Shells brightly coloured/patterned to attract mates?

[ennss]

Why (in a biological and evolutionary sense) are shells brightly coloured/patterned?

Birds and lots of other creatures use colour/pattern to attract mates, flowers apparently use it to attract pollinators, but why are shells colourful?

Is there much or any sexual dimorphism in shells?    I don't really know,  but I don't think there is - so colours/patterns wouldn't be to attract mates (such as in birds) - also, I don't expect that molluscs have the sensory equipment to see the colours anyway.  (Does anyone know if snails can even see colour?)    I expect that some of the predators like cones must have pretty good motion detecting equipment, but can they detect colour?

It's understood that the shells immediate mineral environment can heavily effect the colour but what about the pattern, is it just random?

Any insight appreciated!

[paul monfils]

Hi,

Sexual dimorphism occurs in a few shell-bearing molluscs, like Cassis cornuta and Lambis lambis, but not very many.  And it doesn't involve the color or pattern, just the physical structure of the shell.  It is not likely that the color patterns of mollusc shells are involved in attracting a mate, for the following reasons:
- The eyesight of most mollusks is poor, probably not able to distinguish much more than light vs. dark areas.  Even cones find their prey primarily by smell.
- In life, the shell of most brightly colored species is covered with a fibrous periostracum secreted by the animal, which obscures the color and pattern of the shell.
- Many brightly colored species live in deep water where there is virtually no light.  Even in water of moderate depth the warm colors (red, orange, yellow) are not visible because those colors of light do not penetrate that far into the water.

[ennss]

Thanks for your response.  Obviously, I'm merely a collector and enthusiaist  - I have no background in biology at all -  I didn't even think about the periostracum and lack of light at depth which are totally obvious now that you mention them.    So is there any other proposed purpose to the colour and pattern such as camouflage, or is it strictly a side effect during the deposition of the shell that has no purpose or "cost" to the animal?

The form of the shell on the other hand must be pretty directly related to some evolutionary  advantage, with heavier, thicker or spiny shells having additional protection from predators.  There's obviously a huge difference in "production cost" to the animal between very heavy thick shells and thin light shells...    Are all spines, frills etc. thought to have developed as direct protection from predators?