ASCIDIANS

Ascidians, also referred to as tunicates or sea squirts, give the appearance of being very simple animals. Yet they are very closely related to vertebrates as they do possess a gut and intestine and a rod-like structure similar to the vertebrate backbone. They are either solitary or occur in large communal groups. Some even form large colonial formations.

CONICAL TUNICATE

Aplidium conicum

This ball-shaped structure grows to around 20cm (8in) across and has numerous small apertures all over the body. Coloured almost translucent white to cream, it has less the characteristic of an ascidian and more resembles a sponge.

CIONA

Ciona intestinalis

Usually found on its own, this sea squirt grows to approximately 15cm (6in). It has a white or translucent body with lobed ends to its siphons. These lobes may also be tinged with bright yellow. The inhalant opening is on the top and the exhalant is about a third of the way down on the side. In colder waters, this large species is often associated with Ascidiella aspersa, another similarly sized tunicate. Their range covers the whole of the north Atlantic, but they are found especially in the Scottish sea lochs and southern Norway. Ciona is found from the lower shore, down to 500m (1,650 ft) and grows on rocks, boulder cliffs and algae. It is particularly fond of man-made structures, such as piers, shipwrecks and buoys, and it is thought that its very widespread distribution is due to its propensity to attach itself to ships’ hulls. It reproduces throughout the year once it reaches over 2cm (¾in) in height.

BLUESTRIPED LIGHT BULB TUNICATE

Clavelina dellavallei

This is the largest of a distinctive small group of sea squirts. Growing singularly, but in association with other individuals, it grows to around 2.5cm (1in) high. Tinged with blue lines and a yellow internal ribbing, it is unmistakable.

LIGHT BULB TUNICATE

Clavelina lepadiformis

A very similar species to the Bluestriped, this sea squirt grows in small attached colonies and is around 1.5cm (½in) tall with distinctive, white, light bulb filament markings within the transparent body cavity. It should not be confused with the Ball Tunicate, where the individuals are tightly packed together to form a ball shape; Light Bulb Tunicate colonies are much more loose, almost to the point of having individual zooids. These zooids are cylindrical and smooth, and you may be able to spot either amber- or red-coloured eggs or tadpole-shaped larvae within the body cavity. This species is occasionally found in deeper rock pools, but it is generally more associated with harbour walls, vertical cliffs and ledges.

GLASS BELL TUNICATE

Clavelina nana

The smallest of the bell tunicates at only 7mm (¼in) high, it grows in loose aggregations on algae fronds and around the base of sea fans. Stalked and creamy white in colour, its body has horizontal lines patterned throughout.

DENDRODOA

Dendrodoa spp.

This small orange or red sea squirt, only 8mm (¼in) in size, prefers low light but well-aerated conditions and can form quite large colonial patches. Slightly fuzzy in appearance, both siphons are next to each other. It is commonly associated with various hydroids, which grow amidst the colony. Dendrodoa species are also the tunicates most commonly associated with the sponge Clathrina, where the lacy network formations tend to weave their way amongst the small bells of the tunicate.

BALL TUNICATE

Diozone violacea

This is a tightly packed group of individuals, which resemble Clavelina lepadiformis in structure, but here they are grouped together to form a ball reaching 20cm (8in) in height and over 40cm (1ft 4in) in diameter.

SEA POTATO

Halocinthya papillosa

This is the most distinctive of the ascidians and individuals are usually a deep red colour. Some individuals may be a pale pink in deeper caves, but the siphons are still tinged dark red. It prefers a solitary existence and grows to around 12cm (4¾in).