Large numbers of sea hares — soft-bodied marine mollusks related to sea slugs — have been washing ashore on Hutchinson Island beaches this month, part of an irregular phenomenon that even local experts say remains something of a mystery.
"Sea hares are a marine mollusk. They're a gastropod, not unlike a snail, except they don't have a shell," said Dr. Zack Jud, Director of Education & Exhibits at the Florida Oceanographic Society. "Technically, they're a type of sea slug."
According to Jud, mass sea hare appearances don't happen every year, but when they do occur, they tend to follow a pattern.
"It's a phenomenon that happens regularly, but not annually," Jud said. "Every couple of years, typically, right around this time of year, usually mid to late July, we get these huge eruptions of sea hares along our shoreline."
Scientists aren't certain what triggers the events. Jud said breeding or courtship behavior is one possibility, though egg masses — which would typically accompany spawning — usually aren't present. He suspects water temperature and wind patterns may play a larger role.
"Normally, when we see large numbers of sea hares along our beaches, we also have an upwelling event occurring," Jud said.
West winds during summer months can push warm surface water offshore, pulling cooler water up from the depths — conditions that tend to coincide with sea hare sightings, he said.
"I don't know if it's just a coincidence or whether that's part of the pattern," Jud said, "but it does seem to play out year after year when we have these conditions."
Jud said the sea hares themselves pose no danger to humans who might come into contact with them on the beach. Pets are a different story.
Sea hares release a purple ink for self-defense, and Jud said that ink can be toxic to dogs. He cautioned that he is not a veterinarian, but recommended pet owners keep dogs away from any sea hares washed up on shore.
"If you're walking your dog on the beach, you want to make sure you're keeping your dog away from any sea hares that have washed up on the shoreline," he said.
Jud said these events tend to pass quickly. Based on past occurrences, sea hares are typically present for about a week before disappearing again for an extended stretch.
He noted that recent trips on the water turned up only a handful of sea hares, compared to thousands seen just days earlier — a sign, he said, that this event may be winding down.
Despite the unpredictability of when sea hares will appear, Jud said the timing tends to be remarkably consistent once an event does occur.
"While we don't see sea hares every single summer, when they do appear, it's almost always exactly around the same time," he said. "I think that's a neat pattern. It's one of those scientific stories that we don't have all the answers to."