Acesta excavata
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| ( Fabricius , 1779) |
Acesta excavata is a shell - type from the family of limidae (Limidae). It is an optional part of the Lophelia pertusa community; The hard coral Lophelia pertusa forms cold waterreefsin the north Atlantic together with a typical community of fauna at depths of about 60 to 2100 m water depth.
features
The unequal-sided, but equally-folded, moderately thick case is egg-shaped in outline and higher than it is wide (wide). It reaches a maximum height of 20 centimeters and is slightly extended to the front. The front edge runs obliquely from the vertebra to the ventral edge. There is no anterior ear . The anterior margin is initially slightly concave, later it is moderately convex to the ventral margin. In contrast, a distinct ear is formed on the rear edge , but this is only slightly separated from the housing body. The posterior margin initially runs straight towards the ventral margin and then merges with the ventral margin with a slight curvature. The dorsal margin is straight and wide. The lunula is sunk. The indentation for the byssus is barely visible. The inner edge of the housing is still weakly notched in juvenile housings, this notch disappears with increasing age. The ligament is a large, triangular, slightly sloping resilifer in the center of the lock plate. The lock on either side of the Resilifer is toothless.
The whitish skin is thin but quite firm. It consists of two mineral layers visible to the naked eye, consisting of an inner aragonitic layer and an outer calcitic layer. The lock plate is also made of aragonite. The calcitic position is very often missing in the area of the eddy. The short area between the edge of the case and the surface line consists exclusively of calcite.
The fine and weak ornamentation consists of up to 100 fine, radial and closely spaced lines that start from the vortex and can be followed to the edge of the case. In the middle section of the housing (between the vertebra and the edge of the housing) the ribs are often only weakly developed. The periostracum is very thin, light straw colored to brown and often rubbed off in the middle part of the housing body.
The attachment point of the posterior sphincter muscle and the retractor muscle are located close to each other in the posterior part of the two inner sides of the valve. The surface line is completely without indentation and runs close to the edge of the case.
Geographical distribution, habitat and way of life
The distribution area of Acesta excavata extends from northern Norway to West Africa (Senegal). It also occurs in the Canary Islands , the Azores and in a few places in the Mediterranean . During the Pleistocene, it was even more widespread in the Mediterranean and colonized large areas there. In the western North Atlantic only a single occurrence off Newfoundland (Canada) is known.
Housing was found there in water depths of 33 to 3,200 meters. So far, however, living specimens have only been detected to a depth of around 500 meters. After the death of the animal, the housings can also be carried into deeper areas by currents.
Acesta excavata is often associated with the Lophelia pertusa community on the continental margin of the north-eastern Atlantic . The hard coral Lophelia pertusa , together with a typical community of fauna, forms so-called cold water reefs at depths of 60 to 2100 m on hard floors. Acesta excavata also lives outside these reefs with threads of byssus attached to steep cliffs. Also missing Acesta excavata z. B. in the Lophelia pertusa communities on the continental slope off Ireland, d. H. they are not an obligatory member of this community. So far only larger specimens of Acesta excavata have been found openly on cliffs. Smaller and therefore younger specimens probably live hidden in crevices in the rock. The age can be determined with the help of annual growth in the shell. After about 18 to 22 years they have reached a size of ten centimeters. Overall, Acesta excavata about 50 to 80 years old. The Acesta excavata valves exhibit two modes of growth; H. an allometric growth. The first growth phase is characterized by flat valves and a very oblique ligament pit. In the second phase there is a rapid growth in thickness, i.e. H. the case becomes more inflated (thicker) in proportion to the height. The ligament pit now continues to grow much less obliquely.
Parasitism
The shells of Acesta excavata are frequently attacked by the parasitic foraminifera Hyrrokkin sarcophaga Cedhagen, 1994. Early stages of the foraminifera produce pits 0.5 to 1 mm deep in the shell, which usually only reach into the upper calcitic layer. Older stages penetrate through the shell to the shell of the mussel, which reacts by forming an Aragon callus, through which the hole is closed again. In some cases up to 40 such holes produced by this parasite and their closure by the infected mussel have been observed.
Taxonomy
The taxon was first described by Johann Christian Fabricius as Ostrea excavata . It is the type species of the genus Acesta Adams & Adams, 1858. The World Register of Marine Species lists two synonyms , Lima solida Calcara, 1845 and Acesta excavata sublaevis Nordsieck, 1969.
supporting documents
literature
- Fritz Nordsieck : The European sea shells (Bivalvia). From the Arctic Ocean to Cape Verde, the Mediterranean Sea and the Black Sea. 256 p., Gustav Fischer Verlag, Stuttgart 1969 (p. 22)
- Matthias López Correa, André Freiwald, Jason Hall-Spencer, Marco Taviani: Distribution and habitats of Acesta excavata (Bivalvia: Limidae) with new data on its shell ultrastructure. In: André Freiwald, JM Roberts (Ed.): Cold-water Corals and Ecosystems , pp. 173-205, Springer Verlag, Berlin, Heidelberg 2005 PDF
- Guido Poppe, Yoshihiro Goto: European Seashells Volume 2 (Scaphopoda, Bivalvia, Cephalopoda) . 221 pp., Verlag Christa Hemmen, Wiesbaden 1993 (2000 unc. Reprint), ISBN 3925919104 (pp. 75/6)
- Jon-Arne Sneli, Jørgen Knudsen , Antonia Vedelsby: Johan Christian Fabricius and his molluscan species , Acesta excavata (JC Fabricius, 1779). Steenstrupia, 30 (2): 153-162. Copenhagen, 2009 PDF
On-line
Individual evidence
- ^ Johann Christian Fabricius: Journey to Norway. LXIV + 388 S., Hamburg, Bohn, 1779 Online at Bayerische Staatsbibliothek digital (p. 368)
- ↑ a b World Register of Marine Species: Acesta excavata (H. Adams & A. Adams, 18)
