Biogeography

Seven hundred and twenty ascidian species are now known from Australian waters, of which only 150 are

Figure 27.5 Common cloacal systems of Didemnidae: A, horizontal thoracic cavities (Didemnum, Polysyncraton); B, deep horizontal cavities (Didemnum, Polysyncraton, Lissoclinum); C, three dimensional thoracic and posterior abdominal canals; D, zooids cross extensive cavities between surface and basal layers of test in test connectives (Lissoclinum, Diplosoma); E, flask-shaped colony with central test core surrounded by cavity (Didemnum, Polysyncraton, Lissoclinum); F-C, zooids with atrial siphon and three-dimensional oesophageal and posterior abdominal canals, F, encrusting; C, upright colony lobe (Trididemnum, Leptoclinides). Note: The type of system is often found in, but not exclusive to, the genus indicated. (Figure: Kott 2001.)

Figure 27.5 Common cloacal systems of Didemnidae: A, horizontal thoracic cavities (Didemnum, Polysyncraton); B, deep horizontal cavities (Didemnum, Polysyncraton, Lissoclinum); C, three dimensional thoracic and posterior abdominal canals; D, zooids cross extensive cavities between surface and basal layers of test in test connectives (Lissoclinum, Diplosoma); E, flask-shaped colony with central test core surrounded by cavity (Didemnum, Polysyncraton, Lissoclinum); F-C, zooids with atrial siphon and three-dimensional oesophageal and posterior abdominal canals, F, encrusting; C, upright colony lobe (Trididemnum, Leptoclinides). Note: The type of system is often found in, but not exclusive to, the genus indicated. (Figure: Kott 2001.)

solitary. These are both indigenous species not known beyond Australia and non-indigenous species that almost invariably are tropical, especially recorded from the Great Barrier Reef (GBR), and with a range extending into some part of the wider Indo-West Pacific coralline region. Tropical and temperate locations around the Australian continent contain almost equal numbers of ascidian species. However, they have significantly different biogeographical affinities. Although indigenous and non-indigenous species are found in more or less equal numbers in the tropics, the number of both solitary and colonial indigenous species is more than twice the non-indigenous component of temperate waters. In tropical waters ascidian species are more often non-indigenous. The great species diversity seen in the GBR can be partly attributed to its location in the centre of the vast Indo-West Pacific coralline region. Specia-tion producing a unique fauna is more apparent in waters across the southern part of the Australian continent. Nevertheless, it is probable that the GBR, with its great latitudinal extent, has acted as a bridge between the tropics and the temperate waters of the continent, conveying genetic material from the tropics into the southern waters where it is now incorporated into the indigenous fauna. So far, a reduction in species diversity from north to south in the GBR has not been detected for ascidians. The most conspicuous difference between the ascidian populations of northern and southern reefs is the absence of the great mats of di-demnid/algal symbioses from southern reef flats where diurnal temperature changes in winter may inhibit these species.

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