Major predators of larval acorn barnacles are juvenile fishes and a variety of suspension-feeding invertebrates, including adult barnacles.  Major predators of juvenile barnacles are limpets, whelks, and crabs, and major predators of adults are whelks and sea stars.  Defenses after metamorphosis include a strong outer calcified test and the ability to close up for long periods.

Spat of Chthamalus dalli settled onto the test plates of a
Balanus glandula. The smallest individuals are likely just 1-2d old,
while some older C. dalli can be seen at the 7 o'clock position 8X

A study at Friday Harbor Laboratories, Washington shows that the effect of adult barnacles on consuming larval barnacles depends, in part, on species-specific behaviour of the larvae themselves.  For example, some species prefer to settle onto bare rock, while others favour the lateral shell-plates of the already existing large barnacles.  In some cases where adult Semibalanus cariosus densely populate the rock surfaces, a 65-100% reduction in barnacle settlement can be attributed to larval predation by adults.  Although most years are characterised by low survival of larvae, thus maintaining the bare patches, sometimes in a period of dense settlement of larvae recruitment of barnacles is high because their sheer numbers swamp the filtration ability of the adult Semibalanus.  Navarrete & Wieters 2000 J Exp Mar Biol Ecol 253: 131.

NOTE  this is determined by the presence of nauplius larvae caught up in the cirri and present in the stomach contents of adult S. cariosus

NOTE  these newly recruited juveniles now have to face a gamut of hungry whelks and bulldozing limpets

Five adult barnacles Semibalanus cariosus have created an "oasis" of
settlement for other barnacles (mainly Balanus glandula) on
an otherwise bare rock surface. Note that many Balanus have
settled around the bases and opercular rims of their hosts, and
some even have survived to settle on the opercular plates 0.5X

Studies in Monterey Bay, California on recruitment of barnacles Balanus glandula show a strong negative correlation of recruitment success with midsummer peaks in canopy area of offshore kelp beds Macrocystis pyrifera.  The reason is that juvenile rockfishes Sebastodes spp., known predators of barnacle nauplii and other invertebrate larvae, hang out under the canopy, presumably for their own protection.  In autumn and winter, predation on barnacle and other invertebrate larvae decreases owing to reduced densities of these planktivores.  The authors estimate that predation by the juvenile rockfish reduces recruitment of barnacles to 1/50th of the level that would occur in the predators' absence. Gaines & Roughgarden 1987 Science 235: 479.

NOTE measured as number of fishes seen per minute of SCUBA-diver observation

Shadow-responses, such as from predatory fishes passing overhead, are well known in marine invertebrates such as barnacles, tubeworms, and clams that rely on fast withdrawal into a shell, tube, or burrow to gain protection.  However, withdrawal may incur costs, such as oxygen deprivation or lack of feeding.  Studies in British Columbia show, for example, that barnacles Balanus glandula hide longer in response to a shadow passing overhead the more they have fed prior to the stimulus (see graph on Left). Size appears to have no effect on the hiding response.

Hiding times also decrease in proportion to hunger level (see graph upper Right).  In relation to this, however, the authors find that barnacles living higher on the shore (with less feeding opportunity) do not hide less than do barnacles living lower on the shore, where there is more time to feed.  Finally, what about the relationship between hiding time and time remaining to a receding tide when there is no opportunity to feed?  Would the barnacles exhibit a progressive decrease in hiding time? As seen in the graph on the lower Right, the answer is "yes'. All that remains now is to figure out how the barnacles do it. Dill & Gillett 1991 J Exp Mar Biol Ecol 153: 115.

NOTE  in this study, the time taken to resume fast cirral beating after a shadow is cast upon it

NOTE  here equated to starvation time