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Sea Hares ~ Seahare Slug Facts

Aplysia californica, a California Sea Hare, San Miguel Island CA

Aplysia californica, a California Sea Hare, San Miguel Island CA

Sea hares have two large flaps that fold back on the body to cover their internal shell plate. These wing-like extensions can be used for short bursts of awkward swimming when the slug is threatened. This picture shows the two wings folded up.

I have had a few marine (saltwater) aquariums over the years, and I have enjoyed them immensely. One of them was a 55 gallon live reef tank. This means that live rock (rock from the ocean that contained live organisms such as corals, snails, anemones, limpets, etc. already on it) was used to create a living base reef across the bottom and the back of the tank.

These “live rocks” contained many hitchhikers which then found their way into the aquarium. Some of the hitchhikers were small creatures and pieces of coral. With care and time, these rocks would eventually reveal the treasures, or terrors, they contained.

Although I never attempted to keep sea hares in my aquariums, I was severely tempted. There are mixed opinions as to how easy sea hares are to maintain in an aquarium. To be successful a sufficient amount of algae would need to be available for their consumption. Plus they are very sensitive to nitrates and copper which makes them a little more difficult to keep. Water quality would be very important, and copper could not be used to treat illnesses in the aquarium, therefore limiting your medicine cabinet. defines a sea hare as: “Any of various small marine gastropod mollusks of the subclass Opisthobranchia having projections that resemble rabbit ears.”

Comparing the two pictures of the hare (jack rabbit) above and the sea hare below, it's easy to see the origin of the sea hare's name. Its' oral tentacles have a strong resemblence to a rabbit's ears.

A California sea hare (a type of sea slug) (Aplysia californica) with purple sea urchins (Strongylocentrotus purpuratus). California, Channel Islands NMS.

A California sea hare (a type of sea slug) (Aplysia californica) with purple sea urchins (Strongylocentrotus purpuratus). California, Channel Islands NMS.

The flaps that the sea hare uses to swim can be seen folded up in this picture. See video below of Sea Hare using these flaps to swim!

The flaps that the sea hare uses to swim can be seen folded up in this picture. See video below of Sea Hare using these flaps to swim!

The sea hare is found in the Indo-Pacific region and the British Isles including Britain and Ireland. Sea hares are sometimes referred to as Seahare Slugs. Nudibranchs are referred to as Sea Slugs, and the resemblance between these two creatures is significant. Both of these creatures come in a wide assortment of colors, forms, and sizes.

Nudibranchs are intricate and delicate, but sea hares are mostly large, bulky creatures. Sea hares are generally about 2.8 inches (7 cm) in length. However, the largest species of sea hare, Aplysia vaccaria, can grow to be 30 inches (75 cm) in length. These same creatures can weigh up to 4.4 pounds (2 kilograms)!

Sea hares are mollusks which means they have a shell. A common mollusk is a snail. Sea hares have the typical soft body, but their shell is internal rather than being external.

Sea hares usually have ruffled backs where their wing flaps come together. They use this increased surface area to absorb oxygen during respiration. Most sea hares are a shade of green and have an elongated oval shaped body. They have tentacles that appear to be “rolled” on their heads.

Sea hares are herbivores meaning they eat vegetable matter. They can typically be found munching on seaweed in shallow waters among coral rubble and rock pools. The sea hare’s coloration corresponds with the color of the seaweed it has been eating. This ability camouflages them from their enemies.

Also interesting to note is the fact that young sea hares tend to eat red algae and are red in color. However when the sea hare is ready to mate, it typically consumes algae that is green and brown in color becoming green and brown. Could this be a signal to other sea hares that they are available?

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As a defense mechanism, a sea hare can release ink from its ink glands. This ink serves the sea hare in multiple ways. The first is it acts as a smoke screen allowing the sea hare a means of escape. At the same time, this ink negatively affects the smell sensors of its predators. In fact, in a small environment, this ink can be quite toxic to the inhabitants.

This ink can come in several different colors including white, purple, or red, again depending on the color of the pigments in the seaweed they have consumed. A similar toxin can also be found in their skin making them inedible to most predators.

Sea hares have an extremely keen sense of smell. They can follow even the faintest scent using their well-developed chemoreceptors.

Aplysia dactylomela - Annulated sea hare

Aplysia dactylomela - Annulated sea hare

Sea hares move around using their muscular foot. They also use jet propulsion as a form of locomotion, much the same way as the octopus or squid, but because sea hares lack the brains that these creatures possess, this motion is somewhat erratic.

Just like the nudibranch, the sea hare is hermaphroditic. At times, sea hares form mating chains on shores where they act as males to the hares below them and as females to those above them, all at the same time. Sea hares then lay pink colored eggs that appear threaded.

One of the larger sea hares, Aplysia californica, has proven useful in neurobiological studies because their nerve cells have unusually large axons.

Sea hares are related to nudibranchs, abalone, octopi, squid, scallops, mussels, oysters, clams, chitons, snails, and limpets, all of which are also mollusks.

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Copyright © 2011 Cindy Murdoch (homesteadbound)


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Other Mollusk Articles by this Author:

Other Sealife Articles by this Author:

Video Showing Sea Hare Swimming

Comments: "Sea Hares ~ Seahare Slug"

Cindy Murdoch (author) from Texas on December 07, 2011:

natures47friend - I think that most everything in the ocean is cool. I liked the almost tentative - cold, slimy - followed by the shout of magnificent!

I am pleased that you enjoyed this hub! I bet you will really enjoy the nudibranch one also.

Thanks for stopping by!

natures47friend from Sunny Art Deco Napier, New Zealand. on December 07, 2011:

Hi. I have been wandering all over HP today...and found sea hares curious....aren't they cool...Can imagine some going..."Ooh yuk...gross!" lol

When I took my daughter to Japan (brother in Nagoya) aquarium had a feeling tank. You could stroke sea slugs, we both did...they were cold, slimy and magnificent!

Great have fascinating hubs, so I am going to visit often! Voted up and awesome.

Cindy Murdoch (author) from Texas on September 04, 2011:

Thank you, rosettaartist. They are really beautiful creatures, but I still am fond of the nudibranchs. Thanks for stopping by.

Rosetta Ceesay from United Kingdom on September 04, 2011:

Awesome and love the photographs.

Cindy Murdoch (author) from Texas on September 04, 2011:

Cloverleaf - I hope you are able to see one of these on your next diving experience. Thanks for stopping by. I'm fixing to be busier, I think I finally found a job.

Cloverleaf from Calgary, AB, Canada on September 04, 2011:

I have never seen one of these interesting creatures; I will have to pay more attention when I am around seaweed and rock pools, it would be cool to see one - and now that I have read your hub I will be able to identify it!

Thanks for the hub, you're a very busy lady :-)


Cindy Murdoch (author) from Texas on September 04, 2011:

rockdresses - I am pleased that I was able to introduce you to the marvelous world of sea hares. I am glad that you enjoyed them. Thanks for stopping by.

livelonger - I'm glad that you found the information to be facinating! And yes, you are correct - it is all scaleless creatures that are sensitive to copper. I have had many aquariums, freshwater, saltwater, brackish, South American cichlid and African cichlid. I have enjoyed them all. The saltwater was always the most fascinating, however it was also always the most difficult and expensive to maintain. But I remain fascinated by the creatures that can be found in the oceans. Thanks for stopping by.

Jason Menayan from San Francisco on September 03, 2011:

Fascinating! Thank you for sharing the pictures and information. I know that mollusks are sensitive to copper (so are other scaleless fish, like otocincluses), and the only ones I ever had in my (freshwater) tanks were the unintended regular snails. Saltwater fish and other creatures are always much more interesting looking, as the pictures you've included here make clear. We never had a saltwater tank, but have always marveled at those we've seen.

rockdresses from Turkey on September 03, 2011:

I never see a real sea hares.... Thanks a lot for your kind sharing~ It is really awesome~

Cindy Murdoch (author) from Texas on September 03, 2011:

adeaugustus - thank you so much for your kind words. I like reading your comments - they really make may day! I'm happy to have been able to introduce you to the humble sea hare. Thanks for stopping by and thanks for the vote up! Blessings to you!

adeaugustus on September 03, 2011:

Interesting hub! I have never been able to see a sea hares before. Also during the course of reading, a thought came in my mind, that i should really get an aquarium, and am going to get one, i love to see little animals. Maybe you should write something about it too. Like "AQUARIUM: THE DO'S AND DONT". Voted up as usual friend.

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