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De tere dunschaal Abra tenuis (Montagu, 1803) levend in de voorhaven van Zeebrugge: met beschouwingen over haar schelpkenmerken, verspreiding, ecologie en biologie
Dumoulin, E. (1999). De tere dunschaal Abra tenuis (Montagu, 1803) levend in de voorhaven van Zeebrugge: met beschouwingen over haar schelpkenmerken, verspreiding, ecologie en biologie. De Strandvlo 19(2): 82-100
In: De Strandvlo: Driemaandelijks Tijdschrift van De Strandwerkgroep België. De Strandwerkgroep België: Ursel. ISSN 0773-3542, more
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Keywords
    Biology; Distribution; Ecology; Shells; Abra tenuis (Montagu, 1803) [WoRMS]; ANE, Belgium, Brugge, Zeebrugge [Marine Regions]; Marine

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  • Dumoulin, E., more

Abstract
    In september 1998 empty bivalve shells, often with dryed remnants of flesh, of the tellinid Abra tenuis were found on a tidal flat and in the tidemark of the outer harbour of Zeebrugge. This is probably the first 'recent' (in it's two senses) record for the Belgian coast, although finds of subfossil specimens in 'modem' coastal plain deposits prove that she must have lived here in historical times. Identification features of the species are compared with these of Scrobicularia plana in order to give a tool for easier distinguishing juveniles of the latter from A. tenuis>. Regarding its distribution along tbe continental coast of the North Sea, life occurrence in the sea area boundery S51 (after Seaward, 1990) is demonstrated and bas now to be indicated with status † = live record, post-1950 (in Seaward's atlas). Furthermore, some data from the French channel coast (Brittany), zones S17F and S55, must be amended to status † on the basis of Rullier (1959) and field-observations from Ile des Hébihens (Côte d'Émeraude; 1983) and the Grève de Goulven (Finistère Nord; 1991) (pers. comm. J.-P. Kreps). Tbe northernmost distribution limit of A. tenuis is discussed. So far the coast of Northumberland (East England) is the most likely northern border area of its appearance. Literature data from more northern find-spots: Norway, Lofoten, West-Finmark, Faeroer are considered as rather doubtful. Concerning the ecology of the species we know that the substrate she prefers is fine sand or a mixture of this with soft mud, where she lives burrowed at maximum 2-3 cm depth. Being a small shell and 'deposit feeder' A. tenuis needs the presence of fine sediment. Densities per m2 are given according to Gibbs (1984) and Bachelet (1989). Further, A. tenuis tolerates reduced salinities and fully immersion for a long period. I presume she does not tolerate high energetic environments. One reason may be that the species is not a deep burrower and could easeIy be washed out. She most probably only lives at places where the bottom relief shows a gradual slope. So A. tenuis is indeed a typical inhabitant of estuarine-lagonal habitats where the above mentioned parameters often occur. The species flourishes in the Macoma balthica -community a1 places where also Scrobiculariaplana occurs ('facies' with S. plana ?). Maximum shell lengths of the specimens collected (by hand) at Zeebrugge are under 1 cm (9.60 -9.75 mm). As already stated by van Benthem Jutting (1943) a class magnitude ranging between ± 8 and ± 10 mm seems to be the most appropriate for the mean largest shells of the Southem Bight. Most specimens frorn Zeebrugge show one, sometimes two clear growthline(s) which could be correlated with an interrupted growthstage during the winter period. Small specimens often have no growth checks, but in lager shells their number can vary between two and >five. These additional growthlines are always situated under the winterring(s), on the youngest part of the shell, of which they have not that strong aspect but look more like extra pronounced spiral lines. It may be that these 'pseudo'-growthrings indicate short interruptions in growth due to fluctuating environmental factors, disturbance of the sediment by predators, or spawning stress. But possibly older specimens show a typical more irregular growth pattern. In the Zeebrugge area the main predation pressure is most probably caused by birds. Apart frorn shelducks and lager waders, in this case, also lesser species like sandpipers (Calidris sp.) and plovers (Charadrius sp.) can have A. tenuis on their menu (small species, shallow burrower). A striking character of the species reproduction biology is that she is lecitotrophic with direct development (Gibbs, 1984; Bachelet, 1989). This means that the larval stage completely takes place inside the egg (from fertilized egg to miniature shell). This fact, including the specific demands the species requires from her habitat (salinity, sedirnent, hydrography) makes that the distribution pattern of A. tenuis is discontinuous : suitable places (estuaries, lagoons) sometimes lay far apart, and cannot be colonised via pelagic veliger larves in the water column. Areal extension may take place by way of migrating wildfowl, who can transport eggs or juvenile specimens. In the scope of natural selection, particularly in how organisms select properties and develop strategies to optimize their energy partitioning in an estuarine-lagonal habitat, we see that A. tenuis is in general a r-strategist. However its production of relatively fewer, but lager offspring, indicates a more K -selected character .

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