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White tapered conical shells of planktonic pteropods lying on beach sand

Shells of pteropods on local beaches

Beachgoers may have recently noticed something glistening in the sand, especially in areas where the tide has left debris lines. If they have looked closely at the sand, they may have noticed tiny (less than an inch in length) needle-like cones. These are not fiberglass threads, nor are they spicules from sea sponges. They are actually shells of a group of swimming snails called pteropods, or sea butterflies.

Sea butterfliesA planktonic pteropod showing the two lobes that the animal uses for swimming and feeding

Sea butterflies get their name from the two wing-like flaps that they use for swimming. These flaps can also play a role in capturing food. Sea butterflies are planktonic and are found in ocean waters around the world. They are often in the top 10 meters (32 feet) of the water column. Their shells are made from calcium carbonate.

The pteropods whose shells are currently washing up on beaches in northeast Florida are Creseis acicula. Unfortunately this animal does not have a common name! It produces mucus strings which stick to organic particles in the water. Its food includes bacteria, plant plankton (phytoplankton), other microscopic plankton and detritus. It uses tiny hairs (cilia) on its wing-like flaps to create currents that guide the mucus strands and food to its mouth. Like other snails, the sea butterfly has a special tongue called a radula. The radula is covered by small teeth and is used to help the animal swallow its food. C. acicula are non-migratory pteropods. Field observations report that a two-centimeter long animal can swim at a speed of 12 cm/second. This equates to about a quarter of a mile per hour.

Effects on people?

Pteropods have been reported to cause “sea stings” to swimmers, when the shells penetrate a bathing suit. This generally results in only a slight irritation, unless individuals have high levels of sensitivity. C. acicula is known to have seasonal “swarms,” when large amounts of other zooplankton (animal plankton) are present.


2 Comments on “Shells of pteropods on local beaches

  1. Great explanation, but why are there so many on the beach…I’ve never seen them before…what happened or what is happening?

    • Hi Jack. That’s a great question. The thought seems to be that the population of pteropods increases based on food availability. Winds and currents can affect where the pteropods are located and whether (and when) the shells will get washed up on beaches. It isn’t an uncommon occurrence in Florida (considering the state as a whole), but often occurs over a relatively small stretch of beaches at a time. The shells can wash back into the ocean with subsequent tides, so they may only be apparent on a particular beach for a few days.