A Modern Oyster Shell Midden

Oyster shells in a heap at Whitstable for use as cultch for collecting oyster spat

I have spent a large part of my life studying oyster shells that have been excavated on archaeological excavations of sites in the British Isles dating from the last two thousand years. The shells have been found in a variety of contexts including middens which are heaps of kitchen waste including oyster and other marine mollusc shells. Strangely, I had never seen a modern equivalent until this week when I visited Whitstable on the north Kent coast. I had read all about the famous Whitstable Oyster Fishery but somehow had never got around to visiting the place.

I did not choose a very good day to see Whitstable for the first time. It was very cold, dull, and windy with the choppy sea high up the shingle beach and salt spray continuously misting my camera lens. Nonetheless, I had a great time and made some interesting discoveries – not least of which was my desire to go back ago and explore some more.

Almost the first thing I saw when I hit the shore after a coffee at the Horsebridge Gallery, was the building of the historic Royal Native Oyster Stores belonging to the Whitstable Oyster Company. It included a seafood restaurant closed at the time, and outside were two substantial heaps of empty oyster shells – middens – one against a wall and the other on the shore. A casual observer might wonder why mounds of empty shells had been left lying around and not properly disposed of. There is a good and logical reason.

The shells are being kept for cultch. The youngest form of an oyster is a free-swimming larval stage which needs to find somewhere suitable to settle down and grow. It is very particular about the type of object on which it will land and attach its embryonic shell. It has a limited time, maybe just a couple of weeks if the temperature is optimum, to find just the right place. It likes all sorts of hard substrates but it likes oyster shells best – sometimes empty ones and sometimes live ones.. Traditionally, this preference is catered for by the oyster fishermen who put down quantities of empty oyster shell as cultch on the seabed to encourage the settlement of young spat oysters. They also string old shells together to act as suspended spat catchers in the water. It is interesting to see these historic practices still in operation in an age when many oysters are bred in laboratories before being grown on in metal mesh bags on trestles covered by the tides. They use both old and new methods here.

The pictures in this post show more than one type of oyster shell. The Native British Oyster, also known as the European Flat Oyster (Ostrea edulis) is the type for which Whitstable is most famous. They also use the Pacific or Rock Oyster (Crassostrea gigas) which grows faster and is therefore a good commercial proposition, especially when in recent times the numbers of our native species have reduced.

Kewarra Beach

Goat's Foot Morning Glory flower on the beach

This post represents my first visit to the Australian shoreline. Kewarra Beach, just north of Cairns on the Queensland Coast, is fairly typical of the beaches in the area. You can see from the pictures that it was virtually deserted. Even though the temperature was hot, hot, hot, it was also steamy; for the most part, a dull day with rain clouds tumbling down from the mountains.

Rainforest trees come right down to the sand – and as with the mangroves bordering the river, the tangled networks of roots are exposed. Ideal territory for salt water crocodiles – there is even a notice warning of recent sightings. Thinking that one of the dreaded creatures is possibly lurking somewhere – ready to dart out of concealment for a meal – certainly takes the edge off the idea of paddling or exploring the woods by the shore.

The tide had washed in driftwood, dead fish, coconuts, and strange jawbones. Delicate purple-tinged clams rolled on the surf; oysters clustered on rock; and tiny Sand Bubbler Crabs popped in and out of burrows – scattering small balls of sand in linear patterns on the beach. In this paradise, pink Goat’s Foot Morning Glory flowers decorated the grey rip-rap used as a sea defence.

View of tropical beach with rain clouds

Fish jaw bone on the beach

Trees growing on a sandy beach

Exposed tangled network of tree roots

Australian native rock oysters on a beach boulder

Close-up image of native rock oysters on a boulder

Exposed tree roots on a sandy beach

Driftwood on a sandy Australian beach

Trees with exposed roots growing on a sandy beach

Sign warning of dangerous crocodiles

Dead fish washed up on sandy beach

Sand Bubbler Crab and seashells

Mangrove-lined river flowing onto the beach

Small river estuary with mangroves

Sky, sea, surf, and sand

Small living clam found rolling in the surf

Coconut with husk washed ashore

Tropical trees at the top of a sandy beach

Rip-rap rocks on a tropical beach

Flowering Morning Glory vine on the beach


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Wild Oysters on the Queensland Coast Part 2

Row of wild oysters growing on barnacle-covered rock.

The oysters that I found on the rocks at the northern end of Three Mile Beach in Port Douglas were so different from the ones I had seen at Cape Tribulation that I wondered if they were oysters at all.

The identification of Rock Oysters of the Saccostrea Group in the Indo-West Pacific is a fairly hot topic and some very interesting work was completed a few years ago to try and sort out what is what. See the work of Katherine Lam and Brian Morton.

On the basis of shell morphology, I think the oysters illustrated in this post are Saccostrea mordax which are distinct from the other Saccostrea species in having regularly-spaced grooves radiating from the umbone to the ventral margin of the right valve, the triangular shell shape, and finely plicated valve margin (with regular m-shapes). The left valve is completely attached as in the other species of Saccostrea such as cucullata, glomerata, and kegaki which are all morphologically similar to each other with an oval, deeply cupped left valve and a smaller, relatively flat right valve with slightly plicate, raised margins.

The molecular study by Lam and Morton (2006), based on samples obtained from along the whole of the Australian coastline, clarifies what is known about rock oyster biogeography. The identification of the oysters shown here from Port Douglas tallies with the distribution of Saccostrea mordax that is now thought to have a range from the tropical eastern coast of Australia,  along the northern coast and throughout Western Australia. S. glomerata only occurs on the south-eastern coast of Australia on temperate shores from southern Queensland to New South Wales while S. cucullata shares the geographical range of S. mordax. One exception, based on someone’s personal observation – and not as a result of inclusion in the mitochondrial DNA work – is that putative S. mordax also occurs, but in much lower numbers, on shores dominated by S. glomerata around Moreton Island and Sydney.

Individual living rock oyster at Port Douglas, Queensland, Australia. Possibly Saccostrea mordax.

Group of Rock Oysters, Saccostrea sp. at Port Douglas.

Group of Rock Oysters, Saccostrea sp. at Port Douglas.

Group of Rock Oysters, Saccostrea sp. at Port Douglas.

View looking south along Three Mile Bay, Port Douglas.


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Wild Oysters on the Queensland Coast Part 1

Rock Oysters growing at Cape Tribulation, Queensland

I have written a lot about the natural variations in oyster shells belonging to the British Native, Flat, or European Oyster, Ostrea edulis Linnaeus. However variable these shells may be, it is always possible to identify the shells as belonging to that species, and to distinguish them from other species.

In Australia and the Far East, the oysters that grow wild and naturally on the tropical shores include several species of Saccostrea which can be difficult to differentiate from one another because of the diversity of their outward appearance. The morphologies of Saccostrea glomerata, Saccostrea cucullata, Saccostrea kegaki, and Saccostrea mordax, are so variable and overlapping that is not always possible to tell them apart by eye. As with so many other groups of organism currently being investigated (marine algae for example), it is only by use of mitochondrial-DNA analysis that true identities and relationships can be established (Lam and Morton 2006).

Which brings me to a discussion of the Rock Oysters that I photographed in several locations on the Queensland Coast. The images shown in this Posting were taken at Cape Tribulation in tropical Far North Queensland. Just going by the external characteristics, I suggest that they may be  Saccostrea glomerata – also called the Sydney Rock Oyster. However, the differentiation of that species from Saccostrea cucullata is so problematic at times even for experts that oysters like this are frequently given both names, S. glomerata cucculata.

In following Posts I’ll show oysters growing in Yawarra Bay, Trinity Bay, and Port Douglas for comparison with these from Cape Tribulation. The shells from the rocks at the northern end of Three Mile Bay at Port Douglas look very different from the others and I think that they may be Saccostrea mordax. I’ll also refer in more detail to the Lam and Morton paper:

Lam, K. and Morton B. (2006) Morphological and mitochondrial-DNA analysis of Indo-West Pacific Rock Oysters (Ostreidae: Saccostrea species), Journal of Molluscan Studies (2006) 72: 235 -245, Oxford University Press on behalf of The Malacological Society of London.

Rock Oysters growing at Cape Tribulation, Queensland

Rock Oysters growing at Cape Tribulation, Queensland

Rock Oysters growing at Cape Tribulation, Queensland

Rock Oysters growing on boulders at Cape Tribulation, Queensland


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