Barnacle responses

27 March 1983 J C G Sturdy

Barnacle behaviour

Barnacles normally show a simple cycle of feeding movements, reaching out with their filtering cirri (4,5,6) to catch the plankton and then retracting to flick the food caught into the mouth parts (cirri 1,2,3 and other parts) and then repeat the cycle.

There are four ways in which the barnacle can modify this feeding cycle:

  1. a version in which the movements are smaller (but at the same frequency) which is used for tentative sampling of the water at the commencement of a session of feeding;
  2. grabbing and chewing up macroscopic food particles reaching the filtering cirri;
  3. retracting into the shell and closing the operculum if ``attacked'';
  4. swinging the filtering cirri sideways when they are extended, in response to a probably chemical stimulus.

This last response is entirely self-contained, in that it occurs on reception of the stimulus whenever mechanically possible, ie when the feeding apparatus is extended. The function of this movement is possibly two-fold: (1) grabbing prey to one side; (2) cleaning the sides of the mouthparts when food builds up on them. These attempts to sweep in food from the side work in conjunction with (but do not cause, except if their mechanical action is successful) the operation of putting a ``large'' food particle into the mouth and chewing it. This response is triggered by touching the surface of the inside of the curve of the filtering cirri; the rest of the feeding cirri are not sensitive. For this process, the opening of the operculum gapes wide, the filtering cirri are held at their furthest extension, and the biting mouthparts are extended upwards much more than normal. At least the last two parts of this action are performed by hydrostatic pressure. The animal then makes repeated attempts to bite the object that it has found.

In the early, tentative stages of feeding, the animal will not produce this response, which appears gradually as the feeding movements increase in size. This build-up to the full feeding pattern occurs from the closed state, which is entered when the animal is disturbed (and on drying out). There are two ways of disturbing the barnacle:

  1. moving some of the side plates relative to each other (this is a severe disturbance, and requires considerable pressure compared with that which anything edible to barnacles can exert);
  2. touching the opercular lips of the mantle: 
                  /-------------
/------------/
|                       -------------------\
    /---
   /                  -----------\      -\
   |            ------            \\      \
   |     /------                    \     |  ####   sensitive
   |    /         ----------------  |     |  #### = region of
   |   |    ------################--|     |  ####   mantle
   |   |============================|     |
   |   |    ------################--|     |
   |    \         ----------------  |     |
   |     \------                    /     |
   |            ------            //      /
   \                  -----------/      -/
    |---
|                       -------------------/
\------------\
              \-------------

This arrangement has presumably evolved on the basis of that anything pushing past the feeding apparatus into the animal's shell is not potential food but a potential predator. If this were all that there was to this response, the operculum would be shut as macroscopic food particles are dragged in by the cirri, as they pass into the shell.

To prevent this from happening, the close-on-touch response is blocked by the eating-macroscopic-food response. As the animal becomes more and more ``certain'' that it has got something to get its teeth into, it becomes progressively more reluctant to close up, eventually not responding at all to touching the mantle. This state can be left by:

  1. the object being eaten
  2. the object escaping
  3. (possibly) relative movement of the side plates

The barnacle does not ``give up'' trying to eat an inedible object of biological origin. Once this behaviour has finished, the normal feeding cycle is resumed, and along with it the close-on-touch response reappears.

There are two ways in which the barnacle can close up for protection:

  1. drawing the opercular plates together, leaving the sensory lips of the mantle protruding through a small gap
  2. pulling the mantle lips in and closing the opercular plates hard, to leave no gap for the lips.

(For either of these, the feeding parts are retracted at the same time; avoidance of trapping them appears to be by timing alone, as, if the cirri are held extended experimentally, the operculum will close on them and trap them. The speed of retraction of cirri and of closing of the operculum appear to be regulated together.) Normally (for the close-on- touch response) the first form of this occurs, with a sensory part left protruding. However, if the stimulus is applied with the animal already in this state (but a fresh stimulus is required; the one that caused this first stage of closure must first be removed) the second form (tight closure) occurs. This is also reached directly if the side plates are moved relative to one another. Leaving tight closure always goes to partial closure, and only from there to tentative feeding (or back to tight closure if disturbed again), and is triggered inherently a while after entering it (rather than through any sensory stimulus); this takes a few minutes.

Habituation does not appear to occur except during a particular cycle of events (eating macroscopic objects) in which the animal becomes harder to derail from this behaviour pattern, as the behaviour continues. Removing the stimulus, however, causes the pattern to be left, and any increased tendency to use this pattern just after leaving it is transient (that is, sensitivity remains long enough for the resumption to be taken as part of the same event.)

The behaviour of the barnacle may be summarized thus:

  1. tends to revert to normal feeding cycle when safe and when there is nothing more profitable to do;
  2. hides in shell during periods of probable danger, not re-emerging until safe;
  3. attempting to optimize use of nutrients in water by (a) extending range of catching in likely direction of food; (b) trying to pick up objects touching cirri;
  4. (3b) and (2) interact in that while the animal is busy catching food, the protection mechanism is suppressed, as it could well receive false ``danger'' stimuli as a result of the feeding actions.

It would appear that the design of the barnacle errs on the side of caution, but bearing in mind the limited sensory input, it can be seen that the barnacle's behaviour is set up very finely to make survival as likely as possible given that particular mechanical construction, whilst making very good use of the food supplies available.

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John C. G. Sturdy
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