Monday, 8 August 2016

Oyster Reefs of Far North Queensland

In ecology an oyster reef is the classic example of an open ecosystem.  In open ecosystems nutrients pass through the ecosystem whereas in a closed ecosystem, nutrients are mostly recycled.  There are no truly closed ecosystems that we know of; however some of rainforests would come close.  Larger coral reefs also have a high degree of nutrient recycling so are functionally close to being a closed ecosystem.  Oyster reefs are open ecosystem because the oysters feed on plankton imported by water flowing over the reef and the oysters export frass and sometimes oyster eggs/sperm into the flow.  Nutrients come and go and only negligible amounts are recycled.  Given their conceptual importance, I was surprised that so little attention has been given to oyster reefs.
Mushroom-shaped oyster reefs on bed rock outcrops at White Cliffs, Captain Cook Highway
I noticed that oyster reefs were a distinct ecosystem on tropical Queensland shores about one year ago.  Wikipedia defines an oyster reef as a dense aggregation of oysters that forms a large colonial community.  Oyster reefs have their own ecology and can exist independently of mangroves, coral reefs and other tropical ecosystems.   As oyster reefs are small, ranging from less than a metre across to patchy reefs a few tens of metres across at the most, they are easy to overlook.

Band of oysters on landslide boulders at East Trinity boat ramp (click to enlarge)
Given that oyster reefs are nature’s equivalent to patches broken glass, it also took me a while to want to investigate them.  Oyster shells can easily cut into human skin.  Small pieces of shell often remain embedded in puncture wounds and cause serious infections.  Wading barefoot across a mud flat studded with oyster covered rocks is a testing experience as even the mud between the colonies is full of sharp buried shells.  A common way to have this dreadful experience is to pull up to a beautiful beach with a wide shallow reef flat that forces you to get out of the boat and walk ashore.  Some beaches have oyster covered stones, known as oyster beds, on the reef flat and these are the ones to be careful of.

Stony sand flats such as this one at Giangurra often have oyster beds 
Juvenile oysters cover many of the stones and these have cylindrical spines
One of the worst places I have ever been was a saddle oyster reef at the mouth of Saltwater Creek near Cairns Airport.  Saddle oysters (Isognomon ephippium) are about 12 cm across with thin almost circular shells that when closely stacked look like a bed of axe heads.  Silvery dead shells littered the ground around the reef my first impression of them was of miniature compressed humanoid skulls.  The circular part of shell is offset to one side like the braincase of a skull in cross section and the hinge has a series of deep grooves that resemble tooth sockets.  The saddle oyster reefs stretched right across the bed of a creek that was only millimetres deep a low tide.  I was trapped between a series of these barriers and could not paddle my surf ski out or walk out as the soft mud was as deep as my legs were long and stretched for hundreds of metres.  It was the most vulnerable to crocodile attack I have ever been.  Having no easy exit, I decided to investigate the reef properly so that I would never have to come back, before committing to a two hour slog across the mud flats that involved sitting on the tip of the ski and kicking with both feet to move just 30 cm. 

Saddle oysters reefs consist of tightly packed vertical shells
The surrounding area is covered with their dead shells
Isognomon ephippium
The shells look like weird skulls
A milky oyster reef near Pretty Beach on the Captain Cook Highway lacks the danger but was also a bit depressing.  It is located on the crest of an uplifted coral reef that has very little living coral or other life. The oysters are only present in narrow bands around the margins of the coral platforms.  Oysters normally form a horizontal band on rocky shores that is close to high tide level.  As the entire top of the old coral reef is at oyster level, yet only some places have colonies of contiguous oysters, some other environmental factors must also be important.  I wonder if oyster colonies form much like snowflakes form with edges of the colony that are subject to the most current recruiting the most spat.  Curiously, there is little fouling of living milky oyster reefs by other organisms such as algae and barnacles.
Uplifted coral reef near Pretty Beach, Captain Cook Highway
Looking along the uplifted fringing reef
Not all reefs are ugly and some oyster reefs are pretty enough to be photogenic, particularly the shelves of oysters that form on rocky headlands or on huge boulders that landslides have delivered into the sea.  Unsupported shelves of oysters can extend out for over a metre.  Rough seas probably break off shelves that have grown too wide.  The oysters in the shelves are arranged like a malformed honeycomb into tall roughly shaped polygonal columns.  Most of the column is solid shell and is formed from layer upon layer of shell secreted underneath the living oyster.  Some of the solid columns are more than 10 cm long.  Unfortunately the columns are weakly banded and it is hard to count seasonal ‘growth rings’. 

Massive shelf of oysters near Thala Beach Lodge (White Cliffs)
Same shelf from other direction showing gap where part of reef has broken away
Small oyster reefs in lee of Taylor Point, Cairns
The ultimate oyster reefs in my opinion are the small unattached oyster reefs that lie on sand and mud flats.  These reefs are self-regenerating in that the only place new oysters can settle is on old oysters.  Self-sustaining unattached oyster colonies are not common with only few seen for each kilometre of coastline and they always occur on the landward margins of wide sand or mud flats.  I have also heard that aborigines once returned old shells to the sea so that new oysters could grow and it would be amazing if this was the origin of the unattached oyster reefs that I have observed.

Unattached oyster reef at mouth of Johnstone River, these oysters tend to get eaten by people.
Unattached oyster reefs with milky oysters develop on dead saddle oyster shells
An oyster colony in a midden at Weipa, shellfish were the major food source for local aborigines
All of the oyster reefs prefer relatively semi-sheltered waters or on exposed coasts, positions where waves are broken before impact.  They also prefer large rocks that stand in deeper water and which are probably less prone to sand blasting during rough weather.  It is thought that the lower limit of oyster growth is related to predation.  On floating walkways in marinas, oysters only grow near the water surface and are heavily predated.  Oysters often do much better on the piles within the marina oysters as piles are exposed when the tide recedes. 

Oyster reefs do not like places with high energy wave impacts
When trying to work out why oyster reefs form where they do, it occurred to me that oyster reefs might also be a real world example of DaisyWorld, which is the mother of all ecological theories.  Oyster reefs are mostly bright white yet they live on black rocks.  During summer, the rocks can get super-hot and suffering limpets stand up on their feet like mushrooms to get some air under their shells.  However oysters breathe water and must remain tightly closed and like daisies, they cannot move so heat must be a critical issue.  In Daisy World, white daisies cool the world by reflecting sunlight so could oyster reefs work this way too.  The next step is to look for scientific papers as surely this topic has been researched.  If not, doing some data collection on temperature differences between bare stone and oyster colony would not be too hard.

Oysters seem to do much better in colonies than they do as scattered individuals
One of the few rocks with scattered oysters, showing lots of dead oysters
In other countries and other parts of Australia, the habitat and ecosystem services provided by oyster reefs are highly regarded.  In north Queensland, the rich variety of habitats means that the contribution of oyster reefs to local species diversity is quite low.  However, there are organisations springing up to restore and recreate oyster reefs that have been destroyed over the years, including the Shellfish Reef Restoration Network.  In early Australia many oyster reefs destroyed for various reasons.  Some were mined and the oysters burnt to create lime for concrete.  In many places there has been a major loss of oyster reefs.  

NSW Oyster Fisheries 1883, Source: State Library of Victoria
Oyster reefs provide a complex three dimensional space where creatures could live, however the position of oysters at the top of the tidal range probably limits the number of species that could make use of that potential habitat in north Queensland.  I did not observe much biota making use of the habitat provided by oyster reefs I have examined.  Crabs including stone crabs and half crabs sometimes shelter under unattached oyster reef on tidal flats and algal turf sometimes occurs on old dead shells. 

Well developed oyster reef often has fissures and holes
This older reef has algae turf on the matrix of old dead shell
Large shelfs can collapse, creating oyster boulders that provide habitat for other creatures 
Oyster shells collected from beach that probably come from broken down reefs
1.  Due to crowding the oyster lives in an ice cream cone shaped space inside the column of shell
2.  Column seen from bottom, showing robust outer wall and layers of shell sealing
of the unwanted space below the living oyster
3.  Most columns have a stripe on one side and I assume this is the attachment surface
4.  This column (oyster shell) is 9 cm long, but columns can get a fair bit longer
Oyster drills are the most common visible natural predator of oysters.  In NSW oyster beds that once extended from the high tide line to the bottom of the coastal harbours but sub-tidal colonies were wiped out by introduced mudworms.  Humans are probably the main predator of the remaining oyster beds and reefs in the upper intertidal zone.  Indigenous people and to a lesser extent other people scour most of the coastline near Cairns looking for oysters which they mostly consume on the spot.  In accessible sections of coast, oyster populations are often depressed by over harvesting.  However given the notoriously poor diet of indigenous people, fresh oysters are probably a very important dietary supplement. 

An Oyster Drill - Morula marginalba

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