Research by marine biologists from Wageningen University has shown that feeding on zooplankton by scleractinian corals has been greatly underestimated.
|Written by James Fatherree, M.Sc.|
Coral reefs are home to a vast number of invertebrate species. Many of these are relatively well-known, however others exist which are quite inconspicuous. Most people interested in marine life quickly learn about the more popular invertebrates, such as corals, clams, shrimps, and snails. However, even after having been a diver or marine hobbyist for years, one may know absolutely nothing about a group of animals called “tunicates”.
Figure 1: The Green sea squirt, Didemnum molle.
Tunicates can be seen just about everywhere a SCUBA diver might look during some dives, but despite this, they are much less frequently seen in aquarium shops. Unfortunately, they often starve to death in aquaria, as they depend on feeds that are typically difficult to supply in adequate quantities. Thus, the tunicates, despite their beauty, have not reached the level of popularity that many of the invertebrates which are relatively easy to care for have.
Still, despite this tendency to starve in closed aquarium systems, tunicates are seen for sale regularly, and are occasionally found on quality live rock. Although their husbandry is complicated, long-term survival seem possible. The next section describes the basics of tunicate husbandry.
"Their bodies are always covered by a complex skin, properly called a tunic, which is obviously where their name comes from."
Figure 2: A group of Golden sea squirts, Polycarpa aurata, which can be several centimeters tall. Note the individual in the middle, part of the pharynx is visible inside the atrial siphon.
When tunicates are in their larval phase, they look very similar to a microscopic tadpole. They have what appears to be a fat body and a flattened tail for swimming, and also display a rod-like notochord and hollow nerve cord that runs through their body. Later, the larvae undergo an extensive metamorphosis, and everything that made them look anything like a tadpole is lost in the process.
Tunicates also share numerous other traits and features. For example, all of the tunicates are considered to be marine organisms, although there are some species that may be found in brackish waters as well. Their bodies are always covered by a complex skin, properly called a tunic, which is obviously where their name originates from. This tunic is typically quite thick, tough and leather-like. However, at times it may also be much more delicate and thin, and actually rather transparent. It is flexible and able to expand and contract. The tunic is also strengthened by numerous structural fibers made of a unique compound called tunicin, and may contain carbonate fragments or may incorporate grains of sand, as well.
Figure 3: This colony (likely Botryllus sp.) appeared on the overflow drain of a large live rock holding system and grew to a large mat in a matter of months.
The tunicates are divided into three groups; the thaliaceans, the larvaceans, and the ascidians, with the first two being adapted for a planktonic lifestyle and the latter being bottom-dwellers. The thaliaceans and larvaceans are pelagic and may display weak swimming behaviour, while the ascidians live attached to the seafloor or some other surface. To the best of the author's knowledge, none of the planktonic tunicates have ever been kept in aquaria, and this may prove to be highly difficult. Acsidians remain the best candidates for home and public aquaria.
Individual ascidians can be fairly big, with the largest species reaching a size of several centimeters and resembling a potato. This concerns only one individual sea squirt, though. Ascidians can also form colonies comprised of as many as several thousands of zooids, which can form small mounds, oddly branching structures, or encrusting mats covering large areas of seafloor at times. However, the zooids that make up a colony are especially small compared to solitary ascidians, and they are either connected by common tissue referred to as stolons, or share a common tunic that completely covers all individuals. Encrusting soft corals such as Clavularia sp. display similar morphology.
The majority of ascidians is found in shallow waters, and is typically attached to hard substrates such as live rock, coral rubble, or even live corals. However, there are a few which live on soft bottoms and in deep waters, as well. Of course, specimens from deeper waters are usually not available to the aquarium industry, as they would be difficult for divers to collect.
As far as appearances go, many species are often very colourful and their surface typically has two openings. These are the buccal and atrial siphons, which are the openings where water moves into and out of the body chamber, respectively. When in colonial form, each zooid has its own buccal siphon, but shares a common atrial siphon with other zooids. All in all they look fairly simple on the outside.
Watch a video of a colonial tunicate species, Neptheis fascicularis:
Down inside the siphons things get much more complicated, though. Ascidians have many of the same sorts of organs and systems that other “higher” animals possess, including a heart, a digestive tract, various muscles, and male and female reproductive organs. They also have a specialised gill structure that is also used for feeding, which is called the pharynx.
Ascidians, especially solitary species, are also able to contract tightly and seal up the siphons as a response to disturbances or unwanted matter in the water. At times they can do this rather rapidly, which may result in strong expulsion of water through the siphons. This is where the term sea squirt originates from.
Figure 4: This large cold-water ascidian, Halocynthia roretzi, has some relatively large siphons that are easy to see.
With few exceptions the tunicates are filter feeders that remove particulate matter from sea water as it passes through the pharynx and gills. Tiny hair-like cilia line the gills and beat rapidly, creating a surprisingly strong current of water. This draws water into the buccal siphon and moves it through the pharynx, which resembles a slotted basket. Subsequently, after having passed through the slots, the water moves out of the atrial siphon minus the food particles collected. The concept is that all of the incoming water is strained of food, as all of it must pass through the basket.
The filtration process is quite complex, and involves more than straining the water through slits. Specialised structures produce a thin mucus net lining the pharynx, and when water passes through it, food particles are trapped in the mucus. The mucus net is constantly moved across the gills by cilia, and as food is collected the net is rolled up into a thin cord, which is subsequently moved to the oesophagus.
This entire process allows tunicates to efficiently capture a range of fine particles ranging from small phytoplankton all the way down to bacteria, which are taken into the oesophagus and consumed. The food is then digested and fecal pellets are cast out of the anus, which is located near the atrial siphon. These are removed by the current flowing through the siphon.
Interestingly, some species host microscopic symbionts. Some ascidians house algal and cyanobacterial cells within their tissues, and can thus obtain nutrition from sources other than what is filtered from the environment. However, it is likely tunicates are not able to rely on their symbionts to the degree which reef-building corals and their relatives can.
When diving around reefs, you can find various sea squirts here and there if you look closely, and in some places they are very common. In fact, in some spots they occur literally everywhere, in all sorts of shapes, sizes, and colours. They reside on boat hulls, in tide pools, on rocky shores, and even in drainage canals. As mentioned before, they are sometimes found on live rock imported for aquarists as well, and beautiful specimens may be seen for sale at times. With that in mind, it is tempting to think they are rather resilient and could thrive in a aquaria. This is inaccurate, unfortunately.
In general, sea squirts tend to be very difficult to keep alive in aquaria. Their husbandry has proven to be very challenging, which is the reason they are not as popular as corals and many other invertebrates. If they were easy to care for, they would be seen for sale everywhere.
They are difficult to maintain in aquaria for the same reason that many other filter-feeding animals are. Aquaria generally do not contain sufficient concentrations of proper feeds to keep them alive long-term, and many specimens need more than what we can add to the aquarium safely. This holds especially true if they are large in size, or present in large numbers.
Figure 5: This colony of ascidians displays typical morphology, growing on a chunk of live rock. The zooids are fused together and share a common tunic.
Most of those which are imported on live rock do not live long, and neither will most of the specimens which are purchased separately (Fatherree, pers. obs). Those that are imported on live rock are more likely to die due to rough handling and exposure to air. Exposure seems to be particularly bad for the colonial forms, as they typically disintegrate within a few days after being introduced to an aquarium, while solitary specimens have fared somewhat better. It may be possible that the latter group is able to close down tightly enough to hold in some water and keep air out, however there could be other reasons as well.
"When tunicates are in their larval phase, they look very similar to a microscopic tadpole."
However, with that said, in the last few years, in mature, more heavily stocked aquariums with well-stocked deep sand beds, there has been a significant increase in the survival of tunicates and everything else that consumes plankton and bacteria, including sponges and filter-feeding worms (Fatherree, pers. obs.). Healthy, established systems can produce much more particulate matter in many forms than a new system can, and the inhabitants can take advantage of this.
The careful use of quality phytoplankton feeds may have benefited some species as well. The key is finding a balance between how much food can be added to sustain filter-feeders without adding so much that water quality is affected negatively. That can be a difficult task, but certainly not impossible by any means. It takes some restrain from the aquarist, as overdosing the aquarium with phytoplankton, even if it is alive, can lead to elevated nutrient levels. This will invariably lead to the growth of unwanted algae, which is highly problematic.
With all this in mind, the conclusion can be drawn that well-established, mature systems, and regular addition of quality phytoplankton feeds is required for successful tunicate husbandry. As frustrating as it may be, the fact is that there are still quite a lot of filter-feeding organisms (such as Dendronephthya sp., many sponges, Crinoids, etc.) that have very low survival rates in aquariums, no matter what you may do to try to keep them alive. Maybe the day will come when these animals can be routinely kept alive and well, but we’re not there yet.
As a last note, if you think you have an appropriate system and are willing to do what it takes to properly care for tunicates, it is recommended to start with small specimens. It is preferable to see a small specimen grow into a larger one, instead of starting out with a large animal requiring more food and having a high chance of starving to death. Conduct as much research as possible on any particular species you are interested in before you purchase it – not after.
Figure 6: A large solitary sea squirt, Halocynthia papillosa.
All photographs by James Fatherree. Unauthorized usage of these pictures is strictly prohibited.
Rupert, E.E., R.S. Fox, and R.D. Barnes. 2004. Invertebrate Zoology: A Functional Evolutionary Approach: 7th ed. Brooks, Cole, Thomson, Belmont CA. 963 pp.