Ascidians - The Sea Squirts - Neville Coleman

The basic tunicate or sea squirt is round or sac-like, and is permanently anchored to the substrate. The body is enclosed in a tunic of cellulose (a rare substance in the animal kingdom) which is the source of its name.

Red - brested  Ecteinascidia  Ecteinascidia maxima Kott, 1985

PHYLUM: CHORDATA

CLASS

Ascidiacea (tunicates, sea squirts, colonial ascidians and compound ascidians)

Posy of sea squirts made up from small red Didemnums, Dictyota sp. algae and colour forms of the Stout Rhopalaea Rhopalaea crassa. These rather attractive associative clumps are common in some areas of Papua New Guinea and Indonesia.
(Photo Neville Coleman)

Features 

The Similar Clavelina Clavelina pseudobaudinensis ( Kott, 1976) was recorded from Lord Howe Island during the 10-87 expedition. It was photographed at 15 metres on the side of a wall. The species has a very distinctive white blotch with a dark blue spot in between the inlet and outlet siphons.
(Photo Neville Coleman) 

Sea squirts, or ascidians, may not be the most well-known underwater creatures yet they are the most advanced of invertebrates.
The basic tunicate or sea squirt is round or sac-like, and is permanently anchored to the substrate. The body is enclosed in a tunic of cellulose (a rare substance in the animal kingdom) which is the source of its name.

Almost set in a circle, these Golden Polycarpa Polycarpa aurata (Quoy & Gaimard, 1834) solitary ascidians are an easily recognised species that occurs across the Indo - Pacific. It grows to around 100mm and generally has its inlet siphon higher than the outlet siphon.
(Photo Neville Coleman)

Each individual has two body openings; one is an inlet for water which brings in oxygen for respiration and suspended plankton and detritus for food; the other is an exhalant siphon (usually at the higher end of the body) through which water-bearing carbon dioxide and waste is pumped out of the body.
The inhalant siphon is generally larger and towards the bottom of the body. The large internal gills are ciliated and act as the pump.

More often than not Julin's Phallusia Phallusia julinea -Sluiter, 1915- is found beneath dead coral slabs, or imbedded in coral crevices, or ledges. In this specimen it can be seen that the larger siphon ( lower one) is the inlet siphon and the smaller outlet siphon is in a higher aspect with the entrance turned away from the intake.
Although the body is transparent, the yellow pigmentation is present on most adult specimens and the characteristic pattern makes it recogniseable.
(Photo Neville Coleman)

The name sea squirt generally refers to solitary ascidians but some are colonial with many individuals either connected by a stalk to a common base or with the zooids embedded in a firm, jelly-like matrix, or as an investing sheet of tissue.

Many are beautifully coloured. In some investing compound ascidians each zooid may have a small separate intake siphon, but waste products are channelled out from larger communal exhalant siphons.

The Many - lobed  Botrylloides Botrylloides anceps ( Herdman, 1891) is found along the  Australian coast from central Queensland, south to north Western Australia. It  comes in a number of colour variations, ranging from black, to orange and brown and forms are mostly seen as lobed colonies. This colour form is from Port Stephens New South Wales.
(Photo Neville Coleman) 

Lifestyle 

There are about 1350 types of ascidians in the world's oceans. Ascidians are entirely marine and live wherever they can find their preferred settlement area; some species are specific in their choice of substrate, others are not. They can be found on reefs, rubble, algae, seagrasses, shells or rocks, in caves and under ledges and in sand and mud. There are even pelagic ascidians that drift in ocean currents and have complex life histories.

The Candlegrease Didemnum Didemnum sp. comes on a range of colours and forms depending on where it is growing. In protected habitats on ledges, in caves and along walls it takes on a free flow form. In another more exposed reef habitat it can be flat and encrusting. These fantastic forms were recorded from Milne bay, Papua New Guinea.
(Photo Neville Coleman)

Reproduction  

Most species of ascidians are hermaphrodites. Eggs are released from the exhalant siphon and fertilised by free-swimming sperm in open water, or are held in the exhalant siphon and fertilised by sperm taken in through the inhalant siphon.
The embryos in many species develop quickly, often in a few hours. This may be important in enabling them to settle and develop in suitable situations near their parents.

Brain - like Sycozoa cerebriformis (Quoy & Gaimard, 1834) occur from mid central eastern Australia, south, to mid central Western Australia. The colonies may be up to 200mm across and range in colour from orange, to pink and grey to bright red.
This compound ascidian has vertical double rows of zooids connected to common exhalent openings along the top of the colony.
(Photo Neville Coleman)

They have also been of great value to embryologists and because of their rapid development have been very useful in experimental studies. The larvae are distinctly tadpole-like and have a notochord or rudimentary backbone.
It is the presence of the notochord in the larvae that indicates sea squirts to be related to animals with backbones (vertebrates).

Certainly the most easily recognised common tropical species is the Soft Didemnum Didemnum molle (Herdman, 1886). It can vary in colour from white, to brown or green.
The green colour is produced by the presense of symbiotic Prochlorons. Each globe has a large exhalent opening at the top and hundreds of small inlet siphons around the circumference.
(Photo Neville Coleman)

Associations 

 The main food source for the Rose - ringed Nembrotha Nembrotha rosannulata nudibranch, the Blue - Sigillina Sigillina cyanea ( Herdman, 1899) inhabits areas of sandy bottom with strong current flow. This image was taken off Port Stephens New South Wales. 
(Photo Neville Coleman)

Very few commensal associations have been recorded for ascidians though it is known that they are inhabited by amphipods, shrimps and the pearl fish Carapus sp. (see Australian Fish Behaviour 1993: Coleman).

Identification   

A widespread Indo - Pacific species, the Moluccen Clavelina Clavelina moluccencis ( Sluiter, 1904) may have blue, or transparent colonies.
Both the inlet siphons ( wider ones) and the outlet siphons may be encircled in white and there are 3 dark blue spots in between the siphons. This colonial species can be visually identified by these distinctive characteristics.
(Photo Neville Coleman)

Visual recognition to species level has come a long way over the past 30 years and quite a number of ascidians can be recognised from a good photograph. However, there are many different colours and patterns within many of the investing species and until there is enough photographic evidence cross-referenced with taxonomically identified preserved specimens, many will remain mysteries.

Yellow Didemnum Didemnum sp. is an Indian Ocean species which is not able to be identified to species visually. It lives on coral and rocky reefs in around
8 to 20 metres of water and colonies reach 250 mm across.
A brightly-coloured bulbous growing species, this ascidian did not appear common though it was easy to recognise whenever seen. It is firm to touch and quite spongy with high ridges and minimal attachment to the substrate.
(Photo Neville Coleman)

Rarely seen in other regions in the Indo-Pacific this delicate and attractive colonial ascidian Diazona sp. is particularly common around Sabah, Malaysia and is also present in the Philippines and Indonesia.
( photo: Neville Coleman)

Another colour form of the Candlegrease Didemnum Didemnum sp. from Milne bay Papua New Guinea. The image was taken on the point at Sponge Heaven.
(Photo Neville Coleman)

Found on walls in shady areas with some current flow, the Leopard Diplosoma Diplosoma ferrugeum - Kott, 2001 - is a tropical species which occurs from the Houtman Abrolhos in Western Australia north, around and own the east coast of Australia to at least Lord Howe Island. The pattern and texture is readily determinable for this species and it can be recognised in the field.
(Photo Neville Coleman)

Found in closley knit colonies under overhangs and along walls, the Rock -wall Eusynstyela Eusynstyela latericus ( Sluiter, 1904) may cover around 200mm of area. The species is widely distributed and is known from the Persian Gulf to the Philippines, Indonesia, the Solomon Islands and Australia.

(Photo Neville Coleman)

Living on reef from 20 to 40 metres, these bright green colonies of an unknown ascidian were discovered by my self at 20 metres off Roach island, Lord Howe Island on a research fauna survey during 10/1987.
Since then the new species has been described and published as the Spongy Lissoclinum Lissoclinum spongium Kott, 2001. The small colonies can merge into a sheet measuring over 1 metre across.
(Photo Neville Coleman)

 Well patterned and easily determined, this encrusting species was originally discovered at 20 metres at Heron Island Great Barrier Reef on 16/07/1973.
Now known as the Tabascum Aplidium Aplidium tabascum - Kott, 1992 - it needs to be studied in close up to see its most characteristic identification features.
(Photo Neville Coleman)