Aplysiidae spp.
Spot sea hares year-round from Florida to the Costa Brava. Learn how to recognise these soft-bodied molluscs across 95 countries of diver-logged sightings.
Sea hares are large, slug-like molluscs with two prominent ear-like sensory organs called rhinophores projecting from the head, giving them their name. Body colour varies widely across the family, from mottled brown and olive to deep purple or reddish tones, often with pale spots or a network of fine lines that help camouflage them against algae and rock. They are soft-bodied and fleshy, with folded wing-like flaps (parapodia) along the sides that may undulate gently. Size varies considerably across species in the family, but most are noticeably chunky compared to nudibranchs, and their bulk alone makes them stand out on a reef.
You will most often find sea hares moving slowly along rocky substrate or algae-covered reef, grazing on the algae they depend on for both nutrition and the pigments that tint their skin. They tend to ignore divers entirely and keep crawling at their own unhurried pace, making them very approachable and easy to observe up close. Shallow, algae-rich areas and seagrass beds are the most productive places to look, and they are active during the day.
When threatened, sea hares release a cloud of purple ink from one gland and a sticky white secretion from another, a dual chemical defence that can deter predators and confuse their senses simultaneously. The ink comes from pigments in the red algae they eat, so diet directly shapes their defensive chemistry.
Sea hares are present year-round in both hemispheres with no concentrated season, which matches their broad tolerance for cooler to mild waters observed between roughly 13 and 23 °C. Temperate sites in the Mediterranean or along temperate coastlines of the US and Australia are worth checking in any month.
Sea hares are most commonly logged in the United States, particularly across Florida, the Southeast, and the Pacific Coast, and Australia stands out as a strong second, especially New South Wales and Western Australia. The Mediterranean is a reliable area too, with strong records across Spain's Costa Brava, France's Brittany and Normandy coast, Italy, and Greece. Mexico's Yucatán Peninsula and the Canary Islands also produce consistent sightings, making this a family that rewards divers across both tropical and temperate destinations.
Mapping where Sea Hare is logged…
Sea hares are logged across a wide range of countries, with the strongest diver records in the United States (especially Florida and the Pacific Coast), Australia (New South Wales and Western Australia), and across the Mediterranean in Spain, France, Italy, and Greece. Mexico's Yucatán Peninsula and the Canary Islands are also reliable spots.
Diver records show sea hares present year-round in both the Northern and Southern Hemisphere with no clear peak season. They favour cooler to mild water temperatures, so temperate sites in the Mediterranean or along US and Australian coastlines can be productive in any month.
Look for a large, fleshy, slug-like animal with two prominent ear-like rhinophores on its head and folded flaps running along its sides. Colour varies by species from mottled brown and olive to reddish-purple, often blending with surrounding algae. Their chunky, soft body makes them bulkier than nudibranchs.
No, sea hares pose no danger to divers. They are slow-moving, entirely harmless, and completely uninterested in divers. If stressed they may release a harmless purple ink, but this is a defence aimed at predators, not people.
Size varies considerably across the Aplysiidae family. Some species remain just a few centimetres long, while others, such as the California black sea hare, can exceed 40 cm and weigh several kilograms, making them among the largest gastropod molluscs in the ocean.
Sea hares produce a purple ink from a specialised gland as a chemical defence against predators. The purple pigment comes directly from the red algae they eat, meaning their diet determines the colour and potency of their ink. They can also release a milky white secretion that acts as a separate chemical deterrent.
Site and sighting data is aggregated from diver logs, iNaturalist and GBIF. For taxonomy and conservation status:
Sea Tulip
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