About the Cover
Cover
Many invertebrates form colonies of genetically identical individuals in which some individuals become specialized for different functions. One such animal is Membranipora membranacea, a sheet-like bryozoan commonly found encrusting kelp blades in Puget Sound and many other temperate coastal areas.
The main cover image shows two colonies of M. membranacea, the larger of which is 2 cm in diameter. Each of the rectangular, skeletal boxes in the colony houses an individual zooid that bears a
crown of ciliated tentacles. The tentacles generate water currents that bring suspended food particles to the colony. Groups of zooids form openings (chimneys) that allow water to flow out of the colony after it
has been filtered through the colony's feeding tentacles to capture food. Zooids in the center of chimneys lack feeding tentacles, but the tentacles of individuals surrounding chimneys are raised above those of
their neighbors. Chimneys enhance feeding by preventing fluid pressure build-up within the colony. In the cover picture, chimneys appear as dark patches within the canopy formed by the tentacles.
Because of its simple geometry and rapid growth, M. membranacea provides an excellent opportunity to investigate how fluid flow affects the development of fluid transport systems. Previous
authors suggested that phenotypic plasticity in chimney spacing could be explained if water flow through the colony controls where chimneys form. To test whether water currents affect chimney formation, Michelangelo von Dassow, in a study described on pages 76-82 in this issue, injected water under the canopy of tentacles to enhance flow out the edge of the colony. In the colony shown in the upper portion of the cover image, a sheet of red light was positioned so that only the raised tentacles around chimneys were lit. As the colony grew outward, a new chimney-like opening formed distal to the polyethylene tube through which water was injected (toward the top of the image). The new opening distal to the site of water injection (right-hand tube) formed
sooner than the new opening distal to the control site without water injection (left-hand tube). The ends of the tubes in the upper image were 0.8 cm apart. The observation that enhancing excurrent flow at the
colony edge induced the formation of new chimneys supports the hypothesis that fluid flow controls the development of this fluid transport system. These results provide insight into the role of fluid flow in patterning animal colonies and in the development of fluid transport systems.
Credits: Photos, Michelangelo von Dassow; Cover layout, Beth Liles (Marine Biological Laboratory).
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Copyright © 2006 by the Marine Biological Laboratory.
