The humble ugly algae eating machine. The Sea Hare.

Creatures | 14th December 2009 | Article by Mo | View comments (25)

Heres the deal – the sea hare is a member of cleanup crew member – only really talked about as a last resort.
Difficult to keep alive – but with a worthy reputation.

Sea Hare,Dolabella auricularia. © Mila Zinkova (

Sea Hare, Dolabella auricularia. © Mila Zinkova (

I have a closed loop system. As such I rely on my live rock filtration to do my managing of nasties and goodies in my tank. Recently a spate of careful albeit live food feeding to my corals (don’t do fish) caused PO4 spike that threw my tank out of balance. You know the story, a moment in the tank, a lifetime in the water.

So I started to get very bad hair algae – for the life of me I couldn’t get rid of it. Tried the usual methods – filtration – PO4 remover – more regular water changes – dosing liquid po4 remover. My problems were exaggerated because I run a very stealth Zen like approach to tank keeping. No skimming or reactors – just my external canister filter with 3kg of broken reef plate in the filter trays.

I’d read about Sea Hare’s – natures voracious eaters of green algae – in particular hair algae. I had a lot of hair algae, my theme tune was ‘ the hills are alive with …’ you get my point…

Sea hares – are secretive, ugly, slow moving, gentle, alien, not toilet trained, active sleepy, acrobatic, dull beasts. I purchased my fairly large sea hare from my LFS.

I’d read that sea hare’s are particularly sensitive to introduction into a new tank. In the past I have ignored this and been bitten – so in this case I was careful. I dripped the beasty in over 4 hrs – excessive I know – but I had a higher salinity in my tank to shop so was being extra careful.

The sea hare – dull grey in its pattern – lump like – and soft spiky appendages all over. It has a head with a mouth – two simplistic eye stalks – and in’y and out’y hole  (more on that later). As soon as I was ready I gently drifted the sea hare out of the bag into the corner I had ear marked for it. Main lights were on – I had read it was more a night time beasty so expected it to be in light shock and tank shock and hide in the corner until night time. NOT a BIT OF THAT!

I kid you not – as soon as the sea hare gently rocked down to the corner – its mouth had already started pulling in hair algae. From the moment – no repositioning – not pause – straight out with the head and puling in hair algae in front of me. I realised that the poor thing must have been really hungry. Within an hour it had cleaned the corner it had started in. It then, slug like, started to move around from where it was – slowly stretching its body out to other parts of the rock near it.

Over the course of the afternoon and into evening its didn’t stop eating. It mooched around in the corner for a while where it started but soon started off towards the glass. Most importantly I noticed its soft mouth rasping against the glass. I watched as it like a giant gravity defying slug – pushed itself up the glass towards the top of the tank. Its head coming out of the water surface (the lights were still on and it didn’t care)
It was trying to reach the very top of the tank as if to check its eating boundaries!

Now I noticed its out’y hole – mid way up its body – towards the thicker end away from its head – from it a line of browny flaky pellet shaped poo started to come out. I then realised it had left lots of these little packages where it had started. Sea hare poo! So not only had it not stopped eating – it had digested the hair algae almost instantly into neat brown packages.

So off to bed – and wonder and worry. But cest la vie attitude – what would it do tonight – would it do the whole tank in one night at this rate? Would it knock things off?

Sunday morning I wake up and check – the tank hasn’t been completely house cleaned of algae. Sea hare is snoozing in corner right at bottom of tank.
Rock is looking picked clean in parts.

Over the course of Sunday it followed this pattern which it still does now – sleeping until about 2pm and then mooch around and eat. I noticed Sunday night that it was near some corals – and so watched what it did. It munched up to the coral – around the coral – on the coral – and not once did it knock it off or move it. I realised it was a gentle giant – and very delicate and precise about where it ate – it targeted only the hair algae by spotting it first not just grazing the whole area. When it needed to get to something it would normally elongate its body and stretch to it – or stretch over or squeeze into it (its body of course is just water really).

Within the next week or so – it slowly moved around the tank and only twice knocked over corals that weren’t fixed down (none of my corals are) but these were especially loose as they perched on some of my live rock. It also did acrobatics moving around the tank glass when lights out – either at the top by water line or mid way – stuck to the glass ‘anti gravity’ like. When it ran out of stuff at water line it would do a gentle ‘drop and roll’ and fall from the glass onto the sand.

In my time with it I’ve grown to really find it very alienly cute. Hard worker – never talking back just a bit filthy with all the poo’ing.

I would recommend a sea hare whole heartedly – the best way is to share someone else’s Sea hares are very cheap, £9 (14USD) for my large one. But once you have given it a home the worry is it running out of algae and starving. So you can take it back to LFS – and they may be in the same problem as you after having it back for a while. As you can tell mine was very hungry when I got it. Or you can try and keep it alive with nori sheets or other algae. Or you can borrow one from someone nearby or local fish club or forum ! Like this one.

I’m going to try and keep it alive and then if I have to give it back to the LFS. I’ve grown quite attached to it.


Ed note: We collected a few videos of sea hare’s in action, enjoy.


Have you tried a sea hare in your nano before? What are your thoughts on using a Sea Hare for algae control? Express yourself in the comments section below.

About the author:

Nanoreefer based in London. Currently working on Project Stealth Cube, a 49 ltr/ 10 gallons nano reef consisting of Probably about 8 acans, 1 acan maxima, 3 sets of blastos, assorted zoa's, a Moniti digitata , prizm coral and 2 Yuma's.

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25 Responses on The humble ugly algae eating machine. The Sea Hare.

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  2. john banks says:

    what a cracking article, i have a few questions if thats ok?

    firstly, i understand the need for aclimating but i'm also dubious about adding lfs water into my tank. do you need to keep the seahare submerged and lastly would you be able to keep us updated on its survivably on purchased food?

  3. Curvball says:

    Hi John, I'll try get Mo over here to answer your questions. I myself have never owned a sea hare so I can't help. Thanks for stopping by.

  4. Mo says:

    Hi there – ive just finally read this after it being posted for ages.

    Ill try and be brief in answer to your questions.

    1. nope the sea hare doesnt need to be submerged – quite often mine has its head out the water.

    2. Purchased food – hmm – nope ive never seen it eat anything – even un eaten fish flake food that is on the sand…
    so sorry it would only do the algae.

    Would just mention that you need to be aware the shrimps will pick on the sea hare – and can depending on how often happens drive the sea hare to climb out of the tank – or die if they really pick at it.

    but to temper that statement they are hardy creatures so mostly will just hide and the 2 cleaners shrimp i had get bored easily and it just hides its head until they leave it alone…

    but then it did climb out of the tank once – and thanfully its thought better of that since then …

  5. Miles says:

    I became really attached with my sea hare also! Crazy cute.

    In case you want to keep him, here's what I found he would eat:

    Ulva Lettuce

    Some varieties of Red Gracilaria.

    I order mine from and keep it in a small 10 gallon alge tank, adding some for the sea hare as needed. I've kept mine for 6 months in a solana nano.

  6. wireless_jo says:

    Great article, still really useful 2.5 years on – we have just purchased our first Sea Hare (Daisy) to assist in our ongoing battle with hair algae. She is already a star of our tank and we are very protective of her!

  7. Mo says:

    Good luck Daisy !

  8. Carla says:

    We've had our sea hare for one week in our 28gal nano cube that was quickly overcome with hair algae. It has done a magnificient job with keeping the hair algae closely clipped. I do, however, worry about how much algae it needs to survive. Should nori sheets be added, or the algae pellets to suppliment its diet? We absolutely adore this interesting creature and want to properly care for it. Is it true that if it dies, it will poison everything in the tank?

  9. Trisha says:

    Hi, i have had my sea hare for about 9 months. he has been truly happy in my tank but I think that he is now dying. Yesterday I noticed that down his back he seems to have a spilt and it has a yellow and blue hue! any one any answers?

  10. Diggity says:

    That's about when I noticed mine start to disappear a few years back. Once the split back showed up, it was the beginning of the end. I would take it back to the LFS immediately. That or get his/her tombstone engraved for the memorial service.

    Loved mine too. Became the star of the show immediately.

  11. andy says:

    i had a sea poo years ago when i had my 1st marine tank(3rd time lucky?) and it was overstocked and i didn't know much. had it 4 over a year and only died when its stomach was sucked into my powerhead and ripped its guts out! i tried to remove it and all of sudden its belly split and a huge plume of purple came gushing out thanks to being stuck on said powerhead. threw it strait in the bin and all my fish and corals were fine. i know they're meant to release toxins but i only did a w/c after a few days (4 lack of r.o water) so no, i don't think they're that toxic or as prone to dying as a lot of people say….that said; i bought an xl sea poo today to combat my hair algae and 4 the 1st time after reading this, i'm nervous about losing him/her! please pray 4 it!

  12. Paul says:

    What a great write up,will be getting one tomorrow.

  13. Little turtle says:

    we bought a sea hare last weekend it has now buried itself in the sand at the corner of the tank I took it out and it went right back to the corner should I worry bout him

  14. Taylor says:

    I just recently purchased a sea hare and ever since we got it, it has been standing up vertically. Should we make it go on it's belly or what? He hasn't even eaten.

  15. djfrankn says:

    Hi there, Little turtle. Well, if it has buried in the sand it is most likely that your sea hare is stressed. Do you have shrimps or another creature in your tank that was attacking the new addition? Are your nitrates, PH, salinity within acceptable range? Check those things first and if so, post back.

  16. DAmmons says:

    I had two great Sea Hares, they ate great, adjusted well and were making great progress on my Algae then in one night they both climbed over the intake to my recirculating system and got sucked onto intake grill and died. Any ideas how to keep them off of the intake? I know they can crawl up,down, sideways, upside down, etc.

  17. Lisa says:

    I bought a small sea hare and I cannot find it in my tank at all. I moved rock around looked in the sand and no sea hare what could have hapened to it????

    • Mo says:

      Sounds like bad news , they are hard to find sometimes , can even borrow between sand and rock bottom.

      If you didn’t find it it may have climbed out of tank like mine did once… :(

  18. Raj says:

    Another interesting finding about the sea hare Bursatella leachii purple ink, these not only for the defensive use but also secreted ink contains very useful biomedical compounds for the future research:
    The Bursatella leachii purple fluid has an enzyme system capable of catalyzing the heparin. Heparin-like anticoagulant is known to occur in marine mollusks. Thus, it is not possible for the compound to act as an anticoagulant, since molluscs are devoid of coagulant system as is present in mammals. Several proteins such as protamines and histones can interfere with its anticoagulant activity. The purple fluid showed activity against the heparin like activity, but did not exhibit any haemolytic activity.
    In addition, Bursatella leachii purple fluids contain promising anti-HIV protein called “Bursatellanin-P”(60 kDa).
    CURRENT SCIENCE, 82, 3, 2002, 264-266.
    Mar. Biotechnol. 4, 2002, 447-453.

  19. Evilla says:

    I bought a green sea hare to clean up the red hair algea in my tank and it wouldnt touch it. I have however noticed that it doesnt really eat the green algea either but will gobble up bloodworms and mysis shrimp with gusto and leftover flake food. I still cant figure it out and I had it almost two months now.

  20. rebecca smith says:

    My 100 gallon FWLR is overcome with green hair algae. My LFS tracked down this little critter and it arrived today. I floated the bag about 1 hour then let the Hare in the tank. Within 1 hour it was eating the algae off the sea slug’s shell then promptly went on to a rock and started eating mouthfuls at a time. This bugger is so fascinating. I’m sure it will clean up the tank in no time. I’m going to try and keep it alive as long as I can before I take it back to the LFS.

  21. Moonmoor says:

    I have a sea hare in my nano tank. I didnt buy it it just appeared! It started out quite small and very secretive only coming out after lights out. Tonight it honered us by parading itself along the glass front and what a whopper it is now, it has grown so quickly, or maybe it is another one and the first one is hiding still. I dont know were this one has been before deciding to make its presents known. I have lots and lots of green algy all over my tank so it has lots of food. I have been seeing the brown poo packages and wondered what they were. Lol

  22. steve b says:

    Iv been reading this article over the past week and it has been very helpful iv got a sea hare coming today.
    Going to be swooping it between two tanks to do its job if it does the job as many people have sead it will my tank will be so much better

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