Sea hare

Sea Hares: Facts, Eggs, Ink, and Care

If you’re looking for a challenging, unusual invertebrate to add to your tank, take a look at the sea hares. Due to the possible toxins these sea hares can release and their specific care requirements, the species are considered difficult to keep. It’s recommended that only expert-level hobbyists attempt to take on these sea slugs. It’s even risky to care for them in a community tank! So be careful and weigh the pros and cons carefully before adding one to your tank.

Table of Contents: Sea Hares

Sea hares aren’t for everyone. They look unusual – and that can mean you’ll find them interesting or repulsive. It requires a special kind of person to see the beauty in these grazers. Or someone who relishes a challenge in the invertebrates under their care. So whether you find yourself curious as to why these gastropods present such a fuss, or you want to figure out the finer points of their care, the links below will help you out.

Quick Facts

  • Common Names: Wedge sea hare, Aplysia
  • Scientific Name:  Aplysia, Dolabella, and other species
  • Maximum Size: 16 inches (40.6cm) – Yes, that’s right!
  • Minimum Tank Size: 75 gallons (284L)
  • Aggression Level: Low
  • Care Level: Expert Only
  • Most Active: Night
  • Lifespan: 6 Years
  • Hermaphroditic

Natural Habitat of the Sea Hare

Sea hares get their names from the long ear-like rhinophores on their heads. Rhinophores aren’t ears, though. Sure, it puts people in mind of rabbits (well, if you squint), but they’re used for tasting particles in the water. And since these sea snails don’t carry around an external shell (it’s tiny and kept inside), they’re usually known as sea slugs. (Good thing sea slugs and sea snails are in the same group)  It makes for a confusing classification.

In the wild, Dolabella auricularia and Aplysia spp. sea hare species enjoy living in slow-moving waters throughout the Indian and northwestern Pacific oceans. Strong currents might carry them out to sea, and they’re not strong enough swimmers to get back home. Instead, they prefer to hide in the sand, mud, or patches of seagrass to avoid drawing attention to themselves. Depending on the diet of the particular species, you’ll find colors of green, brown, or a reddish-purple. That coloring (and their usual hangout) helps them blend in with their natural surroundings. They don’t stand out. This allows the sea slug to eat in peace while also improving their odds of NOT becoming food for something else.

They feed on seaweed and algae. Grazing on the plants prevents overgrowth. Yes, plants are important, but when algae get out of control in the ocean (algal blooms), the result often means releasing toxins into the water. Not to mention the oxygen levels take a hit. So sea hares provide a handy gardening service, checking the ocean’s plant life growing in their native regions.

Of course, sea hares also come equipped with a defense system. Their siphons (and their mantle cavity) contain two different inks. (Yup, a little like an octopus) The inks are often toxic, chasing off the lobsters and starfish that find their soft skin a delicious treat. (We’ll discuss that ink in more detail in a moment) These invertebrates are nocturnal, to boot, This helps them avoid many of the active predators hunting for them (but makes them a bit less interesting to observe in the home aquarium).

Sea-hare at Morro Bay

Sea Hare Tank Environment

When you decide to keep one of these gastropods, you have plenty of things to look at for your tank. But perhaps the first, most obvious consideration for proper aquarium conditions is to look at the slug’s maximum size at full maturity. This amazing invertebrate grows up to 16 inches (40.6cm) in length! Think about that. That final size often eludes the casual onlooker at the local fish store because they’re usually sold at much smaller sizes. The minimum tank size to care for a sea hare is a 75-gallon tank (284L).

They do best if you maintain stable reef tank water parameters. You can easily achieve this if you make your water with a recommended reef salt mix, regularly test your water, and perform water changes as necessary to maintain those levels.

Sea hares do well in tanks with a sandy substrate. In particular, a deep sand bed works well because they love to burrow. However, you want to think ahead if you go this route. In the inevitable event your poor gastropod dies, you need to be ready to remove it from the tank IMMEDIATELY. If you don’t, the decaying body can catastrophically contaminate your tank. This is due to the poisonous ink they use to protect themselves. (I promise, we’re going to cover that. For now, though, have a plan in place in case this tricky invertebrate decides to pass away on you)

The tank also needs to be slug-proof. It’s not uncommon to find sea hares getting sucked into a filter. (As you might guess, this isn’t healthy for them) Remember that they prefer slow currents due to poor swimming ability. That means they can’t escape. You’ll want to cover the intake and protect them from any unforeseen accidents.

You will also notice your sea hare trying to redecorate your tank by clumsily knocking things over. It could be rocks, decorations (if you have them), or even coral frags. (Watch out below!) Anchor everything you introduce to the aquarium, so you protect the other tank inhabitants from a clumsy attempt at “furniture moving.”

Like every other invertebrate, sea hares have a sensitivity to copper. If you accidentally introduce ANY copper in your water, you’ll end up killing your poor sea slug.

Aplysia sea hare

Feeding Sea Hares

As mentioned above, sea hares are aggressive algae eaters. Many aquarists often purchase them to clean up problem infestations with hair algae or other notorious troublemakers. And there’s nothing wrong with that! Some shops even rent them out to keep their stock properly fed and help aquarium owners eliminate the vegetation pests. They do a fantastic job of devouring plants – and they love it.

However, you need to think long-term when you bring a sea hare home. They’re NOT small invertebrates. And when presented with a banquet of algae, your gastropod can grow large quite quickly. That can make it difficult for you to grow enough algae to keep them fed. (Yes, you read that correctly: You’ll need to GROW algae) If you intend to keep this animal in your tank, you’ll need to supplement the sea hare DAILY with sheets of dried macroalgae (such as nori).

Unfortunately, some individual sea slugs might not take prepared/dried algae. That means you’re going to need to provide the real deal. Your best bet is to have a refugium set up, producing a healthy macroalgae crop you can readily offer at a moment’s notice. (And it’ll be safe from those chomping jaws while it grows)

Sea Hare Compatibility

Sea hares are docile, non-aggressive invertebrates that shouldn’t cause compatibility issues. After all, they’re algae-munching herbivores, hiding out in the weeds. Other than tumbling the occasional rock (accidentally, always accidentally), they don’t bother anyone around them. It’s the “bunny” part of their nature. (And, no, we’re not talking about the Killer Rabbit of Caerbannog).

However, there’s ONE trait to these amazing gastropods that makes them incompatible with aggressive species that might pick on them. Remember the noxious ink they carry around to irritate and deter would-be predators? You got it. The bioweapon works beautifully – in the wild. While the predator makes a retreat and the ink dissipates in the current, the sea hare has plenty of time to crawl away to safety. (Think of it as a really toxic skunk spray)

But in the confines of a reef tank, there’s nowhere for the ink to go. Potential predators need to wait for the filter systems to clear everything. And the time required exposes them to HIGH levels of the toxin. By the time you walk by and see clear waters, you’ll also notice a dead crustacean or fish. And any time the poor sea slug feels threatened, the process will repeat itself. You need to think through the diet (and nature) or ANY tank mates you might want to add to your aquarium.

Handling an Inking Situation

If (yes, “if” not “when” because odds are it WILL happen at some point) your sea hare inks, you need to act quickly to remove the noxious substance from the water. A charcoal (activated carbon) filter should work chemically if you can get the water through the filter fast enough. Then you need to dispose of the filtration media once the coast is clear.

Once the “coast is clear,” try to identify what caused the inking in the first place. Either remove the problem or closely watch the tank. If the cause of the inking is a semi-aggressive fish, remove them from the tank. You don’t want to cope with a repeat of the problem. (It’d be nice if the fish learned its lesson, but you don’t want to roll the dice on those odds)

And if you want to see what an inking looks like? Take a look at this YouTube video (it’s beautiful, in a toxic way):

SEA HARE BIOWEAPONS – A Colorfully Sticky Defense

Reproduction in Sea Hares

All sea hares are hermaphrodites. This means each individual carries both male and female reproductive organs. So, in theory, that would make establishing a pair easy. However, they are considered extremely difficult to breed in the home aquarium. Part of this is because, even though sea hares are hermaphroditic, they’re NOT self-fertilizing. So you still need two slugs for successful breeding.

In the wild, Aplysia californicus (the California brown sea hare) and other species sometimes mate in chains of up to TWENTY individuals. A large chain often produces up to 80 MILLION eggs at a time. They gather in warm waters and produce long strands of eggs that resemble pink or green spaghetti. Once the eggs are laid, they hatch in 10 to 12 days. The tiny gastropods become free-swimming larvae that spend 30 days as part of the plankton. Then they search out a patch of algae to attach onto. Once attached, they feast and DOUBLE their weight every ten days.

Of course, replicating this system in captivity gets complicated. You need the space for all of those invertebrates (or at least two), as well as the proper conditions for the larvae to develop and survive. As such, most aquarists tend to skip this part.

Purchasing Sea Hares

Sea slugs such as the sea hare group are still relatively rare in the saltwater aquarium hobby. This is mostly because of their specialized eating requirements (well, the amount of the algae anyway) and the dangers of inking. As such, it can be a bit challenging to find them for sale. Your best bet is to phone around to local fish stores (particularly specialized stores) and check out popular online retailers that have them listed on their sites.

Dolabella auricularia - another popular sea hare species

Should You Buy Sea Hare or Not?

Some people put an argument forward that raising a sea hare in captivity is an ethical issue. If you don’t keep up with their algae feedings, they starve. So if you DO decide to add one to your tank strictly for pest control, please have a good plan in mind for what to do once they clean up your tank. These gastropods have big appetites and need LOTS of green stuff to survive. You don’t want to “employ” them and then abandon them once the work’s done.

Another area of warning is the risk of inking. If your sea hare gets spooked and inks your tank, you could suffer pretty significant losses. Make sure you have a high-capacity activated carbon filter running, just in case.

With that said, if you have a massive algae problem or just a soft spot for long-eared sea slugs, hopefully, this article helped you decide if they are something you want to add to your tank.

For More Information

Sea hares fascinate people (or, you know, freak them out). With those floppy tentacles on their head, they draw you in. And even if you don’t feel ready to take one on in your tank, it can’t hurt to learn more about them.

For instance, this YouTube video, where they rank as one of the weirdest in terms of breeding habits (admit it, you thought the chain idea sounded strange):

Underwater Love Chain | World's Weirdest

Maybe you’re having serious issues with hair algae or another problem algae, but you don’t feel you have the ability to handle the demanding needs of a sea hare. Consider getting an emerald crab instead. While far from perfect (all crabs are opportunistic eaters and might take the opportunity to eat something you DON’T want them to), the emerald crab – if it causes problems – won’t cause them as quickly or dramatically as a sea slug.

Or, if you (like me) have decided that the sea hare is fascinating but too risky to add to your tank, consider looking into the fascinating arrow crab as an alternative invertebrate species for your saltwater aquarium.

What to read next

Are you looking for some other interesting invertebrates for your tank? Check out these other invertebrates:





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