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The relationships between initial density of settling larvae, the subsequent density of recruits, and the final density of adult colonies, were examined in the colonial ascidian Podoclavella moluccensis Sluiter (Clavelinidae: Aplousobranchia) at Edithburgh Pier, South Australia, Australia. Recruit mortality was independent of the initial settlement density, consequently the density of recruits showed a strong positive relationship with the initial density of settling larvae. Hence, the number of settling larvae could be reliably inferred from estimates of recruitment, at least 1 month after settlement. In 2 consecutive yr the density of adult colonies was a significant positive function of the density of recruits. Recruit mortality was weakly but significantly correlated with recruit density in the 1st year and was not correlated with density in the 2nd yr. An estimated 6.1% of the larvae released, subsequently settled and only 0.56% survived to adulthood. The initial settlement density of these larvae was an important determinant of the distribution and abundance of adult colonies.

The colonial ascidian Podoclavella moluccensis Sluiter (Aplousobranchia, Polycitoridae) occurs on discrete patches of substrata, separated by uninhabitable sandy seafloor. The potential for exchange between these patches was determined by continuously observing individual larvae from their release from adult colonies until their settlement. Of 270 larvae, 100 were successfully followed to settlement and 80% settled within 2.5 m of their parent colonies. The average distance moved by larvae was 2.2 m after swimming for < 2 min. The distance over which larvae dispersed was a positive function of the time that they spent swimming, but was influenced by current speed and the behaviour of larvae. We conclude that populations of P. moluccensis have extremely limited potential for gene flow and may be regarded as effectively closed systems.

The role of substratum choice and subsequent survival in determining the patterns of recruitment of the colonial ascidian Podoclavella cylindrica (Quoy & Gaimard) (Clavelinidae : Aplousobranchia) were examined. Larval settlement preferences on five substrata were determined by following individual larvae underwater and observing settlement behaviour on their first encounter with a substratum. The survival of settling larvae was monitored daily for 30 consecutive days in quadrats on the five substrata. Larvae settled preferentially on substrata that showed reduced mortality of settlers, hence recruitment was correlated with settlement. In almost all cases larvae rejected the unfouled surfaces of sponges, preferring those with epifauna, and settled in highest densities on bare space. Mortality of new recruits varied markedly between substrata but was very low on bare space. Juvenile mortality resulted from overgrowth by neighbouring invertebrates and from the removal of epifauna, associated with sponges, on which juveniles had settled.

1. 1. Thymidine-5′-carboxylic acid 1 and 2′-deoxyuridine-5′-carboxylic acid 2 , previously only known as synthetic products, have been isolated from the ascidian Aplidium (= Amaroucium ) fuscum (Drasche, 1883) (order Aplousobranchia, family Polyclinidae) of the East Pyrenean coast. 2. 2. These are the first 2′-deoxynucleoside uronic acids isolated from nature, though certain enzymes of bacterial origin are used in the industry to oxidize adenosine to adenosine-5′-carboxylic acid.