Distribution and ecology of the tunicateAscidiella scabra(Müller) in the Skagerak-Kattegat area

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Abstract
The distribution of the typical form of the asci dian Ascidiella scabra in the Skagerak-Kattegat area and its relation to the very complex salinity conditions are described. An account of the relation to temperature conditions and some notes on the relation to substrate and light are given. The species seems to demand an average salinity above 24 ‰ but adults can temporarily stand as low values as 15 ‰. Development from zygotes is possible in salinities down to c. 20 ‰ while larvae endure c.18 ‰. The highest salinities reached in the area, c. 35 ‰ are endured by all developmental stages. The demand for not too low salinity values and certainly to some extent the influence of sub-optimal temperatures cause a marked submergence of the species in the Kattegat area. Different populations have different salinity tolerance ranges, a fact which seems to be mainly phenotypical. Normal development from zygotes occurs at temperatures between 8–10 and 20–22°C. Larvae endure temperatures down to 6°C and the deepest populations, living the whole year at temperatures below 8 °C, are probably recruited from larvae coming down from populations above. There is one generation each year, the average age of the individuals being 12–14 months. The species is protandric. Spawning occurs mainly in June-July and most of the growth is completed during the first summer-autumn. A. scabra is a hard bottom form, sometimes occurring in very large numbers. A slight preference for shady sites is observed.