Types of sea squirts

There are 2 main types of sea squirts: solitary and colonial.  The main features of a solitary tunicate are 2 siphons (branchial and atrial), a generally cylindrical shape, and a leathery or otherwise tough-textured outer covering or tunic.  A few examples of solitary and colonial forms are considered in this section, while INVASIVE SPECIES are dealt with elswhere in the ODYSSEY.

NOTE  lit. “covering or cloak”.  The covering of tunicin is rich in protein, and may represent up to 2/3rds of the body mass in some species.  However, it is too leathery to be edible by humans and this part is discarded in those species that are used as food.  The Japanese “delicacy” maboya, for example, represents the soft parts of a species related to the west-coast sea-peach Halocynthia after the leathery tunic has been discarded.  Maboya is definitely an “acquired taste”



Sea peach Halocynthia aurantium 0.6X

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Research study 1

Solitary tunicates either live on their own, that is, apart from other individuals, or are gregarious, forming large, closely-packed clusters of the same species.  Unlike, other invertebrate aggregations, such as sea anemones Anthopleura elegantissima that are produced asexually, aggregations of sea squirts result mainly from the propensity of the larvae to seek out and to settle near adults of the same species.

photograph of a sea star Evasterias troschelli surrounding a group of solitary tunicates Cnemidocarpa finmarkiensis

Solitary habit in tunicates obviously evolved in response to factors that favoured survival of this feature.  How, then, do we explain the type of aggregating behaviour seen in Cnemidocarpa finmarkiensis and other “solitary” species?  Ideas relating to this suggest a complex balance of costs and benefits, most of which are obvious.  Costs include space and food competition, risk of inbreeding, and possible habitat deterioration.  Benefits include a better chance of fertilisation and more guarantee of living in a favourable area (thus, a higher survival of larvae). Svane & Havenhand 1993 Mar Ecol 14: 53.


A large mottled star Evasterias troschelli surrounds a
group of solitary, but aggregated, tunicates Cnemidocarpa
. Perched at the 2 o'clockposition in the
photo is a juvenile sea cucumber Psolus chitonoides

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Research study 1

drawing of several "zooid systems" of the colonial tunicate Botryllus schlosseri

Colonial sea squirts are either social or compound.  In the former, drawing of a single zooid system of the colonial tunicate Botryllus schlosseriindividuals of the colony have the body shape of solitary forms, but are inter-connected by basal stolons.  In compound forms each member of the colony, known as a zooid, has its own branchial siphon but shares a common atrial siphon with several other zooids usually arranged in a ring around the siphon (known as a "system"). In either case, the colony starts with a single settled larva, and subsequent growth is by asexual budding.  All zooids in a colony are genetically identical clones.  Compound tunicates form smoothly textured, often colourful, growths on intertidal and subtidal rock surfaces.  photograph of a single zooid system of the colonial tunicate Botryllus schlosseri courtesy Stanford University marine collection The zooids in compound forms, such as Botryllus schlosseri, are embedded in a gelatinous matrix and are surrounded by blood vessels and ampullae. Milkman 1967 Biol Bull 132: 229.

NOTE  lit. “animal” G. referring to an individual member of a tunicate colony

Close view of a "zooid system" of the colonial tunicate
Botryllus schlosseri.
Photo courtesy Stanford University marine collection

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Research study 2

photograph of several colour morphs of colonial tunicates Distaplia occidentalis

drawing of several zooids of the colonial tunicate Distaplia occidentalis sharing a common atrial siphonSome colonial species, like Distaplia occidentalis, may share or partly share a common atrial siphon in compound fashion, but are connected by stolons in social fashion.  the different colours of Distaplia in the photo probably represent several different individual colonies. In the drawing on the Right, three zooids share a common atrial opening. Water flow, generated by beating cilia in the branchial basket, moves from outside the zooid, into the branchial siphon, through the branchial basket, and out the atrial siphon. Mackie 1974 Can J Zool 52: 23.

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Research study 3

photograph of the social tunicate Metandrocarpa tayloriBudding is effectively a cloning of new, smaller individual from an adult zooid, each of which has the same genetic makeup of the adult and, thus, of all the members of the colony.  Metandrocarpa taylori is a colonial tunicate of the social kind.  Its members are connected by stolons within which flow nutrients and energy material gained by feeding of any of the members. 

Note in the series below the pattern of budding that occurs after a number of larvae of Metandrocarpa are allowed to settle and metamorphose onto a piece of 15 x 15cm slate in the laboratory, and then placed in the intertidal zone off the California coast.  The colony starts with 18 larvae, or 18 different genotypes.  Growth during each of 4mo in the ocean is indicated.  After 1mo, only 4 new buds have appeared (2 of these during a short stay in the laboratory). Following this, budding becomes geometric. Haven 1971 Biol Bull 140: 400. 

1-month stage in a series showing growth of the social tunicate Metandrocarpa taylori 2-month stage in a series showing growth of the social tunicate Metandrocarpa taylori 3-month stage in a series showing growth of the social tunicate Metandrocarpa taylori 4-month stage in a series showing growth of the social tunicate Metandrocarpa taylori
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