Ascidians (Phylum Tunicata, Class Ascidiacea)
An undescribed Aplidium species collected from the Three Kings Islands. This colonial species illustrates the arrangement of zooids into meandering systems. Photographer Mike Page.
Patricia Kott and Mike Page
General Description
Ascidians, commonly known as sea squirts, are the most diverse class of the Phylum Tunicata. The phylum is characterised by an outer coat of cellulose-like material (the tunic) synthesised by the epidermis, which occurs throughout two of the three classes (absent only in Appendicularia). Tunicates, like chordates, have a dorsal hollow nerve chord and a rod of notochord cells (albeit sometimes only in the larvae), a perforated pharynx (branchial sac) and a ventral pharyngeal endostyle that metabolises iodine (similar to the function of the thyroid gland in chordates). As a result, Tunicata was formerly considered to be a subphylum of the Chordata. However recent genetic evidence confirms its status as a separate Phylum.
Members of the class Ascidiacea are solitary or colonial organisms. Solitary species, common in intertidal habitats, are easily recognised. Generally, they have a tough leathery test with two siphons that squirt water as the animal contracts, if disturbed. The adults are invariably sessile, attached directly to hard substrates, or fixed in sediments by hair-like or branched-root-like extensions of the outer test. Individuals grow up to 10 cm or more. Solitary species in temperate waters appear to live for 12-18 months. However, published information on the life spans of New Zealand or Australian temperate species does not exist, although they may be found to be similar to species in the northern hemisphere.
Colonial species consist of small zooids embedded in a common, usually gelatinous, test with varying degrees of complexity of colonial systems. Many colonial species may live longer than solitary species. Some are known in which the zooids regress during winter months and regenerate in the spring. Colonies result from replication of zooids, connected by, or partially or completely embedded in, the test, and the rate of replication determines the growth rate of the colony. Generally smaller zooids (often less than 1 mm) result from more efficient replicative processes (which interrupt the growth of the individual). Species with prolifically replicating zooids often grow into massive, sheet-like or branched colonies of up to 50 cm or more. Owing to the exponential rate of replication in colonies, one of their advantages is to occupy space more rapidly than other organisms, including solitary ascidian species.
Gene flow in these sessile organisms is affected by tailed larvae, free swimming from two or three days to hours, and often only minutes. Rarely, some forms living on the open ocean floor develop directly-the site-selecting larval stage is lost altogether and juveniles are liberated from the parent. Generally, solitary taxa are externally fertilised, and their gametes and embryos, as well as the free-swimming tailed larvae, are subjected to dispersal. Less subject to dispersal are colonial forms, which are, without exception, internally fertilised and have larvae with relatively short free-swimming duration. Details of the life history are known for only very few species worldwide. Hypsistozoa fasmeriana, an endemic colonial species widely distributed throughout New Zealand, broods its young releasing large and complex larvae in early spring. Subsequent rapid asexual reproduction further insures the success and relatively wide distribution of this species through out New Zealand. However, there is little data available on recruitment for New Zealand fauna.
Externally fertilised (oviparous) solitary species, including Pyura gibbosa from New South Wales (related to the New Zealand P. pachydermatina), are fertile from late autumn to winter and developing embryos are in the water for about 12 hours before the tailed larvae hatch and settle. They begin metamorphosis in the next one or two hours. Individuals become functioning juveniles two to four days after fertilisation. Other solitary species have hatching times from eight to sixty hours; larvae free-swimming for three to twenty-four hours. In all colonial species embryos are brooded internally and tailed larvae, with a free-swimming time of as little as ten minutes, are liberated. Observations on a variety of species in the northern hemisphere indicate that colonies are fertile from spring into summer and that the pattern of development is similar to those indicated for temperate Australian species. Usually neither colonial nor solitary species are self-fertile, for although most species are hermaphrodite, they have strategies that ensure cross-fertilisation, such as first reproducing as males (protandrous) or females (protogynous).
Dispersal threatens the maintenance of populations of sessile organisms and may be the effective pressure that constrains the spread of populations and limits their range. Although it is possible that lack of collecting is the explanation for so many indigenous species of limited range, it could be that the phenomenon is a characteristic of the New Zealand fauna and a consequence of its isolation and hydrological conditions that tend not to disperse reproductive products.
That selective pressures to maintain populations are effective appears to be confirmed in the New Zealand fauna where large and crowded populations of individuals, many up to 10 cm or more in maximum dimension (eg, Pyura pachydermatina and other solitary species), are found in tight aggregates or grow in crowded populations in high-energy habitats such as Foveaux Strait where populations are subject to dispersal. The larval strategies that determine the degree of aggregation of individuals are not known.
Apart from some tropical species that appear to be have symbiotic algal cells living within their tissue (autotrophic) and some abyssal ascidians that appear to be active carnivores, all other ascidians are filter feeders. They obtain their food from water driven through the animal by the cilia lining the pharyngeal perforations. It is strained through micropores of a fine mucous sheet that is moved up over the pharyngeal wall also by ciliary action. Although fertiliser, insecticide and other organic pollutants and suspended sediments in terrestrial runoff could well affect these organisms, their depth range and dispersal of the pollutants in well-irrigated locations is likely to reduce adverse effects. It appears that the provision of maritime and other installations in New Zealand waters associated with tourist yachting, and fish and shellfish cultivation, provide additional habitats for ascidian species, which accordingly are seen as threats to commercial development.
Very little is known about the biogeography of the New Zealand ascidian fauna. Although some workers divided the region into seven latitudinal zones, there really are insufficient data to support this synthesis. Many species are known only from relatively few records of few specimens. Although there is little information, habitat may be more significant than latitude at this scale. Of the 166 species known from New Zealand, 125 are indigenous. Of these, twenty-two are recorded from two or more locations off the South Island and/or subantarctic islands and usually Stewart Island; twenty-eight are also known from the North Island; ten species are known only from Stewart Island; and twenty-five species (including twelve from the Chatham Rise or Chatham Island) are known only from one single location (other than Stewart Island) around the South Island. Twenty-seven species are known from only one or two records off the North Island, suggesting that there has been insufficient sampling, especially at these lower latitudes. There are twelve Antarctic, nine cosmopolitan (possibly introduced) and eight trans-Tasman non-indigenous species. However, the ascidian species that New Zealand has in common with Australia appear to have Gondwana rather than trans-Tasman affinities. In contrast to the Australian fauna (which has a strong tropical affinity and some Southern Ocean elements in temperate regions), the affinities of the unique New Zealand fauna appear to have been derived, predominantly, from a Gondwana fauna. Stewart Island (with sixty species recorded) has the greatest diversity of ascidian species and appears to be an overlap zone between the subantarctic and a New Zealand biogeographic zone, although this distribution pattern may be an artifact of sampling intensity.
Macquarie Island, though not part of the New Zealand exclusive economic zone, has a more recent (volcanic) origin and a rather impoverished fauna of about ten extant subantarctic species and a further five which appear to have become isolated from subantarctic sister species. The Macquarie Island fauna appears to be related more closely to the subantarctic than the New Zealand biogeographic region.
Status
The New Zealand ascidian fauna is of particular biogeographic and phylogenetic interest owing to the high proportion of indigenous species and their possible Gondwana affinities. Insufficient data is available to determine the geographic range of most of these in New Zealand waters. Owing to its indigenous nature, the ascidian fauna could be vulnerable to environmental change, although the deep waters around the continent, the strong currents irrigating the shallow subtidal areas may, at this stage, reduce the risks from pollution and/or sedimentation. Recently documented invasions of foreign ascidians such as Styela clava and Eudistoma elongatum are of concern. These species have potential to out-compete native fauna; however, they generally colonize artificial structures such as marine farms and wharf piles. Potential introduction and invasion of habitat-modifying species (eg, Pyura praeputialis) could also have significant effects on the distribution of New Zealand's indigenous fauna.
Key Locations
Scuba collections from Northland, Spirits Bay, and the Three Kings Islands suggest these are biodiversity hotspots for colonial ascidians. Recent collections from the Kermadec Islands, Fiordland, and Kaikoura detail further species that have yet to be described. Furthermore, recent biodiversity studies of New Zealand seamount fauna have found possible new species. In summary, relatively little is known about the biogeography of New Zealand ascidian fauna.
Summary of Threats
Threats to ascidians in New Zealand include introduced species, competition for resources (particularly in intertidal habitats), habitat degradation/eutrophication of estuarine environments, and destruction of habitat by bottom trawling.
Typical habitats
Ascidians occupy a wide range of benthic habitats from harbours, estuaries, the intertidal zone, subtidal reefs, biogenic reefs, the continental shelf/slope, and seamounts.
State of Information
The New Zealand ascidian fauna has not been described since Brewin's work, 1940-1960 and a subsequent monograph by Millar (1982), based largely on NIWA deepwater dredge collections. Many more new species remain undescribed. For example, Stocker (1985) detailed 19 undescribed colonial ascidians from coastal Northland waters. Furthermore, relatively recent collections of ascidians for isolation of novel pharmaceuticals have demonstrated that there are many more undescribed species in New Zealand's subtidal waters.
Significance for Maori
There is no known use or significance for Ascidians in the Maori world. To the Ngai Tahu people of the South Island, ascidians are termed kaeo (or kaeo, kaio), however their use is unknown.
References
Brewin, B I. 1946. Ascidians in the vicinity of the Portobello Marine Biological Station, Otago Harbour. Transactions and Proceedings of the Royal Society of New Zealand 76(2): 87-131.
覧. 1948. Ascidians of the Hauraki Gulf. Part I. Transactions and Proceedings of the Royal Society of New Zealand 77(1): 115-138.
覧. 1950a. Ascidians of New Zealand. Part IV. Ascidians in the vicinity of Christchurch. Transactions and Proceedings of the Royal Society of New Zealand 78(2-3): 344-353.
覧. 1950b. Ascidians of New Zealand. Part V. Ascidians from the east coast of Great Barrier Island. Transactions and Proceedings of the Royal Society of New Zealand 78(2-3): 354-362.
覧. 1950c. Ascidians from Otago coastal waters. Transactions and Proceedings of the Royal Society of New Zealand 78(1): 54-63.
覧. 1950d. The ascidians of the Sub-antarctic Islands of New Zealand. Cape Expedition Series Bulletin 11: 1-11.
覧. 1951. Ascidians of New Zealand. Part VI. Ascidians of the Hauraki Gulf. Part II. Transactions and Proceedings of the Royal Society of New Zealand 79(1): 104-113.
覧. 1952a. Ascidians of New Zealand. Part VII. Ascidians from Otago Coastal waters. Part II. Transactions and Proceedings of the Royal Society of New Zealand 79(3,4).
覧. 1952b. Ascidians of New Zealand. Part VIII. Ascidians of the East Cape region. Transactions and Proceedings of the Royal Society of New Zealand 80(2): 187-195.
覧. 1956a. Ascidians from the Chatham Islands and the Chatham Rise. Transactions and Proceedings of the Royal Society of New Zealand 84(1): 121-137.
覧. 1960. Ascidians of New Zealand. Part XIII. Ascidians of the Cook Strait region. Transactions and Proceedings of the Royal Society of New Zealand 88(1): 119-120.
Kott, P in press: Sea Squirts In: Gordon, D P (ed.), Phylum Tunicata. The New Zealand Inventory of Biodiversity: A Species 2000 Symposium Review. Canterbury University Press, Christchurch.
Millar, R H. 1982. The marine fauna of New Zealand. New Zealand Oceanographic Institute Memoir 85: 1-117.
Stocker, L J. (1985). An identification guide to some common New Zealand ascidians. University of Auckland Leigh Marine Laboratory Report, Auckland. 74pp.
Strickland, R R. 1990. Nga Tini a Tangaroa, a Maori-English, English-Maori Dictionary of Fish Names. MAF Fisheries, New Zealand. 65pp.
Table 18: Ascidians (Phylum Tunicata) in New Zealand
| Order | Family | Endemic species | Other species | Total species |
|---|---|---|---|---|
| ENTEROGONA | Agneziidae | 2 | 2 | |
| Ascidiidae | 3 | 1 | 4 | |
| Cionidae | 1 | 1 | ||
| Clavelinidae | 1 | 1 | ||
| Corellidae | 1 | 1 | ||
| Didemnidae | 19 | 3 | 22 | |
| Holozoidae | 5 | 2 | 7 | |
| Perophoridae | 1 | 1 | 2 | |
| Polycitoridae | 3 | 1 | 4 | |
| Polyclinidae | 38 | 5 | 43 | |
| Protopolyclinidae | 1 | 1 | ||
| Pseudodistomidae | 4 | 0 | 4 | |
| Pycnoclavellidae | 1 | 1 | 2 | |
| Ritterellidae | 4 | 2 | 6 | |
| 80 | 20 | 100 | ||
| PLEUROGONA | Botryllinae | 4 | 4 | |
| Hexacrobylidae | 1 | 1 | ||
| Molgulidae | 11 | 6 | 17 | |
| Polyzoinae | 10 | 4 | 14 | |
| Pyuridae | 13 | 2 | 15 | |
| Styelinae | 11 | 4 | 15 | |
| 45 | 21 | 66 | ||
| Grand Total | 125 | 41 | 166 |
Figure 48: Transparent sea squirt Ciona intestinalis annual distribution.