Sample Processing - Major Taxa

Major groups

Metazoa are divided into 33 phyla. Of these, 22 have at least some meiobenthic taxa. Some, such as the Gastrotricha, Gnathostomulida, Kinorhyncha, Loricifera and Tardigrada, are exclusively meiobenthic. Some animals, such as the larvae of some macrofaunal species, are referred to as temporary meiofauna, as they are part of the meiobenthos for only a part of their life-cycle. Animals that are meiobenthic throughout their lives are termed permanent meiofauna, and in addition to the five phyla mentioned above these include taxa of Rotifera, Nematoda, Annelida, Platyhelminthes, Hydrozoa, Nemertina, Entoprocta, Mollusca, Brachiopoda, Echinodermata, Tunicata, Priapulida and Sipuncula, and several groups of Arthropoda including Copepoda, Ostracoda, Mystacocarida and Acari.

It is unlikely that the majority of these groups will be found in any one set of samples. For example, certain taxa, such as Gastrotricha and Tardigrada, are typically found interstitially in coarser sediments, whereas others such as Kinorhyncha, which are adapted to a burrowing mode of life, are found in muddier sediments. Meiofauna tend to be termed either 'hard' or 'soft' taxa. 'Hard' meiofauna are those with relatively rigid cuticles or exoskeletons which do not become deformed when they are fixed and preserved in chemicals such as formalin. These include Nematoda, Annelida, Copepoda and Kinorhyncha. 'Soft' meiofauna require specialised collection techniques and have to be examined whilst still alive as their features become deformed when fixed and preserved. Examples include Platyhelminthes, Gastrotricha, Gnathostomulida and Nemertina.

View Meiofaunal Major Taxa Images

In most marine sediments the dominant 'hard' meiofaunal group is the Nematoda. Although all nematodes are unsegmented (figure A,B) cuticular structures give some a segmented appearance (figure C), but close examination will reveal that the animals are not segmented internally. Truly segmented worms, usually with conspicuous chaetae imbedded in the body wall, are Annelida (figure D). Although some are members of the permanent meiofauna, most Annelida are in the macrofaunal size range, and occur in the meiofauna as newly settled juveniles. Also abundant in most marine sediments, and often dominant in phytal habitats, are Copepoda (figure E, F), which have segmented bodies and four pairs of biramous swimming legs. Acari (figure H) have apparently unsegmented bodies, four pairs of legs arising marginally, and may occasionally be abundant in coarse sediments and phytal habitats. Tardigrada (figure H) have four body segments, each with a pair of short legs, and are found interstitially in coarse sediments. Although some are members of the permanent meiofauna, most Amphipoda (figure I) which are found in meiobenthic samples are juveniles of larger forms, and the same is true of Tanaidacea (figure J) and Cumacea (figure K). All Kinorhynchs (figure L) are meiofaunal, have bilaterally symmetrical, superficially segmented bodies armoured with plates, and a retractile head bearing spines. Priapulida larvae (figure M) also have a retractible anterior with spines, but the body is armoured with longitudinal plates. Foraminifera (figure N) are not metazoa, as they are unicellular animals, but their tests are often found in meiobenthic samples and occur in a variety of forms. Recently settled bivalve Mollusca (figure O) are sometimes abundant, and members of other molluscan taxa may also be found. Ostracoda (figure P) also have a bivalve shell, but the enclosed animal has legs. Bivalves and ostracods, and some of the larger arthropods, are not generally extracted efficiently by the decantation and flotation methods outlined the section on Processing samples.

View Meiofaunal Major Taxa Images