Specimen Showcase | Cute Ears in the Sea
- Hong Kong Biodiversity Museum
- Mar 13
- 3 min read
The sea creature, 𝘈𝘱𝘭𝘺𝘴𝘪𝘢 𝘫𝘶𝘭𝘪𝘢𝘯𝘢 (Quoy & Gaimard, 1832), commonly known as the walking sea hare, has a rather unusual appearance. This species is a marine gastropod mollusk, or sea snail, found in shallow marine environments rich in vegetation, particularly tidal pools and seagrass beds with abundant algae. It is widespread in warm oceans all around the world.
As a member of the genus 𝘈𝘱𝘭𝘺𝘴𝘪𝘢, the walking sea hare possesses a rounded shaped body and two long, upward projections from its head, which resemble the ears of a hare. However, the ear-like projections are not for hearing but for “smelling”. The ear-like projections are called rhinophores,chemosensory organs primarily used for distance chemoreception. The rhinophores can detect the scent of chemicals dissolved in the sea water, allowing the sea hares to find and stay close to the food source, as well as to find mates.
Despite looking soft, smooth and round, the walking sea hares do in fact have shells. Unlike other snail-like gastropods, they don’t have a large, heavily calcified external shell allowing them to retreat and hide in. Instead, they have small vestigial shells inside their body, which are responsible for covering the viscera and nominally protecting the heart and other internal organs. Still, the sea hares share some similarities with their snail relatives. For instance, the adult sea hare, just like snails, possesses both male and female reproductive structures that mature at the same time, making them hermaphrodites. During the mating seasons, usually during the warmer period of the year, sea hares would release potent, water-borne sex pheromones, which would be captured by the rhinophores, to attract potential mating partners. During mating, the sea hares are capable of simultaneously delivering sperm while receiving sperm from the others. The fertilized eggs would then be laid in the form of long, stringy egg masses, which somehow looks like a plate of spaghetti in the sea.
Their soft and delicate appearance doesn't mean that sea hares are completely helpless against predators. Most 𝘈𝘱𝘭𝘺𝘴𝘪𝘢 species are capable of secreting reddish-purple ink mixture, from their purple and opaline glands when threatened. The reddish-purple colour comes from a molecule call aplysioviolin, which is a metabolic product originating from the red & purple pigments contained in the red algae eaten by the sea hare. The reddish-purple secretion from the purple gland would mix with the milky secretion from the opaline gland to form the ink mixture that serves to dispel predators with both its taste and smell. It temporarily blinds predator sight with its dark colour, thus granting time for the sea hare to escape. Yet, the walking sea hare, 𝘈. 𝘫𝘶𝘭𝘪𝘢𝘯𝘢, unfortunately, is unable to produce aplysioviolin that makes the mixture reddish-purple, as they do not have a diet of red algae. As a result, the walking sea hare is only capable of releasing milky secretion from the opaline gland. Does that mean the walking sea hare is destined to be eaten? Not exactly. The secretion contains high levels of amino acids and ammonium, making it capable of mimicking the food of the predators. When the predators are enjoying their illusory meal, it will quickly sneak away. How smart!
What makes the walking sea hares more peculiar than other sea hares is their way of walking. Unlike most sea hares, which move around like a slug, the walking sea hare’s posterior end of foot can act as a sucker that anchors itself, allowing great extension of the head and neck to a new position. Just like crawling around in inchworm-style.
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Text: Felix Tan
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