Marine Biology (2+1)

Sedentary or Vagrant epifauna

 

     This group of animals is the largest in terms of both the number of individuals and the diversity of types.  They have two common primary features: all live on the surface of the sea floor and all have at least some ability to move.  They may live on rigid substrate, firm sand or soft mud.  They range in size from microscopic to over a meter in length.  While some move very slowly, others move very quickly. The smallest and probably the slowest of the benthic animals are the single celled animals, which include the foraminifera.  These tiny animals are typically less than a millimeter in diameter with different shaped tests or shells and most of which are multichambered.  Most foraminiferans tests are composed of calcium carbonate, although some are comprised of sediment particles held together by organic material.  They are a significant contributor to marine sediment.  Mobility is nearly nonexistent in foraminiferans; however, the individual foraminiferan can extend its protoplasm through pores in the test and then contract it to pull the test, thus moving slowly.  They feed by engulfing particulate organic matter. Most vagrant benthos are shelled macro-invertebrates, although there are some  worms thatcrawl over the substrate.  The shelled invertebrates are dominated by three groups viz. arthropods (crabs and lobsters), molluscs (clams, gastropods, chitons, and octopuses) and echinoderms (sea urchins, sand dollars, star fishes and sea cucumbers).

            Except the octopuses, the crabs and lobsters are the largest and fastest of the vagrant benthos.  These jointed-leg animals can move rapidly; they use this ability along with their hard exoskeleton for protection from enemies.  In addition, they have some swimming ability, using either their tails and/or specially adapted legs.  Many species in this group live in the shelter of rocks, ledges or other cover.  They are scavengers and will eat almost anything that is available.

               Nearly all vagrant benthic molluscs have external shells and move slowly, on the order of millimeters or centimeters per minute.  Some, such as the chitons, are completely protected by their shell and will move across a hard substrate rasping and scraping food from the surface.  A few gastropods move slowly through the sediment, ingesting whatever material they encounter, with little or no selection.  They digest the nutrient material and excrete the mineral sediment in the form of pellets which become an important contributor to the volume of sediment.  Generally, these animals have thick, heavy shells to protect them from predators and from the enemies.  Most have somewhat bulbous shapes, those that live on soft sediment may have special adaptations in shell morphology to prevent sinking into the mud. e.g. Murex sp with spines.

           The echinoderms, or spiny-skinned animals have bulky shapes, such as the various sea-urchins, sea cucumbers, and many of the starfish.  They have numerous appendages in the form of sucker feet or spines that are used for locomotion.  The sea urchins live on hard substrates where they feed on debris attached to the rocks (or) they may live on the unconsolidated sediment.  Sand dollars have the poorest mobility but they can move slowly by the whisker-like feet that surround their body.  These animals may be partially in faunal in that they can burry themselves just under the sediment surface for protection.