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Het invasieachtig voorkomen in de zuidelijke Noordzee van de hydromedusen Nemopsis bachei L. Agassiz, 1849 en Eucheilota maculata Hartlaub, 1894 in augustus-september 1996 (met aanvullende data voor 1997) (Hydrozoa: Athecata, Thecata)
Dumoulin, E. (1997). Het invasieachtig voorkomen in de zuidelijke Noordzee van de hydromedusen Nemopsis bachei L. Agassiz, 1849 en Eucheilota maculata Hartlaub, 1894 in augustus-september 1996 (met aanvullende data voor 1997) (Hydrozoa: Athecata, Thecata). De Strandvlo 17(4): 102-126
In: De Strandvlo: Driemaandelijks Tijdschrift van De Strandwerkgroep België. De Strandwerkgroep België: Oostende. ISSN 0773-3542, more

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  • Dumoulin, E., more

Abstract
    During the months of August and September 1996 many specimens of the Hydromedusae Nemopsis bachei and Eucheilota maculata were caught at the east-coast of Belgium (Zeebrugge) and some at the adjacent seaside place Cadzand in The Netherlands. E. bachei is the first record in Belgian waters and possibly also a first ascertainment of the species so far south in the Southem Bight of the North Sea. As also appears from literature, it does not seem likely that N. bachei is indigenous in the North Sea. On the other hand settlement of small temporary populations of hydroid colonies, producing fertile medusae, is not impossible. Perhaps old settlements from late fifties till early seventies in several estuaries from the German Bight could have survived unnoticed up to the present and have supplied the Belgian coast (passing through multiple generations along the Dutch coast ?). Another possible explanation for the invasion-like occurrence is a big supply of young medusae from atlantic origin in the northem North Sea. The persisting N to NW -winds during the first half of 1996, together with a heavy NNW -storm at the end of August could then have drifted the species southward. The prevailing late summer plankton-bloom could also have had an influence on the fact that so many mature individuals reached the Southem Bight. But maybe other unknown reasons, such as introduction by way of navigation (hydroid/medusa/larva) or import with commercial oysters (hydroid), could have played a role herein. Remarkable concerning the hydroid stage of N. bachei is, although the medusa is common along the North American east coast, that no appropriate modern description nor a good picture of the former is given in literature. Only Brooks (1883b), Mayer (1910a) and Kühl (1962, 1972) give general descriptions, the latter added with some photographs and drawings of young development stages of the hydroid. These photographs are however not quite usable for determination purposes. The possibly very small dimensions of the animal (about 1 inch high according to Brooks, but only 0.6 mm height and 0.9 mm tentacle length according to Kühl) could be one of the reasons why the species is lacking in monographs regarding Hydroida. Other discrepancies in the observations of Brooks and Kühl concern the number of tentacles of the hydranth (Brooks : 24, Kühl : 6-14) and the nature of the habitus (Brooks : branched sterns, Kühl : solitary hydroid). In an addendum the new records of N. bachei for 1997 along the Belgian coast (Zeebrugge, Oostende) are mentioned. Also recently received complementary data from The Netherlands (Faasse in lit. , 1997) are given. From this country finds of 1993 and 1996 now prove to be known from the Bastem-ScheIdt (Burghsluis, Zierikzee, Neeltje Jans) and of 1996 and 1997 from the cooling-water inlet of the nuclear power station at Borssele along the Western-Scheldt. These data confirm that the appearance of N. bachei in the Southern Bight must have happened a number of years ago, and possibly indicates a more long-term settlement of the hydroid. As postscript some recommendations of Cornelius ( 1995 ), about the use of terminology in literature on hydrozoa, are presented.

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