The sea whelks are large snails that only appear in the episode "Whelk Attack."
A sea whelk is a type of sea snail that is purple with light tangerine shells that are the shape of a cone with normal orange spikes on them. They also have no eyes and a big gaping mouth, that can spit lavender slime bubbles.
During "Whelk Attack," Bikini Bottom sustains 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 a light orange color with a circular series of spikes around each curve on the cone shell. Throughout the episode, these snails are 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.