Free-liv~ng marine nematodes of the genus Ptycl~olaimellus build membranous tubes from detritus bound by released mucus. The mucus is produced by a ventral gland cell opening close to the lips and probably also from hypodermal gland cells along the body wall. The tubes are about 50 ,um in diameter, sinuous, and situated vertically down to l cm depth; they open at the sediment-water interface. These findings suggest that the nematodes may play a significant role at the interfaces by increasing pore water exchange and stabilizing newly sedimented detritus with excreted mucus.
Mass aggregations of the marine free-living nematode Pontonema vulgare containing thousands to milhons of individuals were observed in the summer half-year (May to Oct) above the surface of sublittoral (mainly 6 to 9 m depth) soft bottoms of the inner Flensburg and Kiel fjords. Sedlments were rich in organic matter, reduced, and black nearly to the surface. Oxygen content of the overlying water was very low (1 to 2 m1 O2 I-'). The aggregations occurred on dead and moribund macrofaunal animals, on beds of rothng Ulva sp. and Zostera manna, and near organic wastes from a trout farm. They marked sharply the transition zone between oxlc sites ( 2 4 m1 0, I-' in summer) inhabited by an intact macrofauna, and anoxlc sites (0 to 0.5 m1 O2 1 -I ln summer) lacking any macrofauna (including P. vulgare). It is concluded that these aggregations above (not withm) the surface of the sediment indicate dramatic mortality of the macrofauna livlng there due to decreasing oxygen content and increasing H,S content. These processes are known to occur especially in organically polluted areas. Previous taxonomic descriptions of P. vulgare are supplemented, and P. balticum (Schultz 1932) is suggested to be synonymous with P. vulgare.
The gut content of nearly 2000 specimens of Enoplus brevis was quantitatively analysed. E. brevis is an opportunistic omnivore. Main food categories found were cyanobacteria, diatoms, ohgochaetes, nematodes, and rotifers, With a few exceptions, the diet of females and males was identical. Predominantly, juveniles fed on cyanobacteria and adults on animal prey. Experiments revealed that hying oligochaetes are attacked and preyed upon. The quantities of the food categories eaten by Enoplus varied according to their ambient abundance, both temporarily and spatially on a small scale. At least the nematode prey was strongly selected: some species were significantly preferred and others avoided.
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