Seashells at Cairns 5

Inside of a bivalve seashell tentatively identified as Anadara inaequivalvis (Brugiere)

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.

Outside of a bivalve seashell tentatively identified as Anadara inaequivalvis (Brugiere)

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Seashells at Cairns 4

Seashell thought to be a Trough Shell on the beach near Cairns

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.

Reference

Jansen, P. (1996) Common Seashells of Coastal Northern Queensland, privately published in Townsville, Australia, ISBN 0 646 29824 0.

Seashell thought to be a Trough Shell on the beach near Cairns

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Seashells at Cairns 3

Empty shells of Twisted Ark, Trisidos tortuosa (Linnaeus), at Cairns

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.

Reference

Jansen, P. (1996) Common Seashells of Coastal Northern Queensland, privately published in Townsville, Australia, November 1996. ISBN 0 646 29824 0. 

Empty shells of Twisted Ark, Trisidos tortuosa (Linnaeus), at Cairns

Empty shells of Twisted Ark, Trisidos tortuosa (Linnaeus), at Cairns

Empty shells of Twisted Ark, Trisidos tortuosa (Linnaeus), at Cairns

Seashells at Cairns 2

Common Baler, Giant Baler or Melon Shell (Melo amphora)

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.

Common Baler, Giant Baler or Melon Shell (Melo amphora)

Common Baler, Giant Baler or Melon Shell (Melo amphora)

Common Baler, Giant Baler or Melon Shell (Melo amphora)

Common Baler, Giant Baler or Melon Shell (Melo amphora)

Common Baler, Giant Baler or Melon Shell (Melo amphora)

Common Baler, Giant Baler or Melon Shell (Melo amphora)

Common Baler, Giant Baler or Melon Shell (Melo amphora)

Common Baler, Giant Baler or Melon Shell (Melo amphora)

COPYRIGHT JESSICA WINDER 2013

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