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Silver sandbox

Notes on Ligament article:

The hinge ligament is highly adaptive part of the anatomical structure of a bivalve shell, i.e. the shell of a bivalve mollusk. Bivalves typically have two valves, which are joined together by a strong, flexible, and elastic fibrous organic material situated on the hinge line at the dorsal edge of the shell. In life, the shell needs to be able to open a little to allow the foot and siphons to protrude, and then close again, without the valves moving out of alignment with one another. The ligament is usually brown to black in color and connects the two bivalve shells at the hinge line and functions like a spring to open the valves when the adductor muscles relax.

Composition of the ligament
The ligament is an uncalcified elastic structure comprised of minimally two layers, a lamellar layer and a fibrous layer. The lamellar layer consists entirely of organic material (a protein and collagen matrix), generally brown in color, and is elastic in response to both compressional and tensional stresses. The fibrous layer is made of aragonite fibers and organic material, lighter in color and often iridescent, and is elastic only under compressional stress. The protein responsible for the elasticity of the ligament is abductin, which has enormous elastic resiliency that cause the valves of the bivalve mollusk to open when the adductor muscles relax. Ligaments that are simple morphologically have a central fibrous layer between anterior and posterior lamellar layers. Repetitive ligaments are morphologically more complex and display additional, repeated layers. A recent study using scanning electron microscopy (SEM), X-ray diffraction (XRD), and infrared spectroscopy (FTIR), have found that some bivalve mollusks have a third type of fibrous layer in the ligament (located in the middle) which has a unique spring-like protein fiber (ca. 120 nm in diameter) structure, stretching continuously from the left to right valve.

Elastic opening of valves
When the adductor muscles of a bivalve mollusk contract the valves close which compresses the ligament. The elastic resiliency of the ligament reopens the shell when the adductor muscles relax. Interestingly, scallops (Pectinidae), which swim by repeatedly clapping (opening and closing) their valves, recover a greater fraction of the work done through their abductin than do other more sedentary clams.

Taxonomic use of differences in hinge ligaments
The hinge ligament of a bivalve shell can be either internal, external, or both, and is an example of complex development. Various types of hinge ligaments have been found in living species (i.e. extant species), and the ligaments can be reconstructed in most fossil bivalves from their sites of attachment on the shell. The taxonomic distribution of ligament types among families of bivalves has been used by paleontologists and malacologists as a means of inferring phylogenic evolution.

Hinge ligament types
closed. An internal ligament is usually called a resilium and attaches to a resilifer or chrondophore which is a depression or pit inside the shell near the umbo.
 * EXTERNAL: dorsal to the hinge teeth, visible and under tension when the valves are closed. External ("dorsal") ligaments can be: (1) simple, planivincular, parivincular, duplivincular, alivincular, or multivincular; (2) external or submarginal; (3) opisthodetic, prosodetic, or amphidetic; and (4) set on gutters (simple only), fossettes (simple only), nymphs, or pseudonymphs.
 * INTERNAL: ventral to the hinge teeth, not visible and under compression when the valves are

General references
E.R. Trueman, General features of Bivalvia. In: Moore R.C., editor. Bivalvia. Ligament. In: Treatise on invertebrate paleontology. Vol. 2. Geological Society of America and University of Kansas Press; 1969. p. N58-N64. Part N - Mollusca, Bivalvia Vol. 6.

T.R. Waller, ''The evolution of ligament systems in the Bivalvia. In: Morton B., editor. Proceedings of a Memorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. Hong Kong: Hong Kong University Press; 1990. p. 49-71.

J. G. Carter, Evolutionary significance of shell microstructure in the Paleotaxodonta, Pteriomorphia and Isofilibranchia (Bivalvia: Mollusca). In: Carter J.G., editor. Skeletal biomineralization: patterns, processes, and evolutionary trends. New York: Van Nostrand Reinhold; 1990. p. 135-296.