Settlement of competent cyprids of goose barnacles Pollicipes polymerus is stimulated by the presence of healthy adult peduncles of the same species.  In the field, juveniles are often abundant on the peduncles or stalks of adults, especially in the upper parts. Lewis 1975 Mar Biol 32: 127; Lewis 1975 Mar Biol 32: 141; see also Hoffman 1988 Pac Sci 42: 154.

Gregarious settlement of Pollicipes polymerus has mixed blessings.  On the one hand, the juveniles are afforded protection from predators, desiccation, and strong wave action but, on the other, their presence as they grow may increase competitive pressure within the clusters.  Examination of goose barnacles along the Washington-state coastline reveals that 80% bear juveniles.  The distribution of juveniles on adults is not random.  Rarely, for example, are they present on the capitulum or shell-plates, possibly because limpets sometimes live there and may bulldoze them off.  On the peduncle or stalk, a few large juveniles, usually less than 5, of 6-7mm rostral-carinal length, live close to the substratum surface.  Proximally, near the capitulum, reside many small juveniles, of 0.5-1mm length. These may number in the low hundreds, possibly attracted there because the cuticle in this region is newest. 

However, the author thinks that in the proximity of the capitulum, these small individuals may be within the cirral “screen” of the adult and, with less food available for growth, may tend to remain small.  The larger, distally sited juveniles are able to move onto the rock substratum from the peduncle by developing “peduncular extensions”.  These are bulb-like expansions, possibly formed by high internal hemocoelic pressures.  The expansions are often bright red owing to the thinness of the cuticle and to the hemoglobin contained within.  They appear to carry ducts from the cement gland.  Although the author notes that the adults act as a conduit for movement of juveniles onto the primary rock substratum, it is not clear whether the transfer actually works in the conveyor-belt fashion as described here, and this may be worth investigating.  Hoffman 1984 Crustaceana 46: 295. Photograph courtesy Gary McDonald, Long Marine Laboratory, Santa Cruz, California.

NOTE  growth of the peduncle occurs mainly at the attachment point of the capitulum.  The freshly deposited cuticle in this region is thin and likely transmits more chemical signals than does older cuticle

NOTE  as in all barnacles, the cement glands remain active throughout life.  The author has even observed detached adults reattaching themselves through secretion of fresh cement from the basal disc

Goose barnacle Pollicipes polymerus with many juveniles
attached to its peduncle. A limpet is crawling on the shell
plates. Note that neither are the smallest individuals
clustering close to the capitulum, nor are all the
largest juveniles nearest the base of the adult 1.6X

Around La Jolla, California goose barnacles Pollicipes polymerus settle year-round, but with a peak in early spring.  As noted in Research Study 1 above the cyprids prefer to settle on the peduncles of adults, and survival is highest there in comparison with other substrata.  The juveniles remain on the adults until they reach about 9mm rostral-carinal length (see drawing) and then move to the rock substratum via peduncular extensions.  Settlement on adults is thought to afford the juveniles a refuge from predation and desiccation.  Hoffman 1989 J Exp Mar Biol Ecol 125: 83. Drawing courtesy DFO Science Stock Status Report C6-O6 1999, Government of Canada.

When barnacle larvae settle, do they settle randomly or do they settle in an aggregated way? This is investigated for 3 barnacle species in Monterey Bay, California, including goose barnacles Pollicipes polymerus, and acorn barnacles Balanus glandula and Chthamalus dalli. The researchers set out settling plates for an 18-mo period and analyse settling patterns every few weeks.

Results show that the spatial array of settling goose barnacles Pollicipes polymerus differs significantly from that of acorn barnacles in that it is strongly aggregated, while that of the acorn-barnacle settlers is more random.  Note in the graphs that the dispersion index for the acorn barnacles is close to 1, indicating randomness, while that for goose barnacles is significantly <1 indicating aggregated settlement. Note also that degree of aggregation of goose-barnacle settlers appears to increase as their density increases.  The data suggest that costs of aggregation, such as intraspecific competition for space, are greater than benefits, such as increased probability of finding mates, for acorn barnacles than for goose barnacles.  While is is known that Pollicipes cyprids preferentially settle onto the peduncles of adults, the attraction of cyprids of Pollicipes when settling in the absence of adults suggests how the characteristic clumps of goose barnacles could form initially.  Satchell & Farrell 1993 Mar Biol 116: 241.

NOTE  dispersion index is obtained by measuring mean nearest-neighbour distanceand dividing that by the expected mean nearest-neighbour distance given random settlement (this is a theoretical number taken from anotherpublication).  Thus, a value near 1 indicates random settlement, a value >1 indicates uniform settlement, and a value <1 indicates aggregated settlement

Settlement of goose barnacles: random, uniform, or aggregated?

Settlement of cyprids of goose barnacles Pollicipes polymerus and acorn barnacles Chthamalus sp. at 5 sites at La Jolla, California located within a few hundred meters of one another are temporally and spatially correlated, suggesting common onshore larval transport events.  The transporting mechanism involves trains of large internal tidal bores with diurnal or semidiurnal periodicity, lasting for several days.  The peaks in settlement during several consecutive days are a result of this periodicity.  Settlement of Pollicipes is more variable than Chthamalus at the different sites, thought by the author to reflect a stronger behavioral component affecting settling in the former barnacle.  Detailed results show that settlement of Pollicipes is periodic during May-June, with a heavy pulse in July.  The upper graph shows settlement patterns at 4 of the sites, with a 5th, more heavily settled site being featured in the lower graph.  A noteworthy aspect of settlement of both types of barnacles is their abrupt cessation from one day to the next, thought by the author to owe to sinking of the thermocline.  The cyprids are massed in the colder water below the thermocline and, as this is pushed onshore by the tidal bores, it tilts upwards, feeding the larvae into surface waters, which then wash onto the shore.  This creates a settlement pulse, which then may decrease abruptly as the cold water sinks and is replaced by warmer, less dense and larval deficient surface water.  Pineda 1994 Mar Ecol Progr Ser 107: 125.

NOTE  only data for goose barnacles Pollicipes are presented here

NOTE because the cyprids of Pollicipes tend to settle on or near the peduncles of adults (see Research Studies 1 & 2 above), and because the author sited the experimental settlement plates some distance away from established adults, there may have been competing settlement stimuli for the larvae