Marginal tentacles of the corallimorpharian Rhodactis rhodostoma. 2. Induced development and long-term effects on coral competitors |
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Authors: | O Langmead N E Chadwick-Furman |
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Institution: | (1) School of Ocean Sciences, The University of Wales, Bangor, Menai Bridge Marine Science Laboratory, Gwynedd, LL 59 5EY, United Kingdom, GB;(2) Interuniversity Institute for Marine Science, P.O. Box 469, Eilat, Israel, and Faculty of Life Sciences, Bar Ilan University, Ramat Gan, Israel, IL |
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Abstract: | Polyps of the corallimorpharian Rhodactis rhodostoma (Ehrenberg, 1934) form aggregations that monopolise patches of space on the shallow reef flats of some Red Sea coral reefs.
Some of these polyps bear specialised bulbous marginal tentacles (BMTs) where they contact cnidarian competitors. BMTs differ
from the normally filiform marginal tentacles (FMTs) of R. rhodostoma, and appear to develop from them. However, their morphogenesis and long-term impacts on spatial competition with reef corals
are unknown. We experimentally induced contacts between R. rhodostoma polyps and colonies of the branching stony coral Acropora eurystoma on a shallow coral reef at Eilat, northern Red Sea. During the first 24 d of contact, the A. eurystoma colonies extruded mesenterial filaments that damaged the tissues of the corallimorpharian polyps. After 18 d,>90% of R. rhodostoma individuals had developed BMTs, which resulted in a reversal in the direction of competitive damage. During the subsequent
1.5 years of observation, the corallimorpharians maintained well-developed BMTs, unilaterally damaged the tissues of A. eurystoma, and in some cases moved onto the stony coral skeletons and partially overgrew them. BMTs developed from FMTs in a series
of four distinct stages, accompanied by significant changes in their morphology, cnidom, and density of nematocysts. Isolated
control polyps did not develop BMTs or show any signs of damage. In contrast, corallimorpharian polyps transplanted into contact
with colonies of the massive stony coral Platygyra daedalea began to develop sporadic BMTs, but were unilaterally and severely damaged by the corals, and started to disappear within
21 d, after the corals developed sweeper tentacles. We conclude that long-term outcomes of competition between R. rhodostoma and reef-building corals depend largely on the relative aggressive reach of the competitive mechanisms developed by each
species. As a consequence, this corallimorpharian is an intermediate competitor in the aggressive hierarchy among Indo-Pacific
reef corals. This study confirms that R. rhodostoma polyps may actively damage and overgrow some stony corals, leading to the formation of an almost continuous blanket of polyps
in large patches of some shallow reef flats.
Received: 15 July 1998 / Accepted: 24 March 1999 |
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