![]() ![]() via occasional upwelling of high saline waters containing larvae that originate from the Kattegat (Hansen et al. rubens in this region is driven allochthonously, i.e. It is unclear at present whether the recruitment of A. rubens from Kiel Fjord has been reported by Kowalski ( 1955) at a salinity of 15, but no studies have dealt with the complete development and the inter-annual variation of larval abundances and settlement success in this area. ![]() From the blastula stage, the larva develops through a gastrula and bipinnaria stage to a brachiolaria larva which attains the competence to attach itself onto a suitable hard substrate and to metamorphose into a juvenile sea star (Barker and Nichols 1983 Nichols and Barker 1984 Gondolf 2000 Haesaerts et al. rubens lasts for about 9–12 weeks at a temperature of ~14 ☌. rubens is primarily achieved by passive transport of the planktonic larval stages, the species’ distribution mainly depends on the larval stages which are, moreover, most vulnerable to abiotic stress, and therefore, represent a bottleneck to recruitment (McEdward and Janies 1993 Sameoto and Metaxas 2008). rubens population density may therefore have large effects on the entire benthic ecosystem in this region of the Baltic Sea. rubens can eliminate up to 77 % of the yearly Mytilus edulis recruitment on soft bottoms, and thus, prevent the habitat from becoming a blue mussel monoculture (Nauen 1978 Reusch and Chapman 1997). With a biomass of about 32,000 tons in Kiel Bay, A. In Kiel Fjord, Asterias rubens can occur in high abundances (up to 120 ind m −2) and is, along with cod ( Gadus morhua) and the green crab ( Carcinus maenas), one of the most important predators of the benthic community (Anger et al. rubens population from Kiel Fjord, Germany (54☁9.8′N 10☉.0′E), lives close to the species’ absolute border of distribution, as no adults have been observed east of the island of Ruegen, Germany, (54☂2.9′N 12☁7.8′E) with average salinities as low as 8 (Binyon 1961). High river run-off causes a salinity decrease from west to east and a large vertical gradient between the low saline surface water and high saline bottom water (Hjalmarsson et al. The Baltic Sea is characterized by a strong salinity gradient. rubens and a much shorter spawning season compared with North Sea populations (Kowalski 1955). Hyposaline stress in this area, however, is correlated with reduced maximum size of A. rubens is the only echinoderm species to form large populations in the western parts of the brackish Baltic Sea. This osmoconforming species is one of the most euryhaline echinoderm species, tolerating salinities between 8 and 36 (Binyon 1961, 1962 Wolff 1968 Nauen 1978). The echinoderm Asterias rubens is a widely distributed benthic predator of the boreal area. ![]() rubens in the western Baltic Sea north-westwards and may lead to local extinction of a keystone species of the benthic ecosystem. Projected desalination of the Baltic Sea could shift the distribution of A. It appears that only years with high and stable salinities permit recruitment of A. rubens settlement rates only in 2009, the year when salinity was the highest and least variable during the period of spawning and larval development. rubens larvae and settled juveniles were also observed in Kiel Fjord and correlated to salinity values measured from March until June during 6 years (2005–2010). At higher salinities, larvae developed normally and metamorphosed into juvenile sea stars. At a salinity of 12, no larvae reached metamorphosis. At a salinity of 9, development ceased prior to the blastula stage. Sea star eggs were fertilized in vitro, and development was monitored in the laboratory at four salinities (9, 12, 15 and 18) for 10 weeks. In this study, we combined field and laboratory investigations to test whether the salinity of Kiel Fjord is high enough to enable successful development of A. However, Kiel Fjord with an average salinity of about 15 is located close to the eastern distribution boundary of A. rubens is the only echinoderm species and one of the main benthic predators controlling blue mussel ( Mytilus edulis) abundance. In Kiel Fjord, located in the western Baltic Sea, A. Salinity strongly influences development and distribution of the sea star Asterias rubens. ![]()
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