Title:
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Biological and ecological studies on benthic foraminfera from the bathyal and abyssal Northeast Atlantic
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In samples from 4095 m and 398 m in the Porcupine Seabight
foraminiferans constitute 58.4 - 79.3~ of the total~meiofauna.
Nematodes followed by harpacticoid copepods are the most abundant
meiofaunal metazoan taxa. Foraminiferal densities (967 - 1331
individuals 10 cm-2 ) and diversities (68 - 95 species; Shannon-
Wiener index = 3.21 - 4.15) are higher than recorded in most
previous studies. This is attributed to the detailed sorting
procedure adopted which also provides information on the sizestructure
and vertical distribution of foraminiferal assemblages in
the top 5 cm of sediment. Most specimens were smaller than 150 ~m
and some were infaunal. Komokiaceans only occurred at the deeper
site. Allogromiids, hormosinaceans and unilocular agglutinating
forms had increased relative abundances at the deeper site while
the Rotaliina were relativelY,less common. The composition of
these assemblages is discussed in relation to changes in the
sediment structure, as observed in resin impregnated sediment
cores.
The morphology and fine structure of two large agglutinating
foraminiferans, Rhizammina al&aeformis Brady and Pelosina aff.
aborescens Pearcy, was examined using light microscopy, SEM and
TEM. In R. al&aeformis the test is a long, branching tube which
sometimes forms a tangled mass on the sediment surface. It
contains a protoplasmic strand and extracellular mass of
stercomata. The protoplasm is deeply invaginated to produce a
system of labyrinthine spaces communicating with the tube lumen and
possibly providing sites for extracellular digestion. The
stercomata are enveloped in an organic sheet-like system and
contain clay minerals. In f. aff. arborescens the tree-like test
projects above the sediment surface. Protoplasm occurs in the
central lumen. in fine branching canals which ramify through the
thick. muddy. test wall. as well as in the distal branching
complex. In TEM the protoplasm has a highly vacuolated appearance.
There are no stercomata. Differences between the two species seem
to reflect different feeding strategies; R. alKaeform1s 1s probably
a deposit feeder and f. aff. arbor esc ens a suspension feeder.
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