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The life and death of perforate corals at Palmyra Atoll, USA: micro-community structure within the skeleton.

  • Author(s): Furby, Kathryn A
  • Advisor(s): Sandin, Stuart A
  • et al.
Abstract

Coral reefs have been part of the earth’s oceans since the Mesozoic Era, over 200 million years ago. In recent decades, however, reefs have been progressively suffering as a result of human activities. However, coral species vary in their response to ecological disturbance. The work described here examines aspects of the biology of two central Pacific species: Porites superfusa, a small encrusting coral, and Acropora cytherea, a dynamic, massive table coral. Both species are perforate corals, with skeletons permeated with an extensive canal system partially lined with living coral tissue. Thus far, the question of whether the perforate coral condition conveys functional advantages lacking in corals with imperforate (relatively dense) skeletons has been overlooked. The present work supported with field work and sample collections at Palmyra Atoll, Line Islands, USA, during approximately annual visits of several weeks each from 2012 to 2016. The reef is protected from fishing and local pollution but still exposed to wide-scale disturbances like climate change (especially evident in warm water bleachings in 2009, 2015, 2016). This dissertation explores the importance of coral regrowth, endoliths and adjacent epiliths to reef recovery dynamics. Long-term changes in coverage of Porites superfusa were followed in a time series of high-resolution photoquadrats that demonstrated new settlement as well as changes in existing colonies (growth, partial mortality, death, and “death” followed by resurrection). Partial mortality and survival was usually observed in larger individuals, whereas smaller individuals tended to grow progressively or apparently die out completely. However, quite often a new small individual would appear in the area where a colony had died previously in the time series. Although settlement of planula larvae from the water column could not be completely ruled out, the source of new growth could have been from a small amount of living tissue cryptically surviving in or on the “dead” colony. Further work would be required to see if new growth was seeded by small regions of viable tissue. For the other perforate coral in this dissertation, Acropora cytherea, the distribution of living tissues was studied in the canals permeating the skeleton in healthy and evidently dead (algae-covered) regions of the colony. Core samples were studied by scanning electron and light microscopy. In healthy regions, the living coral tissues lined only the intraskeletal canals to a depth of several millimeters from the surface of the colony. In healthy parts of the colony, the canals more than a few millimeters deep in the colony were bounded not by living coral tissues, but with calcareous skeleton. Although the coral skeleton was riddled with endoliths (algae, fungi and bacteria), they were relatively rarely observed in the canal space. In regions where the living part of the colony was covered over with turf algae and other invading organisms, these endoliths were detected, along with abundant sediment, packing the most superficial intracellular canals; surprisingly the canals deeper beneath the overgrown region were free of such extraneous material. In the light of these results, one can speculate that fluid (driven by flagella in the relatively superficial healthy part of the colony) might percolate throughout the intraskeletal canal system, even in regions underlying places overgrown with algae and other organisms. Such a flow could conceivably distribute nutrients or coral cells to regions where they might influence the repair of the locally overgrown parts of the colony overlying them. Light microscopic and molecular techniques (internal transcribed spacers, ITS and 18S cDNA) were combined to provide an overview of the overgrowing and endolithic organisms associated with A. cytherea. The sampled regions of the colony were living, recently dead (near living coral tissue) or long dead (far from living coral tissue). The data permitted mostly genus-level identification of macroalgae, endolithic and epilithic algae, and endolithic fungi. A comparison between living, recently dead, and long dead samples demonstrated a succession in community composition of the algae. Even so, there were notable similarities in the genera of endolithic fungi present in the skeleton of living and dead regions of the colony. The endolithic communities within the coral may be a link between live and dead coral in the calcium carbonate structures. This might reflect a uniformity and connectivity maintained by the deep circulation within the unobstructed lumens of the intraskeletal canal system throughout the colony, even beneath superficially overgrown areas. In sum, the results of the different parts of this dissertation point to the need for further studies of the recently neglected perforate skeleton character and connecting endoliths and their possible relationship to resilience of some perforate corals. Understanding partial survival and dead coral skeleton may hold additional hints at maintaining and protecting coral reef ecosystems.

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