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You are here: Home / Invertebrates / Tunicates / Tunicates in the Marine Aquarium

Tunicates in the Marine Aquarium

by greg.chernoff

Aquarium hobbyists, and reef aquarists in particular, are anything but conformists. If truth be told, they love to stand out from the crowd. There are all sorts of ways to break from the pack–experimenting with novel equipment or methodologies, building a supremely aesthetic aquascape, and so on. But the one sure way to garner attention is to successfully keep (if not propagate) a rare, unusual and difficult species.

Different species are challenging for different reasons. Some highly specialized or highly sensitive creatures truly do belong in expertly maintained “species tanks.” In quite few instances, however, an animal’s only major husbandry issue stems from feeding. Such issues are often encountered in efforts to keep sessile planktivores such as tunicates.

It could seem ironic (perhaps hilariously so) that with all the emphasis reef aquarists place on limiting algal growth, this would be an impediment to keeping any kind of animal. In actuality, the problem lies in providing the right type of algae in the right amount and manner. Where excessive dissolved nutrient concentrations are the norm, several problems arise. Benthic algae proliferates and eventually smothers sessile invertebrates. Under these adverse conditions, the wrong types of phytoplankton may prevail (noxious cyanobacteria, dinoflagellates, etc.). Moreover, planktonic blooms may become a boom-and-bust affair, forcing phytoplanktivores to starve between water changes, replacement of chemical filter media, and so on.

[Phyto]

It’s important to understand that on typical coral reefs (which are quite nutrient-poor), phytoplankton productivity is mainly allochthonous; in other words, it grows in, and washes in from, elsewhere. And not very much washes in, either. Organic carbon obtained from drifting oceanic phytoplankton contributes only 0.3%-13% to a reef’s gross production. This makes phyto a very, very precious food source (particularly for certain essential vitamins and fatty acids). Even marine snow many contribute considerable nourishment.

Filter-feeding animals such as tunicates are well-adapted to efficiently harvest phytoplankton and bacterioplankton but are also utterly dependent upon them. Low-quality dead or artificial invert additives just won’t cut it for these guys! Providing a very steady supply of nutritionally balanced live phyto is critical to maintaining their long-term health in captivity.

 

Living Phyto Filters

The tunicates (otherwise known as the urochordates or “tail-chordates”) are a sizeable group at around 1,500 species. They are so-named for the tough, cellulose-based tunic that covers and protects the animal. Sharing ancestry with fishes (and therefore all vertebrates), some are free-swimming (i.e. the larvaceans and thaliaceans) and are indeed fairly fish-like. But it is the ascidians that are of the most interest to reef aquarists.

Members of the urochordate class Ascidiacea (charmingly referred to as the sea squirts) are overwhelmingly sessile as adults. They may be solitary (e.g. Polycarpa), colonial (e.g. Clavelina) or compound (e.g. Botryllus). However, because of their lifestyle as sedentary filter-feeders, they all must actively draw water (along with suspended food particles) into and out of their bodies in order to eat. This is carried out mainly by ciliary action. Water is taken in at the mouth through an incurrent siphon and passed at the gill slits through an excurrent siphon. While each individual ascidian usually possesses its own incurrent siphon, the excurrent siphon may be shared in compound species.

Mucus in the gills captures food. Cilia on the pharynx spread the mucus in a sheet. As food particles are captured in the mucus net, they are moved by cilia into stomach. Digestion takes place in the midgut. Wastes are expelled from the anus, which is located near the excurrent opening.

 

Tunicates in Captivity

To date, there have been few successes keeping tunicates in aquaria. Even the beautiful Didemnum molle, which grows with the aid of symbiotic algae, has yet to become a reef tank regularity. To be fair, there have been relatively few earnest efforts to keep ascidians. Part of this can be blamed on the poor availability of quality live planktonic aquarium foods.

A few species appear with some regularity in the aquarium trade. Polycarpa aurata and Neptheis fascicularis are but a couple examples of routinely collected sea squirts. Various ascidians may be obtained as hitchhikers on high-quality, recently harvested live rock.

While they do not necessarily enjoy being continuously blasted by strong, direct currents, sea squirts are best placed in an area of vigorous, chaotic water movement. With the rare exception of the few species that harbor symbiotic algae, there is no need to place an ascidian under bright light. In fact, doing so can actually encourage nuisance algae to grow on their tunic.

Ascidians struggle in captivity enough as it is; poor water quality will only push them over the cliff. Any system that houses these delicate animals should be mature and stable and have excellent water conditions. Though nobody really knows why, they store enormous amounts of trace minerals such as vanadium in their bodies; therefore, trace elements should be replenished regularly and as directed.

Finally, they should be fed very well! When keeping sea squirts, do not settle for anything less than the best foods. Live phytoplanktonic and bacterioplanktonic feeds (e.g. PNS Probio™) should be added frequently (once or even twice daily!). Mechanized dosing is ideal. Heavy feeding brings us right back to water quality; regularly monitor key parameters such as nitrate, pH, alkalinity, etc. and perform large water changes routinely to ensure that the aggressive feeding does not foul the aquarium water!

With so few reports of their captive and care and with acceptable live planktonic foods just now becoming widely available, ascidian husbandry is hardly an exact science. There remains much about them to be learned. Still, with a few more tests and trials, these beautiful and fascinating creatures could certainly become prevalent in the hobby in coming years.

[Phyto]

Reader Interactions

Comments

  1. Asad Khan says

    November 25, 2019 at 2:48 pm

    I didn’t even know what tunicates were prior to reading this. Thanks for expanding my knowledge.

    Reply
  2. Kelly Tompkins says

    November 25, 2019 at 3:23 pm

    Sea squirt is fun to say! I wonder how often we get tunicates on live rock and frag plugs. I get sponges sometimes too.

    http://bit.ly/2PTnrN2

    Reply
  3. Kelly Tompkins says

    November 25, 2019 at 3:24 pm

    Tunicate sounds like a first aid term. Sea squirt is fun to say! I wonder how often we get tunicates on live rock and frag plugs. I get sponges sometimes too.

    http://bit.ly/2PTnrN2

    Reply
  4. Kelly Tompkins says

    November 25, 2019 at 3:26 pm

    I have never seen them for sale at my LFS.

    http://bit.ly/2PTnrN2

    Reply
  5. Kelly Tompkins says

    November 25, 2019 at 3:26 pm

    I have never seen them for sale at my LFS

    Reply
  6. Kelly Tompkins says

    November 25, 2019 at 3:27 pm

    Besides food related mortality. Are they hearty?

    Reply
  7. Jonathan says

    November 25, 2019 at 6:13 pm

    Don’t see them too often. Some fish love them, so you need to be careful.

    Reply
  8. Mike Mijarez says

    November 25, 2019 at 8:35 pm

    Can they come on lfs bought items?

    Reply
  9. robert vice says

    November 25, 2019 at 9:58 pm

    I dont believe I’ve seen those before

    Reply
  10. Kinue says

    November 25, 2019 at 11:35 pm

    Can’t say I’ve ever seen tunicates at any LFS. Great article.

    Reply
  11. Nickn says

    November 26, 2019 at 3:36 am

    I havent seen any available

    Reply
  12. dartagnon_1965 says

    November 26, 2019 at 4:16 am

    Never seen these

    Reply
  13. dartagnon_1965 says

    November 26, 2019 at 4:34 am

    These do not appeal to me.

    Reply
  14. F Fudge says

    November 26, 2019 at 6:25 am

    Solid info

    Reply
  15. Joe Car says

    November 26, 2019 at 6:56 am

    Thank you for the info, never seen them at my LFS.

    Reply
  16. Becky says

    November 26, 2019 at 7:07 am

    I have not seen any of these at my LFS. Maybe a good hitch hiker on some live rock?

    Reply
  17. Wilfredo Robles says

    November 26, 2019 at 12:08 pm

    never seen them

    Reply
  18. Cassandre-Leigh Klaasen says

    November 26, 2019 at 12:21 pm

    Ha ha – sea squirt !

    Reply
  19. Richard Baer says

    November 26, 2019 at 1:20 pm

    I’ve seen them as hitchhikers, but never for sale at my LFS that I’ve noticed.

    Reply
  20. phyllis owens says

    November 26, 2019 at 2:11 pm

    haven’t seen or known they exist, thanks for the article

    Reply
  21. polygonreef says

    November 26, 2019 at 5:18 pm

    So interested!

    Reply
    • Mark Spears says

      July 13, 2020 at 3:03 pm

      I have thousands in my system from 1 mm to over an inch long. These are white. Hairy mouth.

      Reply
  22. Michgander reefer says

    November 26, 2019 at 5:38 pm

    Never seen them out here in sd

    Reply
  23. Chris S. Buswell says

    November 26, 2019 at 7:36 pm

    At the risk of sounding negative, tunicates may be more trouble than they are worth.

    Reply
  24. Matt Blefeld says

    November 26, 2019 at 7:57 pm

    Very interesting

    Reply
  25. Steven Rodriguez says

    November 26, 2019 at 8:35 pm

    Very cool would love to have one

    Reply
  26. liesle_memmott says

    November 26, 2019 at 9:03 pm

    No luck in my world of seeing them. Would be interesting even if I can’t keep them.

    Reply
  27. Christopher Burns says

    November 26, 2019 at 9:52 pm

    i like them but not for my tank

    Reply
  28. Dallas Tippie says

    November 27, 2019 at 12:39 am

    Super cool organism!

    Reply
  29. daroberts87 says

    November 27, 2019 at 7:17 am

    https://www.algaebarn.com/

    Reply
  30. Brett says

    November 27, 2019 at 8:39 am

    Cool!

    Reply
  31. whwhunt says

    November 27, 2019 at 9:56 am

    Great read.

    Reply
  32. Liz McDaneld says

    November 27, 2019 at 4:01 pm

    Interesting read

    Reply
  33. Liz McDaneld says

    November 27, 2019 at 4:01 pm

    Interesting read

    Reply
  34. Tim Klieber says

    November 27, 2019 at 4:48 pm

    Learned something new today!

    Reply
  35. Nicholas Steele says

    November 28, 2019 at 5:28 am

    I’ve seen them while diving and it’s probably one of the many things I don’t have an interest in keeping in my tank!

    Reply
  36. Tim Kubajak says

    November 28, 2019 at 4:53 pm

    I just learned something new!

    Reply
  37. Echo says

    November 28, 2019 at 9:16 pm

    I’ve seen some tunicates but they seem like they’d be a bit high-maintenance. I might revisit the idea when I have more experience.

    Reply
  38. dcallahan56789 says

    November 29, 2019 at 8:40 am

    i personally wonder if this is something better left in the ocean until we better understand what we need to give it to survive. However, how do we figure it out until we try??
    There was a video released recently showing a diver releasing dye near a sea squirt. Truly amazing how much water passed through the animal.

    Reply
  39. rcedeno2 says

    November 29, 2019 at 7:14 pm

    Didn’t even know what tunicates was…

    Reply
  40. Wesley Spangler says

    November 29, 2019 at 9:11 pm

    I’ve only ever seen in photos.

    Reply
  41. Luis Aceves says

    November 30, 2019 at 3:08 pm

    Great info. Especially for beginners like me. Thanks.

    Reply
  42. James says

    November 30, 2019 at 7:57 pm

    Not something I want to worry about trying to keep.

    Reply
  43. James says

    December 1, 2019 at 10:37 am

    I don’t think keeping trunicates is worth the effort, I would hate to see something die because I didn’t do well enough.

    Reply

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