Sea Whelks are large snails that only appeared in the episode "Whelk Attack."
Sea Whelks are a type of sea snail. During "Whelk Attack,"Bikini Bottom sustained devastation due to giant whelks. It is later revealed that germs caused the whelks to be aggravated.
They resemble a regular sea snail without the stems leading up to the eyes and the eyes themselves. While the bodies are purple, their shells feature an orange color with a circular series of spikes around each curve on the cone shell. Throughout the episode, these snails have been depicted as giants, but they actually can fit in the palm of a fish's hand.
Sea whelks have a disease (most likely a bunghole parasite) that aggravates the whelks, causing a rampage while being huge in size.