''Didemnum vexillum'' is a species of

colonial Colonial or The Colonial may refer to: * Colonial, of, relating to, or characteristic of a colony or colony (biology) Architecture * American colonial architecture * French Colonial * Spanish Colonial architecture Automobiles * Colonial (1920 a ...

tunicate A tunicate is a marine invertebrate animal, a member of the subphylum Tunicata (). It is part of the Chordata, a phylum which includes all animals with dorsal nerve cords and notochords (including vertebrates). The subphylum was at one time ...

in the

family Family (from la, familia) is a group of people related either by consanguinity (by recognized birth) or affinity (by marriage or other relationship). The purpose of the family is to maintain the well-being of its members and of society. Idea ...

Didemnidae Didemnidae, or Didemnidæ, is a family of colonial tunicates in the order Enterogona. These marine animals are found in shallow water on the seabed. Members of this family have small zooids that form encrusting colonies. The body of each zooid i ...

. It is commonly called sea vomit, marine vomit, pancake batter tunicate, or carpet sea squirt. It is thought to be native to Japan, but it has been reported as an

invasive species An invasive species otherwise known as an alien is an introduced organism that becomes overpopulated and harms its new environment. Although most introduced species are neutral or beneficial with respect to other species, invasive species adv ...

in a number of places in Europe, North America and New Zealand. It is sometimes given the nickname "''D. vex''" because of the vexing way in which it dominates marine ecosystems when introduced into new locations, however the

species epithet Specific name may refer to: * in Database management systems, a system-assigned name that is unique within a particular database In taxonomy, either of these two meanings, each with its own set of rules: * Specific name (botany) A botanical name ...

vexillum The ''vexillum'' (; plural ''vexilla'') was a flag-like object used as a military standard by units in the Ancient Roman army. Use in Roman army The word ''vexillum'' is a derivative of the Latin word, ''velum'', meaning a sail, which con ...

'' actually derives from the

Latin Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...

word for flag, and the species was so named because of the way colonies' long tendrils appear to wave in the water like a flag. ''D. vexillum'' can grow on most hard

substrate Substrate may refer to: Physical layers *Substrate (biology), the natural environment in which an organism lives, or the surface or medium on which an organism grows or is attached ** Substrate (locomotion), the surface over which an organism lo ...

s, including cobble, boulders, and artificial structures including boats, dock pilings, and aquaculture equipment. It can also grow over other organisms, including sessile invertebrates,

algae Algae ( , ; : alga ) are any of a large and diverse group of photosynthetic, eukaryotic organisms. The name is an informal term for a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from ...

, and

eelgrass Eelgrass is a common name for several plants and may refer to: * ''Zostera'', marine eelgrass * ''Vallisneria ''Vallisneria'' (named in honor of Antonio Vallisneri) is a genus of freshwater aquatic plant, commonly called eelgrass, tape grass or ...

. Its appearance has given rise to the common names of sea vomit or marine vomit. It was first described from New Zealand as ''Didemnum vexillum'' by Patricia Kott in 2002. The species had been discovered previously in its native region of Japan.

Description

A colony of ''Didemnum vexillum'' consists of a number of sac-shaped

zooid A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooid ...

s connected by a common tunic. Each zooid is about long and has a buccal siphon through which water is drawn into the interior. The water then passes into a shared cavity from which it is pumped out through an atrial siphon. The surface of the colony is smooth, leathery, and often veined in appearance; the buccal siphons appear as numerous fine pores, and the atrial siphons as a smaller number of larger holes. The colony is firmly attached to a hard surface from which it can be difficult to detach. ''D. vexillum'' has different forms in different locations. It can form a thin or thick encrusting mat, or form large or small lobes. The colour can be orange, pink, tan, creamy yellow or greyish-white and the tunic is sparsely strengthened by stellate spicules with nine to eleven rays. Where there is little water movement, the colonies may dangle in ropey masses from hard substrates, such as cables, docks, and the hulls of vessels. These stringy formations have led to it being colloquially referred to as "Sea Cheese" in Canada. In places with stronger currents, they cover the surface of rocks, boulders, pebbles, gravel, and oysterbeds in a thin, encrusting layer.

Distribution

''Didemnum vexillum'' appears to be native to Japan, where it was recorded in

Mutsu Bay is a bay located within Aomori Prefecture, in the northern Tōhoku region of northern Japan. It has an east-west distance of approximately and a north-south distance of approximately at its eastern end, with a total area of approximately . N ...

in 1926. It is still common there and, as well as growing on rock surfaces and seagrasses (''

Zostera ''Zostera'' is a small genus of widely distributed seagrasses, commonly called marine eelgrass, or simply seagrass or eelgrass, and also known as seaweed by some fishermen and recreational boaters including yachtsmen. The genus ''Zostera'' con ...

''), it grows as a fouling organism on cultured

bivalves Bivalvia (), in previous centuries referred to as the Lamellibranchiata and Pelecypoda, is a class of marine and freshwater molluscs that have laterally compressed bodies enclosed by a shell consisting of two hinged parts. As a group, biv ...

, net cages, pilings and other man-made structures. Its depth range is from the intertidal zone down to a depth of about . The Portuguese oyster (''Crassostrea angulata'') cultivated off the Atlantic coast of France and Portugal was largely killed by an iridoviral disease in 1969. To replace the stock, large quantities of

Pacific oyster The Pacific oyster, Japanese oyster, or Miyagi oyster (''Magallana gigas''), is an oyster native to the Pacific coast of Asia. It has become an introduced species in North America, Australia, Europe, and New Zealand. Etymology The genus ...

(''Crassostrea gigas'') spat were imported from Japan. Some efforts were made to kill any fouling organisms that might be associated with the spat by immersing it for two periods, each of one hour, in fresh water. However, in the early 1970s, there were reports of a brown ''Didemnum ''

ascidian Ascidiacea, commonly known as the ascidians, tunicates (in part), and sea squirts (in part), is a polyphyletic class in the subphylum Tunicata of sac-like marine invertebrate filter feeders. Ascidians are characterized by a tough outer "tunic ...

on oyster beds in the region. Around the same time, oyster spat from Japan was introduced into the Pacific coastal regions of Canada and the USA and soon afterwards an ascidian was observed there, and later identified as ''D. vexillum''. By 2014, this tunicate was present in the eastern Atlantic, North Sea and Mediterranean Sea in the Netherlands, the United Kingdom, Ireland, France and Italy. On the east coast of North America its range extended from New Jersey, New York, Connecticut, Rhode Island, Massachusetts and New Hampshire, to Maine. In the eastern Pacific its range extends from Alaska to California. It is also present in New Zealand, where it was first observed in 2001, its only known encroachment into the Southern Hemisphere. Because this ascidian broods its embryos and the larvae are free-swimming for only a brief time, natural dispersal of ''D. vexillum'' is limited in extent. Dispersal to new habitats in far flung parts of the world is likely to be through its accidental conveyance on the hulls of boats or other floating structures, or its introduction to aquaculture installations, perhaps with commercial oyster spat, seed mussels or aquaculture equipment.

Biology

Like other colonial ascidians, ''Didemnum vexillum'' is a

suspension feeder Filter feeders are a sub-group of suspension feeding animals that feed by straining suspended matter and food particles from water, typically by passing the water over a specialized filtering structure. Some animals that use this method of feedin ...

. Water is drawn in through the buccal siphon of each zooid, the

plankton Plankton are the diverse collection of organisms found in water (or air) that are unable to propel themselves against a current (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they provide a cr ...

and fine particles of

detritus In biology, detritus () is dead particulate organic material, as distinguished from dissolved organic material. Detritus typically includes the bodies or fragments of bodies of dead organisms, and fecal material. Detritus typically hosts commu ...

are filtered out, and the water and waste products leave through a common atrial siphon. Each individual

is a

hermaphrodite In reproductive biology, a hermaphrodite () is an organism that has both kinds of reproductive organs and can produce both gametes associated with male and female sexes. Many taxonomic groups of animals (mostly invertebrates) do not have ...

. Sperm is liberated into the sea and may get drawn into another zooid with the water current, fertilisation taking place internally. The

embryo An embryo is an initial stage of development of a multicellular organism. In organisms that reproduce sexually, embryonic development is the part of the life cycle that begins just after fertilization of the female egg cell by the male sperm ...

s have yolks from which they derive nourishment while they are brooded inside the colonial tunic. When they hatch, after about two weeks, the

larva A larva (; plural larvae ) is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle. Th ...

e have a short free-living stage lasting up to a few hours, before undergoing metamorphosis into a zooid ready to found a new colony. The new colony grows by asexual reproduction, with new zooids

budding Budding or blastogenesis is a type of asexual reproduction in which a new organism develops from an outgrowth or bud due to cell division at one particular site. For example, the small bulb-like projection coming out from the yeast cell is kno ...

off existing ones. A fragment of a colony may become detached (perhaps by "dripping" off a floating structure), adhere to a new

and found a new colony. Such colonies can grow rapidly, with a six- to eleven-fold increase in colony size having been observed over a fifteen-day period.

Invasive impact

''Didemnum vexillum'' is capable of forming large colonies which can overgrow rocks and gravel, smother

benthic The benthic zone is the ecological region at the lowest level of a body of water such as an ocean, lake, or stream, including the sediment surface and some sub-surface layers. The name comes from ancient Greek, βένθος (bénthos), meaning "t ...

organisms and change the marine balance of the seafloor community. In some places in the Netherlands it covers 95% of the seabed and there is a marked decrease in populations of the

sea urchin Sea urchins () are spiny, globular echinoderms in the class Echinoidea. About 950 species of sea urchin live on the seabed of every ocean and inhabit every depth zone from the intertidal seashore down to . The spherical, hard shells (tests) ...

'' Psammechinus miliaris'' and the

brittle star Brittle stars, serpent stars, or ophiuroids (; ; referring to the serpent-like arms of the brittle star) are echinoderms in the class Ophiuroidea, closely related to starfish. They crawl across the sea floor using their flexible arms for locom ...

Ophiothrix fragilis ''Ophiothrix fragilis'' is a species of brittle star in the order Ophiurida. It is found around the coasts of western Europe and is known in Britain as the common brittle star. It is also found along the coast of South Africa where it is known ...

''. On the

Georges Bank Georges Bank (formerly known as St. Georges Bank) is a large elevated area of the sea floor between Cape Cod, Massachusetts (United States), and Cape Sable Island, Nova Scotia (Canada). It separates the Gulf of Maine from the Atlantic Ocean. T ...

, Massachusetts, the seabed community is also changed, with some invertebrates decreasing but others, such as two species of

polychaete worm Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are ma ...

, increasing in abundance. Here, ''D. vexillum'' restricts the potential settling sites for larvae of the

bay scallop ''Argopecten irradians'', formerly classified as ''Aequipecten irradians'', common names Atlantic bay scallop or bay scallop, is a species of scallop in the family Pectinidae. An edible saltwater clam, it is native to the northwest Atlantic ...

(''Argopecten irradians'') and the

sea scallop ''Placopecten magellanicus'', previously listed as ''Pecten tenuicostatus'' and as ''Pecten grandis'' and once referred to as the "giant scallop", common names Atlantic deep-sea scallop, deep sea scallop, North Atlantic sea scallop, American se ...

(''Placopecten magellenaicus''). It exudes a toxic substance which discourages

predators Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...

and prevents the larvae of other species from settling on it. In New Zealand it overgrows

mussel Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, whic ...

s and aquaculture equipment, but has not had quite such a devastating effect on the

ecology Ecology () is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms at the individual, population, community, ecosystem, and biosphere level. Ecology overl ...

as anticipated.

Genome assembly

In 2016 the draft genome of ''Didemnum vexillum'' reported the non-coding RNA annotation complement. To this end a combination of several computational approaches, including blast searches with a wide range of parameters, and secondary structured-centred survey with infernal. The resulting candidate set was curated extensively to produce a high-quality ncRNA annotation of the first draft of the ''D. vexillum'' genome. It comprises 57 miRNA families, 4 families of ribosomal RNAs, 22 isoacceptor classes of tRNAs (of which more than 72 % of loci are pseudogenes), 13 snRNAs, 12 snoRNAs, and 1 other RNA family. Additionally, 21 families of mitochondrial tRNAs and 2 of mitochondrial ribosomal RNAs and 1 long non-coding RNA. A new version of ''D. vexillum'' genome is online available. This new version combines PacBio technology and the Illumina raw data used to assembly the first draft genome version published in BMC genomics.

References

External links

Species Profile - Sea Squirt (''Didemnum vexillum'')
National Invasive Species Information Center,

United States National Agricultural Library The United States National Agricultural Library (NAL) is one of the world's largest agricultural research libraries, and serves as a national library of the United States and as the library of the United States Department of Agriculture. Located ...

. Lists general information and resources for Sea Squirt. {{Taxonbar, from=Q4917479 Enterogona Animals described in 2002