Abstract. Artificial reefs in marine environments are in most cases submerged structures consisting of dumped waste material or specific constructions made with the purpose of enriching the local fish populations and other marine life to the benefit of recreational and commercial fisheries. Such structures are susceptible to fouling and will successively develop assemblages, which may or may not resemble epibioses on natural substrata. Studies of artificial reefs have focused predominantly on fish assemblages and have largely disregarded the development of sessile biota and their structural and functional relationships. In addition, most studies are from tropical or subtropical environments. To manage and understand artificial reefs, a whole‐ecosystem approach is necessary, incorporating studies of all aspects of hard substratum ecology including both structural and functional variables. This review is an attempt to evaluate the present knowledge of ecological aspects of artificial reefs, emphasising the role of sessile hard substratum biota.

Abstract. Oviparous solitary ascidians, such as Ciona intestinalis (L.), generally spawn freely into the water‐column where the eggs are fertilized and develop into free‐swimming non‐feeding larvae. In the laboratory, both spawning and settlement are controlled by light. It has been suggested that both spawning and development time are synchronized so that larval settlement takes place under optimal light conditions in the middle of the day when larvae are best able to locate shaded habitats. Our observations of field populations, however, have shown that Ciona may spawn and settle at any time of the day. Freely spawned eggs, which are negatively buoyant and slightly adhesive, may become attached to any available substratum. Ova can also be released in mucus strings which are particularly adhesive and will readily adhere to adult Ciona and other surfaces. In all cases, fertilization and development proceed normally. Laboratory investigations have shown that newly hatched larvae may either escape to the plankton or be retained in the mucus string until settlement. Thus, the dense aggregations of both adults and newly settled Ciona which are commonly observed in the field could be explained by abbreviated dispersal due to mucus string spawning and epibenthic retention of eggs. Summary In the field, Ciona intestinalis embryos at different developmental stages are found simultaneously, indicating that spawning is not confined to a specific time of day. Free eggs are negatively buoyant and slightly adhesive. Eggs are spawned in mucus strings, where fertilization and development takes place. Such strings are retained epibenthically. Settlement and subsequent metamorphosis takes place in the mucus strings, but in the laboratory 40 to 60 % of the larvae escape to the plankton. The aggregation of both adult and juvenile Ciona observed in the field can be explained by mucus string spawning and epibenthic retention of eggs.

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