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What’s up with those Whelks? | Sanibel Sea School Blog

What’s up with those Whelks?

If you’ve ever visited Sanibel, it is almost certain that you have been dazzled with the beauty of one of our common mollusk species – the Lightning Whelk (Busycon contrarium). This gastropod is a sea snail with a beautiful pattern that resembles lightning bolts cascading down the shell. We often refer to the shell of a Lightning Whelk as ‘left-handed’ or sinistral, which means that their shell coils to the left – a very rare characteristic. Most other gastropods possess a ‘right-handed’ or dextral shell, where the shell spirals to the right.

Male and female exhibiting sexual dimorphism.

Not only do Lightning Whelks have a unique shell, but they also have a fascinating method of reproduction. Lately, we have been finding large (8-12″) whelks in consort with a group of smaller (3-4″) individuals in San Carlos Bay. It is not uncommon for males and females of the same species to differ; this is called sexual dimorphism. A common dimorphism is size, where one gender is larger than the other. Lightning Whelks are a strongly dimorphic species and in this case, the females are much larger than the male.

Mating season for whelks occurs from October-January, so we are currently at the tailĀ end of the season. After copulation, female whelks begin producing spiral structures (egg cases) containing many connected capsules each of which contains about 30 embryos. In the spring on Sanibel, we see innumerable egg cases that have detached from the substrate washing up in our wrack line. Remember how we have been seeing one female with multiple mates? It turns out that these egg cases, host thousands of eggs, which may have paternity of up to 7 different males. It may seem a little strange to us, but this is an efficient, and not uncommon method of increasing the whelk’s genetic diversity, ensuring stable populations for the future.

Lightning Whelk egg case washed up in the wrack line

Sure is great, what we find in the shallows of San Carlos Bay.

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