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On the ecology of Acoela (Turbellaria) in the German Bight

1968; Springer Nature; Volume: 18; Issue: 1-2 Linguagem: Inglês

10.1007/bf01611668

0017-9957

Jürgen Dörjes,

Marine Biology and Ecology Research

Ecological investigations on the turbellarian order Acoela were carried out from 1962 to 1965 in List (Sylt), Helgoland, Wilhelmshaven and Juist. The sediments in the eulittoral of the Frisian Islands (Juist) and the Jade are structurally rather similar; sediment mobility or muddy components prevent establishment of an interstitial fauna. Helgoland possesses a great variety of different biotopes; this island harbors the highest number of species in the southern North Sea. In contrast to Juist, Sylt offers predominantly sandy biotopes; in addition to the mudliving species of the epifauna, a number of typical interstitial species was recorded. Of 82 species of Turbellaria Acoela identified, only 1 lives in greater sediment depths; all others are confined to the uppermost centimeters or millimeters of the sediment. Besides the sediment boundaries, the low water line forms an important population barrier. The differences in the sediments can be related to specific morphological and habitual adaptations of their inhabitants. Acoela living in interstitial spaces possess mostly a long and slender body form, which, in turn is correlated to internal organization. Most Acoela species which exist in the spaces between sand grains have an unpaired ovary, shifted from near the statocyst to the area behind the mouth opening. Big vacuoles in the peripheral parenchym counteract mechanical deformation by the sediment. Fixation mechanisms occur in form of tail plates, adhesive glands or caudal organs. The body of mud-living Acoela is usually short and stout. Special structural adaptations are absent. The determining factors in the colonization of mesopsammal biotopes are structure and composition of the sediment. Quantitative investigations at List on Sylt reveal the Acoela speciesHaplogonaria syltensis andPseudaphanostoma psammophilum as being stenotope and stenobath.H. syltensis reaches population densities under favorable circumstances of up to 1500 individuals per 2.5 cm3. InHaplogonaria syltensis andPseudaphanostoma psammophilum annual population fluctuations are not attributable to their reproductive phases which depend upon the season; population decline during summer and displacement from optimal living spaces is due to the vigorous development of competing oligochaetes and archiannelides.