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Related FAQs: Sea Squirts, Ascidians 2, Ascidian ID, Ascidian Behavior, Ascidian Compatibility, Ascidian Selection, Ascidian Systems, Ascidian Feeding, Ascidian Disease, Ascidian Reproduction,

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/The Conscientious Reef Aquarist

Almost Us!? Sea Squirts, Tunicates, Ascidians, Subphylum Urochordata, Phylum Chordata Pt. 5

To: Pt. 1, Pt. 2, Pt. 3, Pt. 4,

By Bob Fenner

Unidentified Blue Ascidians Costa Rica (Pacific side), 2011

Genus Rhopalaea:

Rhopalaea sp. (likely R. crassa) Translucent cool colored groupings. Common throughout the tropical Indo-Pacific, reef flats and slopes. Predated by Nembrotha nudibranchs. Vertical whitish or brown lines are their sperm ducts. N. Sulawesi pix. 

Verticals (Full/Cover Page Sizes Available
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Rhopalaea abdominalis. Reef Tunicate.  3/4 to 1.5 inch. Dark brown to purple in colour. Cozumel 2008.

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Genus Symplegma:

Symplegma viride, Encrusting Social Tunicate. Variably colored, tunic sharing colonies. Roatan 2019.

Genus Synoicum:

Synoicum sp. N. Sulawesi pic. 

Genus Trematooecia:

Trematooecia aviculifera, Bleeding Teeth Bryozoan. Caribbean. 1/2-3 inch colonies. Encrust in protected areas. Turks image.
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Genus Trididemum:

Trididemum solidum, the Overgrowing Mat Tunicate. 3-12 inches. Tropical West Atlantic. Small individuals embedded into a heavy communal mass. Gray, blue, green or white. Cozumel images by Di. 

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Many unidentified Ascidians:

A nice, unknown species of blue/black Ascidian I found in Mabul, Malaysia. Included just because I thought it beautiful.

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General Biology:

Tunicates are made up of individuals termed zooids. The sessile (permanently attached, as opposed to the pelagic species) ones are attached to various substrates and have two siphons (incurrent, excurrent) in which they pump water through the gill net in their bodies. In fact, their ability to contract their siphons is the one easy trait to separate the ascidians from the at-times similar Sponges (phylum Porifera) which have permanent openings. 

    The basic body plan of tunicates varies from simple to compound with single individuals clustering to degrees, to sharing tunics to having collective larger siphons in common within a colony. Simple ascidians may live only a year. Larger, colonial types may persist for several years. 

    There are about two thousand described species. 


Worldwide, in shallows to abyssal depths. 


Zooids come in one millimeter to some five inches in length.

Aquarium Husbandry:

    Most folks get their ascidians for "free" as explants with their live rock, though they can be procured through specialty shops and suppliers on the Net. They vary tremendously in their hardiness/capacity to be kept let alone propagated. Happily, many that come into the trade as "recruits" are not as sensitive to temperature change, edgy and changeable water quality et al.  due to the sorts of environments their substrate is collected from. 

Selection, Acclimation, Placement:

     Like Sponges, ascidians should not be lifted into the air. Best to carefully place them with their substrate in a bag of just adequate size, with plenty of water, and in turn double, treble this with other bagging outside to support the inner bag. Some species sponsor endo-symbiotic species of algae, but many are cryptic, living in the dark or at least the shade. Careful observation of the other life on, near them will grant you insight as to what type of environment yours favors. You are encouraged nonetheless to provide plenty of water circulation about these animals. 


     All ascidians are plankton feeders. Well-seasoned systems with refugiums, not-over-vigorous skimming, and daily addition of phytoplankton preparations, blended (as in a blender) foods (either solids and/or liquids, and/or fresh/frozen material... plankton infusoria, newly hatched brine shrimp, minced worms, flakes, bivalve juice...), with the filter pumps cycled off (for fifteen minutes via timers is best...) are good ways of assuring nutrifying these animals. 

Symbioses, including Predator-Prey Relations:

    There are whole groups of crustaceans, worms and other phyla that make their lives with/in ascidians. Some are mutualistic, others tend to parasitism. Know that there are Sea Stars, Urchins, Crabs, Hermits, Snails (including some nudibranchs) and some fishes that are predatory on ascidians. Best to keep so-called "reef-safe" fishes, stony and soft corals, gorgonians, mushrooms, zoanthids, sponges, tubeworms and bivalves with these animals. BTW, folks with large marine Angels et al. feeders on Sea Squirts might save some money shopping for these foodstuffs in the Korean Foods section of your food store. 


    These animals are hermaphroditic, possessing both functional male and female structures, but cross-fertilization is the rule, with sperm from one animal triggering release of eggs and subsequent fertilization by another individual. Additionally these animals reproduce asexually in a number of ways. 


    Interesting? You bet... for such apparently simple, attached animals, the ascidians are complex animals with digestive, nervous, reproductive/developmental and circulatory systems (closed, with a heart, but alternating in pumping direction every few hours!) that point up their relationship with the "higher" chordates. Useful? Again, for sure. If you have healthy live rock, you assuredly have Sea Squirts. These cryptic animals are there, either too small, too camouflaged to be seen, or under, within your LR. Filtering away, keeping your water clean, perhaps adding their gametes occasionally to the mix of filter-feeding materials.

    You won't see their free-swimming young in your system, but if you look closely you will find these close relations to the vertebrates in/on your live rock. 


Bibliography/Further Reading: http://www.ascidians.com/

Marine Hitchhiker/Critter ID (Maughmer, Toonen, Tompkins)

Allen, Gerald R. & Roger Steene. 1994. Indo-Pacific Coral Reef Field Guide. Tropical Reef Research, Singapore.

Erhardt, Harry & Horst Moosleitner. 1998. Marine Atlas, v.3 Invertebrates. MERGUS, Germany. 1,326pp.

Gosliner, Terrence M, Behrens, David W. & Gary C. Williams. 1996. Coral Reef Animals of the Indo-Pacific. Sea Challengers, Monterey, California. 314pp.

Mercier, Annie & Jean-Francois Hamel. 1998. Tunicates. A crucial step in evolution. FAMA 3/98.

Volkart, Bill. 1990. Sea squirts- tubular wonders. TFH 10/90.

To: Pt. 1, Pt. 2, Pt. 3, Pt. 4,

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