Tunicates in the Marine Aquarium

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).

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]