I seem to remember picking up this shell from the strand-line at Trinity Beach – which is just north along the coast from Cairns in Queensland. I photographed it against an improvised background of my black trousers – first showing the inside and then the outside of the shell. I think it is a shell of Anadara inaequivalvis (Bruguière) (see CIESM The Mediterranean Science Commission Atlas). This species is found along the coast from Northern Territory to central Queensland as well as being an accidentally introduced species in other parts of the world.
Apparently, when the mollusc is younger, the two hinged valves of the shell are different sizes (inequivalve) but as the mollusc matures the shells become equal in size (equivalve). The number of ribs is important for distinguishing between the different species of Anadara. A. inaequivalvis has between 31 and 34 radial ribs (I can count 31 in this particular specimen. A similar species, A. polii (Lamark) has only 26 – 28 radial ribs. The length of A. inaequivalvis ranges from 70 – 80 mm in mature specimens with a height of upto 61 mm. Unfortunately, I had no ruler to photograph with the shell to indicate scale but it was a fairly large shell.
I’m puzzling over this shell at the moment. If I recall correctly, it is a live specimen that rolled up the beach with the tide a bit north of Cairns itself, at Yawarra Beach. It looked fairly ordinary and plain until I turned it round to view the edge and saw beautiful delicate growth rings and lovely purple tinged beaks or umbones. I think it is a Mactra. Possibly Mactra dissimilis Reeve.
This group of shells is fairly large with triangular shells; and the animals live in sand. M. dissimilis is the most common species of this family in northern Queensland. It has a sculpture of concentric growth rings but overall is generally smooth in appearance. It is white and tinged with purple and is about 50 mm long. It is found from the Northern Territory to northern New South Wales.
Jansen, P. (1996) Common Seashells of Coastal Northern Queensland, privately published in Townsville, Australia, ISBN 0 646 29824 0.
The angularity of this bivalved shell made it immediately noticeable on the sandy beach at Cairns. It is called a Twisted Ark because of the strange configuration of the shell which is strongly twisted with a sharp ridge from the top to the margin. The Latin name is Trisidos tortuosa (Linnaeus), formerly Arca tortuosa. It varies from 60 – 100 mm in length and occurs on the Australian coast from the Northern Territory to Queensland.
Jansen, P. (1996) Common Seashells of Coastal Northern Queensland, privately published in Townsville, Australia, November 1996. ISBN 0 646 29824 0.
I only found the one empty beach-worn shell of this Common Baler (Melo amphora Lightfoot) on the beach at Cairns. It is also known as the Giant Baler or Melon Shell. It has spines around the whorls at the anterior end. The pattern on the shell is now indistinct but a fresh shell would have characteristic variable orange-brown zig-zag marks. This species is apparently fairly common in the Indian Ocean, on the Queensland Coast of Western Australia, and off shore New Guinea. The largest recorded size is just under 500 mm in length but this one was only 145 mm. It is quite a heavy shell, and in life it has a thin brown periostracum layer which wears off eventually. The huge curved aperture or mouth of the shell can be used to bale out boats – but has broken on this shell.
Cairns doesn’t have a beach as you would normally understand it. Most of the area offshore is a vast acreage of mudflats on which it is forbidden to walk. However, there is a narrow fringe of sand along the northern part, accessible from the Esplanade that runs along the whole of the seafront. It is here, randomly dispersed, that assorted small seashells lie in natural drifts on the beach at Cairns, Queensland, Australia.
The photographs in this post show how these marine gastropod and bivalve shells look in situ, just as they were found, on the shore. Identifying them all to species is more problematical – particularly as it is not permitted to collect shells and take them out of the country! I did take lots of pictures while I was there, some in close-up and against the plain background of sand, rock, clothing, or even a plastic dish, so that I could have a go at putting a name to them later. And that is what I am about to do over this series of postings on the seashells I saw in Cairns and other Queensland localities when I was on holiday in 2011.