Saturday, August 19, 2017

Low tide living

June 19, 2011 by  
Filed under blog, CICBP, New Content

A clump of Goniastrea corals exposed by extreme low sprig tides.

The spring tides of the mid winter months generate the lowest daytime tides of the year. In my part of the world, Townsville, northern Australia, it provides a great opportunity to view the Great Barrier Reef glinting for a couple of hours under clear winter skies. I recently used these lowest astronomical daytime tides (the summer equivalents occur at night) to photograph a suite of specialist inter-tidal corals from the genus Goniastea.  The photos below were taken at Nelly Bay, Magnetic Island.  In Great Barrier Reef terms Magnetic Island is a large inshore granite island fringed by turbid water coral communities.

As is often the case it is the things right under your nose that are the last to get done.  For years now I have been planning to photograph the Magnetic Island low tide daytime exposed corals. In particular I was after three Goniastrea species: G. retifomis, G. favulus and G.aspera. The reef flat has other inter-tidal corals (e.g. some Platygyra, Montipora and Porites species) but it is Goniastrea which emerges first and submerges last on these extreme tides.  As a niche its a pretty hostile place to be as it maximises the risk from exposure to air, sun and rain.  As a result these corals often show deformities of the colony – by definition the tops of the highest colonies are often dead – but what is most striking is the way the these corals protect themselves by generating a protective mucus.  Taking macro-photos standing around on a reef at low tide is challenging enough without the pervasive glistening reflections coming from the coral mucus.

I suppose it should come as no surprise that life is so in sync with the rhythms of the Earth, but the way these three Gonisatrea species cluster on this ecological veneer always amazes. When you have geological time as your coach then evolution naturally tends to fine tune things.  My interest in taking these photos is to fill a blind spot in my coverage of the genus Goniastrea.  So expect to see an update in the genus index in the near future.  For now check out the slide show below. Goniastrea is best recognised by the presence of paliform lobes which are not always easy to see in the field (especially when the coral is hunkered down under low tide stress!) but I did manage to capture some useful field examples (see arrows in the slideshow).  The Coral Finder Toolkit explains the use of paliform lobes at length in Coral Finder Toolkit Training Movie 6 – enjoy!

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