A good introduction to the features and conditions of the high-level habitat of barnacles Balanus glandula and a co-occurring red alga Endocladia muricata on west-coast shores is the dissertation of a researcher from Stanford University on shores around Pacific Grove, California.  One of many points of interest in the study is the fact that 93 sedentary and sessile species, representing 15 major taxa, are present.  Thirty-four relatively common species inhabit a variety of microhabitats including holdfast branchlets and blades of algae, and in around dead tests of barnacles.  Food resources are partitioned by these 34 species as follows: 7 filter-feeders, 10 herbivorous browsers/scrapers, 6 macroalgivores, 5 ominores/scavengers, and 6 carnivores.  The report provides a wealth of data on community composition and seasonal changes in this surprisingly rich upper-level intertidal area.  Glynn 1965 Beaufortia 12 (148): 1.

NOTE at 198 published pages, the article appears to represent most if not all of the author’s PhD thesis, published by the journal as a separate number for the year 1965

High- mid-intertidal region showing acorn
barnacles Balanus glandula and seaweeds,
mostly Endocladia muricata 0.2X

Note the fairly even spacing of individuals in the population of Balanus glandula shown in the photo. The spacing regularity owes to the unique behaviour of the cyprid in its last movements prior to settling.  It moves forwards and backwards as though assessing how much room is available, then settles (see diagram on Right). The strategy is to be close enough to your neighbours to leave as little room as possible for competing animals and plants to settle and grow, yet still leave enough room so that conditions don’t become too crowded later.

NOTE the large individuals in the photo are Balanus glandula, while the smaller darker individuals are Chthamalus dalli. As will be seen in the section on INTERSPECIFIC COMPETITION, non-conspecific settlers generally respond to their own "rules" for spacing - in this case, more or less where they please

The large evenly spaced individuals are Balanus glandula,
while the smaller ones are mostly Chthamalus dalli 1X

When insufficient space is available, individual barnacles crowd one another and compete for space by growing vertically.  This forms typical “hummock” shapes.  Because of the thin shells and weak attachments associated with this type of crowded growth, hummock formation can lead to wholesale mortality through storms and/or battering logs.

NOTE  readers interested in the dynamics of hummock-formation in barnacles, in this case in the Atlantic-coast species Semibalanus balanoides, are directed to a study done by researchers from Brown University, Rhode Island and UC Santa Barbara, California on beaches in Rhode Island. As well as reiterating the negative effects of crowding, the authors point out such advantageous features of hummocking as: 1) more effective feeding, 2) less energy and materials allocated to structural support, 3) buffering from temperature and desiccation stress, and increased reproductive output.  Bertness et al. 1998 Ecology 79 (4): 1382

Balanus glandula showing
etiolated growth through
overcrowding 1X

Gregariousness is a feature of many sessile marine invertebrates, but perhaps none more than what is demonstrated by  barnacles.  What benefit accrues to barnacles from the gregarious settling of their larvae? Think about the options given below, then CLICK HERE for explanations.

By settling in the same area as the adults, they are assured of hospitable environmental conditions in which to live. 

High densities enable cooperative feeding. 

High densities are necessary for reproduction. 

Higher density increases overall defense against predators. 

Many barnacles lead to greater attachment strength. 

Instraspecific competition in barnacles seems to centre around availability of space rather than food.  Under severe conditions of crowding, it would be expected that extra energy would be channeled into production of shell than into production of gametes, because the shells grow excessively large.  To see if this is so, scientists at the West Vancouver Laboratory, British Columbia create variable degrees of crowding by drilling depressions into Plexiglas plates into which barnacles Balanus glandula are allowed to settle during springtime abundance of cyprid larvae.  Densities include “isolated” (1.75cm separation distance), “pairs” (0.75cm separation), “medium crowded” (1458 individuals on a 0.6m² plate, also 0.75cm separation), and “crowded” (7500 individuals on a 0.6m² plate, equivalent to natural densities, and also 0.75cm separation).  At 1.75cm separation distance or less all barnacles are within penis-length distribution of at least one other individual, so all are capable of reproduction. 

Results over 6mo of growth show that there is a trade-off between production of eggs, somatic-tissues, and shell.  Thus, under more crowded conditions, there is an decrease in egg:body tissue ratio (see graph on Left) and an increase in shell:body tissue ratio (see graph on Right).  More energy is channeled into egg production and less into shell growth; the opposite is the case under less crowded conditions.  Wu et al. 1977 Can J Zool 55: 643.

NOTE   in a related study the authors confirm that crowding in experimentally manipulated populations of barnacles Balanus glandula significantly reduces individual food consumption, growth, and reproduction, with production of eggs being most sensitive to the effects of crowding.  Wu 1980 Can J Zool 58: 559.

NOTE  for quickest comprehension of these data, just look at the Nov set of bars for each relationship. Note that no statistical analyses appear to have been done on the dat

Similar results are obtained in a study on effects of crowding on fecundity in barnacles Balanus glandula in Sinclair Inlet and Mukkaw Bay, Washington. Here, uncrowded or "low density" 2yr+ individuals produce slightly larger clutches of eggs than crowded or "high density" ones (see graph on Left). In contrast, 2yr+ Chthamalus dalli produce slightly larger clutches of eggs when in high densities in comparison with when in low densities (graph on Right).  In discussing the different responses of the 2 species to crowding, the author suggests that possession of a membranous base by Chthamalus may allow it to slide away from competitors under crowded conditions, thus maintaining larger relative space for incubation of eggs.  By contrast, Balanus has a calcareous base and is permanently fixed in position on the rock.  Under crowded conditions in Balanus, selection may favour a thicker shell for its crushing resistance, thus reducing the relative space available for housing eggs.  These ideas are speculative, but should be easily testable.  Wethey 1984 Can J Zool 62: 1788.

NOTE  uncrowded conditions are created by scraping barnacles from experimental plots to produce the required spacing (e.g., high density= 3 individuals ^.  cm^-2 and low density= 0.8 individuals ^.  cm^-2

NOTE  the data are highly variable and, though statistically significant for each species, they hardly inspires confidence especially for smaller-sized individuals

The degree of this preferential settlement by Balanus varies with habitat. For example, it is not as common on wave-exposed open coasts, nor is it found in calm-water habitats. It also varies with intertidal height.

Data collected during August provide information on settlement of Balanas glandula and Chthamalus dalli spat onto the following types of microhabitats in August at 3 different tidal heights:  living B. glandula, algae, bare rock, basal-plate scars of B. glandula, and empty tests of B. glandula.

The upper-Left graph shows recruitment data for the low intertidal zone (1m above MLLW). Notable values are highlighted, namely, avoidance of  Chthamalus for living Balanus, and recruitment of Balanus onto basal-plate scars and empty shells of conspecifics. 

The other graphs show comparable data for the mid-intertidal (1.4m above MLLW) and high intertidal (1.8m above MLLW) zones. Note that preference of Balanus for basal-plate scars and test-plates of conspecifics increases from the low to high intertidal zones.  In the high zone, recruitment of Balanus within the shell plates of conspecifics is almost 90 times greater than predicted on the basis of available microhabitat area. As the barnacles grow, competition for space intensifies within the confines of the barnacle-test microhabitats and, later in life, weaker individuals begin to die.  The authors note that the preferential settlement in empty barnacle tests is a trade-off between reduced mortality during the early vulnerable stages, for example, from predatory whelks Nucella spp. and bulldozing limpets, and increased mortality once the available space is filled.  Schubart et al. 1995 J Exp Mar Biol Ecol 186: 143.

NOTE the graphs are read as follows: the X-axis shows the ratio of found versus expected percentages of B. glandula recruitment in a certain microhabitat. For example, if bare rock comprises 3% of the available microhabitat in a study plot and if it is populated by 3% of the total number of settled spat, then the bar on the histogram would read 1.  Thus, a value greater than 1 indicates a preference for a certain microhabitat, while a value less than 1 indicates avoidance

NOTE  experimental removal of the side walls of such enclosures, especially at high intertidal levels, results in higher mortality of the enclosed juveniles than in neighbouring enclosures left intact