Why these giant, shell-less sea slugs are named after the common hare

Nature · Published 3 October 2026 Swipe more like this →

In short: Sea hares are bulky, herbivorous marine mollusks that look nothing like rabbits, yet they share a name. Named for the two upward-projecting rhinophores that mimic a hare's ears, these creatures are masters of chemical defense, using toxic ink and slime to deter predators in their shallow-water homes.

The order Aplysiida, or sea hares, comprises medium to very large opisthobranch gastropods. Their name, lepus marinus, dates back to Roman times, reflecting their rounded bodies and distinctive sensory organs. While older classifications placed them in the suborder Aplysiida, taxonomic debates have seen them shifted into the Aplysiomorpha clade, though recent literature continues to favor the original Aplysiida designation established by Paul Henri Fischer in 1883 and later refined by Johannes Thiele in 1925.

These mollusks are remarkable for their size and biology. The largest species, Aplysia vaccaria, can grow up to 99 centimeters and weigh 14 kilograms, making it a contender for the largest gastropod. They are hermaphroditic and possess a highly developed sense of smell, using their sensitive rhinophores to track food. Their diet of seaweed dictates their coloration, which acts as natural camouflage. When threatened, they release toxic, multi-colored ink to disrupt a predator's sense of smell, or secrete a clear, defensive slime that physically clogs the olfactory receptors of creatures like lobsters.

Beyond their ecological role, sea hares have significant cultural and scientific importance. In regions like Hawaii, the Philippines, Samoa, Kiribati, and Fiji, they and their egg strands are consumed as traditional food, with the egg masses often compared to noodles. In the scientific community, Aplysia californica has become a cornerstone of neurobiology. Because of their unusually large axons, these slugs were instrumental in the work of Nobel Laureate Eric Kandel, who used them to study the fundamental mechanisms of learning and memory through the gill and siphon withdrawal reflex.

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