Biology Question #795

Terry Hare, a 50 year old male from Oshawa asks on May 17, 2002,

Why are most scallops white and some are yellowish brown to almost an orange color?

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The answer

Peter Fankboner answered on May 27, 2002

The sexes are separate in scallops, and this difference is expressed in the colour of their gonads with the female's being orange to red, and the male's white to yellow. The origin of this pigmentation is the microscopic plant food (phytoplankton) they filter from sea water which contain photosynthetic pigments including the greenish chlorophylls and the orange-red carotenoids. The latter, which may include astaxanthins and xanthophylls, contribute to the orange-red colouration of the yolk material in female scallop's eggs. Male scallop sperm lacks pigmented nutrients.

The scallops (see photo) eyes are partially surrounded by light-blocking pigmentation (often brownish melanins) which gives directional discrimination to light reaching the retina. Other soft parts such as the mantle tissues, which support the eyes and form the siphonal openings, may also be pigmented. The blue colour that one often sees in the scallop mantle tissues is often due to red carotenoid pigments conjugated with protein.

The shell colouration in scallop shells is genetically determined and is expressed via the protein glue (conchiolin) that binds the calcium carbonate crystals of the shell together (much like the mortar binds bricks in a wall). Melanins and porphyrins, pigments derived directly or indirectly from filtered phytoplankton food, may be incorporated into the shell's conchiolin glue and may thus provide the yellow-tan-brown-orange-red cast to the shell and shell markings of scallops.

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