Reef-bryozoans and bryozoan-microreefs: Control factor evidence from the philippines and other regions |
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Authors: | Dr Joachim Scholz Prof Dr Gero Hillmer |
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Institution: | (1) Department of Geology and Paleontology, University of Hamburg, Bundesstr. 55, D-20146 Hamburg |
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Abstract: | Summary In this paper, a preliminary concept on the interplay of local, regional and global control factors of bryozoan diversity
and distribution pattern is introduced. Recent bryozoans from the Philippines, New Zealand and the Gulf of Aqaba are compared
to the selected fossil specimens from the Oxfordian and Santonian.
Reef bryozoan skeletons are studied in order to separate local control within the substrate-water interface from regional
control. The latter originate mainly from the transport function of the water column (e.g. sediment load, wave energy, vagile
predators). This is true especially for erect (tree-like) and massive (multilaminar) bryozoans which are subjected to the
dynamics of the water body in the littoral area. This regional control, affects simultaneously several structural and substrate
zones of a reef. Early life history of vertically growing reef bryozoans reflect local control, while older zoarial structures
reflect the signals of regional influence. Three types of multiserial nodular bryozoans are cited: self-overgrowing sheets
(‘S-Nodule’, derived from ‘S-Sheet’), circumlaterally budding colonies (‘C-Nodules’), and fungiform bryozoans.
Bryozoan growth form selection allows the separation of two types of regional controls, long range control which favours the
selection of specialized sheets, and slow rate control documented in the growth form modification of sheets changing into
nodules.
In the domain of local control epibiontic microorganisms, microbial mats and biofilms on hard substrata represent probably
the most important elements, aside from the limited substrate space. Symbiotic and/or competitive bryozoan-microorganism interrelationships
result in the distinct adaptations of bryozoan growth. It is apparent that bryozoan modifications of substrate micro-topography
influence the character of epibiontic microbial settlement. The peak occurrence of microbial settlement on prominent external
bryozoan skeleton parts is discussed as an effect of feeding currents. In contrast, smooth and unelevated exoskeletons are
less favourable for micro-epibiontic colonization. Due to the high level of order in the spatial zonation patterns of epibionts
on calcifying bryozoans, the term ‘bryozoan-microreef’ is introduced. Bryozoan-microreefs are locally controlled reefs. They
are e.g. characterized by higher competitive abilities than bryozoans without ‘reefdwellers’. Local control has a fast rate
of change and is reflected in morphologies of individual zooids and/or single zooid generations.
Regional control has a slower rate but a higher range. It is important for growth modifications of the whole zoaria. The very
slow rate of global control cannot be recorded within the life span of bryozoan zoaria unless it is expressed through regional
control (such as monsoons). Nevertheless, global control is paleoecologically important because it is traceable in bryozoan
(paleo-)biogeography. For practical purpose, we suggest to define those control factors of bryozoans as global which affect
simultaneously at least one tropical and one non-tropical bryozoan community.
This reportis dedicated to the memory of our friend and research colleague Mr. Joselito G. Santiago of the University of San
Carlos, Marine Biology Section. Tiago (as we fondly call him) has started within the scope of the Philippine Bryozoan Project
an applied research on fish abundance and productivity around a new artificial reef model. This prototype model was conceptualized
with him. His promising study remained unfinished; he died May 13, 1993, while sampling inside the reef cave off Marigondon. |
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Keywords: | Bryozoa Ecology Evolution Reefs Control Factors Microbial Mats Microreefs Nodular growth Philippines Recent |
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