Difference between Sea Hare and Nudibranch

Snails, abalones, garden slugs, and nudibranch are gastropods that belong to the phylum mollusca. The gastropoda class usually includes snails and slugs that belong to freshwater, seawater or the land. This article will focus on sea hare and nudibranch, and the differences between them.

Sea Hare

Sea hare is a marine snail that is characterised by a shell reduced to a flat plate on its body. Its classification is as follows:

Kingdom: Animalia

Phylum: Mollusca

Class: Gastropoda

Family: Aplysiidae

The most well-known species of sea hare are Aplysia californica, Aplysia kurodai, and Aplysia dactylomela.

Their physical characteristics include:

  • An underdeveloped shell that is reduced to a flat plate
  • Distinct tentacles
  • Body that is either smooth or full of warts

The sea hares thrive in shallow water on seaweeds. They mostly feed on seaweeds, and the colour of seaweeds that they eat gives colour to their body, that is, red, green and brown. Sea hares are sluggish in nature and cannot use speed to run away from their fast predators. Their lack of well developed shells also adds to their misery. However, they have evolved other means of protection such as crypsis. Crypsis is the phenomenon of hiding from the predator by blending themselves into the environment by changing their colour, shape and pattern.

Other mechanisms include chemical defences. Inking is a chemical defence where the sea hare produces ink and opaline from separate glands that drive the predators such as lobsters, crabs, fish and anemones away. The ink is a purple-coloured secretion and opaline is a clear viscous fluid. The mixture of both forms an aversive compound, and is released from their siphon in the direction of their predator.

Sea hares are mostly hermaphrodite, meaning acting as both males and females during reproduction. They mate in chains and lay eggs in threads that are pink in colour. They are used in Chinese delicacies, and are also used in neurobiological studies because of their unusually long axons.

Refer: Phylum Mollusca – Characteristics, Classification and Examples


Nudibranch also known as sea rabbits or sea slugs are marine gastropods that are extraordinarily coloured organisms. Their classification is as follows:

Kingdom: Animalia

Phylum: Mollusca

Class: Gastropoda

Order: Nudibranchia

Well-known species of nudibranch include Tethys fimbria, Glaucus atlanticus (blue sea slug), Dendronotus frondosus (black sea slug), and Doris odhneri.

The word nudibranch (originated in Latin and Greek) means naked gills. The naked gill refers to the breathing apparatus on their bodies. They are soft-bodied and are of striking colours. They thrive in seawater at shallow depths. They feed on invertebrates such as sea anemone.

Their physical characteristics include:

  • They lack shells and true gills
  • Have a radula as feeding organ
  • Have outgrowths called cerata that fulfils defensive and respiratory needs
  • A chemosensory organ called rhinophore arises from the head that serves as olfactory organs.

Nudibranchs are carnivorous in nature as they feed on algae, sponges, anemones and other sea animals. The rhinophores help in identifying their prey. They are hermaphrodites, meaning they act as both males and females during mating. They lay their eggs in gelatinous spirals that resemble a ribbon.

For their defence mechanism, they don’t have shells to protect themselves from the enemy. They mimic their surroundings by camouflaging to get rid of the predators. Another mechanism is an interesting one, where they shoot stinging cells onto the predators. The nudibranchs eat stinging animals of the Cnidarian phylum. The stinging cells, also called nematocysts, are passed unharmed into the organism’s digestive system. The nematocysts mature here and are used by the organism during the time of attack.

Commercially they are used in food and decorations.

Sea Hare vs Nudibranch

Sea Hare



Sea hares are gastropods that have an underdeveloped shell.

Nudibranchs are gastropods that do not have shells.

Feeding Habits

They feed on seaweeds, and hence are herbivorous in nature.

They feed on algae, sponges, anemones and other sea animals. They are carnivorous in nature.

Presence of Shell

They have underdeveloped shells that are sometimes located internally.

They don’t have any shell-like structure.


Their gills are present inside the body.

Their gills are naked and external.


They release eggs in a thread-like appearance.

They release eggs in a ribbon-like appearance.

Defensive Mechanism

They secrete purple ink and viscous opaline to form an aversive compound that drives the predators away.

They shoot the predators with stinging cells to drive them away.


  • Aplysia californica
  • Aplysia kurodai
  • Aplysia dactylomela
  • Tethys fimbria
  • Glaucus atlanticus
  • Dendronotus frondosus
  • Doris odhneri

Commercial Use

They are eaten as Chinese delicacies, and are also used in neurobiological studies.

They are used in food and decorations.

Explore BYJU’S Biology for more related topics.

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Frequently Asked Questions on Difference between Sea Hare and Nudibranch

Why are nudibranchs so colourful?

Nudibranchs get their colour from the food they eat. Their coloured bodies are an advantage as it helps them to camouflage from predators.

Are sea hares poisonous?

No, sea hares are not poisonous, but they secrete a toxin that saves them from the predators.

Are sea hares edible?

Yes, sea hares are eaten as a delicacy in China.

Are sea hares fast?

No, sea hares are sluggish in nature. That is why they had to develop chemical defensive techniques to save themselves from predators.

What makes nudibranch unique?

Nudibranchs are striking in colour, and have rhinophores as their olfactory organs near their mouth. Additionally, they have outgrowths called cerata on their body that helps in respiration and defence.

What does the term nudibranch mean?

The term nudibranch is derived from the Latin and Greek dictionary and translates to naked gills.

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