Tardigrades may be carnivorous, herbivorous, or bacteri-ovorous. Furthermore, a few marine tardigrade species are parasites on other marine invertebrates. Tetrakentron synaptae is found on the holothurian, Leptosynapta galliennei, where it punctures the epidermal cells of the holothurian and sucks out the cell contents. This species is the only tardigrade that has true adaptations for parasitism. It is dorso-ventrally flattened and all the sensory structures are reduced. As well, the claws are armed with three large hooks that are used to penetrate the epidermis of the holothurian. Females in particular are less mobile and are located in small depressions in the tegument of the holothurian. Another parasitic species is Echiniscoides hoepneri that lives on the embryos of the barnacle, Semibalanus balanoides.
The feeding ecology of the many marine species is not fully understood, but it is known that several species do not eat at all for long periods. Some species have symbiotic bacteria in special head vesicles. The genus Wingstrandarctus that lives in coral sand has three head vesicles containing thiobacteria (sulfur bacteria). The bacteria may give the tardigrade dissolved organic matter (DOM) products such as amino acids and glucose. In the deep-sea family Coronarctidae, the mid-gut may be filled with a white amorphous content that is very similar to gut contents found in bacteriovorous tardigrades.
The feeding ecology of terrestrial and freshwater tardi-grades is much better known. The heterotardigrades of the family Echiniscidae seems to be adapted to suck out the cell contents of mosses. Many species have very long stylets to penetrate the thick cellulose walls of mosses. Large eutardigrade species such as Milnesium tardigradum, Macrobiotus richtersi, and Amphibolus nebulosus are carnivores and eat nematodes, rotifers, and other tardigrades. Smaller bryophilous species of eutardigrades may not always suck out the moss cells, but instead may eat the epiphytic diatoms and bacteria that live on the moss. Small eutardigrades living in soil or in the rhizoids of mosses have very thin and narrow buccal tubes. In the genus Diphascon, the buccal tube is flexible with spiral rings like a vacuum cleaner tube. This genus is also found in cryoconite ("star dust") on the Greenland ice cap. The species, Diphascon recamieri, has a rusty colored mid-gut and probably feeds on iron bacteria in the cryoconite.
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