title used in an account of west-coast marine invertebrates entitled A Snail's Odyssey
  Predators & defenses
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  Sea stars
  In this section sea stars as predators are considered, while studies on FISHES, LIMPETS, WHELKS, and INSECTS are found in other sections.
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Research study 1

graph showing times that barnacles Balanus glandula remain closed when stimulated by different species of predatorsIf disturbed while feeding, most barnacles species quickly withdraw their cirri and close up their protective opercular plates.  Withdrawal may be a response to shadows passing overhead, to physical contact (animate or inanimate), or to chemical emanations of a predator.  Studies at the Bamfield Marine Sciences Centre, British Columbia reveal that barnacles Balanus glandula are actually able to distinguish between predatory and non-predatory species of sea stars and snails, and withdraw significantly longer when exposed to the predatory types. The trials involve holding a test animal in contact with a barnacle’s opercular plates for 60sec, and recording the length of time the barnacle pulls into its test. 

Results show a significant separation of non-predators, such as the sea star Henricia leviuscula, snail Promartynia (Tegula) pulligo, and brown alga Fucus disticus, from predators, such as sea stars Pycnopodia helianthoides and Leptasterias hexactis, and whelks Nucella ostrina and N. lamellosa, with the former eliciting average withdrawal times of 145sec and the latter, of 356sec (see graph).  Note that the response is not a general one to gastropods or asteroids, but is specific to potential predatory species.  In this regard, note also the significant separation of the sea star P. helianthoides from the whelk N. lamellosa; the former rarely feeds on B. glandula, while the latter regularly feeds on it.  If, by remaining closed the barnacle becomes less chemically “visible” to a predator, then why not close up for a longer period?  The explanation may be that barnacles need to be open to feed, exchange gases, and to eliminate urine and feces, and a longer time would compromise these functions.  With this in mind, the authors note a worthwhile follow-up research project, that is, to test whether higher-level barnacles would remain closed for a shorter time.  As to whether withdrawal actually prevents a barnacle from being eaten, the authors are uncertain, but note that withdrawal photograph of a juvenile sunflower star Pycnopodia helianthoides humped up and digestingis likely to reduce the chance of this happening.  Palmer et al. 1982 Mar Biol 67: 51.

NOTE  before being used in the experiment, all barnacles are subjected to a shadow that causes them to withdraw.  If withdrawal exceeds 10-11sec, the barnacles are discarded.  This acts to “standardise" behaviour (i.e., sets a standard for the propensity of a barnacle to withdraw), and also takes into account the effect of the shadow of test predators as they are held against the opercular plates

NOTE  the horizontal lines indicate statistically homogenous subsets of data.  Thus, the mean withdrawal time for Pycnopodia is significantly different from that for Nucella


Juvenile sunflower star Pycnopodia helianthoides
humped up and perhaps digesting a barnacle 1X