Marine Biology

Infaunal Organisms

This group includes various meiofauna and macrofauna such as snails, clams, worms, sea urchins, and crustaceans. Some groups are entirely infaunal, such as the tusk shells (scaphopods). Infaunal organisms occupy two different models of life. Some graze or plow through the sediment (sediment destabilizers) and others construct extensive burrow complexes that they occupy and in which they move about (sediment stabilizers). There are also those that burrow or bore near the surface and simply occupy that place; they do not move from place to place unless uprooted by waves, currents, or other organisms.

Grazing or plowing organisms include some sea urchins, snails, and clams. These organisms have shells that are stream lined for this type of activity. In the case of the sea urchins, they have short, stubby spines. Such animals ingest large quantities of sediment, extract the organic debris, and them excrete the sediment in pellet form.

A few types like the ghost shrimp, have great burrowing abilities and may be found over 2 m beneath the substrate. They take in suspended particles and digest the organics, then excrete the mineral sediment. This feeding style is also used by the numerous clams that burrow near the surface. They have an inhalant siphon and an exhalent siphon that are used for circulating the water through their digestive systems. Numerous varieties of worms also occupy this mode of life. These types of infaunal organisms typically move only when they are exhumed from their burrow.

The types of meiofaunal organisms are represented by a broad range of invertebrate phyla. These meiofaunal organisms have a size range similar to that of some of the smaller mesoplankton and the microplankton. These include the members of the phyla Ciliophora, turbellarians of the Platyhelminthes, Gastrotricha, Kinorhyncha, Tardigrada, Annelida, and Arthropoda. These organisms are very abundant only in the intertidal beaches and their biomass decreases with increasing depth in the oceans. The abundant groups are nematodes and harpacticoid copepods. These meiofauna forms a very good food source to most of the macrofaunal deposit feeders like larger polychaetes, holothurids, fishes such as young ones flat fishes, gobies and mullets. The role of these as food of the macrofaunal organisms mainly depends on the nature of sediments. That is the muddy sediment is known to harbour more meiofaunal biomass in the top layer, which is more accessible to the predators than the sandy sediment.

These infaunal meiobenthos are also known to exhibit a variety of feeding habits viz. herbivores- feeds on the attached diatoms; detritus feeders, suspension feeders and predators. Suspension feeders are quite rare, due to lack of plankton availability. They prefer to feed on bacteria and microalgae attached to the sand grains. Large bodied animals and sessile benthos are poorly represented in the marine meiobenthos, for example, members of the phyla Echinodermata and Cnidaria. The members of the phyla such as Phoronida, Pogonophora, Porifera, Ctenophora, Hemichordata and Chaetognatha are totally absent in the meiobenthic communities.

There are some organisms, such as certain clams and sponges that can bore into solid rock or shells. This is done through a combination of physical rasping and chemical reaction between substances secreted by the organisms and the substrate.