Bizarre and Surprising, Sea Squirt Care is a New Type of Challenge

Common Name(s): Sea Squirts, Tunicates

Taxonomy:

Domain: Eukarya

Kingdom: Animalia

Phylum: Chordata

Subphylum: Urochordata

Class: Ascidiacea, Thaliacea, Larvacea

Order: Numerous

Family: Numerous

Genera: Numerous

Description:

Sea squirts, or tunicates, are unusual and unique creatures that are definitely unlike anything else you can find in a marine aquarium. They are such crazy animals that it is almost difficult to know where to start when describing them. Though they appear to be simple, they are, in fact, the most advanced aquarium organism you can keep that is not a fish! They are a shining example of how appearances can be deceiving.

In the aquarium environment, adult tunicates are most often encountered as misidentified sponges. Though similar in their porous, amorphous structures, sponges and sea squirts are not at all closely related. More often than not, the aquarist discovers the tunicate colony on a newly imported piece of live rock, or finds that is has been growing unseen in the tank all along. Hitchhiking tunicates are a welcome bonus in a reef tank, though they can also become invasive fouling organisms in the wild, if introduced to non-native waters. Tunicates frequently come in colonial masses that look like lace surrounded by a firm gelatinous membrane. The colonial mass is punctuated with holes, the incurrent and excurrent siphons, and may be colorful, black, or even transparent.

The first thing you must know about tunicates is that they are chordates. Being chordates means they share certain similarities with all other chordates on Earth (including us). Namely, at some point in its development, every chordate will exhibit a hollow dorsal nerve chord, pharyngeal gill slits, a flexible notochord (providing dorsal support), a post-anal tail, and a ventral heart. During the development of most animals from phylum Chordata, the notochord grows to surround the dorsal nerve chord, to form a bony or cartilaginous spine, or vertebral column (the backbone). These are the vertebrates, and they are the most evolutionarily advanced animals on the planet. Mammals, birds, reptiles, and fish are the most familiar kinds of vertebrates. Tunicates, on the other hand, demonstrate a seemingly backwards maturation process because they are some of the few invertebrate chordates. Unlike most others, they lose their dorsal nerve and notochord as they become adults, or zooids. In this manner, the larval and zooid forms are vastly different. Larvae are free-swimming and resemble tad-poles. They have a mouth, called the incurrent siphon, as well as an excurrent siphon and an anus. They possess internal gill structures, a muscular tail, a dorsal nerve, and a notochord. Surprisingly, they even have a heart, a stomach, and an eye. Adult tunicates, on the other hand, are mainly sessile (attached) filter feeders. Their bodies, which can range from very colorful, to drab, to clear, may even house photosynthetic bacteria called Prochloron, which helps to provide nutrition for its host. Mature sea squirts lose much of their vertebrate likenesses, by taking on a barrel-like body shape and eliminating the rudimentary spine and eye. Their siphons become much more pronounced, as does the pharynx (the filtering mechanism/stomach). Adults maintain a small nerve ganglion, grow gonads, and develop a firm layer of leathery outer tissue know as the tunic (from which the name tunicate is derived).

Natural Habitat and Ecology:

Tunicates are found worldwide, in both shallow and deep waters. In coastal waters, they are known to be ample contributors to fouling growth, and their tough tunic makes them difficult or impossible to remove. They are extremely efficient filter feeders, and a single zooid may be capable of filtering up to 100 gallons of water each day. The extreme dietary needs of tunicates may be one reason why they tend to thrive in the nutrient rich waters of harbors and marinas. Due in part to their mobile larvae, and in part to global ship traffic, tunicates spread easily to new areas. In some regions of the world, invasive species like the club tunicate (Styela clava) cause significant problems for endemic species and marine equipment.

Tunicates are usually colonial, and the vast majority of the three thousand known species come in one of two varieties: sessile and free-swimming. When sessile, they are known as sea squirts (class Ascidiacea). Once fixed in place, most sea squirts never move again. However, at least two aquarium species of barrel tunicate, Atriolum robustum and Didemnum molle, are known to move across the substrate.

Those tunicates that are not sessile are called salps (class Thaliacea). Through the use of their siphons, they are able propel themselves through the water, and can even control buoyancy by adjusting the ionic compositions of their body fluids. Most salps form large floating colonies, and some are capable of creating unusually bright displays of bioluminescence.

There also exists a third small group of tunicates, about 70 species in all, termed larvaceans (class Appendicularia). Larvaceans are free-swimming like salps, but unlike all other tunicates they retain their notochord throughout their life. They also cast a large mucus net, referred to as a house, to ensnare small food particles.

Tunicates use broadcast spawning to reproduce sexually, and most are hermaphroditic. In some species, there is an internal gestation period which may last for more than a month. Many can also reproduce asexually through the process of budding. New animals grow from the tunic of older zooids, while a network of blood vessels connects the entire colony.

Tunicates are interesting in other ways, as well. For instance, the symbiotic bacteria tunicates house are very similar to plant and algal chloroplastsa possible indication of an ancient evolutionary relationship. In fact, the consensus scientific opinion is that the cellular organelles chloroplasts and mitochondria, originated as symbiotic bacteria similar to Procholoron. In addition, tunicates have been tagged as potentially valuable pharmaceutical animals. Ongoing genetic research has also suggested that calling tunicates chordates may not even tell the entire story of what is a tunicate. They are truly amazing creatures!

Aquarium Care:

Sea squirt care varies to some degree. As previously mentioned, some hardy varieties survive the collection and transfer of their live rock homes, only to thrive in the home aquarium. Other species can be more difficult to care for because they have very demanding nutritional needs. Their filtering capacity allows them to sift through large quantities of water in a short time, removing much of the bacterio-, zoo-, and phytoplankton that they feed upon. Photosynthetic species, like the aforementioned green barrel tunicates, may have higher survival rates because of their symbiotic bacteria. Tunicates will benefit from direct daily feedings of brine shrimp nauplii, live rotifers, and a splash of mollusk juice. Feeding can be achieved using turkey baster or a syringe. You can also cut off the end of a plastic water bottle, set it inverted over the colony, and squirt the food in underneath so that it is trapped by the plastic. Be sure to remove the end of the bottle after about 5 minutes.

The great thing about tunicates is that their water quality needs are less demanding than many other aquarium invertebrates. They can handle minor drops in water quality, but it is generally a good idea to maintain parameters that are similar to those needed by mushroom polyps and zoanthids. Lighting needs will vary according to species, ranging from none to brightly lit. For the sake of your other tank inhabitants, it is advisable to maintain fairly strong filtration to offset the extra feeding needed by sea squirts. Some species even secrete toxins, so good water flow and protein skimming will also benefit your other livestock. The trick is to balance the amount of feeding and skimming, so that toxins and nutrients are removed by sufficient planktonic animals remain.

Sea squirt care is generally considered difficult. However, that assertion is greatly dependant upon the species you possess. Surviving hitchhikers will need no special attention. If purchased from the store, it is likely that specialized attention will be needed. Good candidates for captive care include the solitary Giant Pacific tunicate (Polycarpa aurata) and Cobalt tunicates (Rhopalaea spp. ), as well as the colonial squirts of genus Clavelina. Adding one or more of these tunicate species will certainly add a twist to your reef tank. The striking color patterns, the weird ethereal translucence, and the unusual ecology of the sea squirt is more than enough reason to give them a try. Fulfilling the extra care needs will only make you a better marine aquarist anyway!

Quick Notes:

Tunicates are one of the examples of invertebrate chordates.

Tunicates are highly efficient filter feeders capable of filtering 100 gallons per day.

Sea Squirts are sessile animals that often enter aquaria as hitchhikers on live rock.

They come in all kinds of colors and color patterns, including transluscent.

Feeding demands are high, so direct daily feeding is a must.

Works Cited:

Huber, Peter Castro and Michael E. Marine Biology 5th Ed. Boston: McGraw Hill, 2005.

Jennings, Greg. The New Encyclopedia of the Saltwater Aquarium. Buffalo: Firefly books Ltd., 2007.

Shimek, Ronald L. Marine Invertebrates. Neptune City : T.F.H Publications, 2004.

Shimek, Ronald L. Tunicates or Sea Squirts: A Wet Link. Reefkeeping Magazine Online. Mar. 2005, 4:2. 2005. URL:< http://www.reefkeeping.com/issues/2005-03/rs/>

Tullock, John H. Natural Reef Aquariums; Simplified Approaches to Creating Living Saltwater Microcosms. Neptune City: T.F.H Publications, 2001.

Photo Credits:

Cline, Linda. Dancingfish Online. 2006. URL: < http://www.dancingfish.com/news/2006/06/>

Pearson Education, Inc. Publishing as Benjamin Cummings.