By the Numbers: Tunicates, Different from Sponges

Not everything that looks like a sponge is a sponge. Covered by the “tunics” that give them their names, outwardly, tunicates resemble sponges and can be hard to tell apart from them. But their internal architecture is much more complex than sponges. 

Colonial tunicates, species undetermined, at Bonaire. 

IT’S THE “TUNIC” PART OF TUNICS’ ANATOMY THAT GIVES THEM THEIR SUPERFICIAL RESEMBLANCE TO SPONGES. The stiff, cellulose coverings that conceal tunicates’ inner workings may be very different from sponges’ walls of individual cells. But sometimes you have to look to discern the differences.

The similarities are a little ironic because tunicates and sponges are found at the opposite end of invertebrates’ evolutionary journey. Whatever, tunicates are probably among the more underappreciated denizens of reef life.


1  Tunicates are actually complex animals that start off with characteristics of animals with spines and, in a feat of “reverse evolution” end up as more-or-less card-carrying invertebrates.

2  Whereas sponges lack organs and tissues, in their free-swimming larval stages tunicate bodies feature tails, pharyngeal gill slits, dorsal central nerve cords and notocords – structures found in animals with backbones. They have digestive tracts, hearts and circulatory systems that move pale green blood around.

3  During the transition to adulthood, however, the tails, nerve cords and notocords disappear and they settle down as sessile invertebrates.

Which doesn’t keep some experts from referring to them as “Chordates,” or at least “Proto-chordates.” Another term used to be “urochordates,” but in recent years that’s become passé.

5  Actually, terminology for tunicates is a little vague and subject to interpretation by the scientist doing the interpretation. For example, “Urochordates” is still easy to find in tunicate research.

A “simple” Ascidian, species undetermined – an individual animal with straightforward inflow and outflow siphon openings, on the Great Barrier Reef.

6  The most important group of tunicates are the some 2,300 species in Class Ascidiacea (meaning “Little Bottle”).

7  From free-swimming larvae, as adults Ascidians secure themselves to the to the substrate – corals, rocks, shells or other surfaces – and live sessile lives capturing plankton from the passing currents.


8  To be clear, tunicates are called tunicates because the Ascidian varieties develop stiff outer covering called “tunics.” They’re often called “Sea Squirts” because, if disturbed (for example, brought to the surface), they have a habit of squirting out water.

9  Individual animals are called zooids. They may be solitary, social (clumped groups that are attached at their bases) or compound communities of many zooids that can form large colonies.

10  They can be tube- or bottle-shaped or encrusting, growing over and mimicking the surfaces beneath. Colors can vary widely as diverse as red, green, black, purple and deep blue.

11  Sizes can range from tiny to several feet tall.

Bluebell Tunicates (Clavelina puertosecensis), at Utila Island, Honduras. 

12  Tunicates feed by filtering plankton from the surrounding water, drawing it in through buccal siphons (in-current openings), filtering it though their digestive systems and expelling it through atrial siphons (out-current openings).

13  Ascidian siphon openings are normally relatively few per animal and much larger than the numerous pores of sponges…

14 Although some encrusting species draw water in through numerous small opening and exhale it through a single excurrent siphon.

At left, an encrusting Ascidian, species undetermined, at Roatan Island, Honduras; at right, an encrusting Orange Sheath Tunicate, at Cape Ann, Mass.

15  Also, the siphons are generally found in pairs – as in, inflowing and outflowing. An individual Ascidian may have its own siphons but some community-based groups may share a single siphon arrangement at their common base.

16  A significant tunicate tell, often visible inside the siphons, are stiff tissues that support the internal organs and help the Ascidians hold their shapes.


A chain of Salps in the Red Sea, their internal organs clearly visible within transparent bodies.

17  The other significant group are in Class Thaliacea (“to bloom”), free-swimming open-water animals.

18  The most notable of these are some 40 species of Salps.

19   Salps are basically barrel-shaped with incurrent openings at one end and outflow openings at the other.

20  They’ve been described as resembling transparent jet engines…which fits, since they can achieve a sort of jet propulsion by pushing water through their bodies.

21  Mostly, however, they’re planktonic creatures that go where the currents take them.

22  Transparent their gelatinous bodies may be, but within them the internal organs are  highly visible, sometimes colorful.

Sometimes, Salps link up in circular chains.

23  Salps reproduce by aesexual budding, which makes it easy for them to join together to form long chains, sometimes in circular shapes.

24  While their gelatinous makeup might seem jellyfish-like, they don’t sting.

PRINCIPAL SOURCES: Reef Coral Identification, Florida, Caribbean, Bahamas, Paul Humann, Ned DeLoach; Marine Life, Caribbean, Bahamas, Florida, Marty Snyderman & Clay Wiseman; Marine Biology, Peter Castro, Michael Huber; Tunicate, Ascidiacea, Wikipedia.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.