Meiofauna of Sandy Beaches
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The surface of sandy beaches is permanently moving under the action of the tides, wind and currents. It is a reason why a sandy shoreline provides no place for attachment of surface-growing seaweeds and no inviting crevices, but it can hold water between small grains. Beneath the water surface, the environment is unaffected by the weather. Sandy beaches appear dead and inhospitable, like a desert. However, a wide variety of organisms inhabit the space between the sediment particles, in damp sand, on sandy shore, among them most abundant – meiofauna communities.
Meiofauna
The term „Meiofauna“ is related to microscopically small benthic invertebrates that live in both marine and fresh water environments. Meiofauna formally defined as a group of organisms by their size, larger than microfauna but smaller than macrofauna. In practice these are metazoan (some researchers include protozoan as well) animals that can pass unharmed through a 0.5 – 1 mm mesh but will be retained by a 30 μm mesh but the exact dimensions will vary from researcher to researcher. Nowadays the term meiofauna is used interchangeably with meiobenthos. Meiofauna is mainly found in and on soft sediments, but also on underwater algae and higher plants as well as on other hard substrates. The heterogeneity of meiofaunal habitats is so large and meiobenthic taxa so diverse.
Meiofauna of sandy beaches
Practically most classes of the Metazoa are represented in the meiofauna of sandy beaches, while the Protozoa are represented by their largest forms, e.g., Foraminifera and Ciliata. The smaller protozoans are generally considered as microfauna. On most beaches, the interstitial fauna is rich and diverse, even exceeding the macrofauna biomass. There may be as many as 25 meiofauna species for every macrofauna species (Armonies and Reise 2000). The reson is simple – greater stability and complexity of the interstitial habitat.
Most common metazoan phyla represented in the benthic meiofauna:
- Cnidaria - Hydroida, Scyphozoa, Anthozoa
- Platyhelminthes – Turbellaria
- Gnathifera – Gnathostomulida, Rotifera, Rotatoria
- Nemertinea
- Nemathelminthes – Nematoda, Kinorhyncha, Priapulida, Loricifera, Gastrotricha
- Tardigrada
- Crustacea – Cephalocarida, Ostracoda, Mystacocarida, Copepoda (e.g. Harpacticoida, Cyclopoida), Malacostraca (e. g. Tanaidacea, Isopoda, Amphipoda)
- Chelicerata – Acari
- Annelida – Polychaeta, Oligochaeta
- Sipuncula
- Mollusca – Aplacophora, Gastropoda
- Tentaculata – Bryozoa
- Echinodermata - -Holothuroidea
- Chaetognatha
- Tunicata – Ascidiacea
This list of meiofauna taxa is far from being complete and have to be supplemented in the future, particulate since recent investigations of high-ranking taxa have brought meiofauna into the mainstream of invertebrate phylogeny (Giere 2009).
Meiofauna is the best-studied component of the interstitial biota. These small creatures are considered as temporary meiofauna if they are larval stages of macrofauna and permanent meiofauna if their entire life cycle is spent in previous mentioned size category (McLachlan & Brown 2006). McIntyre (1969) used the term permanent members (or permanent meiofauna) for species belonging to the meiofauna during the whole of their life cycle in marine systems in contrast to temporary meiofauna. The dominant taxa of sandy beach meiofauna are nematodes and harpacticoid copepods, with other important group including turbelarians, oligochaetes, ostracods or gastrotrichs.
Adaptations to Sandy Biotope
Sediment consist of sand particles which are mostly round due to abrasion. Usually spaces between them occupy 30-40% of the sediment volume. It creates a system of interstices which is filed with water, air, detritus and organisms. The smallest animals living in the sediment, the interstitial communities, move through the sediment, using the film of water which surrounds individual particles of sand. Interstitial animals therefore display numerous adaptations to resist one of the harshest environment for life on the surface of the planet. Organisms living here must be able to withstand the marine conditions of inundation by salt water which alternates with exposure to terrestrial conditions. Common sand meiofaunal adaptations by Giere (2009): to narrow spaces – miniaturization, elongation and flexibility to the mobile environment – adhesion, special locomotion and reinforcing structures to the three-dimensional dark conditions – static organs, reduction of pigments and eyes Additionally interstitial organisms show specific adaptation related to reproduction and development – the production of only few eggs, direct sperm transfer or internal fertilization, brood protection, abbreviated larval life and restricted propagation (Giere 2009).
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