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 actually 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.