Invertebrates
Sea Hare
Aplysia · also called sea hares
A sea hare looks like a soft, unassuming grazing slug, right up until something threatens it and it releases a cloud of purple ink alongside a second, separate milky secretion, a double chemical defence in one animal. Its real claim to fame, though, is hidden inside its body: nerve cells large enough to see with the naked eye, which helped researchers win a Nobel Prize for uncovering how memory actually works at the cellular level.
Overview
Sea hares, genus Aplysia, are large, soft-bodied sea slugs found grazing algae in shallow coastal waters worldwide. Named for the rabbit-like rhinophores on their head, they retain a thin, greatly reduced internal shell, a remnant of their snail ancestry.
A defence in two parts, a legacy in neuroscience
When threatened, a sea hare releases both a cloud of purple ink and a separate milky secretion called opaline, working through sight and taste and smell together. Its unusually large nerve cells made it a foundational model organism for Nobel Prize-winning research into how learning and memory work at the cellular level.
Status
Least Concern — common and widespread across shallow coastal waters worldwide, supported by an abundant algal food supply.
Sea Hare: common questions
Why is it called a sea hare?
The name comes from a pair of long, upright, rolled sensory structures on the head called rhinophores, which along with the animal's generally rounded, soft body gave early observers the impression of a rabbit's or hare's head with its ears raised, even though a sea hare is a marine mollusk with no actual relationship to hares or rabbits. The rhinophores themselves function as chemosensory organs, helping the sea hare detect chemical cues in the surrounding water rather than serving any hearing-related purpose despite their ear-like appearance.
Why do sea hears make Nobel Prize-winning research subjects?
Sea hares, and particularly the species Aplysia californica, possess unusually large individual nerve cells, some large enough to see and manipulate directly under a microscope or even with the naked eye, which made them an exceptionally convenient model for studying how individual nerve cells function and communicate. Research building on this system was central to uncovering basic cellular mechanisms of learning and memory, work recognised with a Nobel Prize in Physiology or Medicine for research that used the sea hare's uniquely large, accessible neurons as its experimental foundation.
Does a sea hare have two different defensive secretions?
Yes, in addition to the more visually obvious purple or reddish ink released from one gland, many sea hares also produce a separate, milky white secretion called opaline from a different gland, and the two are frequently released together when the animal is seriously disturbed. While the ink is thought to work partly through visual confusion, the opaline appears to act more through taste and smell, together giving the sea hare a combined, multi-pronged chemical defence against attacking predators.
Can a sea hare's colour actually change based on what it eats?
Yes, because the pigments responsible for both a sea hare's body colouration and its defensive ink are derived directly from the specific marine algae it has recently been eating, individuals feeding on different types or colours of algae can display noticeably different shades, and the exact colour of the ink released can shift accordingly as well. This diet-linked colouration is a distinctive feature of sea hare biology not seen in quite the same direct way in many other marine animals.