Locomotion at Argyle

    Animals move for various needs involved in feeding, reproduction, and protection.  Because there are no completely pelagic organisms at Argyle, we will concentrate on the challenges of locomotion over a substrate.  The substrate at Argyle varies in its composition, including combinations of boulders, large smooth pebble, coarse pebble, sand, anoxic mud, other mud and clay, dead clam and barnacle shells, and live oyster and barnacle shells.   Locomotion over a substrate is not limited to a two-dimensional surface, as rocks and sediment create structure and permit vertical movement.  In Argyle, we defined substrates by their position on rock surfaces: epifaunal, cryptofaunal, and infaunal.  We then used these designations to survey the diversity and distribution of different organisms.

Three substrates found in Argyle: (1) shell, (2) shell and pebble, (3) pebble.

Different modes of locomotion (or the absence of locomotion) can be identified for animals at Argyle Creek:

    These different modes of locomotion are useful over different spatial and temporal scales: sessile organisms interact with the most restricted area of substrate, while walking and swimming organisms can cover the largest area. 
Lighter color indicates location of an oyster bed.

    Given the high velocity flow environment of Argyle Creek, many animals are sessile. The diversity of organisms that are sessile span five phyla, including sponges, bryozoans, arthropod barnacles, annelid tube worms, and molluscan oysters.  Sessile organisms cement themselves onto substrates such as rocks, or other organisms, as is the case with bryozoans and sponges that can grow over the shells of barnacles and oysters.  The choices that sessile organisms make at the time of settlement are extremely important, because they will only ever interact--in terms of feeding, reproduction, and protection--with their immediate environment, and with water that flows past that particular location.
    Influx of ocean water provides food for filter feeders, along with a means of waste removal, gas exchange and gamete dispersal. Many sessile adults have planktonic larvae, such as barnacle nauplii, which swim using setose appendages and choose, with sensory input, the substratum to which the adult is permanently attached.


    Semi-sessile organisms, like the dark shelled mussel Mytilus pictured to the left, are temporarily attached to a substrate, but have the ability to detach and reattach.  Mussels continually produce and dissolve byssal threads from a specialized gland on the foot, which they use to attach to a hard substrate, sometimes another mussel.  Unlike rocky intertidal mussels, which aggregate attached to rocks, mussels at Argyle were found to form large mats with small rocks tied together by byssal threads, similar to mussels found in bays.  Mats could be lifted intact from the pebble surface, which were held in place by the weight of the mat.
    Anemones can crawl or drop from their substrate, and reattach elsewhere.  Evidence of this comes from our observation that certain sessile organisms typically found on the top of rocks, such as barnacles, occasionally were found inappropriately on the underside of rocks, suggesting that the rock had been turned over.  Small (0.25 cm to 1.0 cm) orange anemones, on the other hand, were nearly always on the underside, suggesting that the anemones had moved consistently in response to rock turning.


     Burrowing organisms include bivalve clams, gastropod snails, annelid worms, and echinoderm brittle stars, which all use different methods to burrow.  Clams burrow using a muscular foot, which is expanded through the application of hydraulic pressure to a fluid filled cavity called a hemocoel.  The burrowing process occurs as follows: the siphons close to prevent sediment from entering the mantle cavity, and the foot probes downward.  Pressure is then increased in the hemocoel, creating a "terminal anchor," or bulge at the tip of the foot.  With the foot wedged into place, retractor muscles then pull the shell downward.  The adductor muscles holding the shell closed then relax, allowing the shell to open and to remain anchored in the sediment.  This burrowing cycle is then repeated.  
    Small, tall-spired gastropods, Bittium eschrichtii, were predominantly found in the sediment of Zone 1, where they burrowed using the muscular foot. Their shells were found downstream from this site, primarily as "shell hash" and as homes adopted by small hermit crabs.  We found small brittle stars (Amphipholis squamata, measuring 1 cm arm tip to arm tip) both cryptofaunally and in the sediment below surface rocks, where arms appear to operate in burrowing, by scraping out a hole while other appendages afford anchorage or propel the animal forward.

 Animals that use creeping locomotion, involving mucous and muscular contractions, include various molluscs (limpets, chitons, gastropod snails, and nudibranchs).  The muscular foot uses hydrostatic pressure of the hemocoel to produce waves of expansion, contraction, and contact between the mucous-coated foot and the substrate.  Animals can move using "retrograde" waves or "direct" waves of contraction, where the wave moves either opposite to or in the same direction as the movement of the animal, respectively.  The two sides of the limpet foot can be controlled independently, so that waves can move in opposite directions on either half of the foot, allowing the animal to turn around without moving forward.

    Crabs walk sideways using their jointed thoracic appendages as levers by both extending, which pushes the body off the substrate, and retracting, which pulls the body in the direction of motion.  Two species of crabs were observed, and these species moved over different spatial scales, related to crab size.  Hemigrapsus (2 cm) were found walking on, between, and under small (2-3 cm) pebbles.  The  larger crabs (Cancer magister), found strictly in zone 4, covered much larger distances, walking over larger parts of the substratum over several meters.
    Swimming was used by sculpin fish, which swam in the water column hovering above the substrate.

 Cancer productus in field
 Cancer productus close-up view