Secrets of a Sea Necklace

Abstract

Beachcomers in the Middle Atlantic states frequently count among their treasures a a string of parchment egg cases made by whelks of the genus Busycon. Besides being a chewy rattle for the enterprising beach baby, a sea necklace provides an ideal teaching tool for a study of reproductive potential. The following technique has proven effective in the classroom. It is presented as an activity guide such as students might receive before doing the exercise. Materials. The activity requires a dried sea necklace (fig. 1), shells of adult knobbed and channeled whelks, Petri dish halves, dissecting microscopes or hand lenses. Introduction. The channeled whelk (Busycon canaliculatum) and the knobbed whelk (B. carica) are marine gastropods found along the Atlantic Coast from Massachusetts to Florida. They are often found intertidally, where they hunt at night and partially bury themselves in mud or sand by day. The long chains of parchment-like pouches that wash up on eastern beaches are the egg cases of these organisms. A female adult snail digs a hole in the sand and anchors her first eggs to a pebble, then she secretes a fluid that forms a tough casing around them. To this casing, she joins a second one, continuing the process until she has made a long string. The female can make ten to fifteen egg cases in a single day. She ordinarily produces only one chain of cases per year. Procedure. (Parenthetical comments serve as a teacher's guide.) 1. Each team of students should break off one egg case from a sea necklace, twisting it loose from the connecting cord without squeezing the pouch itself. 2. The egg cases of the knobbed whelk have a flattened edge similar to the edge of a coin; those of the channeled whelk have sides that meet in a single keel-like edge. Make a preliminary identification of which species of Busycon produced the egg case you have. 3. The eggs are sealed in the pouches during autumn spawning, and the egg strings develop during the winter, hatching in the spring. Whelks have a veliger form of larva which, though adapted for free swimming, remains in the egg case. There it changes into the juvenile form, which is recognizable as a gastropod with a real shell. 4. Tear the egg case open. Notice how tough the translucent membrane really is. Name two other organisms that surround their eggs with some type of protective coating. (This is a common survival mechanism for many animals whose eggs develop outside the mother.) 5. Pour the contents of the case into half of a Petri dish and observe the shape and size of the juvenile snails. Make a sketch of one of the snail shells and indicate its size in millimeters. Was your preliminary species identification correct?