockweed in the Atlantic, and on kelps and other seaweeds in the Pacific. Abundance Relative abundance: M. tuberculata species are among the best-known marine bryozoans, and are commonly found in oceans around the world. Heavy encrusting of kelp fronds with the bryozoan Membranipora is common in kelp forests. Species Description: The kelp encrusting bryozoan, Membranipora tuberculata, is a colony form-ing a white crust, up to several centimeters in diameter. It consists of a single layer of zooids, which have a fine reticulate honeycomb appearance. Individual colonies are 0.5-0.8 mm tall, over 76 mm wide, rectangular, and covered with a lightly calcified mem-brane, with a heavily calcified rim, bearing calcified tubercles at the distal corners, as well as tiny spines projecting from the side of the rim, to the center of an individual. Distinctive features: The membraniporids are the only members of the order Cheilostomata known to have a planktotrophic cyphonautes larva, a free swimming, plankton feeding stage with a triangular bivalve shell. This settles and meta-morphoses into an ancestrula (often double or twinned), which buds to form a flat, en-crusting colony. Key Feature M. tuberculata is known for encrusting the kelp Macrocystis; under favorable conditions colonies grow rapidly and may coalesce to cover entire kelp blades in 3-4 weeks. Overgrown blades often bear 0.5-1 kg of bryozoans per square meter of blade surface, and occasionally perhaps ten times that density. Size: Colony Height: 0.5-0.8 mm Width: more than 76 mm (3 in.) Natural History General natural history: Bryozoans are an ancient group, with a fossil record extending from the early Pleistocene. In the phylum Bryozoa there are about 4,000 species, which makes it one of the major phyla. Over 250 species have been recorded just in California. Forms of the colony vary among different species, which range from flat encrusted sheets, to folded leaf-like bushes. Bryozoans use microscopic mobile pincers called “avicularia” to pluck off any settlers that try to land on them. If pieces of a bryozoan colony break off, the pieces can continue to grow and form new colonies. Colonies increase in size asexually by budding. New colonies are created by hermaphroditic sexual reproduction. The twin shelled larvae of Membranipora, known as cyphonautes, feed in the plankton for several weeks, usually in surface waters, before they settle and metamor-phose. The larvae appear regularly in plankton tows; while drifting freely they fall prey to many pelagic invertebrate predators as well as fishes. Next the cyphonautes settle on a blade of giant kelp and metamorphose into twinned zooid ancestrulas of Membranipora. From this twinned ancestrula, this young colony grows by budding. Within about three weeks, such colonies may spread from this tiny beginning to sheets that completely encase sections of kelp blades and floats; they can add a row of zooids every 18 hours. Infrequently, colonies with spines at the corners of some zooid houses may occur natural-ly; more often the spines are absent. However, in most colonies lacking spines, the spines will develop as a predator induced defense, a response to predation such as from nudi-brancs. Before this induced defense was recognized, spined colonies were thought to be a wholly different species. In laboratory experiments, the predatory defense is triggered in less than one hour, in water that bears chemical cues from predators, which ceases if the predators are removed. The spines greatly impede predation by these predators. Bryozoans are tiny invertebrates, which expand from one, to a colony of thousands, which can encrust an entire kelp blade. Individual bryozoans, called zooids, live within box-shaped compartments made of calcium carbonate, and chitin, material that is found in crab shells. Zooids are extremely small, perhaps smaller than 1/32 of an inch. These tiny larval bryozoans are clam-like swimmers, in bivalve shells. Bryozoans parachute down onto clean kelp blades by opening their shells like umbrellas and drifting down. Always alert for chemical cues, bryozoans test the surface, and then cement themselves to the kelp blades with a sort of sticky glue. Younger bryozoans settle in place and change into adult form, captive within their own shelled rectangular house. After becoming established on the kelp, the lone settlers begin to multiply. Budding off clones in neat rows, a colonies fan out, frosting the blade with a crust of the tiny animals. Predator(s): Bryozoan colonies are important food sources for some nudibrancs, sea urchins and fish Prey: Bacteria, phytoplankton, small organic detritus, and other small organisms. Feeding behavior: (click on site) Feeding behavior notes Bryozoans have a unique feeding appendage, called a lophophore. A lophophore, is a U-shaped, circular ring of tentacles that are ciliated, and used for filter feeding. By extend-ing a crown of tentacles above its shell, bryozoans flick their tentacles through the water to catch small bits of food. Reproduction Feeding and spawning may take place simultaneously. The lophophoral tentacles surround the mouth, which leads to a U-shaped gut. The anus opens just outside the lophophore. There is a lot going on here, but most of it is only revealed under a dissecting microscope. At intervals, small groups of adjacent zooids suddenly retract their lophophores in unison and then suddenly retract them, suggesting a neural connection between zooids. Zooids that are full with ripe eggs, take on an orange color, due to carotenoid pigments from ingested diatoms. Spawned zygotes (fertilized eggs), emerge from the intertenticular organ (the ITO, a short tube lying between two tentacles at the lophophores rim). They bounce from one lophophore to another as they are caught and then rejected in the feed-ing currents of different zooids until they finally reach open water at the edge of the colo-ny. Bryozoan zooids are hermaphroditic. Sperm are released by testes into a zooids body cav-ity, or coelom, and emitted in packets, tail first, through the pore in one of two specia-lized tentacles. As they emerge from the tentacles, many of the sperm packets are swept into exhalent currents and carried away from the colony for possible cross-fertilization in other colonies. Other sperm packets may be drawn into lophophores of sister zooids in the same colony. In either case, the sperm wiggle vigorously amid the lophophores tentacles; a few may be eaten, but many somehow enter the ITO head first and pass into the recipient’s coelom. It isn’t known if a chemo-attractant is involved. Almost 100 percent of the eggs are fertilized as they enter the coelom through the oviduct. The resultant zy-gotes then enter the inner end of the ITO, pass out into the lophophores discharge current, and soon are in the open sea. Embryonic development does not commence until four days after the zygotes are released. Seasonal Behavior Spring and summer Seasonal kelp habitat In spring and summer, habitat for bryozoans is replenished by kelp growth, but later it is reduced and even wholly removed by the loss of canopy from autumn die-back and win-ter storms. This cycle places many constraints on the community. Since kelp blades only live for an average of three months, the sessile animals that colonize them must conform to an exacting timetable of rapid growth, early sex, and a short lifespan. Conservation Issues Bryozoans are studied in great detail, by biochemical scientists. The relatively transparent living colonies are excellent for observations under a microscope. Some bryozoans could have potential medical uses, as they produce a large variety of chemical compounds.
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