Aantekeningen over de bij de zandopspuiting te Scheveningen in 1975 aangevoerde mollusken. Deel 2

Notes on molluscs from dredged sand used for the raising of the beach at Scheveningen, The Netherlands, province of Zuid-Holland, in 1975. During the summer of 1975 the North Sea beach at Scheveningen was raised with sand, dredged from three slightly different positions in the North Sea, some 20 km NW of Scheveningen (viz. 52.13.30 N/4.03.10 E, 52.14.10 N/4.00.00 E, 52.11.13 N/4.03.10 E), at a depth of 20 to 26 meters (fig. 1). Altogether some 700.000 m³ of sand were transported to the beach and in this way a large number of marine organisms – especially molluscs – was brought ashore. In this material there were species that never before were recorded at Scheveningen. The mollusc material was collected during the raising of the beach and for some months after- wards. A survey of this material is given here. For each species the questions are asked whether or not it was a part of the living fauna at or near the dredging area and what its future influence might be on the distribution pattern of mollusc material washing ashore at Scheveningen in a natural way. The material A list of mollusc species is given in tab. 2. It contains 77 species which may be divided into four groups: a. ‘Off-shore species’ (Eisma, 1966). These are species that hardly ever are found washed ashore on the Dutch coast. The following species are regarded as such (if only one or a few specimens were found the species is indicated with an asterisk): Striarca lactea, Glycymeris glycymeris *, Aequipecten opercularis, Diplodonta rotundata *, Montacuta substriata *, Spisula elliptica, Angulus donacinus *, Ensis magnus *, Angulus pygmaeus, Gari fervensis, Abra prismatica, Mysia undata, Alvania lactea, Oenopota turricula, Actaeon tornatilis *, b. Species also occurring in the coastal region: Mysella bidentata, Tellimya ferruginosa, Spisula solida, Ensis arcuatus, Ensis phaxoides, Donax vittatus, Chamelea gallina striatula, Euspira poliana. Fresh material of Tellimya ferruginosa, Spisula solida, Donax vittatus and Euspira catena is only very rarely washed ashore nowadays. Many specimens of Spisula solida, Ensis arcuatus, E. phaxoides and Chamelea gallina striatula reached dimensions which are never found in the normal beach material. Such specimens may be indicated as ‘off-shore specimens’. c. Fossil material. Fossil material is well-represented as to the number of species. The age of the material is presumably Eemian or younger. Rare species (one or two valves/fragments) are indicated with an asterisk: Mytilus edulis, Chlamys varia, Anomia ephippium, Ostrea edulis, Lucinella divaricata, Acanthocardia tuberculata, Cerastoderma edule, C. glaucum, Mactra corallina cinerea, Mactra glauca *, Spisula subtruncata, Lutraria lutraria, Solen marginatus *, Macoma balthica, Scrobicularia plana, Venerupis aurea senescens, Corbula gibba, Pholas dactylus *, Zirfaea crispata, Gibbula cineraria, Littorina littorea, Bittium reticulatum *, Ceratostoma erinaceum *, and the freshwater mollusc Corbicula fluminalis. Of the species mentioned Cerastoderma edule, Mactra corallina cinerea, Spisula subtruncata and Macoma balthica are regularly found alive on the beach of Scheveningen. d. A small number of freshwater and terrestrial molluscs: Pisidium sp., Sphaerium sp., Theodoxus fluviatilis, Valvata piscinalis and Succinea sp., which might be regarded as (?Holocene) fossils. The North Sea fauna. The following 15 species are considered to form a part of the fauna living within the dredging area, those found with soft parts, animal or even still alive, are marked with an asterisk: Striarca lactea, Aequipecten opercularis, Mysella bidentata, Tellimya ferruginosa, Spisula elliptica * Spisula solida *, Ensis arcuatus *, Ensis magnus *, Ensis phaxoides *, Angulus pygmaeus, Abra prismatica * Donax vittatus *, Chamelea gallina striatula * Mysia undata, and Euspira poliana *. It seems likely, that Gari fervensis, Alvania lactea and Oenopota turricula with respectively 10, 13 and 10 valves or specimens of a quite fresh condition should be added to this, which would bring the number up to 18. Perhaps the same holds true for Actaeon tornatilis with two fresh specimens. About Angulus fabulus, Angulus tenuis and Abra alba we do not feel quite sure. Some dozens of apparently fresh valves of the first two species and 14 fresh valves of Abra alba were found, but there was no double-valved material (the valves of Abra alba get easily separated, but the ligament of the other two is very persistent), either with or without soft parts. They could well occur in the neighbourhood of the dredging area, but all three are also commonly washed ashore and thus could have been mixed, at least in part, with the dredged material. Ensis minor was found in many fragments of almost fresh, and in a few cases quite fresh, appearance and with the epidermis still present, but, contrary to Ensis arcuatus and Ensis phaxoides, no double-valved specimens with animal or soft parts were found. Spisula subtruncata occupies a doubtful position, even though fresh or nearly fresh valves were found. Such valves of about 0.5-1.5 cm length are commonly washed ashore, though never one corresponding double-valved specimen was found, with or without soft parts. Several other species were found in a fresh condition, but their numbers were too small for any conclusion as to whether they live in the dredging area or not: Glycymeris glycymeris, Diplodonta rotundata, Montacuta substriata, Ensis arcuatus ensoides, Laevicardium crassum, Phaxas pellucidus, Angulus donacinus, Turneria jeffreysi, Thracia papyracea, Tornus subcarinatus, Euspira catena, and Buccinum undatum. The most interesting species in this fauna are Ensis magnus (4 large posterior fragments, one of them double with soft parts), up to now only known from the Faroes and Norway, and Alvania lactea (13 fresh specimens), of which there is no evidence of its occurring in the North Sea as far as we can find in the literature. The influence of the dredged material on the natural situation. This is mainly a matter of local importance and has been discussed in full elsewhere. Since collectors from abroad may visit Scheveningen, however, it is thought useful to give some indications. In general all molluscs with natural colours from the off-shore category (mentioned above) should be regarded as originating from the dredging area in the North Sea. The most striking case is that of Spisula elliptica of which it is supposed that at least some 70 million valves were brought on the beach during the sand-transport. Since fossil valves of this species are commonly washed ashore at Scheveningen, there seems to be reason for careful discrimination. The following material of this species should be regarded as originating from the sand-transport: a. all light-coloured valves, usually tinged with pinkish brown; b. strongly banded valves with 5-7 broad colour-lines and rather pronounced umbones; c. fresh valves with parts of epidermis; however, these are scarce now if to be found at all. Natural beach material from Scheveningen and adjacent regions is characterised by its dark shades in brown or blue and lacks the features, or rather combination of features, mentioned above. A short survey will be given here of the more important material that finds its origin in transport from the dredging area (see also table 1): Striarca lactea: valves with natural colour; Aequipecten opercularis: ± all young valves with natural colours; Lucinella divaricata: ± all valves; Tellimya ferruginosa *: most of the light coloured valves; Goodallia triangularis: ± all valves; Spisula elliptica *: see above; Spisula solida *: fresh valves with or without epidermis; Ensis ensis *: most of the light coloured fragments; Angulus pygmaeus *: valves with natural colour; Donax vittatus: large and ± all other valves with natural colour; Abra prismatica *: fresh valves; Chamelea gallina striatula *: large valves with natural colours; Corbula gibba: ± all valves; Alvania lactea: light coloured, fresh or ± fresh specimens; Euspira poliana *: most of the specimens with natural colour pattern, when not quite fresh and shiny; Oenopota turricula: ± all white specimens. The indication ± is used for: nearly, about, or with a few exceptions and means that a similar specimen may be washed ashore (very) occasionally in a natural way. Species that were frequent in the transported sand are marked with an asterisk, nine in total. The other seven species were present in small numbers only and their chance of being found is therefore restricted.