There are two common species of whelk in this area: the Channeled Whelk (Busycon canaliculatum) and the Knobbed Whelk (Busycon carica). They represent our largest beach snails, and their shells can grow to be 7 to 9 inches long.  As living snails, or as empty shells on the beach, they are fairly easy to distinguish, and are appropriately named. The Channeled Whelk has a groove, or channel, running along the spiraled portion of its shell, while the Knobbed whelk has distinctly developed knobs around the the spiraled portion of its shell.

A knobbed whelk shell

Comparison of Channeled and Knobbed Whelk Shells from the Peterson Guide to the Atlantic Seashore

These two snail species can also be distinguished by their egg cases. Mating occurs during the spring and fall, after which females lay their eggs in protected rounded capsules joined to form a chain. Each capsule can contain up to 100 eggs, and a single chain can contain over 100 capsules. Once the chain is complete, the female whelk will bury one end in the substrate to keep the eggs from washing on shore and drying out.  If successful, the whelks will hatch as fully formed snails, approximately 4 mm long.  Unfortunately, their efforts to secure these egg cases to the ocean floor are not always successful, and chains of whelk egg cases occasionally wash up on shore like the one pictured below that I found on a Falmouth beach this weekend.  To determine which of the two whelk species produced the egg cases, examine the outer edge of each capsule: Knobbed whelk egg cases have flatted edges to each of the capsules, while the outer rim of Channeled Whelk egg cases are pinched to a narrow point.

Note the flattened edges of each capsule of the Knobbed Whelk egg case

Baby knobbed whelks from the above egg case

Whelks feed mainly on bivalves, which they are uniquely adapted to open – if you’ve ever tried to pry open a living quahog or other clam with your hands, you already know this is not an easy feat. But the whelk’s shell is specifically adapted to pry clam shells apart. They do this by inserting the narrow end of their shell between the two valves of a clam and pry the shell open just enough to insert their proboscis, which the whelk uses to consume the clam from within its own shell.  Like all snails, whelks have a radula, a rough tongue like organ, that it uses to scrape and consume its prey.